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@@ -195,26 +195,28 @@ For all the macros, the first parameter is the name of the class/struct. The `DE
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```mermaid
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flowchart TD
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A["choosing a NLOHMANN_DEFINE_* macro"] --> B{"adding fields to a base class?"}
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A["choosing a NLOHMANN_DEFINE_* macro"] --> B{{"adding fields to a base class?"}}
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B -->|"yes"| C["...DERIVED_TYPE..."]
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B -->|"no"| D["...TYPE..."]
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C --> E{"need access to private members?"}
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C --> E{{"need access to private members?"}}
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D --> E
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E -->|"yes"| F["...INTRUSIVE... (used inside the class)"]
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E -->|"no"| G["...NON_INTRUSIVE... (used in the namespace)"]
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F --> H{"only serializing, never parsing back?"}
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F --> H{{"only serializing, never parsing back?"}}
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G --> H
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H -->|"yes"| I["...ONLY_SERIALIZE"]
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H -->|"no"| J{"allow missing keys when parsing?"}
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H -->|"no"| J{{"allow missing keys when parsing?"}}
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J -->|"yes"| K["...WITH_DEFAULT"]
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J -->|"no"| L["plain (missing keys throw)"]
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I --> M{"need custom JSON key names?"}
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I --> M{{"need custom JSON key names?"}}
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K --> M
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L --> M
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M -->|"yes"| N["...WITH_NAMES"]
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M -->|"no"| O["done"]
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```
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<p class="nj-caption" markdown>Decision tree for choosing a `NLOHMANN_DEFINE_*` macro: answer the questions from top to bottom; each answer picks one part of the macro name, and the parts are combined in this order. The table below lists the resulting macros.</p>
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| Need access to private members | Need only serialization | Allow missing values when de-serializing | macro |
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|------------------------------------------------------------------|------------------------------------------------------------------|------------------------------------------------------------------|--------------------------------------------------------------------------------------------------------------|
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| <div style="color: green;">:octicons-check-circle-fill-24:</div> | <div style="color: red;">:octicons-x-circle-fill-24:</div> | <div style="color: red;">:octicons-x-circle-fill-24:</div> | [**NLOHMANN_DEFINE_TYPE_INTRUSIVE**](../api/macros/nlohmann_define_type_intrusive.md) |
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@@ -224,6 +226,8 @@ flowchart TD
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| <div style="color: red;">:octicons-x-circle-fill-24:</div> | <div style="color: red;">:octicons-x-circle-fill-24:</div> | <div style="color: green;">:octicons-check-circle-fill-24:</div> | [**NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_DEFAULT**](../api/macros/nlohmann_define_type_non_intrusive.md) |
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| <div style="color: red;">:octicons-x-circle-fill-24:</div> | <div style="color: green;">:octicons-check-circle-fill-24:</div> | <div style="color: grey;">:octicons-skip-fill-24:</div> | [**NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE**](../api/macros/nlohmann_define_type_non_intrusive.md) |
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<p class="nj-table-caption" markdown>Macros for classes and structs without a base class. A green check means yes, a red cross means no, and a grey skip sign means the column does not apply, as a macro that only serializes never reads values.</p>
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For _derived_ classes and structs, use the following macros
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| Need access to private members | Need only serialization | Allow missing values when de-serializing | macro |
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@@ -235,12 +239,33 @@ For _derived_ classes and structs, use the following macros
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| <div style="color: red;">:octicons-x-circle-fill-24:</div> | <div style="color: red;">:octicons-x-circle-fill-24:</div> | <div style="color: green;">:octicons-check-circle-fill-24:</div> | [**NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_WITH_DEFAULT**](../api/macros/nlohmann_define_derived_type.md) |
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| <div style="color: red;">:octicons-x-circle-fill-24:</div> | <div style="color: green;">:octicons-check-circle-fill-24:</div> | <div style="color: grey;">:octicons-skip-fill-24:</div> | [**NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE**](../api/macros/nlohmann_define_derived_type.md) |
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<p class="nj-table-caption" markdown>Macros for derived classes and structs, read as Table 1.</p>
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!!! info "Implementation limits"
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- The current macro implementations are limited to at most 63 member variables. If you want to serialize/deserialize
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types with more than 63 member variables, you need to define the `to_json`/`from_json` functions manually.
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- For the `WITH_NAMES` variants the limit is halved to 31 member variables.
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To pick the macro for your own type and get code to start from, answer the questions below.
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<div class="nj-macro-chooser" data-variant="full" markdown>
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<div class="nj-mc-fallback" markdown>
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- Members are private: use an `INTRUSIVE` macro inside the class ([reference](../api/macros/nlohmann_define_type_intrusive.md){ data-nj-mc="intrusive" }).
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- All members are public, or you cannot edit the class: use a `NON_INTRUSIVE` macro in the class's namespace ([reference](../api/macros/nlohmann_define_type_non_intrusive.md){ data-nj-mc="non_intrusive" }).
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- The class adds members to a serializable base class: use a `DERIVED_TYPE` macro with the base class as second argument ([reference](../api/macros/nlohmann_define_derived_type.md){ data-nj-mc="derived" }).
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- Missing keys should keep their default values: append `_WITH_DEFAULT`; otherwise a missing key throws.
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- You only convert to JSON: append `_ONLY_SERIALIZE` instead.
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- JSON keys differ from the member names: append `_WITH_NAMES` and write `"key", member` pairs ([reference](../api/macros/nlohmann_define_type_with_names.md){ data-nj-mc="with_names" }).
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- Private members of a class you cannot edit: no macro works; write the conversion by hand ([third-party types](#how-do-i-convert-third-party-types){ data-nj-mc="third_party" }).
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- The type is not default constructible and is converted in both directions: no macro works; specialize `adl_serializer` with a `from_json` that returns the value, or append `_ONLY_SERIALIZE` if you only convert to JSON ([non-default-constructible types](#how-can-i-use-get-for-non-default-constructiblenon-copyable-types){ data-nj-mc="non_default" }).
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</div>
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</div>
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<p class="nj-caption" markdown>Macro chooser: each answer selects one part of the macro name; the generated program defines the type, calls the macro and converts a value to JSON and back. Combinations that no macro supports show the alternative instead of code.</p>
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??? example "Example: using the `NLOHMANN_DEFINE_TYPE_*` macros"
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The `to_json`/`from_json` functions for the `person` struct above can be created with:
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@@ -321,7 +346,7 @@ For _derived_ classes and structs, use the following macros
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The `NLOHMANN_DEFINE_TYPE_*`/`NLOHMANN_DEFINE_DERIVED_TYPE_*` macros are generic over any `basic_json`
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specialization, including `nlohmann::ordered_json`. Simply use `ordered_json` as the target type and members
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are serialized in declaration order -- no separate macro or extra code is needed.
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are serialized in declaration order — no separate macro or extra code is needed.
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```cpp
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namespace ns {
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@@ -354,7 +379,7 @@ For _derived_ classes and structs, use the following macros
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There is currently no `NLOHMANN_DEFINE_TYPE_*`-style macro for types that are not
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[DefaultConstructible](https://en.cppreference.com/w/cpp/named_req/DefaultConstructible). This is not an
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intentional omission of documentation -- no such macro exists yet; see
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intentional omission of documentation — no such macro exists yet; see
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[How can I use `get()` for non-default constructible/non-copyable types?](#how-can-i-use-get-for-non-default-constructiblenon-copyable-types)
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for the manual pattern to use instead.
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@@ -370,6 +395,8 @@ flowchart LR
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B -->|"user specialization replaces the default"| E["user's adl_serializer specialization for T"]
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```
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<p class="nj-caption" markdown>How a conversion between `json` and a type `T` is dispatched: the default `adl_serializer` calls the free `to_json`/`from_json` functions found by argument-dependent lookup, while a user specialization of `adl_serializer` replaces this path.</p>
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The library uses **JSON Serializers** to convert types to JSON.
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The default serializer for `nlohmann::json` is `nlohmann::adl_serializer` (ADL means [Argument-Dependent Lookup](https://en.cppreference.com/w/cpp/language/adl)).
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@@ -563,7 +590,7 @@ For smart pointers, class hierarchies, `std::variant`, and `std::chrono`, see
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## Why can't I convert to/from `std::any`?
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`std::any` is intentionally excluded from `get<T>()`/generic conversion support, so `get<std::any>()` and
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containers like `std::map<std::string, std::any>` fail to compile by design -- there is no way to know, from a
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containers like `std::map<std::string, std::any>` fail to compile by design — there is no way to know, from a
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`json` value alone, which concrete type to store inside the `std::any`. To work with heterogeneous JSON values,
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dispatch on the value's type manually and construct the `std::any` (or extract from it) yourself:
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@@ -595,7 +622,7 @@ what the library does instead.
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## Why does `std::wstring` convert or dump incorrectly?
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The library assumes UTF-8 encoding internally, so `std::wstring` is not supported out of the box -- see the FAQ
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The library assumes UTF-8 encoding internally, so `std::wstring` is not supported out of the box — see the FAQ
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entry on [wide string handling](../home/faq.md#wide-string-handling) for why, and for a UTF-8 conversion recipe.
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## Can I write my own serializer? (Advanced use)
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@@ -661,3 +688,12 @@ struct bad_serializer
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- [Specializing enum conversion](enum_conversion.md) - map enums to JSON strings instead of integers
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- [Supported macros](macros.md) - reference for `NLOHMANN_DEFINE_TYPE_*` and related macros
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- [`adl_serializer`](../api/adl_serializer/index.md) - the default `JSONSerializer` used in the conversion dispatch
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- [`NLOHMANN_DEFINE_TYPE_INTRUSIVE`](../api/macros/nlohmann_define_type_intrusive.md) - generate the conversion functions inside the type
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- [`NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE`](../api/macros/nlohmann_define_type_non_intrusive.md) - generate the conversion functions outside the type
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- [`NLOHMANN_DEFINE_TYPE_WITH_NAMES`](../api/macros/nlohmann_define_type_with_names.md) - use JSON keys that differ from the member names
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- [`NLOHMANN_DEFINE_DERIVED_TYPE`](../api/macros/nlohmann_define_derived_type.md) - generate the conversion functions for derived classes
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- [`NLOHMANN_JSON_SERIALIZE_ENUM`](../api/macros/nlohmann_json_serialize_enum.md) - map enum values to JSON values
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- [`get`](../api/basic_json/get.md) - convert a JSON value to a C++ type
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- [`get_to`](../api/basic_json/get_to.md) - convert a JSON value into an existing variable
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- [`json_serializer`](../api/basic_json/json_serializer.md) - the `JSONSerializer` template parameter
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@@ -1,4 +1,4 @@
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<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/arbitrary_types/" rel="canonical"><link href=../ rel=prev><link href=../type_recipes/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>Arbitrary Type Conversions - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.ab4e12ef.min.css><link rel="stylesheet" href="../../assets/external/fonts.googleapis.com/css.61a430c9.css"><style>:root{--md-text-font:"Roboto";--md-code-font:"JetBrains Mono"}</style><link rel=stylesheet href=../../css/custom.css><script>__md_scope=new URL("../..",location),__md_hash=e=>[...e].reduce(((e,_)=>(e<<5)-e+_.charCodeAt(0)),0),__md_get=(e,_=localStorage,t=__md_scope)=>JSON.parse(_.getItem(t.pathname+"."+e)),__md_set=(e,_,t=localStorage,a=__md_scope)=>{try{t.setItem(a.pathname+"."+e,JSON.stringify(_))}catch(e){}}</script></head> <body dir=ltr data-md-color-scheme=default data-md-color-primary=indigo data-md-color-accent=indigo> <input class=md-toggle data-md-toggle=drawer type=checkbox id=__drawer autocomplete=off> <input class=md-toggle data-md-toggle=search type=checkbox id=__search autocomplete=off> <label class=md-overlay for=__drawer></label> <div data-md-component=skip> <a href=#arbitrary-type-conversions class=md-skip> Skip to content </a> </div> <div data-md-component=announce> </div> <header class=md-header data-md-component=header> <nav class="md-header__inner md-grid" aria-label=Header> <a href=../.. title="JSON for Modern C++" class="md-header__button md-logo" aria-label="JSON for Modern C++" data-md-component=logo> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 8a3 3 0 0 0 3-3 3 3 0 0 0-3-3 3 3 0 0 0-3 3 3 3 0 0 0 3 3m0 3.54C9.64 9.35 6.5 8 3 8v11c3.5 0 6.64 1.35 9 3.54 2.36-2.19 5.5-3.54 9-3.54V8c-3.5 0-6.64 1.35-9 3.54"/></svg> </a> <label class="md-header__button md-icon" for=__drawer> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M3 6h18v2H3zm0 5h18v2H3zm0 5h18v2H3z"/></svg> </label> <div class=md-header__title data-md-component=header-title> <div class=md-header__ellipsis> <div class=md-header__topic> <span class=md-ellipsis> JSON for Modern C++ </span> </div> <div class=md-header__topic data-md-component=header-topic> <span class=md-ellipsis> Arbitrary Type Conversions </span> </div> </div> </div> <form class=md-header__option data-md-component=palette> <input class=md-option data-md-color-media="(prefers-color-scheme: light)" data-md-color-scheme=default data-md-color-primary=indigo data-md-color-accent=indigo 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6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> </form> <script>var palette=__md_get("__palette");if(palette&&palette.color){if("(prefers-color-scheme)"===palette.color.media){var media=matchMedia("(prefers-color-scheme: light)"),input=document.querySelector(media.matches?"[data-md-color-media='(prefers-color-scheme: light)']":"[data-md-color-media='(prefers-color-scheme: dark)']");palette.color.media=input.getAttribute("data-md-color-media"),palette.color.scheme=input.getAttribute("data-md-color-scheme"),palette.color.primary=input.getAttribute("data-md-color-primary"),palette.color.accent=input.getAttribute("data-md-color-accent")}for(var[key,value]of Object.entries(palette.color))document.body.setAttribute("data-md-color-"+key,value)}</script> <label class="md-header__button md-icon" for=__search> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M9.5 3A6.5 6.5 0 0 1 16 9.5c0 1.61-.59 3.09-1.56 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<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=description content="JSON for Modern C++ is a header-only C++11 library that makes JSON a first-class data type."><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/arbitrary_types/" rel="canonical"><link href=../ rel=prev><link href=../type_recipes/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>Arbitrary Type Conversions - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.ab4e12ef.min.css><link rel=stylesheet href=../../css/custom.css><link rel=stylesheet href=../../css/brand.css><link rel=stylesheet href=../../css/fig-sax.css><link rel=stylesheet href=../../css/fig-jsonview.css><link rel=stylesheet href=../../css/fig-exceptions.css><link rel=stylesheet href=../../css/fig-pointer.css><link rel=stylesheet href=../../css/fig-charts.css><link rel=stylesheet href=../../css/fig-integration.css><link rel=stylesheet href=../../css/fig-macros.css><script>__md_scope=new URL("../..",location),__md_hash=e=>[...e].reduce(((e,_)=>(e<<5)-e+_.charCodeAt(0)),0),__md_get=(e,_=localStorage,t=__md_scope)=>JSON.parse(_.getItem(t.pathname+"."+e)),__md_set=(e,_,t=localStorage,a=__md_scope)=>{try{t.setItem(a.pathname+"."+e,JSON.stringify(_))}catch(e){}}</script></head> <body dir=ltr data-md-color-scheme=default data-md-color-primary=custom data-md-color-accent=custom> <input class=md-toggle data-md-toggle=drawer type=checkbox id=__drawer autocomplete=off> <input class=md-toggle data-md-toggle=search type=checkbox id=__search autocomplete=off> <label class=md-overlay for=__drawer></label> <div data-md-component=skip> <a href=#arbitrary-type-conversions class=md-skip> Skip to content </a> </div> <div data-md-component=announce> </div> <header class=md-header data-md-component=header> <nav class="md-header__inner md-grid" aria-label=Header> <a href=../.. title="JSON for Modern C++" class="md-header__button md-logo" aria-label="JSON for Modern C++" data-md-component=logo> <img src=../../images/wordmark-header.svg alt=logo> </a> <label class="md-header__button md-icon" for=__drawer> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M3 6h18v2H3zm0 5h18v2H3zm0 5h18v2H3z"/></svg> </label> <div class=md-header__title data-md-component=header-title> <div class=md-header__ellipsis> <div class=md-header__topic> <span class=md-ellipsis> JSON for Modern C++ </span> </div> <div class=md-header__topic data-md-component=header-topic> <span class=md-ellipsis> Arbitrary Type Conversions </span> </div> </div> </div> <form class=md-header__option data-md-component=palette> <input class=md-option data-md-color-media="(prefers-color-scheme: light)" data-md-color-scheme=default data-md-color-primary=custom data-md-color-accent=custom aria-label="Switch to dark mode" type=radio name=__palette id=__palette_0> <label class="md-header__button md-icon" title="Switch to dark mode" for=__palette_1 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 8a4 4 0 0 0-4 4 4 4 0 0 0 4 4 4 4 0 0 0 4-4 4 4 0 0 0-4-4m0 10a6 6 0 0 1-6-6 6 6 0 0 1 6-6 6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> <input class=md-option data-md-color-media="(prefers-color-scheme: dark)" data-md-color-scheme=slate data-md-color-primary=custom data-md-color-accent=custom aria-label="Switch to light mode" type=radio name=__palette id=__palette_1> <label class="md-header__button md-icon" title="Switch to light mode" for=__palette_0 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 18c-.89 0-1.74-.2-2.5-.55C11.56 16.5 13 14.42 13 12s-1.44-4.5-3.5-5.45C10.26 6.2 11.11 6 12 6a6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> </form> <script>var palette=__md_get("__palette");if(palette&&palette.color){if("(prefers-color-scheme)"===palette.color.media){var media=matchMedia("(prefers-color-scheme: light)"),input=document.querySelector(media.matches?"[data-md-color-media='(prefers-color-scheme: light)']":"[data-md-color-media='(prefers-color-scheme: dark)']");palette.color.media=input.getAttribute("data-md-color-media"),palette.color.scheme=input.getAttribute("data-md-color-scheme"),palette.color.primary=input.getAttribute("data-md-color-primary"),palette.color.accent=input.getAttribute("data-md-color-accent")}for(var[key,value]of Object.entries(palette.color))document.body.setAttribute("data-md-color-"+key,value)}</script> <label class="md-header__button md-icon" for=__search> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M9.5 3A6.5 6.5 0 0 1 16 9.5c0Line truncated
|
||||
<span class=w> </span><span class=c1>// a simple struct to model a person</span>
|
||||
<span class=w> </span><span class=k>struct</span><span class=w> </span><span class=nc>person</span><span class=w> </span><span class=p>{</span>
|
||||
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>string</span><span class=w> </span><span class=n>name</span><span class=p>;</span>
|
||||
@@ -122,24 +122,24 @@
|
||||
<span class=p>}</span>
|
||||
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code>Ned Flanders (60) lives in 744 Evergreen Terrace
|
||||
</code></pre></div> </details> <h2 id=simplify-your-life-with-macros>Simplify your life with macros<a class=headerlink href=#simplify-your-life-with-macros title="Permanent link">¶</a></h2> <p>If you just want to serialize/deserialize some structs, the <code>to_json</code>/<code>from_json</code> functions can be a lot of boilerplate.</p> <p>There are several macros to make your life easier as long as you want to use a <abbr title="JavaScript Object Notation">JSON</abbr> object as serialization. The macros are following the naming pattern, and you can choose the macro based on the needed features:</p> <ul> <li>All the macros start with <code>NLOHMANN_DEFINE</code>.</li> <li>If you want a macro for the derived object, use the <a href=../../api/macros/nlohmann_define_derived_type/ ><code>DERIVED_TYPE</code></a> variant, otherwise use <code>TYPE</code>.<ul> <li>The <code>DERIVED_TYPE</code> variant requires an additional parameter of a base type, which should have the <code>to_json</code>/<code>from_json</code> functions defined. For instance, with a macro of its own.</li> </ul> </li> <li>If you need access to the private fields use <a href=../../api/macros/nlohmann_define_type_intrusive/ ><code>INTRUSIVE</code></a> variant, otherwise use <a href=../../api/macros/nlohmann_define_type_non_intrusive/ ><code>NON_INTRUSIVE</code></a>.<ul> <li>The <code>INTRUSIVE</code> macro should be defined <strong>inside</strong> the target class/struct, <code>NON_INTRUSIVE</code> should be defined within the same namespace.</li> </ul> </li> <li>If you want to deserialize the incomplete JSONs, use the <code>WITH_DEFAULTS</code> variant, which will use the default values for the member variables absent in <abbr title="JavaScript Object Notation">JSON</abbr>, the variant without <code>WITH_DEFAULTS</code> will raise an exception.</li> <li>If you do not need deserialization at all and only interested in <code>to_json</code> function, you can use the <code>ONLY_SERIALIZE</code> variant.</li> <li>If you want to use the custom <abbr title="JavaScript Object Notation">JSON</abbr> names for member variables, use <a href=../../api/macros/nlohmann_define_type_with_names/ ><code>WITH_NAMES</code></a> variant, otherwise the <abbr title="JavaScript Object Notation">JSON</abbr> name of the variable will be the same as its regular name. </li> </ul> <p>For all the macros, the first parameter is the name of the class/struct. The <code>DERIVED_TYPE</code> macros require a second parameter of a base class. All the remaining parameters name the member variables. The <code>WITH_NAMES</code> macros require a <abbr title="JavaScript Object Notation">JSON</abbr> name before each of the variables.</p> <pre class=mermaid><code>flowchart TD
|
||||
A["choosing a NLOHMANN_DEFINE_* macro"] --> B{"adding fields to a base class?"}
|
||||
A["choosing a NLOHMANN_DEFINE_* macro"] --> B{{"adding fields to a base class?"}}
|
||||
B -->|"yes"| C["...DERIVED_TYPE..."]
|
||||
B -->|"no"| D["...TYPE..."]
|
||||
C --> E{"need access to private members?"}
|
||||
C --> E{{"need access to private members?"}}
|
||||
D --> E
|
||||
E -->|"yes"| F["...INTRUSIVE... (used inside the class)"]
|
||||
E -->|"no"| G["...NON_INTRUSIVE... (used in the namespace)"]
|
||||
F --> H{"only serializing, never parsing back?"}
|
||||
F --> H{{"only serializing, never parsing back?"}}
|
||||
G --> H
|
||||
H -->|"yes"| I["...ONLY_SERIALIZE"]
|
||||
H -->|"no"| J{"allow missing keys when parsing?"}
|
||||
H -->|"no"| J{{"allow missing keys when parsing?"}}
|
||||
J -->|"yes"| K["...WITH_DEFAULT"]
|
||||
J -->|"no"| L["plain (missing keys throw)"]
|
||||
I --> M{"need custom JSON key names?"}
|
||||
I --> M{{"need custom JSON key names?"}}
|
||||
K --> M
|
||||
L --> M
|
||||
M -->|"yes"| N["...WITH_NAMES"]
|
||||
M -->|"no"| O["done"]</code></pre> <table> <thead> <tr> <th>Need access to private members</th> <th>Need only serialization</th> <th>Allow missing values when de-serializing</th> <th>macro</th> </tr> </thead> <tbody> <tr> <td><div style="color: green;"><span class=twemoji><svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M1 12C1 5.925 5.925 1 12 1s11 4.925 11 11-4.925 11-11 11S1 18.075 1 12m16.28-2.72a.75.75 0 0 0-.018-1.042.75.75 0 0 0-1.042-.018l-5.97 5.97-2.47-2.47a.75.75 0 0 0-1.042.018.75.75 0 0 0-.018 1.042l3 3a.75.75 0 0 0 1.06 0Z"/></svg></span></div></td> <td><div style="color: red;"><span class=twemoji><svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M1 12C1 5.925 5.925 1 12 1s11 4.925 11 11-4.925 11-11 11S1 18.075 1 12m8.036-4.024a.75.75 0 0 0-1.042.018.75.75 0 0 0-.018 1.042L10.939 12l-2.963 2.963a.749.749 0 0 0 .326 1.275.75.75 0 0 0 .734-.215L12 13.06l2.963 2.964a.75.75 0 0 0 1.061-1.06L13.061 12l2.963-2.964a.749.749 0 0 0-.326-1.275.75.75 0 0 0-.734.215L12 10.939Z"/></svg></span></div></td> <td><div style="color: red;"><span class=twemoji><svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M1 12C1 5.925 5.925 1 12 1s11 4.925 11 11-4.925 11-11 11S1 18.075 1 12m8.036-4.024a.75.75 0 0 0-1.042.018.75.75 0 0 0-.018 1.042L10.939 12l-2.963 2.963a.749.749 0 0 0 .326 1.275.75.75 0 0 0 .734-.215L12 13.06l2.963 2.964a.75.75 0 0 0 1.061-1.06L13.061 12l2.963-2.964a.749.749 0 0 0-.326-1.275.75.75 0 0 0-.734.215L12 10.939Z"/></svg></span></div></td> <td><a href=../../api/macros/nlohmann_define_type_intrusive/ ><strong>NLOHMANN_DEFINE_TYPE_INTRUSIVE</strong></a></td> </tr> <tr> <td><div style="color: green;"><span class=twemoji><svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M1 12C1 5.925 5.925 1 12 1s11 4.925 11 11-4.925 11-11 11S1 18.075 1 12m16.28-2.72a.75.75 0 0 0-.018-1.042.75.75 0 0 0-1.042-.018l-5.97 5.97-2.47-2.47a.75.75 0 0 0-1.042.018.75.75 0 0 0-.018 1.042l3 3a.75.75 0 0 0 1.06 0Z"/></svg></span></div></td> <td><div style="color: red;"><span class=twemoji><svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M1 12C1 5.925 5.925 1 12 1s11 4.925 11 11-4.925 11-11 11S1 18.075 1 12m8.036-4.024a.75.75 0 0 0-1.042.018.75.75 0 0 0-.018 1.042L10.939 12l-2.963 2.963a.749.749 0 0 0 .326 1.275.75.75 0 0 0 .734-.215L12 13.06l2.963 2.964a.75.75 0 0 0 1.061-1.06L13.061 12l2.963-2.964a.749.749 0 0 0-.326-1.275.75.75 0 0 0-.734.215L12 10.939Z"/></svg></span></div></td> <td><div style="color: green;"><span class=twemoji><svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M1 12C1 5.925 5.925 1 12 1s11 4.925 11 11-4.925 11-11 11S1 18.075 1 12m16.28-2.72a.75.75 0 0 0-.018-1.042.75.75 0 0 0-1.042-.018l-5.97 5.97-2.47-2.47a.75.75 0 0 0-1.042.018.75.75 0 0 0-.018 1.042l3 3a.75.75 0 0 0 1.06 0Z"/></svg></span></div></td> <td><a href=../../api/macros/nlohmann_define_type_intrusive/ ><strong>NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_DEFAULT</strong></a></td> </tr> <tr> <td><div style="color: green;"><span class=twemoji><svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M1 12C1 5.925 5.925 1 12 1s11 4.925 11 11-4.925 11-11 11S1 18.075 1 12m16.28-2.72a.75.75 0 0 0-.018-1.042.75.75 0 0 0-1.042-.018l-5.97 5.97-2.47-2.47a.75.75 0 0 0-1.042.018.75.75 0 0 0-.018 1.042l3 3a.75.75 0 0 0 1.06 0Z"/></svg></span></div></td> <td><div style="color: green;"><span class=twemoji><svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M1 12C1 5.925 5.925 1 12 1s11 4.925 11 11-4.925 11-11 11S1 18.075 1 12m16.28-2.72a.75.75 0 0 0-.018-1.042.75.75 0 0 0-1.042-.018l-5.97 5.97-2.47-2.47a.75.75 0 0 0-1.042.018.75.75 0 0 0-.018 1.042l3 3a.75.75 0 0 0 1.06 0Z"/></svg></span></div></td> <td><div style="color: grey;"><span class=twemoji><svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M1 12C1 5.925 5.925 1 12 1s11 4.925 11 11-4.925 11-11 11S1 18.075 1 12m16.333-4.167a.825.825 0 0 0-1.166-1.166l-9.5 9.5a.825.825 0 0 0 1.166 1.166Z"/></svg></span></div></td> <td><a href=../../api/macros/nlohmann_define_type_intrusive/ ><strong>NLOHMANN_DEFINE_TYPE_INTRUSIVE_ONLY_SERIALIZE</strong></a></td> </tr> <tr> <td><div style="color: red;"><span class=twemoji><svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M1 12C1 5.925 5.925 1 12 1s11 4.925 11 11-4.925 11-11 11S1 18.075 1 12m8.036-4.024a.75.75 0 0 0-1.042.018.75.75 0 0 0-.018 1.042L10.939 12l-2.963 2.963a.749.749 0 0 0 .326 1.275.75.75 0 0 0 .734-.215L12 13.06l2.963 2.964a.75.75 0 0 0 1.061-1.06L13.061 12l2.963-2.964a.749.749 0 0 0-.326-1.275.75.75 0 0 0-.734.215L12 10.939Z"/></svg></span></div></td> <td><div style="color: red;"><span class=twemoji><svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M1 12C1 5.925 5.925 1 12 1s11 4.925 11 11-4.925 11-11 11S1 18.075 1 12m8.036-4.024a.75.75 0 0 0-1.042.018.75.75 0 0 0-.018 1.042L10.939 12l-2.963 2.963a.749.749 0 0 0 .326 1.275.75.75 0 0 0 .734-.215L12 13.06l2.963 2.964a.75.75 0 0 0 1.061-1.Line truncated
|
||||
M -->|"no"| O["done"]</code></pre> <p class=nj-caption>Decision tree for choosing a <code>NLOHMANN_DEFINE_*</code> macro: answer the questions from top to bottom; each answer picks one part of the macro name, and the parts are combined in this order. The table below lists the resulting macros.</p> <table> <thead> <tr> <th>Need access to private members</th> <th>Need only serialization</th> <th>Allow missing values when de-serializing</th> <th>macro</th> </tr> </thead> <tbody> <tr> <td><div style="color: green;"><span class=twemoji><svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M1 12C1 5.925 5.925 1 12 1s11 4.925 11 11-4.925 11-11 11S1 18.075 1 12m16.28-2.72a.75.75 0 0 0-.018-1.042.75.75 0 0 0-1.042-.018l-5.97 5.97-2.47-2.47a.75.75 0 0 0-1.042.018.75.75 0 0 0-.018 1.042l3 3a.75.75 0 0 0 1.06 0Z"/></svg></span></div></td> <td><div style="color: red;"><span class=twemoji><svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M1 12C1 5.925 5.925 1 12 1s11 4.925 11 11-4.925 11-11 11S1 18.075 1 12m8.036-4.024a.75.75 0 0 0-1.042.018.75.75 0 0 0-.018 1.042L10.939 12l-2.963 2.963a.749.749 0 0 0 .326 1.275.75.75 0 0 0 .734-.215L12 13.06l2.963 2.964a.75.75 0 0 0 1.061-1.06L13.061 12l2.963-2.964a.749.749 0 0 0-.326-1.275.75.75 0 0 0-.734.215L12 10.939Z"/></svg></span></div></td> <td><div style="color: red;"><span class=twemoji><svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M1 12C1 5.925 5.925 1 12 1s11 4.925 11 11-4.925 11-11 11S1 18.075 1 12m8.036-4.024a.75.75 0 0 0-1.042.018.75.75 0 0 0-.018 1.042L10.939 12l-2.963 2.963a.749.749 0 0 0 .326 1.275.75.75 0 0 0 .734-.215L12 13.06l2.963 2.964a.75.75 0 0 0 1.061-1.06L13.061 12l2.963-2.964a.749.749 0 0 0-.326-1.275.75.75 0 0 0-.734.215L12 10.939Z"/></svg></span></div></td> <td><a href=../../api/macros/nlohmann_define_type_intrusive/ ><strong>NLOHMANN_DEFINE_TYPE_INTRUSIVE</strong></a></td> </tr> <tr> <td><div style="color: green;"><span class=twemoji><svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M1 12C1 5.925 5.925 1 12 1s11 4.925 11 11-4.925 11-11 11S1 18.075 1 12m16.28-2.72a.75.75 0 0 0-.018-1.042.75.75 0 0 0-1.042-.018l-5.97 5.97-2.47-2.47a.75.75 0 0 0-1.042.018.75.75 0 0 0-.018 1.042l3 3a.75.75 0 0 0 1.06 0Z"/></svg></span></div></td> <td><div style="color: red;"><span class=twemoji><svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M1 12C1 5.925 5.925 1 12 1s11 4.925 11 11-4.925 11-11 11S1 18.075 1 12m8.036-4.024a.75.75 0 0 0-1.042.018.75.75 0 0 0-.018 1.042L10.939 12l-2.963 2.963a.749.749 0 0 0 .326 1.275.75.75 0 0 0 .734-.215L12 13.06l2.963 2.964a.75.75 0 0 0 1.061-1.06L13.061 12l2.963-2.964a.749.749 0 0 0-.326-1.275.75.75 0 0 0-.734.215L12 10.939Z"/></svg></span></div></td> <td><div style="color: green;"><span class=twemoji><svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M1 12C1 5.925 5.925 1 12 1s11 4.925 11 11-4.925 11-11 11S1 18.075 1 12m16.28-2.72a.75.75 0 0 0-.018-1.042.75.75 0 0 0-1.042-.018l-5.97 5.97-2.47-2.47a.75.75 0 0 0-1.042.018.75.75 0 0 0-.018 1.042l3 3a.75.75 0 0 0 1.06 0Z"/></svg></span></div></td> <td><a href=../../api/macros/nlohmann_define_type_intrusive/ ><strong>NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_DEFAULT</strong></a></td> </tr> <tr> <td><div style="color: green;"><span class=twemoji><svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M1 12C1 5.925 5.925 1 12 1s11 4.925 11 11-4.925 11-11 11S1 18.075 1 12m16.28-2.72a.75.75 0 0 0-.018-1.042.75.75 0 0 0-1.042-.018l-5.97 5.97-2.47-2.47a.75.75 0 0 0-1.042.018.75.75 0 0 0-.018 1.042l3 3a.75.75 0 0 0 1.06 0Z"/></svg></span></div></td> <td><div style="color: green;"><span class=twemoji><svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M1 12C1 5.925 5.925 1 12 1s11 4.925 11 11-4.925 11-11 11S1 18.075 1 12m16.28-2.72a.75.75 0 0 0-.018-1.042.75.75 0 0 0-1.042-.018l-5.97 5.97-2.47-2.47a.75.75 0 0 0-1.042.018.75.75 0 0 0-.018 1.042l3 3a.75.75 0 0 0 1.06 0Z"/></svg></span></div></td> <td><div style="color: grey;"><span class=twemoji><svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M1 12C1 5.925 5.925 1 12 1s11 4.925 11 11-4.925 11-11 11S1 18.075 1 12m16.333-4.167a.825.825 0 0 0-1.166-1.166l-9.5 9.5a.825.825 0 0 0 1.166 1.166Z"/></svg></span></div></td> <td><a href=../../api/macros/nlohmann_define_type_intrusive/ ><strong>NLOHMANN_DEFINE_TYPE_INTRUSIVE_ONLY_SERIALIZE</strong></a></td> </tr> <tr> <td><div style="color: red;"><span class=twemoji><svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M1 12C1 5.925 5.925 1 12 1s11 4.925 11 11-4.925 11-11 11S1 18.075 1 12m8.036-4.024a.75.75 0 0 0-1.042.018.75.75 0 0 0-.018 1.042L10.939 12l-2.963 2.963a.749.749 0 0 0 .326 1.275.75.75 0 0 0 .734-.215L12 13.06l2.963 2.964a.75.75 0 0 0 1.061-1.06L13.061 12l2.963-2.964a.749.749 0 0 0-.326-1.275.75.75 0 0 0-.734.215L12 10.939Z"/></svg></span></div></td> <td><div style="color: red;"><span class=twemoji><svg xmlns=http://www.w3.org/2000/svg viewboxLine truncated
|
||||
<span class=w> </span><span class=n>NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE</span><span class=p>(</span><span class=n>person</span><span class=p>,</span><span class=w> </span><span class=n>name</span><span class=p>,</span><span class=w> </span><span class=n>address</span><span class=p>,</span><span class=w> </span><span class=n>age</span><span class=p>)</span>
|
||||
<span class=p>}</span>
|
||||
</code></pre></div> <p>If you want to inherit the <code>person</code> struct and add a field to it, it can be done with:</p> <div class=highlight><pre><span></span><code><span class=k>namespace</span><span class=w> </span><span class=nn>ns</span><span class=w> </span><span class=p>{</span>
|
||||
@@ -173,7 +173,7 @@
|
||||
<span class=w> </span><span class=s>"postcode"</span><span class=p>,</span><span class=w> </span><span class=n>m_postcode</span><span class=p>)</span>
|
||||
<span class=w> </span><span class=p>};</span>
|
||||
<span class=p>}</span>
|
||||
</code></pre></div> </details> <div class="admonition warning"> <p class=admonition-title>Overriding conversions for natively-supported types</p> <p>The library already provides built-in <code>to_json</code>/<code>from_json</code> conversions for <abbr title="Standard Template Library">STL</abbr> containers such as <code>std::vector</code>, <code>std::array</code>, and <code>std::map</code>. Defining your own free-function <code>to_json</code>/<code>from_json</code> overload for one of these container types directly (instead of for your own type) can conflict with the built-in overload during overload resolution, producing compiler errors ("no matching overloaded function", "call is ambiguous") that vary by compiler and library version. If you need different conversion behavior for a container type the library already handles, wrap it in your own type (or use <code>adl_serializer</code> specialization, as shown <a href=#how-do-i-convert-third-party-types>above</a> for <code>boost::optional</code>) instead of trying to re-specialize <code>to_json</code>/<code>from_json</code> for the container type itself.</p> </div> <div class="admonition warning"> <p class=admonition-title>Raw C-style arrays</p> <p>Members declared as raw C-style arrays (e.g., <code>char buf[1024]</code>) do not round-trip safely through <code>NLOHMANN_DEFINE_TYPE_*</code> macros or the default (de)serializers: <code>to_json</code> serializes any <code>char</code> array as a <abbr title="JavaScript Object Notation">JSON</abbr> <em>string</em> (matching the <code>std::string</code>-constructible overload), but the <code>from_json</code> overload for fixed-size arrays expects a <abbr title="JavaScript Object Notation">JSON</abbr> <em>array</em> and iterates it element-wise, which fails with a <code>type_error</code> when given a string. Use <code>std::string</code>, <code>std::array<char, N></code>, or a manually written <code>to_json</code>/<code>from_json</code> pair for such members instead.</p> </div> <div class="admonition note"> <p class=admonition-title>Macros and <code>nlohmann::ordered_json</code></p> <p>The <code>NLOHMANN_DEFINE_TYPE_*</code>/<code>NLOHMANN_DEFINE_DERIVED_TYPE_*</code> macros are generic over any <code>basic_json</code> specialization, including <code>nlohmann::ordered_json</code>. Simply use <code>ordered_json</code> as the target type and members are serialized in declaration order -- no separate macro or extra code is needed.</p> <div class=highlight><pre><span></span><code><span class=k>namespace</span><span class=w> </span><span class=nn>ns</span><span class=w> </span><span class=p>{</span>
|
||||
</code></pre></div> </details> <div class="admonition warning"> <p class=admonition-title>Overriding conversions for natively-supported types</p> <p>The library already provides built-in <code>to_json</code>/<code>from_json</code> conversions for <abbr title="Standard Template Library">STL</abbr> containers such as <code>std::vector</code>, <code>std::array</code>, and <code>std::map</code>. Defining your own free-function <code>to_json</code>/<code>from_json</code> overload for one of these container types directly (instead of for your own type) can conflict with the built-in overload during overload resolution, producing compiler errors ("no matching overloaded function", "call is ambiguous") that vary by compiler and library version. If you need different conversion behavior for a container type the library already handles, wrap it in your own type (or use <code>adl_serializer</code> specialization, as shown <a href=#how-do-i-convert-third-party-types>above</a> for <code>boost::optional</code>) instead of trying to re-specialize <code>to_json</code>/<code>from_json</code> for the container type itself.</p> </div> <div class="admonition warning"> <p class=admonition-title>Raw C-style arrays</p> <p>Members declared as raw C-style arrays (e.g., <code>char buf[1024]</code>) do not round-trip safely through <code>NLOHMANN_DEFINE_TYPE_*</code> macros or the default (de)serializers: <code>to_json</code> serializes any <code>char</code> array as a <abbr title="JavaScript Object Notation">JSON</abbr> <em>string</em> (matching the <code>std::string</code>-constructible overload), but the <code>from_json</code> overload for fixed-size arrays expects a <abbr title="JavaScript Object Notation">JSON</abbr> <em>array</em> and iterates it element-wise, which fails with a <code>type_error</code> when given a string. Use <code>std::string</code>, <code>std::array<char, N></code>, or a manually written <code>to_json</code>/<code>from_json</code> pair for such members instead.</p> </div> <div class="admonition note"> <p class=admonition-title>Macros and <code>nlohmann::ordered_json</code></p> <p>The <code>NLOHMANN_DEFINE_TYPE_*</code>/<code>NLOHMANN_DEFINE_DERIVED_TYPE_*</code> macros are generic over any <code>basic_json</code> specialization, including <code>nlohmann::ordered_json</code>. Simply use <code>ordered_json</code> as the target type and members are serialized in declaration order — no separate macro or extra code is needed.</p> <div class=highlight><pre><span></span><code><span class=k>namespace</span><span class=w> </span><span class=nn>ns</span><span class=w> </span><span class=p>{</span>
|
||||
<span class=w> </span><span class=n>NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE</span><span class=p>(</span><span class=n>person</span><span class=p>,</span><span class=w> </span><span class=n>name</span><span class=p>,</span><span class=w> </span><span class=n>address</span><span class=p>,</span><span class=w> </span><span class=n>age</span><span class=p>)</span>
|
||||
<span class=p>}</span>
|
||||
|
||||
@@ -188,11 +188,11 @@
|
||||
|
||||
<span class=n>ns</span><span class=o>::</span><span class=n>marker</span><span class=w> </span><span class=n>m</span><span class=p>{};</span>
|
||||
<span class=n>nlohmann</span><span class=o>::</span><span class=n>json</span><span class=w> </span><span class=n>j</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>m</span><span class=p>;</span><span class=w> </span><span class=c1>// {}</span>
|
||||
</code></pre></div> </div> <div class="admonition note"> <p class=admonition-title>No macro for non-default-constructible types</p> <p>There is currently no <code>NLOHMANN_DEFINE_TYPE_*</code>-style macro for types that are not <a href="https://en.cppreference.com/w/cpp/named_req/DefaultConstructible">DefaultConstructible</a>. This is not an intentional omission of documentation -- no such macro exists yet; see <a href=#how-can-i-use-get-for-non-default-constructiblenon-copyable-types>How can I use <code>get()</code> for non-default constructible/non-copyable types?</a> for the manual pattern to use instead.</p> </div> <h2 id=how-do-i-convert-third-party-types>How do I convert third-party types?<a class=headerlink href=#how-do-i-convert-third-party-types title="Permanent link">¶</a></h2> <p>This requires a bit more advanced technique. But first, let us see how this conversion mechanism works:</p> <pre class=mermaid><code>flowchart LR
|
||||
</code></pre></div> </div> <div class="admonition note"> <p class=admonition-title>No macro for non-default-constructible types</p> <p>There is currently no <code>NLOHMANN_DEFINE_TYPE_*</code>-style macro for types that are not <a href="https://en.cppreference.com/w/cpp/named_req/DefaultConstructible">DefaultConstructible</a>. This is not an intentional omission of documentation — no such macro exists yet; see <a href=#how-can-i-use-get-for-non-default-constructiblenon-copyable-types>How can I use <code>get()</code> for non-default constructible/non-copyable types?</a> for the manual pattern to use instead.</p> </div> <h2 id=how-do-i-convert-third-party-types>How do I convert third-party types?<a class=headerlink href=#how-do-i-convert-third-party-types title="Permanent link">¶</a></h2> <p>This requires a bit more advanced technique. But first, let us see how this conversion mechanism works:</p> <pre class=mermaid><code>flowchart LR
|
||||
A["construct json j = t, or call j.get() for T"] --> B["JSONSerializer for T: to_json / from_json"]
|
||||
B -->|"default JSONSerializer"| C["adl_serializer for T: to_json / from_json"]
|
||||
C -->|"unqualified call, found via ADL"| D["free to_json(j, t) / from_json(j, t) in T's namespace"]
|
||||
B -->|"user specialization replaces the default"| E["user's adl_serializer specialization for T"]</code></pre> <p>The library uses <strong><abbr title="JavaScript Object Notation">JSON</abbr> Serializers</strong> to convert types to <abbr title="JavaScript Object Notation">JSON</abbr>. The default serializer for <code>nlohmann::json</code> is <code>nlohmann::adl_serializer</code> (<abbr title="Argument-dependent lookup">ADL</abbr> means <a href="https://en.cppreference.com/w/cpp/language/adl">Argument-Dependent Lookup</a>).</p> <p>It is implemented like this (simplified):</p> <div class=highlight><pre><span></span><code><span class=k>template</span><span class=w> </span><span class=o><</span><span class=k>typename</span><span class=w> </span><span class=nc>T</span><span class=o>></span>
|
||||
B -->|"user specialization replaces the default"| E["user's adl_serializer specialization for T"]</code></pre> <p class=nj-caption>How a conversion between <code>json</code> and a type <code>T</code> is dispatched: the default <code>adl_serializer</code> calls the free <code>to_json</code>/<code>from_json</code> functions found by argument-dependent lookup, while a user specialization of <code>adl_serializer</code> replaces this path.</p> <p>The library uses <strong><abbr title="JavaScript Object Notation">JSON</abbr> Serializers</strong> to convert types to <abbr title="JavaScript Object Notation">JSON</abbr>. The default serializer for <code>nlohmann::json</code> is <code>nlohmann::adl_serializer</code> (<abbr title="Argument-dependent lookup">ADL</abbr> means <a href="https://en.cppreference.com/w/cpp/language/adl">Argument-Dependent Lookup</a>).</p> <p>It is implemented like this (simplified):</p> <div class=highlight><pre><span></span><code><span class=k>template</span><span class=w> </span><span class=o><</span><span class=k>typename</span><span class=w> </span><span class=nc>T</span><span class=o>></span>
|
||||
<span class=k>struct</span><span class=w> </span><span class=nc>adl_serializer</span><span class=w> </span><span class=p>{</span>
|
||||
<span class=w> </span><span class=k>static</span><span class=w> </span><span class=kt>void</span><span class=w> </span><span class=nf>to_json</span><span class=p>(</span><span class=n>json</span><span class=o>&</span><span class=w> </span><span class=n>j</span><span class=p>,</span><span class=w> </span><span class=k>const</span><span class=w> </span><span class=n>T</span><span class=o>&</span><span class=w> </span><span class=n>value</span><span class=p>)</span><span class=w> </span><span class=p>{</span>
|
||||
<span class=w> </span><span class=c1>// calls the "to_json" method in T's namespace</span>
|
||||
@@ -305,7 +305,7 @@
|
||||
<span class=w> </span><span class=p>}</span>
|
||||
<span class=w> </span><span class=p>};</span>
|
||||
<span class=p>}</span>
|
||||
</code></pre></div> <h2 id=why-isnt-my-to_json-or-from_json-found>Why isn't my <code>to_json</code> or <code>from_json</code> found?<a class=headerlink href=#why-isnt-my-to_json-or-from_json-found title="Permanent link">¶</a></h2> <p>The library calls <code>to_json</code> and <code>from_json</code> without qualification, so the compiler finds them by <a href="https://en.cppreference.com/w/cpp/language/adl">argument-dependent lookup</a> in the namespace of the type, or in the namespaces of its template arguments. The usual reasons for a function that is "not found" are:</p> <ul> <li><strong>The type is an alias of a standard type.</strong> After <code class=highlight><span class=k>using</span><span class=w> </span><span class=n>Hash</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>array</span><span class=o><</span><span class=n>std</span><span class=o>::</span><span class=kt>uint8_t</span><span class=p>,</span><span class=w> </span><span class=mi>6</span><span class=o>></span><span class=p>;</span></code> the type <em>is</em> <code>std::array<std::uint8_t, 6></code>, which lives in namespace <code>std</code>. A <code>to_json(json&, const Hash&)</code> in your namespace is never selected. Either give the type its own identity, for example <code class=highlight><span class=k>struct</span><span class=w> </span><span class=nc>Hash</span><span class=w> </span><span class=o>:</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>array</span><span class=o><</span><span class=n>std</span><span class=o>::</span><span class=kt>uint8_t</span><span class=p>,</span><span class=w> </span><span class=mi>6</span><span class=o>></span><span class=w> </span><span class=p>{};</span></code> (the function is then found in <code>Hash</code>'s namespace), or specialize <a href=#how-do-i-convert-third-party-types><code>adl_serializer<Hash></code></a>.</li> <li><strong>The function is in the wrong namespace.</strong> It must be in the namespace of the type (a global type needs a global function), not in the namespace of the code that calls the conversion. Specializations of <code>adl_serializer</code> belong in namespace <code>nlohmann</code>.</li> <li><strong>It is declared too late.</strong> A conversion can only use what has been declared when it is instantiated. If <code class=highlight><span class=n>json</span><span class=w> </span><span class=n>j</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>vector</span><span class=o><</span><span class=n>third_party</span><span class=o>::</span><span class=n>string</span><span class=o>></span><span class=p>{...};</span></code> appears before the <code>adl_serializer</code> specialization, the compiler reports an error such as "explicit specialization of <code>adl_serializer<...></code> after instantiation". Put the specialization in a small header that every file includes <strong>before</strong> it uses the type with <code>json</code>.</li> <li><strong>It is a template.</strong> For <code>template <class T> struct box</code>, write <code>template <class T> void to_json(json&, const box<T>&)</code>, not an overload for one <code>T</code>.</li> <li><strong>It worked by accident.</strong> Some versions found <code>to_json</code> overloads in <code>nlohmann::detail</code> through argument-dependent lookup of the base class of <code>basic_json</code>. Do not rely on this; it is not part of the interface.</li> </ul> <p>For smart pointers, class hierarchies, <code>std::variant</code>, and <code>std::chrono</code>, see <a href=../type_recipes/ >Recipes for common types</a>.</p> <h2 id=why-cant-i-convert-tofrom-stdany>Why can't I convert to/from <code>std::any</code>?<a class=headerlink href=#why-cant-i-convert-tofrom-stdany title="Permanent link">¶</a></h2> <p><code>std::any</code> is intentionally excluded from <code>get<T>()</code>/generic conversion support, so <code>get<std::any>()</code> and containers like <code>std::map<std::string, std::any></code> fail to compile by design -- there is no way to know, from a <code>json</code> value alone, which concrete type to store inside the <code>std::any</code>. To work with heterogeneous <abbr title="JavaScript Object Notation">JSON</abbr> values, dispatch on the value's type manually and construct the <code>std::any</code> (or extract from it) yourself:</p> <div class=highlight><pre><span></span><code><span class=n>std</span><span class=o>::</span><span class=n>any</span><span class=w> </span><span class=nf>value_to_any</span><span class=p>(</span><span class=k>const</span><span class=w> </span><span class=n>json</span><span class=o>&</span><span class=w> </span><span class=n>j</span><span class=p>)</span><span class=w> </span><span class=p>{</spanLine truncated
|
||||
</code></pre></div> <h2 id=why-isnt-my-to_json-or-from_json-found>Why isn't my <code>to_json</code> or <code>from_json</code> found?<a class=headerlink href=#why-isnt-my-to_json-or-from_json-found title="Permanent link">¶</a></h2> <p>The library calls <code>to_json</code> and <code>from_json</code> without qualification, so the compiler finds them by <a href="https://en.cppreference.com/w/cpp/language/adl">argument-dependent lookup</a> in the namespace of the type, or in the namespaces of its template arguments. The usual reasons for a function that is "not found" are:</p> <ul> <li><strong>The type is an alias of a standard type.</strong> After <code class=highlight><span class=k>using</span><span class=w> </span><span class=n>Hash</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>array</span><span class=o><</span><span class=n>std</span><span class=o>::</span><span class=kt>uint8_t</span><span class=p>,</span><span class=w> </span><span class=mi>6</span><span class=o>></span><span class=p>;</span></code> the type <em>is</em> <code>std::array<std::uint8_t, 6></code>, which lives in namespace <code>std</code>. A <code>to_json(json&, const Hash&)</code> in your namespace is never selected. Either give the type its own identity, for example <code class=highlight><span class=k>struct</span><span class=w> </span><span class=nc>Hash</span><span class=w> </span><span class=o>:</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>array</span><span class=o><</span><span class=n>std</span><span class=o>::</span><span class=kt>uint8_t</span><span class=p>,</span><span class=w> </span><span class=mi>6</span><span class=o>></span><span class=w> </span><span class=p>{};</span></code> (the function is then found in <code>Hash</code>'s namespace), or specialize <a href=#how-do-i-convert-third-party-types><code>adl_serializer<Hash></code></a>.</li> <li><strong>The function is in the wrong namespace.</strong> It must be in the namespace of the type (a global type needs a global function), not in the namespace of the code that calls the conversion. Specializations of <code>adl_serializer</code> belong in namespace <code>nlohmann</code>.</li> <li><strong>It is declared too late.</strong> A conversion can only use what has been declared when it is instantiated. If <code class=highlight><span class=n>json</span><span class=w> </span><span class=n>j</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>vector</span><span class=o><</span><span class=n>third_party</span><span class=o>::</span><span class=n>string</span><span class=o>></span><span class=p>{...};</span></code> appears before the <code>adl_serializer</code> specialization, the compiler reports an error such as "explicit specialization of <code>adl_serializer<...></code> after instantiation". Put the specialization in a small header that every file includes <strong>before</strong> it uses the type with <code>json</code>.</li> <li><strong>It is a template.</strong> For <code>template <class T> struct box</code>, write <code>template <class T> void to_json(json&, const box<T>&)</code>, not an overload for one <code>T</code>.</li> <li><strong>It worked by accident.</strong> Some versions found <code>to_json</code> overloads in <code>nlohmann::detail</code> through argument-dependent lookup of the base class of <code>basic_json</code>. Do not rely on this; it is not part of the interface.</li> </ul> <p>For smart pointers, class hierarchies, <code>std::variant</code>, and <code>std::chrono</code>, see <a href=../type_recipes/ >Recipes for common types</a>.</p> <h2 id=why-cant-i-convert-tofrom-stdany>Why can't I convert to/from <code>std::any</code>?<a class=headerlink href=#why-cant-i-convert-tofrom-stdany title="Permanent link">¶</a></h2> <p><code>std::any</code> is intentionally excluded from <code>get<T>()</code>/generic conversion support, so <code>get<std::any>()</code> and containers like <code>std::map<std::string, std::any></code> fail to compile by design — there is no way to know, from a <code>json</code> value alone, which concrete type to store inside the <code>std::any</code>. To work with heterogeneous <abbr title="JavaScript Object Notation">JSON</abbr> values, dispatch on the value's type manually and construct the <code>std::any</code> (or extract from it) yourself:</p> <div class=highlight><pre><span></span><code><span class=n>std</span><span class=o>::</span><span class=n>any</span><span class=w> </span><span class=nf>value_to_any</span><span class=p>(</span><span class=k>const</span><span class=w> </span><span class=n>json</span><span class=o>&</span><span class=w> </span><span class=n>j</span><span class=p>)</span><span class=w> </span><span class=p>{</spaLine truncated
|
||||
<span class=w> </span><span class=k>if</span><span class=w> </span><span class=p>(</span><span class=n>j</span><span class=p>.</span><span class=n>is_boolean</span><span class=p>())</span><span class=w> </span><span class=p>{</span><span class=w> </span><span class=k>return</span><span class=w> </span><span class=n>j</span><span class=p>.</span><span class=n>get</span><span class=o><</span><span class=kt>bool</span><span class=o>></span><span class=p>();</span><span class=w> </span><span class=p>}</span>
|
||||
<span class=w> </span><span class=k>if</span><span class=w> </span><span class=p>(</span><span class=n>j</span><span class=p>.</span><span class=n>is_number_integer</span><span class=p>())</span><span class=w> </span><span class=p>{</span><span class=w> </span><span class=k>return</span><span class=w> </span><span class=n>j</span><span class=p>.</span><span class=n>get</span><span class=o><</span><span class=kt>int</span><span class=o>></span><span class=p>();</span><span class=w> </span><span class=p>}</span>
|
||||
<span class=w> </span><span class=k>if</span><span class=w> </span><span class=p>(</span><span class=n>j</span><span class=p>.</span><span class=n>is_number_float</span><span class=p>())</span><span class=w> </span><span class=p>{</span><span class=w> </span><span class=k>return</span><span class=w> </span><span class=n>j</span><span class=p>.</span><span class=n>get</span><span class=o><</span><span class=kt>double</span><span class=o>></span><span class=p>();</span><span class=w> </span><span class=p>}</span>
|
||||
@@ -321,7 +321,7 @@
|
||||
<span class=w> </span><span class=k>if</span><span class=w> </span><span class=p>(</span><span class=n>a</span><span class=p>.</span><span class=n>type</span><span class=p>()</span><span class=w> </span><span class=o>==</span><span class=w> </span><span class=k>typeid</span><span class=p>(</span><span class=n>std</span><span class=o>::</span><span class=n>string</span><span class=p>))</span><span class=w> </span><span class=p>{</span><span class=w> </span><span class=k>return</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>any_cast</span><span class=o><</span><span class=n>std</span><span class=o>::</span><span class=n>string</span><span class=o>></span><span class=p>(</span><span class=n>a</span><span class=p>);</span><span class=w> </span><span class=p>}</span>
|
||||
<span class=w> </span><span class=k>return</span><span class=w> </span><span class=nb>nullptr</span><span class=p>;</span>
|
||||
<span class=p>}</span>
|
||||
</code></pre></div> <h2 id=why-does-serializing-a-stdmapstdunordered_map-with-non-string-keys-produce-an-array>Why does serializing a <code>std::map</code>/<code>std::unordered_map</code> with non-string keys produce an array?<a class=headerlink href=#why-does-serializing-a-stdmapstdunordered_map-with-non-string-keys-produce-an-array title="Permanent link">¶</a></h2> <p>A <code>std::map</code>/<code>std::unordered_map</code> whose key type is not string-like (e.g., <code>std::map<int, std::string></code>) cannot be serialized as a <abbr title="JavaScript Object Notation">JSON</abbr> object, because <abbr title="JavaScript Object Notation">JSON</abbr> object keys must be strings. See <a href=../types/#converting-maps-with-non-string-keys>Converting maps with non-string keys</a> in the types article for what the library does instead.</p> <h2 id=why-does-stdwstring-convert-or-dump-incorrectly>Why does <code>std::wstring</code> convert or dump incorrectly?<a class=headerlink href=#why-does-stdwstring-convert-or-dump-incorrectly title="Permanent link">¶</a></h2> <p>The library assumes <abbr title="Unicode Transformation Format">UTF</abbr>-8 encoding internally, so <code>std::wstring</code> is not supported out of the box -- see the <abbr title="Frequently Asked Questions">FAQ</abbr> entry on <a href=../../home/faq/#wide-string-handling>wide string handling</a> for why, and for a <abbr title="Unicode Transformation Format">UTF</abbr>-8 conversion recipe.</p> <h2 id=can-i-write-my-own-serializer-advanced-use>Can I write my own serializer? (Advanced use)<a class=headerlink href=#can-i-write-my-own-serializer-advanced-use title="Permanent link">¶</a></h2> <p>Yes. You might want to take a look at <a href="https://github.com/nlohmann/json/blob/develop/tests/src/unit-udt.cpp"><code>unit-udt.cpp</code></a> in the test suite, to see a few examples.</p> <p>If you write your own serializer, you will need to do a few things:</p> <ul> <li>use a different <code>basic_json</code> alias than <code>nlohmann::json</code> (the last template parameter of <code>basic_json</code> is the <code>JSONSerializer</code>)</li> <li>use your <code>basic_json</code> alias (or a template parameter) in all your <code>to_json</code>/<code>from_json</code> methods</li> <li>use <code>nlohmann::to_json</code> and <code>nlohmann::from_json</code> when you need <abbr title="Argument-dependent lookup">ADL</abbr></li> </ul> <p>Here is an example, without simplifications, that only accepts types with a size <= 32, and uses <abbr title="Argument-dependent lookup">ADL</abbr>.</p> <div class=highlight><pre><span></span><code><span class=c1>// You should use void as a second template argument</span>
|
||||
</code></pre></div> <h2 id=why-does-serializing-a-stdmapstdunordered_map-with-non-string-keys-produce-an-array>Why does serializing a <code>std::map</code>/<code>std::unordered_map</code> with non-string keys produce an array?<a class=headerlink href=#why-does-serializing-a-stdmapstdunordered_map-with-non-string-keys-produce-an-array title="Permanent link">¶</a></h2> <p>A <code>std::map</code>/<code>std::unordered_map</code> whose key type is not string-like (e.g., <code>std::map<int, std::string></code>) cannot be serialized as a <abbr title="JavaScript Object Notation">JSON</abbr> object, because <abbr title="JavaScript Object Notation">JSON</abbr> object keys must be strings. See <a href=../types/#converting-maps-with-non-string-keys>Converting maps with non-string keys</a> in the types article for what the library does instead.</p> <h2 id=why-does-stdwstring-convert-or-dump-incorrectly>Why does <code>std::wstring</code> convert or dump incorrectly?<a class=headerlink href=#why-does-stdwstring-convert-or-dump-incorrectly title="Permanent link">¶</a></h2> <p>The library assumes <abbr title="Unicode Transformation Format">UTF</abbr>-8 encoding internally, so <code>std::wstring</code> is not supported out of the box — see the <abbr title="Frequently Asked Questions">FAQ</abbr> entry on <a href=../../home/faq/#wide-string-handling>wide string handling</a> for why, and for a <abbr title="Unicode Transformation Format">UTF</abbr>-8 conversion recipe.</p> <h2 id=can-i-write-my-own-serializer-advanced-use>Can I write my own serializer? (Advanced use)<a class=headerlink href=#can-i-write-my-own-serializer-advanced-use title="Permanent link">¶</a></h2> <p>Yes. You might want to take a look at <a href="https://github.com/nlohmann/json/blob/develop/tests/src/unit-udt.cpp"><code>unit-udt.cpp</code></a> in the test suite, to see a few examples.</p> <p>If you write your own serializer, you will need to do a few things:</p> <ul> <li>use a different <code>basic_json</code> alias than <code>nlohmann::json</code> (the last template parameter of <code>basic_json</code> is the <code>JSONSerializer</code>)</li> <li>use your <code>basic_json</code> alias (or a template parameter) in all your <code>to_json</code>/<code>from_json</code> methods</li> <li>use <code>nlohmann::to_json</code> and <code>nlohmann::from_json</code> when you need <abbr title="Argument-dependent lookup">ADL</abbr></li> </ul> <p>Here is an example, without simplifications, that only accepts types with a size <= 32, and uses <abbr title="Argument-dependent lookup">ADL</abbr>.</p> <div class=highlight><pre><span></span><code><span class=c1>// You should use void as a second template argument</span>
|
||||
<span class=c1>// if you don't need compile-time checks on T</span>
|
||||
<span class=k>template</span><span class=o><</span><span class=k>typename</span><span class=w> </span><span class=nc>T</span><span class=p>,</span><span class=w> </span><span class=k>typename</span><span class=w> </span><span class=nc>SFINAE</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=k>typename</span><span class=w> </span><span class=nc>std</span><span class=o>::</span><span class=n>enable_if</span><span class=o><</span><span class=k>sizeof</span><span class=p>(</span><span class=n>T</span><span class=p>)</span><span class=w> </span><span class=o><=</span><span class=w> </span><span class=mi>32</span><span class=o>>::</span><span class=n>type</span><span class=o>></span>
|
||||
<span class=k>struct</span><span class=w> </span><span class=nc>less_than_32_serializer</span><span class=w> </span><span class=p>{</span>
|
||||
@@ -357,4 +357,4 @@
|
||||
<span class=w> </span><span class=n>value</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>j</span><span class=p>.</span><span class=k>template</span><span class=w> </span><span class=n>get</span><span class=o><</span><span class=n>T</span><span class=o>></span><span class=p>();</span><span class=w> </span><span class=c1>// oops!</span>
|
||||
<span class=w> </span><span class=p>}</span>
|
||||
<span class=p>};</span>
|
||||
</code></pre></div> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">¶</a></h2> <ul> <li><a href=../type_recipes/ >Recipes for common types</a> - smart pointers, class hierarchies, <code>std::variant</code>, and <code>std::chrono</code></li> <li><a href=../conversions/ >Converting values</a> - the general overview of <code>get</code>/<code>get_to</code> and implicit conversions</li> <li><a href=../enum_conversion/ >Specializing enum conversion</a> - map enums to <abbr title="JavaScript Object Notation">JSON</abbr> strings instead of integers</li> <li><a href=../macros/ >Supported macros</a> - reference for <code>NLOHMANN_DEFINE_TYPE_*</code> and related macros</li> <li><a href=../../api/adl_serializer/ ><code>adl_serializer</code></a> - the default <code>JSONSerializer</code> used in the conversion dispatch</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 25, 2026 11:07:51 UTC">October 25, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2025 Fonticons, Inc.--><path d="M416 32H31.9C14.3 32 0 46.5 0 64.3v383.4C0 465.5 14.3 480 31.9 480H416c17.6 0 32-14.5 32-32.3V64.3c0-17.8-14.4-32.3-32-32.3M135.4 416H69V202.2h66.5V416zM102.2 96a38.5 38.5 0 1 1 0 77 38.5 38.5 0 1 1 0-77m282.1 320h-66.4V312c0-24.8-.5-56.7-34.5-Line truncated
|
||||
</code></pre></div> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">¶</a></h2> <ul> <li><a href=../type_recipes/ >Recipes for common types</a> - smart pointers, class hierarchies, <code>std::variant</code>, and <code>std::chrono</code></li> <li><a href=../conversions/ >Converting values</a> - the general overview of <code>get</code>/<code>get_to</code> and implicit conversions</li> <li><a href=../enum_conversion/ >Specializing enum conversion</a> - map enums to <abbr title="JavaScript Object Notation">JSON</abbr> strings instead of integers</li> <li><a href=../macros/ >Supported macros</a> - reference for <code>NLOHMANN_DEFINE_TYPE_*</code> and related macros</li> <li><a href=../../api/adl_serializer/ ><code>adl_serializer</code></a> - the default <code>JSONSerializer</code> used in the conversion dispatch</li> <li><a href=../../api/macros/nlohmann_define_type_intrusive/ ><code>NLOHMANN_DEFINE_TYPE_INTRUSIVE</code></a> - generate the conversion functions inside the type</li> <li><a href=../../api/macros/nlohmann_define_type_non_intrusive/ ><code>NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE</code></a> - generate the conversion functions outside the type</li> <li><a href=../../api/macros/nlohmann_define_type_with_names/ ><code>NLOHMANN_DEFINE_TYPE_WITH_NAMES</code></a> - use <abbr title="JavaScript Object Notation">JSON</abbr> keys that differ from the member names</li> <li><a href=../../api/macros/nlohmann_define_derived_type/ ><code>NLOHMANN_DEFINE_DERIVED_TYPE</code></a> - generate the conversion functions for derived classes</li> <li><a href=../../api/macros/nlohmann_json_serialize_enum/ ><code>NLOHMANN_JSON_SERIALIZE_ENUM</code></a> - map enum values to <abbr title="JavaScript Object Notation">JSON</abbr> values</li> <li><a href=../../api/basic_json/get/ ><code>get</code></a> - convert a <abbr title="JavaScript Object Notation">JSON</abbr> value to a C++ type</li> <li><a href=../../api/basic_json/get_to/ ><code>get_to</code></a> - convert a <abbr title="JavaScript Object Notation">JSON</abbr> value into an existing variable</li> <li><a href=../../api/basic_json/json_serializer/ ><code>json_serializer</code></a> - the <code>JSONSerializer</code> template parameter</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../ class="md-footer__link md-footer__link--prev" aria-label="Previous: Features"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> Features </div> </div> </a> <a href=../type_recipes/ class="md-footer__link md-footer__link--next" aria-label="Next: Recipes for Common Types"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> Recipes for Common Types </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blankLine truncated
|
||||
@@ -190,26 +190,28 @@ For all the macros, the first parameter is the name of the class/struct. The `DE
|
||||
|
||||
```
|
||||
flowchart TD
|
||||
A["choosing a NLOHMANN_DEFINE_* macro"] --> B{"adding fields to a base class?"}
|
||||
A["choosing a NLOHMANN_DEFINE_* macro"] --> B{{"adding fields to a base class?"}}
|
||||
B -->|"yes"| C["...DERIVED_TYPE..."]
|
||||
B -->|"no"| D["...TYPE..."]
|
||||
C --> E{"need access to private members?"}
|
||||
C --> E{{"need access to private members?"}}
|
||||
D --> E
|
||||
E -->|"yes"| F["...INTRUSIVE... (used inside the class)"]
|
||||
E -->|"no"| G["...NON_INTRUSIVE... (used in the namespace)"]
|
||||
F --> H{"only serializing, never parsing back?"}
|
||||
F --> H{{"only serializing, never parsing back?"}}
|
||||
G --> H
|
||||
H -->|"yes"| I["...ONLY_SERIALIZE"]
|
||||
H -->|"no"| J{"allow missing keys when parsing?"}
|
||||
H -->|"no"| J{{"allow missing keys when parsing?"}}
|
||||
J -->|"yes"| K["...WITH_DEFAULT"]
|
||||
J -->|"no"| L["plain (missing keys throw)"]
|
||||
I --> M{"need custom JSON key names?"}
|
||||
I --> M{{"need custom JSON key names?"}}
|
||||
K --> M
|
||||
L --> M
|
||||
M -->|"yes"| N["...WITH_NAMES"]
|
||||
M -->|"no"| O["done"]
|
||||
```
|
||||
|
||||
Decision tree for choosing a `NLOHMANN_DEFINE_*` macro: answer the questions from top to bottom; each answer picks one part of the macro name, and the parts are combined in this order. The table below lists the resulting macros.
|
||||
|
||||
| Need access to private members | Need only serialization | Allow missing values when de-serializing | macro |
|
||||
| ------------------------------ | ----------------------- | ---------------------------------------- | ---------------------------------------------------------------------------------------------------------------------------------------- |
|
||||
| | | | [**NLOHMANN_DEFINE_TYPE_INTRUSIVE**](https://json.nlohmann.me/api/macros/nlohmann_define_type_intrusive/index.md) |
|
||||
@@ -219,6 +221,8 @@ flowchart TD
|
||||
| | | | [**NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_DEFAULT**](https://json.nlohmann.me/api/macros/nlohmann_define_type_non_intrusive/index.md) |
|
||||
| | | | [**NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE**](https://json.nlohmann.me/api/macros/nlohmann_define_type_non_intrusive/index.md) |
|
||||
|
||||
Macros for classes and structs without a base class. A green check means yes, a red cross means no, and a grey skip sign means the column does not apply, as a macro that only serializes never reads values.
|
||||
|
||||
For *derived* classes and structs, use the following macros
|
||||
|
||||
| Need access to private members | Need only serialization | Allow missing values when de-serializing | macro |
|
||||
@@ -230,11 +234,26 @@ For *derived* classes and structs, use the following macros
|
||||
| | | | [**NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_WITH_DEFAULT**](https://json.nlohmann.me/api/macros/nlohmann_define_derived_type/index.md) |
|
||||
| | | | [**NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE**](https://json.nlohmann.me/api/macros/nlohmann_define_derived_type/index.md) |
|
||||
|
||||
Macros for derived classes and structs, read as Table 1.
|
||||
|
||||
Implementation limits
|
||||
|
||||
- The current macro implementations are limited to at most 63 member variables. If you want to serialize/deserialize types with more than 63 member variables, you need to define the `to_json`/`from_json` functions manually.
|
||||
- For the `WITH_NAMES` variants the limit is halved to 31 member variables.
|
||||
|
||||
To pick the macro for your own type and get code to start from, answer the questions below.
|
||||
|
||||
- Members are private: use an `INTRUSIVE` macro inside the class ([reference](https://json.nlohmann.me/api/macros/nlohmann_define_type_intrusive/index.md)).
|
||||
- All members are public, or you cannot edit the class: use a `NON_INTRUSIVE` macro in the class's namespace ([reference](https://json.nlohmann.me/api/macros/nlohmann_define_type_non_intrusive/index.md)).
|
||||
- The class adds members to a serializable base class: use a `DERIVED_TYPE` macro with the base class as second argument ([reference](https://json.nlohmann.me/api/macros/nlohmann_define_derived_type/index.md)).
|
||||
- Missing keys should keep their default values: append `_WITH_DEFAULT`; otherwise a missing key throws.
|
||||
- You only convert to JSON: append `_ONLY_SERIALIZE` instead.
|
||||
- JSON keys differ from the member names: append `_WITH_NAMES` and write `"key", member` pairs ([reference](https://json.nlohmann.me/api/macros/nlohmann_define_type_with_names/index.md)).
|
||||
- Private members of a class you cannot edit: no macro works; write the conversion by hand ([third-party types](#how-do-i-convert-third-party-types)).
|
||||
- The type is not default constructible and is converted in both directions: no macro works; specialize `adl_serializer` with a `from_json` that returns the value, or append `_ONLY_SERIALIZE` if you only convert to JSON ([non-default-constructible types](#how-can-i-use-get-for-non-default-constructiblenon-copyable-types)).
|
||||
|
||||
Macro chooser: each answer selects one part of the macro name; the generated program defines the type, calls the macro and converts a value to JSON and back. Combinations that no macro supports show the alternative instead of code.
|
||||
|
||||
Example: using the `NLOHMANN_DEFINE_TYPE_*` macros
|
||||
|
||||
The `to_json`/`from_json` functions for the `person` struct above can be created with:
|
||||
@@ -301,7 +320,7 @@ Members declared as raw C-style arrays (e.g., `char buf[1024]`) do not round-tri
|
||||
|
||||
Macros and `nlohmann::ordered_json`
|
||||
|
||||
The `NLOHMANN_DEFINE_TYPE_*`/`NLOHMANN_DEFINE_DERIVED_TYPE_*` macros are generic over any `basic_json` specialization, including `nlohmann::ordered_json`. Simply use `ordered_json` as the target type and members are serialized in declaration order -- no separate macro or extra code is needed.
|
||||
The `NLOHMANN_DEFINE_TYPE_*`/`NLOHMANN_DEFINE_DERIVED_TYPE_*` macros are generic over any `basic_json` specialization, including `nlohmann::ordered_json`. Simply use `ordered_json` as the target type and members are serialized in declaration order — no separate macro or extra code is needed.
|
||||
|
||||
```
|
||||
namespace ns {
|
||||
@@ -330,7 +349,7 @@ nlohmann::json j = m; // {}
|
||||
|
||||
No macro for non-default-constructible types
|
||||
|
||||
There is currently no `NLOHMANN_DEFINE_TYPE_*`-style macro for types that are not [DefaultConstructible](https://en.cppreference.com/w/cpp/named_req/DefaultConstructible). This is not an intentional omission of documentation -- no such macro exists yet; see [How can I use `get()` for non-default constructible/non-copyable types?](#how-can-i-use-get-for-non-default-constructiblenon-copyable-types) for the manual pattern to use instead.
|
||||
There is currently no `NLOHMANN_DEFINE_TYPE_*`-style macro for types that are not [DefaultConstructible](https://en.cppreference.com/w/cpp/named_req/DefaultConstructible). This is not an intentional omission of documentation — no such macro exists yet; see [How can I use `get()` for non-default constructible/non-copyable types?](#how-can-i-use-get-for-non-default-constructiblenon-copyable-types) for the manual pattern to use instead.
|
||||
|
||||
## How do I convert third-party types?
|
||||
|
||||
@@ -344,6 +363,8 @@ flowchart LR
|
||||
B -->|"user specialization replaces the default"| E["user's adl_serializer specialization for T"]
|
||||
```
|
||||
|
||||
How a conversion between `json` and a type `T` is dispatched: the default `adl_serializer` calls the free `to_json`/`from_json` functions found by argument-dependent lookup, while a user specialization of `adl_serializer` replaces this path.
|
||||
|
||||
The library uses **JSON Serializers** to convert types to JSON. The default serializer for `nlohmann::json` is `nlohmann::adl_serializer` (ADL means [Argument-Dependent Lookup](https://en.cppreference.com/w/cpp/language/adl)).
|
||||
|
||||
It is implemented like this (simplified):
|
||||
@@ -512,7 +533,7 @@ For smart pointers, class hierarchies, `std::variant`, and `std::chrono`, see [R
|
||||
|
||||
## Why can't I convert to/from `std::any`?
|
||||
|
||||
`std::any` is intentionally excluded from `get<T>()`/generic conversion support, so `get<std::any>()` and containers like `std::map<std::string, std::any>` fail to compile by design -- there is no way to know, from a `json` value alone, which concrete type to store inside the `std::any`. To work with heterogeneous JSON values, dispatch on the value's type manually and construct the `std::any` (or extract from it) yourself:
|
||||
`std::any` is intentionally excluded from `get<T>()`/generic conversion support, so `get<std::any>()` and containers like `std::map<std::string, std::any>` fail to compile by design — there is no way to know, from a `json` value alone, which concrete type to store inside the `std::any`. To work with heterogeneous JSON values, dispatch on the value's type manually and construct the `std::any` (or extract from it) yourself:
|
||||
|
||||
```
|
||||
std::any value_to_any(const json& j) {
|
||||
@@ -539,7 +560,7 @@ A `std::map`/`std::unordered_map` whose key type is not string-like (e.g., `std:
|
||||
|
||||
## Why does `std::wstring` convert or dump incorrectly?
|
||||
|
||||
The library assumes UTF-8 encoding internally, so `std::wstring` is not supported out of the box -- see the FAQ entry on [wide string handling](https://json.nlohmann.me/home/faq/#wide-string-handling) for why, and for a UTF-8 conversion recipe.
|
||||
The library assumes UTF-8 encoding internally, so `std::wstring` is not supported out of the box — see the FAQ entry on [wide string handling](https://json.nlohmann.me/home/faq/#wide-string-handling) for why, and for a UTF-8 conversion recipe.
|
||||
|
||||
## Can I write my own serializer? (Advanced use)
|
||||
|
||||
@@ -604,3 +625,11 @@ struct bad_serializer
|
||||
- [Specializing enum conversion](https://json.nlohmann.me/features/enum_conversion/index.md) - map enums to JSON strings instead of integers
|
||||
- [Supported macros](https://json.nlohmann.me/features/macros/index.md) - reference for `NLOHMANN_DEFINE_TYPE_*` and related macros
|
||||
- [`adl_serializer`](https://json.nlohmann.me/api/adl_serializer/index.md) - the default `JSONSerializer` used in the conversion dispatch
|
||||
- [`NLOHMANN_DEFINE_TYPE_INTRUSIVE`](https://json.nlohmann.me/api/macros/nlohmann_define_type_intrusive/index.md) - generate the conversion functions inside the type
|
||||
- [`NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE`](https://json.nlohmann.me/api/macros/nlohmann_define_type_non_intrusive/index.md) - generate the conversion functions outside the type
|
||||
- [`NLOHMANN_DEFINE_TYPE_WITH_NAMES`](https://json.nlohmann.me/api/macros/nlohmann_define_type_with_names/index.md) - use JSON keys that differ from the member names
|
||||
- [`NLOHMANN_DEFINE_DERIVED_TYPE`](https://json.nlohmann.me/api/macros/nlohmann_define_derived_type/index.md) - generate the conversion functions for derived classes
|
||||
- [`NLOHMANN_JSON_SERIALIZE_ENUM`](https://json.nlohmann.me/api/macros/nlohmann_json_serialize_enum/index.md) - map enum values to JSON values
|
||||
- [`get`](https://json.nlohmann.me/api/basic_json/get/index.md) - convert a JSON value to a C++ type
|
||||
- [`get_to`](https://json.nlohmann.me/api/basic_json/get_to/index.md) - convert a JSON value into an existing variable
|
||||
- [`json_serializer`](https://json.nlohmann.me/api/basic_json/json_serializer/index.md) - the `JSONSerializer` template parameter
|
||||
@@ -167,4 +167,8 @@ result in undefined behavior. Since version 3.12.0, this library checks for `nul
|
||||
|
||||
## See also
|
||||
|
||||
- [JSON_ASSERT](../api/macros/json_assert.md) - control behavior of runtime assertions
|
||||
- [`JSON_ASSERT`](../api/macros/json_assert.md) - control behavior of runtime assertions
|
||||
- [Supported Macros](macros.md) - all supported macros
|
||||
- [Working Without Exceptions](no_exceptions.md) - using the library without exceptions
|
||||
- [Exceptions](../home/exceptions.md) - all exceptions and their error codes
|
||||
- [Macros](../api/macros/index.md) - reference of all macros
|
||||
@@ -1,4 +1,4 @@
|
||||
<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/assertions/" rel="canonical"><link href=../parsing/untrusted_input/ rel=prev><link href=../serialization/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>Runtime Assertions - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.ab4e12ef.min.css><link rel="stylesheet" href="../../assets/external/fonts.googleapis.com/css.61a430c9.css"><style>:root{--md-text-font:"Roboto";--md-code-font:"JetBrains Mono"}</style><link rel=stylesheet href=../../css/custom.css><script>__md_scope=new 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15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> </form> <script>var palette=__md_get("__palette");if(palette&&palette.color){if("(prefers-color-scheme)"===palette.color.media){var media=matchMedia("(prefers-color-scheme: light)"),input=document.querySelector(media.matches?"[data-md-color-media='(prefers-color-scheme: light)']":"[data-md-color-media='(prefers-color-scheme: dark)']");palette.color.media=input.getAttribute("data-md-color-media"),palette.color.scheme=input.getAttribute("data-md-color-scheme"),palette.color.primary=input.getAttribute("data-md-color-primary"),palette.color.accent=input.getAttribute("data-md-color-accent")}for(var[key,value]of Object.entries(palette.color))document.body.setAttribute("data-md-color-"+key,value)}</script> <label class="md-header__button md-icon" for=__search> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M9.5 3A6.5 6.5 0 0 1 16 9.5c0 1.61-.59 3.09-1.56 4.23l.27.27h.79l5 5-1.5 1.5-5-5v-.79l-.27-.27A6.52 6.52 0 0 1 9.5 16 6.5 6.5 0 0 1 3 9.5 6.5 6.5 0 0 1 9.5 3m0 2C7 5 5 7 5 9.5S7 14 9.5 14 14 12 14 9.5 12 5 9.5 5"Line truncated
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12s-1.44-4.5-3.5-5.45C10.26 6.2 11.11 6 12 6a6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> </form> <script>var palette=__md_get("__palette");if(palette&&palette.color){if("(prefers-color-scheme)"===palette.color.media){var media=matchMedia("(prefers-color-scheme: light)"),input=document.querySelector(media.matches?"[data-md-color-media='(prefers-color-scheme: light)']":"[data-md-color-media='(prefers-color-scheme: dark)']");palette.color.media=input.getAttribute("data-md-color-media"),palette.color.scheme=input.getAttribute("data-md-color-scheme"),palette.color.primary=input.getAttribute("data-md-color-primary"),palette.color.accent=input.getAttribute("data-md-color-accent")}for(var[key,value]of Object.entries(palette.color))document.body.setAttribute("data-md-color-"+key,value)}</script> <label class="md-header__button md-icon" for=__search> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M9.5 3A6.5 6.5 0 0 1 16 9.5c0 1.6Line truncated
|
||||
|
||||
<span class=k>using</span><span class=w> </span><span class=n>nlohmann</span><span class=o>::</span><span class=n>json</span><span class=p>;</span>
|
||||
|
||||
@@ -54,4 +54,4 @@
|
||||
<span class=w> </span><span class=p>}</span>
|
||||
<span class=p>}</span>
|
||||
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code>[json.exception.parse_error.101] parse error: attempting to parse an empty input; check that your input string or stream contains the expected JSON
|
||||
</code></pre></div> </details> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">¶</a></h2> <ul> <li><a href=../../api/macros/json_assert/ >JSON_ASSERT</a> - control behavior of runtime assertions</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 25, 2026 11:07:51 UTC">October 25, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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|
||||
</code></pre></div> </details> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">¶</a></h2> <ul> <li><a href=../../api/macros/json_assert/ ><code>JSON_ASSERT</code></a> - control behavior of runtime assertions</li> <li><a href=../macros/ >Supported Macros</a> - all supported macros</li> <li><a href=../no_exceptions/ >Working Without Exceptions</a> - using the library without exceptions</li> <li><a href=../../home/exceptions/ >Exceptions</a> - all exceptions and their error codes</li> <li><a href=../../api/macros/ >Macros</a> - reference of all macros</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../parsing/untrusted_input/ class="md-footer__link md-footer__link--prev" aria-label="Previous: Parsing Untrusted Input"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> Parsing Untrusted Input </div> </div> </a> <a href=../serialization/ class="md-footer__link md-footer__link--next" aria-label="Next: Serialization"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> Serialization </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2025 Fonticons, Inc.--><path d="M173.9 397.4c0 2-2.3 3.6-5.2 3.6-3.3.3-5.6-1.3-5.6-3.6 0-2 2.3-3.6 5.2-3.6 3-.3 5.6 1.3 5.6 3.6m-31.1-4.5c-.7 2 1.3 4.3 4.3 4.9 2.6 1 5.6 0 6.2-2s-1.3-4.3-4.3-5.2c-2.6-.7-5.5.3-6.2 2.3m44.2-1.7c-2.9.7-4.9 2.6-4.6 4.9.3 2 2.9 3.3 5.9 2.6 2.9-.7 4.9-2.6 4.6-4.6-.3-1.9-3-3.2-5.9-2.9M252.8 8C114.1 8 8 113.3 8 252c0 110.9 69.8 205.8 169.5 239.2 12.8 2.3 17.3-5.6 17.3-12.1 0-6.2-.3-40.4-.3-61.4 0 0-70 15-84.7-29.8 0 0-11.4-29.1-27.8-36.6 0 0-22.9-15.7 1.6-15.4 0 0 24.9 2 38.6 25.8 21.9 38.6 58.6 27.5 72.9 20.9 2.3-16 8.8-27.1 16-33.7-55.9-6.2-112.3-14.3-112.3-110.5 0-27.5 7.6-41.3 23.6-58.9-2.6-6.5-11.1-33.3 2.6-67.9 20.9-6.5 69 27 69 27 20-5.6 41.5-8.5 62.8-8.5s42.8 2.9 62.8 8.5c0 0 48.1-33.6 69-27 13.7 34.7 5.2 61.4 2.6 67.9 16 17.7 25.8 31.5 25.8 58.9 0 96.5-58.9 104.2-114.8 110.5 9.2 7.9 17 22.9 17 46.4 0 33.7-.3 75.4-.3 83.6 0 6.5 4.6 14.4 17.3 12.1C436.2 457.8 504 362.9 504 252 504 113.3 391.5 8 252.8 8M105.2 352.9c-1.3 1-1 3.3.7 5.2 1.6 1.6 3.9 2.3 5.2 1 1.3-1 1-3.3-.7-5.2-1.6-1.6-3.9-2.3-5.2-1m-10.8-8.1c-.7 1.3.3 2.9 2.3 3.9 1.6 1 3.6.7 4.3-.7.7-1.3-.3-2.9-2.3-3.9-2-.6-3.6-.3-4.3.7m32.4 35.6c-1.6 1.3-1 4.3 1.3 6.2 2.3 2.3 5.2 2.6 6.5 1 1.3-1.3.7-4.3-1.3-6.2-2.2-2.3-5.2-2.6-6.5-1m-11.4-14.7c-1.6 1-1.6 3.6 0 5.9s4.3 3.3 5.6 2.3c1.6-1.3 1.6-3.9 0-6.Line truncated
|
||||
@@ -152,4 +152,8 @@ Output:
|
||||
|
||||
## See also
|
||||
|
||||
- [JSON_ASSERT](https://json.nlohmann.me/api/macros/json_assert/index.md) - control behavior of runtime assertions
|
||||
- [`JSON_ASSERT`](https://json.nlohmann.me/api/macros/json_assert/index.md) - control behavior of runtime assertions
|
||||
- [Supported Macros](https://json.nlohmann.me/features/macros/index.md) - all supported macros
|
||||
- [Working Without Exceptions](https://json.nlohmann.me/features/no_exceptions/index.md) - using the library without exceptions
|
||||
- [Exceptions](https://json.nlohmann.me/home/exceptions/index.md) - all exceptions and their error codes
|
||||
- [Macros](https://json.nlohmann.me/api/macros/index.md) - reference of all macros
|
||||
+144
-3
@@ -53,11 +53,152 @@ See [binary values](../binary_values.md) for more information.
|
||||
| UBJSON (size) | 58.6 % | 92.3 % | 86.8 % | 97.4 % |
|
||||
| UBJSON (size+type) | 55.9 % | 92.3 % | 85.0 % | 95.0 % |
|
||||
|
||||
Sizes compared to minified JSON value; the smallest size per file is printed in bold.
|
||||
<p class="nj-table-caption" markdown>Size of each test file in each format, in percent of the minified JSON value; the smallest size per file is printed in bold. The (size) and (size+type) rows are written with the `use_size` and `use_type` options of [`to_bjdata`](../../api/basic_json/to_bjdata.md) and [`to_ubjson`](../../api/basic_json/to_ubjson.md).</p>
|
||||
|
||||
<div class="nj-charts-grid-wrap" markdown>
|
||||
<div class="nj-fig-wrap" markdown>
|
||||
<svg class="nj-fig nj-charts nj-charts--sizes" viewBox="0 0 340 220" role="img" aria-labelledby="nj-charts-bs-canada nj-charts-bs-canada-d" xmlns="http://www.w3.org/2000/svg">
|
||||
<title id="nj-charts-bs-canada">Size of canada.json in each binary format relative to minified J​SON</title>
|
||||
<desc id="nj-charts-bs-canada-d">B​JData 53.2 %; B​ON8 50.5 %; B​SON 85.8 %; C​BOR 50.5 %; MessagePack 50.5 %; U​BJSON 53.2 %. Smallest: B​ON8 and C​BOR and MessagePack.</desc>
|
||||
<text class="nj-charts-panel" x="8" y="18">canada.json</text>
|
||||
<line class="nj-charts-grid" x1="104" y1="40" x2="104" y2="196"/>
|
||||
<text class="nj-charts-tick" x="104" y="211">0 %</text>
|
||||
<line class="nj-charts-grid" x1="186.727" y1="40" x2="186.727" y2="196"/>
|
||||
<text class="nj-charts-tick" x="186.727" y="211">50 %</text>
|
||||
<text class="nj-charts-rowlabel nj-charts-fmt" x="96" y="58.5">B​JData</text>
|
||||
<path class="nj-charts-c1 nj-charts-dim" d="M104 46 H188.022 Q192.022 46 192.022 50 V58 Q192.022 62 188.022 62 H104 Z"><title>canada.json, B​JData: 53.2 % of minified J​SON</title></path>
|
||||
<text class="nj-charts-rowlabel nj-charts-fmt nj-charts-best" x="96" y="84.5">B​ON8</text>
|
||||
<path class="nj-charts-c2" d="M104 72 H183.555 Q187.555 72 187.555 76 V84 Q187.555 88 183.555 88 H104 Z"><title>canada.json, B​ON8: 50.5 % of minified J​SON (smallest)</title></path>
|
||||
<text class="nj-charts-rowlabel nj-charts-fmt" x="96" y="110.5">B​SON</text>
|
||||
<path class="nj-charts-c3 nj-charts-dim" d="M104 98 H241.96 Q245.96 98 245.96 102 V110 Q245.96 114 241.96 114 H104 Z"><title>canada.json, B​SON: 85.8 % of minified J​SON</title></path>
|
||||
<text class="nj-charts-rowlabel nj-charts-fmt nj-charts-best" x="96" y="136.5">C​BOR</text>
|
||||
<path class="nj-charts-c4" d="M104 124 H183.555 Q187.555 124 187.555 128 V136 Q187.555 140 183.555 140 H104 Z"><title>canada.json, C​BOR: 50.5 % of minified J​SON (smallest)</title></path>
|
||||
<text class="nj-charts-rowlabel nj-charts-fmt nj-charts-best" x="96" y="162.5">MessagePack</text>
|
||||
<path class="nj-charts-c5" d="M104 150 H183.555 Q187.555 150 187.555 154 V162 Q187.555 166 183.555 166 H104 Z"><title>canada.json, MessagePack: 50.5 % of minified J​SON (smallest)</title></path>
|
||||
<text class="nj-charts-rowlabel nj-charts-fmt" x="96" y="188.5">U​BJSON</text>
|
||||
<path class="nj-charts-c6 nj-charts-dim" d="M104 176 H188.022 Q192.022 176 192.022 180 V188 Q192.022 192 188.022 192 H104 Z"><title>canada.json, U​BJSON: 53.2 % of minified J​SON</title></path>
|
||||
<line class="nj-charts-ref" x1="269.455" y1="36" x2="269.455" y2="196"/>
|
||||
<text class="nj-charts-tick nj-charts-tick--r" x="271.455" y="32">minified J​SON = 100 %</text>
|
||||
<text class="nj-charts-tick" x="269.455" y="211">100 %</text>
|
||||
<text class="nj-charts-val" x="197.022" y="58.5">53.2 %</text>
|
||||
<text class="nj-charts-val nj-charts-best" x="192.555" y="84.5">50.5 %</text>
|
||||
<text class="nj-charts-val" x="250.96" y="110.5">85.8 %</text>
|
||||
<text class="nj-charts-val nj-charts-best" x="192.555" y="136.5">50.5 %</text>
|
||||
<text class="nj-charts-val nj-charts-best" x="192.555" y="162.5">50.5 %</text>
|
||||
<text class="nj-charts-val" x="197.022" y="188.5">53.2 %</text>
|
||||
</svg>
|
||||
|
||||
</div>
|
||||
<div class="nj-fig-wrap" markdown>
|
||||
<svg class="nj-fig nj-charts nj-charts--sizes" viewBox="0 0 340 220" role="img" aria-labelledby="nj-charts-bs-twitter nj-charts-bs-twitter-d" xmlns="http://www.w3.org/2000/svg">
|
||||
<title id="nj-charts-bs-twitter">Size of twitter.json in each binary format relative to minified J​SON</title>
|
||||
<desc id="nj-charts-bs-twitter-d">B​JData 91.1 %; B​ON8 83.8 %; B​SON 95.2 %; C​BOR 86.3 %; MessagePack 86 %; U​BJSON 91.3 %. Smallest: B​ON8.</desc>
|
||||
<text class="nj-charts-panel" x="8" y="18">twitter.json</text>
|
||||
<line class="nj-charts-grid" x1="104" y1="40" x2="104" y2="196"/>
|
||||
<text class="nj-charts-tick" x="104" y="211">0 %</text>
|
||||
<line class="nj-charts-grid" x1="186.727" y1="40" x2="186.727" y2="196"/>
|
||||
<text class="nj-charts-tick" x="186.727" y="211">50 %</text>
|
||||
<text class="nj-charts-rowlabel nj-charts-fmt" x="96" y="58.5">B​JData</text>
|
||||
<path class="nj-charts-c1 nj-charts-dim" d="M104 46 H250.729 Q254.729 46 254.729 50 V58 Q254.729 62 250.729 62 H104 Z"><title>twitter.json, B​JData: 91.1 % of minified J​SON</title></path>
|
||||
<text class="nj-charts-rowlabel nj-charts-fmt nj-charts-best" x="96" y="84.5">B​ON8</text>
|
||||
<path class="nj-charts-c2" d="M104 72 H238.651 Q242.651 72 242.651 76 V84 Q242.651 88 238.651 88 H104 Z"><title>twitter.json, B​ON8: 83.8 % of minified J​SON (smallest)</title></path>
|
||||
<text class="nj-charts-rowlabel nj-charts-fmt" x="96" y="110.5">B​SON</text>
|
||||
<path class="nj-charts-c3 nj-charts-dim" d="M104 98 H257.513 Q261.513 98 261.513 102 V110 Q261.513 114 257.513 114 H104 Z"><title>twitter.json, B​SON: 95.2 % of minified J​SON</title></path>
|
||||
<text class="nj-charts-rowlabel nj-charts-fmt" x="96" y="136.5">C​BOR</text>
|
||||
<path class="nj-charts-c4 nj-charts-dim" d="M104 124 H242.787 Q246.787 124 246.787 128 V136 Q246.787 140 242.787 140 H104 Z"><title>twitter.json, C​BOR: 86.3 % of minified J​SON</title></path>
|
||||
<text class="nj-charts-rowlabel nj-charts-fmt" x="96" y="162.5">MessagePack</text>
|
||||
<path class="nj-charts-c5 nj-charts-dim" d="M104 150 H242.291 Q246.291 150 246.291 154 V162 Q246.291 166 242.291 166 H104 Z"><title>twitter.json, MessagePack: 86 % of minified J​SON</title></path>
|
||||
<text class="nj-charts-rowlabel nj-charts-fmt" x="96" y="188.5">U​BJSON</text>
|
||||
<path class="nj-charts-c6 nj-charts-dim" d="M104 176 H251.06 Q255.06 176 255.06 180 V188 Q255.06 192 251.06 192 H104 Z"><title>twitter.json, U​BJSON: 91.3 % of minified J​SON</title></path>
|
||||
<line class="nj-charts-ref" x1="269.455" y1="36" x2="269.455" y2="196"/>
|
||||
<text class="nj-charts-tick nj-charts-tick--r" x="271.455" y="32">minified J​SON = 100 %</text>
|
||||
<text class="nj-charts-tick" x="269.455" y="211">100 %</text>
|
||||
<text class="nj-charts-val" x="259.729" y="58.5">91.1 %</text>
|
||||
<text class="nj-charts-val nj-charts-best" x="247.651" y="84.5">83.8 %</text>
|
||||
<text class="nj-charts-val" x="266.513" y="110.5">95.2 %</text>
|
||||
<text class="nj-charts-val" x="251.787" y="136.5">86.3 %</text>
|
||||
<text class="nj-charts-val" x="251.291" y="162.5">86.0 %</text>
|
||||
<text class="nj-charts-val" x="260.06" y="188.5">91.3 %</text>
|
||||
</svg>
|
||||
|
||||
</div>
|
||||
<div class="nj-fig-wrap" markdown>
|
||||
<svg class="nj-fig nj-charts nj-charts--sizes" viewBox="0 0 340 220" role="img" aria-labelledby="nj-charts-bs-citm_catalog nj-charts-bs-citm_catalog-d" xmlns="http://www.w3.org/2000/svg">
|
||||
<title id="nj-charts-bs-citm_catalog">Size of citm_catalog.json in each binary format relative to minified J​SON</title>
|
||||
<desc id="nj-charts-bs-citm_catalog-d">B​JData 78.1 %; B​ON8 63.5 %; B​SON 95.8 %; C​BOR 68.4 %; MessagePack 68.5 %; U​BJSON 78.2 %. Smallest: B​ON8.</desc>
|
||||
<text class="nj-charts-panel" x="8" y="18">citm_catalog.json</text>
|
||||
<line class="nj-charts-grid" x1="104" y1="40" x2="104" y2="196"/>
|
||||
<text class="nj-charts-tick" x="104" y="211">0 %</text>
|
||||
<line class="nj-charts-grid" x1="186.727" y1="40" x2="186.727" y2="196"/>
|
||||
<text class="nj-charts-tick" x="186.727" y="211">50 %</text>
|
||||
<text class="nj-charts-rowlabel nj-charts-fmt" x="96" y="58.5">B​JData</text>
|
||||
<path class="nj-charts-c1 nj-charts-dim" d="M104 46 H229.22 Q233.22 46 233.22 50 V58 Q233.22 62 229.22 62 H104 Z"><title>citm_catalog.json, B​JData: 78.1 % of minified J​SON</title></path>
|
||||
<text class="nj-charts-rowlabel nj-charts-fmt nj-charts-best" x="96" y="84.5">B​ON8</text>
|
||||
<path class="nj-charts-c2" d="M104 72 H205.064 Q209.064 72 209.064 76 V84 Q209.064 88 205.064 88 H104 Z"><title>citm_catalog.json, B​ON8: 63.5 % of minified J​SON (smallest)</title></path>
|
||||
<text class="nj-charts-rowlabel nj-charts-fmt" x="96" y="110.5">B​SON</text>
|
||||
<path class="nj-charts-c3 nj-charts-dim" d="M104 98 H258.505 Q262.505 98 262.505 102 V110 Q262.505 114 258.505 114 H104 Z"><title>citm_catalog.json, B​SON: 95.8 % of minified J​SON</title></path>
|
||||
<text class="nj-charts-rowlabel nj-charts-fmt" x="96" y="136.5">C​BOR</text>
|
||||
<path class="nj-charts-c4 nj-charts-dim" d="M104 124 H213.171 Q217.171 124 217.171 128 V136 Q217.171 140 213.171 140 H104 Z"><title>citm_catalog.json, C​BOR: 68.4 % of minified J​SON</title></path>
|
||||
<text class="nj-charts-rowlabel nj-charts-fmt" x="96" y="162.5">MessagePack</text>
|
||||
<path class="nj-charts-c5 nj-charts-dim" d="M104 150 H213.336 Q217.336 150 217.336 154 V162 Q217.336 166 213.336 166 H104 Z"><title>citm_catalog.json, MessagePack: 68.5 % of minified J​SON</title></path>
|
||||
<text class="nj-charts-rowlabel nj-charts-fmt" x="96" y="188.5">U​BJSON</text>
|
||||
<path class="nj-charts-c6 nj-charts-dim" d="M104 176 H229.385 Q233.385 176 233.385 180 V188 Q233.385 192 229.385 192 H104 Z"><title>citm_catalog.json, U​BJSON: 78.2 % of minified J​SON</title></path>
|
||||
<line class="nj-charts-ref" x1="269.455" y1="36" x2="269.455" y2="196"/>
|
||||
<text class="nj-charts-tick nj-charts-tick--r" x="271.455" y="32">minified J​SON = 100 %</text>
|
||||
<text class="nj-charts-tick" x="269.455" y="211">100 %</text>
|
||||
<text class="nj-charts-val" x="238.22" y="58.5">78.1 %</text>
|
||||
<text class="nj-charts-val nj-charts-best" x="214.064" y="84.5">63.5 %</text>
|
||||
<text class="nj-charts-val" x="267.505" y="110.5">95.8 %</text>
|
||||
<text class="nj-charts-val" x="222.171" y="136.5">68.4 %</text>
|
||||
<text class="nj-charts-val" x="222.336" y="162.5">68.5 %</text>
|
||||
<text class="nj-charts-val" x="238.385" y="188.5">78.2 %</text>
|
||||
</svg>
|
||||
|
||||
</div>
|
||||
<div class="nj-fig-wrap" markdown>
|
||||
<svg class="nj-fig nj-charts nj-charts--sizes" viewBox="0 0 340 220" role="img" aria-labelledby="nj-charts-bs-jeopardy nj-charts-bs-jeopardy-d" xmlns="http://www.w3.org/2000/svg">
|
||||
<title id="nj-charts-bs-jeopardy">Size of jeopardy.json in each binary format relative to minified J​SON</title>
|
||||
<desc id="nj-charts-bs-jeopardy-d">B​JData 96.6 %; B​ON8 87.5 %; B​SON 106.7 %; C​BOR 88 %; MessagePack 87.9 %; U​BJSON 96.6 %. Smallest: B​ON8.</desc>
|
||||
<text class="nj-charts-panel" x="8" y="18">jeopardy.json</text>
|
||||
<line class="nj-charts-grid" x1="104" y1="40" x2="104" y2="196"/>
|
||||
<text class="nj-charts-tick" x="104" y="211">0 %</text>
|
||||
<line class="nj-charts-grid" x1="186.727" y1="40" x2="186.727" y2="196"/>
|
||||
<text class="nj-charts-tick" x="186.727" y="211">50 %</text>
|
||||
<text class="nj-charts-rowlabel nj-charts-fmt" x="96" y="58.5">B​JData</text>
|
||||
<path class="nj-charts-c1 nj-charts-dim" d="M104 46 H259.829 Q263.829 46 263.829 50 V58 Q263.829 62 259.829 62 H104 Z"><title>jeopardy.json, B​JData: 96.6 % of minified J​SON</title></path>
|
||||
<text class="nj-charts-rowlabel nj-charts-fmt nj-charts-best" x="96" y="84.5">B​ON8</text>
|
||||
<path class="nj-charts-c2" d="M104 72 H244.773 Q248.773 72 248.773 76 V84 Q248.773 88 244.773 88 H104 Z"><title>jeopardy.json, B​ON8: 87.5 % of minified J​SON (smallest)</title></path>
|
||||
<text class="nj-charts-rowlabel nj-charts-fmt" x="96" y="110.5">B​SON</text>
|
||||
<path class="nj-charts-c3 nj-charts-dim" d="M104 98 H276.54 Q280.54 98 280.54 102 V110 Q280.54 114 276.54 114 H104 Z"><title>jeopardy.json, B​SON: 106.7 % of minified J​SON</title></path>
|
||||
<text class="nj-charts-rowlabel nj-charts-fmt" x="96" y="136.5">C​BOR</text>
|
||||
<path class="nj-charts-c4 nj-charts-dim" d="M104 124 H245.6 Q249.6 124 249.6 128 V136 Q249.6 140 245.6 140 H104 Z"><title>jeopardy.json, C​BOR: 88 % of minified J​SON</title></path>
|
||||
<text class="nj-charts-rowlabel nj-charts-fmt" x="96" y="162.5">MessagePack</text>
|
||||
<path class="nj-charts-c5 nj-charts-dim" d="M104 150 H245.435 Q249.435 150 249.435 154 V162 Q249.435 166 245.435 166 H104 Z"><title>jeopardy.json, MessagePack: 87.9 % of minified J​SON</title></path>
|
||||
<text class="nj-charts-rowlabel nj-charts-fmt" x="96" y="188.5">U​BJSON</text>
|
||||
<path class="nj-charts-c6 nj-charts-dim" d="M104 176 H259.829 Q263.829 176 263.829 180 V188 Q263.829 192 259.829 192 H104 Z"><title>jeopardy.json, U​BJSON: 96.6 % of minified J​SON</title></path>
|
||||
<line class="nj-charts-ref" x1="269.455" y1="36" x2="269.455" y2="196"/>
|
||||
<text class="nj-charts-tick nj-charts-tick--r" x="271.455" y="32">minified J​SON = 100 %</text>
|
||||
<text class="nj-charts-tick" x="269.455" y="211">100 %</text>
|
||||
<text class="nj-charts-val" x="268.829" y="58.5">96.6 %</text>
|
||||
<text class="nj-charts-val nj-charts-best" x="253.773" y="84.5">87.5 %</text>
|
||||
<text class="nj-charts-val" x="285.54" y="110.5">106.7 %</text>
|
||||
<text class="nj-charts-val" x="254.6" y="136.5">88.0 %</text>
|
||||
<text class="nj-charts-val" x="254.435" y="162.5">87.9 %</text>
|
||||
<text class="nj-charts-val" x="268.829" y="188.5">96.6 %</text>
|
||||
</svg>
|
||||
|
||||
</div>
|
||||
</div>
|
||||
<p class="nj-charts-caption" markdown>The table above as charts: size per base format relative to minified JSON (vertical line at
|
||||
100 %); the smallest format per file is drawn in a stronger colour than the others. The size and size+type variants of BJData and UBJSON are
|
||||
omitted here; see the table.</p>
|
||||
|
||||
## See also
|
||||
|
||||
- [`cbor_tag_handler_t`](../../api/basic_json/cbor_tag_handler_t.md) - how CBOR tags are handled when parsing
|
||||
- [`JSON_STRICT_BINARY_UTF8`](../../api/macros/json_strict_binary_utf8.md) - reject ill-formed UTF-8 in strings
|
||||
- [Binary values](../binary_values.md) - binary values and subtypes
|
||||
- [**to_cbor**](../../api/basic_json/to_cbor.md) / [**from_cbor**](../../api/basic_json/from_cbor.md), [**to_msgpack**](../../api/basic_json/to_msgpack.md) / [**from_msgpack**](../../api/basic_json/from_msgpack.md), [**to_bson**](../../api/basic_json/to_bson.md) / [**from_bson**](../../api/basic_json/from_bson.md), [**to_ubjson**](../../api/basic_json/to_ubjson.md) / [**from_ubjson**](../../api/basic_json/from_ubjson.md), [**to_bjdata**](../../api/basic_json/to_bjdata.md) / [**from_bjdata**](../../api/basic_json/from_bjdata.md), [**to_bon8**](../../api/basic_json/to_bon8.md) / [**from_bon8**](../../api/basic_json/from_bon8.md) - the conversion functions
|
||||
- [Parsing untrusted input](../parsing/untrusted_input.md)
|
||||
- [`to_cbor`](../../api/basic_json/to_cbor.md) - / [`from_cbor`](../../api/basic_json/from_cbor.md), [`to_msgpack`](../../api/basic_json/to_msgpack.md) / [`from_msgpack`](../../api/basic_json/from_msgpack.md), [`to_bson`](../../api/basic_json/to_bson.md) / [`from_bson`](../../api/basic_json/from_bson.md), [`to_ubjson`](../../api/basic_json/to_ubjson.md) / [`from_ubjson`](../../api/basic_json/from_ubjson.md), [`to_bjdata`](../../api/basic_json/to_bjdata.md) / [`from_bjdata`](../../api/basic_json/from_bjdata.md), [`to_bon8`](../../api/basic_json/to_bon8.md) / [`from_bon8`](../../api/basic_json/from_bon8.md) - the conversion functions
|
||||
- [Parsing untrusted input](../parsing/untrusted_input.md) - harden an application that parses untrusted binary data
|
||||
- [SAX interface](../parsing/sax_interface.md) - decode binary formats without building a value
|
||||
|
||||
@@ -430,3 +430,12 @@ The library maps BJData types to JSON value types as follows:
|
||||
}
|
||||
|
||||
```
|
||||
|
||||
## See also
|
||||
|
||||
- [Binary formats](index.md) - overview and comparison of the supported binary formats
|
||||
- [Binary values](../binary_values.md) - how binary values and subtypes map to BJData
|
||||
- [`to_bjdata`](../../api/basic_json/to_bjdata.md) - serialize a JSON value to BJData
|
||||
- [`from_bjdata`](../../api/basic_json/from_bjdata.md) - parse a JSON value from BJData
|
||||
- [`JSON_STRICT_BINARY_UTF8`](../../api/macros/json_strict_binary_utf8.md) - reject ill-formed UTF-8 in strings
|
||||
- [SAX interface](../parsing/sax_interface.md) - decode binary formats without building a value
|
||||
@@ -1,4 +1,4 @@
|
||||
<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/binary_formats/bjdata/" rel="canonical"><link href=../ rel=prev><link href=../bon8/ rel=next><link rel=icon href=../../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>BJData - JSON for Modern C++</title><link rel=stylesheet href=../../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../../assets/stylesheets/palette.ab4e12ef.min.css><link rel="stylesheet" href="../../../assets/external/fonts.googleapis.com/css.61a430c9.css"><style>:root{--md-text-font:"Roboto";--md-code-font:"JetBrains Mono"}</style><link rel=stylesheet href=../../../css/custom.css><script>__md_scope=new URL("../../..",location),__md_hash=e=>[...e].reduce(((e,_)=>(e<<5)-e+_.charCodeAt(0)),0),__md_get=(e,_=localStorage,t=__md_scope)=>JSON.parse(_.getItem(t.pathname+"."+e)),__md_set=(e,_,t=localStorage,a=__md_scope)=>{try{t.setItem(a.pathname+"."+e,JSON.stringify(_))}catch(e){}}</script></head> <body dir=ltr data-md-color-scheme=default data-md-color-primary=indigo data-md-color-accent=indigo> <input class=md-toggle data-md-toggle=drawer type=checkbox id=__drawer autocomplete=off> <input class=md-toggle data-md-toggle=search type=checkbox id=__search autocomplete=off> <label class=md-overlay for=__drawer></label> <div data-md-component=skip> <a href=#bjdata class=md-skip> Skip to content </a> </div> <div data-md-component=announce> </div> <header class=md-header data-md-component=header> <nav class="md-header__inner md-grid" aria-label=Header> <a href=../../.. title="JSON for Modern C++" class="md-header__button md-logo" aria-label="JSON for Modern C++" data-md-component=logo> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 8a3 3 0 0 0 3-3 3 3 0 0 0-3-3 3 3 0 0 0-3 3 3 3 0 0 0 3 3m0 3.54C9.64 9.35 6.5 8 3 8v11c3.5 0 6.64 1.35 9 3.54 2.36-2.19 5.5-3.54 9-3.54V8c-3.5 0-6.64 1.35-9 3.54"/></svg> </a> <label class="md-header__button md-icon" for=__drawer> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M3 6h18v2H3zm0 5h18v2H3zm0 5h18v2H3z"/></svg> </label> <div class=md-header__title data-md-component=header-title> <div class=md-header__ellipsis> <div class=md-header__topic> <span class=md-ellipsis> JSON for Modern C++ </span> </div> <div class=md-header__topic data-md-component=header-topic> <span class=md-ellipsis> BJData </span> </div> </div> </div> <form class=md-header__option data-md-component=palette> <input class=md-option data-md-color-media="(prefers-color-scheme: light)" data-md-color-scheme=default data-md-color-primary=indigo data-md-color-accent=indigo aria-label="Switch to dark mode" type=radio name=__palette id=__palette_0> <label class="md-header__button md-icon" title="Switch to dark mode" for=__palette_1 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 8a4 4 0 0 0-4 4 4 4 0 0 0 4 4 4 4 0 0 0 4-4 4 4 0 0 0-4-4m0 10a6 6 0 0 1-6-6 6 6 0 0 1 6-6 6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> <input class=md-option data-md-color-media="(prefers-color-scheme: dark)" data-md-color-scheme=slate data-md-color-primary=indigo data-md-color-accent=indigo aria-label="Switch to light mode" type=radio name=__palette id=__palette_1> <label class="md-header__button md-icon" title="Switch to light mode" for=__palette_0 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 18c-.89 0-1.74-.2-2.5-.55C11.56 16.5 13 14.42 13 12s-1.44-4.5-3.5-5.45C10.26 6.2 11.11 6 12 6a6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> </form> <script>var palette=__md_get("__palette");if(palette&&palette.color){if("(prefers-color-scheme)"===palette.color.media){var media=matchMedia("(prefers-color-scheme: light)"),input=document.querySelector(media.matches?"[data-md-color-media='(prefers-color-scheme: light)']":"[data-md-color-media='(prefers-color-scheme: dark)']");palette.color.media=input.getAttribute("data-md-color-media"),palette.color.scheme=input.getAttribute("data-md-color-scheme"),palette.color.primary=input.getAttribute("data-md-color-primary"),palette.color.accent=input.getAttribute("data-md-color-accent")}for(var[key,value]of Object.entries(palette.color))document.body.setAttribute("data-md-color-"+key,value)}</script> <label class="md-header__button md-icon" for=__search> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M9.5 3A6.5 6.5 0 0 1 16 9.5c0 1.61-.59 3.09-1.56 4.23l.27.27h.79l5 5-1.5 1.5-5-5v-.79l-.27-.27A6.52 6.52 0 0 1 9.5 16 6.5 6.5 0 0 1 3 9.5 6.5 6.5 0 0 1 9.5 3m0 2C7 5 5 7 5 9.5S7 14 9.5 14 14 12 14 9.5 12 5 9.5 5"/></svg> </label> <div class=md-searcLine truncated
|
||||
<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=description content="JSON for Modern C++ is a header-only C++11 library that makes JSON a first-class data type."><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/binary_formats/bjdata/" rel="canonical"><link href=../ rel=prev><link href=../bon8/ rel=next><link rel=icon href=../../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>BJData - JSON for Modern C++</title><link rel=stylesheet href=../../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../../assets/stylesheets/palette.ab4e12ef.min.css><link rel=stylesheet href=../../../css/custom.css><link rel=stylesheet href=../../../css/brand.css><link rel=stylesheet href=../../../css/fig-sax.css><link rel=stylesheet href=../../../css/fig-jsonview.css><link rel=stylesheet href=../../../css/fig-exceptions.css><link rel=stylesheet href=../../../css/fig-pointer.css><link rel=stylesheet href=../../../css/fig-charts.css><link rel=stylesheet href=../../../css/fig-integration.css><link rel=stylesheet href=../../../css/fig-macros.css><script>__md_scope=new URL("../../..",location),__md_hash=e=>[...e].reduce(((e,_)=>(e<<5)-e+_.charCodeAt(0)),0),__md_get=(e,_=localStorage,t=__md_scope)=>JSON.parse(_.getItem(t.pathname+"."+e)),__md_set=(e,_,t=localStorage,a=__md_scope)=>{try{t.setItem(a.pathname+"."+e,JSON.stringify(_))}catch(e){}}</script></head> <body dir=ltr data-md-color-scheme=default data-md-color-primary=custom data-md-color-accent=custom> <input class=md-toggle data-md-toggle=drawer type=checkbox id=__drawer autocomplete=off> <input class=md-toggle data-md-toggle=search type=checkbox id=__search autocomplete=off> <label class=md-overlay for=__drawer></label> <div data-md-component=skip> <a href=#bjdata class=md-skip> Skip to content </a> </div> <div data-md-component=announce> </div> <header class=md-header data-md-component=header> <nav class="md-header__inner md-grid" aria-label=Header> <a href=../../.. title="JSON for Modern C++" class="md-header__button md-logo" aria-label="JSON for Modern C++" data-md-component=logo> <img src=../../../images/wordmark-header.svg alt=logo> </a> <label class="md-header__button md-icon" for=__drawer> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M3 6h18v2H3zm0 5h18v2H3zm0 5h18v2H3z"/></svg> </label> <div class=md-header__title data-md-component=header-title> <div class=md-header__ellipsis> <div class=md-header__topic> <span class=md-ellipsis> JSON for Modern C++ </span> </div> <div class=md-header__topic data-md-component=header-topic> <span class=md-ellipsis> BJData </span> </div> </div> </div> <form class=md-header__option data-md-component=palette> <input class=md-option data-md-color-media="(prefers-color-scheme: light)" data-md-color-scheme=default data-md-color-primary=custom data-md-color-accent=custom aria-label="Switch to dark mode" type=radio name=__palette id=__palette_0> <label class="md-header__button md-icon" title="Switch to dark mode" for=__palette_1 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 8a4 4 0 0 0-4 4 4 4 0 0 0 4 4 4 4 0 0 0 4-4 4 4 0 0 0-4-4m0 10a6 6 0 0 1-6-6 6 6 0 0 1 6-6 6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> <input class=md-option data-md-color-media="(prefers-color-scheme: dark)" data-md-color-scheme=slate data-md-color-primary=custom data-md-color-accent=custom aria-label="Switch to light mode" type=radio name=__palette id=__palette_1> <label class="md-header__button md-icon" title="Switch to light mode" for=__palette_0 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 18c-.89 0-1.74-.2-2.5-.55C11.56 16.5 13 14.42 13 12s-1.44-4.5-3.5-5.45C10.26 6.2 11.11 6 12 6a6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> </form> <script>var palette=__md_get("__palette");if(palette&&palette.color){if("(prefers-color-scheme)"===palette.color.media){var media=matchMedia("(prefers-color-scheme: light)"),input=document.querySelector(media.matches?"[data-md-color-media='(prefers-color-scheme: light)']":"[data-md-color-media='(prefers-color-scheme: dark)']");palette.color.media=input.getAttribute("data-md-color-media"),palette.color.scheme=input.getAttribute("data-md-color-scheme"),palette.color.primary=input.getAttribute("data-md-color-primary"),palette.color.accent=input.getAttribute("data-md-color-accent")}for(var[key,value]of Object.entries(palette.color))document.body.setAttribute("data-md-color-"+key,value)}</script> <label class="md-header__button md-icon" for=__search> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M9.5 3A6.5 6.5 0 0 1 16 9.5c0 1.61-.59 3.09-1.Line truncated
|
||||
<span class=w> </span><span class=nt>"id"</span><span class=p>:</span><span class=w> </span><span class=mi>300</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"name"</span><span class=p>:</span><span class=w> </span><span class=s2>"json"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"pi"</span><span class=p>:</span><span class=w> </span><span class=mf>3.14</span><span class=p>,</span>
|
||||
@@ -104,4 +104,4 @@
|
||||
<span class=w> </span><span class=nt>"compact"</span><span class=p>:</span><span class=w> </span><span class=kc>true</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"schema"</span><span class=p>:</span><span class=w> </span><span class=mi>0</span>
|
||||
<span class=p>}</span>
|
||||
</code></pre></div> </details> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 26, 2026 17:25:46 UTC">October 26, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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|
||||
</code></pre></div> </details> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">¶</a></h2> <ul> <li><a href=../ >Binary formats</a> - overview and comparison of the supported binary formats</li> <li><a href=../../binary_values/ >Binary values</a> - how binary values and subtypes map to <abbr title="Binary JData">BJData</abbr></li> <li><a href=../../../api/basic_json/to_bjdata/ ><code>to_bjdata</code></a> - serialize a <abbr title="JavaScript Object Notation">JSON</abbr> value to <abbr title="Binary JData">BJData</abbr></li> <li><a href=../../../api/basic_json/from_bjdata/ ><code>from_bjdata</code></a> - parse a <abbr title="JavaScript Object Notation">JSON</abbr> value from <abbr title="Binary JData">BJData</abbr></li> <li><a href=../../../api/macros/json_strict_binary_utf8/ ><code>JSON_STRICT_BINARY_UTF8</code></a> - reject ill-formed <abbr title="Unicode Transformation Format">UTF</abbr>-8 in strings</li> <li><a href=../../parsing/sax_interface/ ><abbr title="Simple API for XML">SAX</abbr> interface</a> - decode binary formats without building a value</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../ class="md-footer__link md-footer__link--prev" aria-label="Previous: Binary Formats"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> Binary Formats </div> </div> </a> <a href=../bon8/ class="md-footer__link md-footer__link--next" aria-label="Next: BON8"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> BON8 </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2025 Fonticons, Inc.--><path d="M173.9 397.4c0 2-2.3 3.6-5.2 3.6-3.3.3-5.6-1.3-5.6-3.6 0-2 2.3-3.6 5.2-3.6 3-.3 5.6 1.3 5.6 3.6m-31.1-4.5c-.7 2 1.3 4.3 4.3 4.9 2.6 1 5.6 0 6.2-2s-1.3-4.3-4.3-5.2c-2.6-.7-5.5.3-6.2 2.3m44.2-1.7c-2.9.7-4.9 2.6-4.6 4.9.3 2 2.9 3.3 5.9 2.6 2.9-.7 4.9-2.6 4.6-4.6-.3-1.9-3-3.2-5.9-2.9M252.8 8C114.1 8 8 113.3 8 252c0 110.9 69.8 205.8 169.5 239.2 12.8 2.3 17.3-5.6 17.3-12.1 0-6.2-.3-40.4-.3-61.4 0 0-70 15-84.7-29.8 0 0-11.4-29.1-27.8-36.6 0 0-22.9-15.7 1.6-15.4 0 0 24.9 2 38.6 25.8 21.9 38.6 58.6 27.5 72.9 20.9 2.3-16 8.8-27.1 16-33.7-55.9-6.2-112.3-14.3-112.3-110.5 0-27.5 7.6-41.3 23.6-58.9-2.6-6.5-11.1-33.3 2.6-67.9 20.9-6.5 69 27 69 27 20-5.6 41.5-8.5 62.8-8.5s42.8 2.9 62.8 8.5c0 0 48.1-33.6 69-27 13.7 34.7 5.2 61.4 2.6 67.9 16 17.7 25.8 31.5 25.8 58.9 0 96.5-58.9 104.2-114.8 110.5 9.2 7.9 17 22.9 17 46.4 0 33.7-.3 Line truncated
|
||||
@@ -333,3 +333,12 @@ Output:
|
||||
"schema": 0
|
||||
}
|
||||
```
|
||||
|
||||
## See also
|
||||
|
||||
- [Binary formats](https://json.nlohmann.me/features/binary_formats/index.md) - overview and comparison of the supported binary formats
|
||||
- [Binary values](https://json.nlohmann.me/features/binary_values/index.md) - how binary values and subtypes map to BJData
|
||||
- [`to_bjdata`](https://json.nlohmann.me/api/basic_json/to_bjdata/index.md) - serialize a JSON value to BJData
|
||||
- [`from_bjdata`](https://json.nlohmann.me/api/basic_json/from_bjdata/index.md) - parse a JSON value from BJData
|
||||
- [`JSON_STRICT_BINARY_UTF8`](https://json.nlohmann.me/api/macros/json_strict_binary_utf8/index.md) - reject ill-formed UTF-8 in strings
|
||||
- [SAX interface](https://json.nlohmann.me/features/parsing/sax_interface/index.md) - decode binary formats without building a value
|
||||
@@ -250,3 +250,12 @@ of the message are reported as [parse_error.110](../../home/exceptions.md#jsonex
|
||||
}
|
||||
|
||||
```
|
||||
|
||||
## See also
|
||||
|
||||
- [Binary formats](index.md) - overview and comparison of the supported binary formats
|
||||
- [Binary values](../binary_values.md) - how binary values and subtypes map to BON8
|
||||
- [`to_bon8`](../../api/basic_json/to_bon8.md) - serialize a JSON value to BON8
|
||||
- [`from_bon8`](../../api/basic_json/from_bon8.md) - parse a JSON value from BON8
|
||||
- [`JSON_STRICT_BINARY_UTF8`](../../api/macros/json_strict_binary_utf8.md) - reject ill-formed UTF-8 in strings
|
||||
- [SAX interface](../parsing/sax_interface.md) - decode binary formats without building a value
|
||||
@@ -1,4 +1,4 @@
|
||||
<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/binary_formats/bon8/" rel="canonical"><link href=../bjdata/ rel=prev><link href=../bson/ rel=next><link rel=icon href=../../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>BON8 - JSON for Modern C++</title><link rel=stylesheet href=../../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../../assets/stylesheets/palette.ab4e12ef.min.css><link rel="stylesheet" href="../../../assets/external/fonts.googleapis.com/css.61a430c9.css"><style>:root{--md-text-font:"Roboto";--md-code-font:"JetBrains Mono"}</style><link rel=stylesheet href=../../../css/custom.css><script>__md_scope=new URL("../../..",location),__md_hash=e=>[...e].reduce(((e,_)=>(e<<5)-e+_.charCodeAt(0)),0),__md_get=(e,_=localStorage,t=__md_scope)=>JSON.parse(_.getItem(t.pathname+"."+e)),__md_set=(e,_,t=localStorage,a=__md_scope)=>{try{t.setItem(a.pathname+"."+e,JSON.stringify(_))}catch(e){}}</script></head> <body dir=ltr data-md-color-scheme=default data-md-color-primary=indigo data-md-color-accent=indigo> <input class=md-toggle data-md-toggle=drawer type=checkbox id=__drawer autocomplete=off> <input class=md-toggle data-md-toggle=search type=checkbox id=__search autocomplete=off> <label class=md-overlay for=__drawer></label> <div data-md-component=skip> <a href=#bon8 class=md-skip> Skip to content </a> </div> <div data-md-component=announce> </div> <header class=md-header data-md-component=header> <nav class="md-header__inner md-grid" aria-label=Header> <a href=../../.. title="JSON for Modern C++" class="md-header__button md-logo" aria-label="JSON for Modern C++" data-md-component=logo> <svg 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<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=description content="JSON for Modern C++ is a header-only C++11 library that makes JSON a first-class data type."><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/binary_formats/bon8/" rel="canonical"><link href=../bjdata/ rel=prev><link href=../bson/ rel=next><link rel=icon href=../../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>BON8 - JSON for Modern C++</title><link rel=stylesheet href=../../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../../assets/stylesheets/palette.ab4e12ef.min.css><link rel=stylesheet href=../../../css/custom.css><link rel=stylesheet href=../../../css/brand.css><link rel=stylesheet href=../../../css/fig-sax.css><link rel=stylesheet href=../../../css/fig-jsonview.css><link rel=stylesheet 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data-md-component=announce> </div> <header class=md-header data-md-component=header> <nav class="md-header__inner md-grid" aria-label=Header> <a href=../../.. title="JSON for Modern C++" class="md-header__button md-logo" aria-label="JSON for Modern C++" data-md-component=logo> <img src=../../../images/wordmark-header.svg alt=logo> </a> <label class="md-header__button md-icon" for=__drawer> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M3 6h18v2H3zm0 5h18v2H3zm0 5h18v2H3z"/></svg> </label> <div class=md-header__title data-md-component=header-title> <div class=md-header__ellipsis> <div class=md-header__topic> <span class=md-ellipsis> JSON for Modern C++ </span> </div> <div class=md-header__topic data-md-component=header-topic> <span class=md-ellipsis> BON8 </span> </div> </div> </div> <form class=md-header__option data-md-component=palette> <input class=md-option data-md-color-media="(prefers-color-scheme: light)" data-md-color-scheme=default 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12s-1.44-4.5-3.5-5.45C10.26 6.2 11.11 6 12 6a6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> </form> <script>var palette=__md_get("__palette");if(palette&&palette.color){if("(prefers-color-scheme)"===palette.color.media){var media=matchMedia("(prefers-color-scheme: light)"),input=document.querySelector(media.matches?"[data-md-color-media='(prefers-color-scheme: light)']":"[data-md-color-media='(prefers-color-scheme: dark)']");palette.color.media=input.getAttribute("data-md-color-media"),palette.color.scheme=input.getAttribute("data-md-color-scheme"),palette.color.primary=input.getAttribute("data-md-color-primary"),palette.color.accent=input.getAttribute("data-md-color-accent")}for(var[key,value]of Object.entries(palette.color))document.body.setAttribute("data-md-color-"+key,value)}</script> <label class="md-header__button md-icon" for=__search> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M9.5 3A6.5 6.5 0 0 1 16 9.5c0 1.61-.59 3.09-1.5Line truncated
|
||||
<span class=w> </span><span class=nt>"id"</span><span class=p>:</span><span class=w> </span><span class=mi>300</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"name"</span><span class=p>:</span><span class=w> </span><span class=s2>"json"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"pi"</span><span class=p>:</span><span class=w> </span><span class=mf>3.14</span><span class=p>,</span>
|
||||
@@ -56,4 +56,4 @@
|
||||
<span class=w> </span><span class=nt>"format"</span><span class=p>:</span><span class=w> </span><span class=s2>"BON8"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"schema"</span><span class=p>:</span><span class=w> </span><span class=mi>0</span>
|
||||
<span class=p>}</span>
|
||||
</code></pre></div> </details> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 26, 2026 17:25:46 UTC">October 26, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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||||
</code></pre></div> </details> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">¶</a></h2> <ul> <li><a href=../ >Binary formats</a> - overview and comparison of the supported binary formats</li> <li><a href=../../binary_values/ >Binary values</a> - how binary values and subtypes map to <abbr title="Binary Object Notation 8">BON8</abbr></li> <li><a href=../../../api/basic_json/to_bon8/ ><code>to_bon8</code></a> - serialize a <abbr title="JavaScript Object Notation">JSON</abbr> value to <abbr title="Binary Object Notation 8">BON8</abbr></li> <li><a href=../../../api/basic_json/from_bon8/ ><code>from_bon8</code></a> - parse a <abbr title="JavaScript Object Notation">JSON</abbr> value from <abbr title="Binary Object Notation 8">BON8</abbr></li> <li><a href=../../../api/macros/json_strict_binary_utf8/ ><code>JSON_STRICT_BINARY_UTF8</code></a> - reject ill-formed <abbr title="Unicode Transformation Format">UTF</abbr>-8 in strings</li> <li><a href=../../parsing/sax_interface/ ><abbr title="Simple API for XML">SAX</abbr> interface</a> - decode binary formats without building a value</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../bjdata/ class="md-footer__link md-footer__link--prev" aria-label="Previous: BJData"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> BJData </div> </div> </a> <a href=../bson/ class="md-footer__link md-footer__link--next" aria-label="Next: BSON"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> BSON </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2025 Fonticons, Inc.--><path d="M173.9 397.4c0 2-2.3 3.6-5.2 3.6-3.3.3-5.6-1.3-5.6-3.6 0-2 2.3-3.6 5.2-3.6 3-.3 5.6 1.3 5.6 3.6m-31.1-4.5c-.7 2 1.3 4.3 4.3 4.9 2.6 1 5.6 0 6.2-2s-1.3-4.3-4.3-5.2c-2.6-.7-5.5.3-6.2 2.3m44.2-1.7c-2.9.7-4.9 2.6-4.6 4.9.3 2 2.9 3.3 5.9 2.6 2.9-.7 4.9-2.6 4.6-4.6-.3-1.9-3-3.2-5.9-2.9M252.8 8C114.1 8 8 113.3 8 252c0 110.9 69.8 205.8 169.5 239.2 12.8 2.3 17.3-5.6 17.3-12.1 0-6.2-.3-40.4-.3-61.4 0 0-70 15-84.7-29.8 0 0-11.4-29.1-27.8-36.6 0 0-22.9-15.7 1.6-15.4 0 0 24.9 2 38.6 25.8 21.9 38.6 58.6 27.5 72.9 20.9 2.3-16 8.8-27.1 16-33.7-55.9-6.2-112.3-14.3-112.3-110.5 0-27.5 7.6-41.3 23.6-58.9-2.6-6.5-11.1-33.3 2.6-67.9 20.9-6.5 69 27 69 27 20-5.6 41.5-8.5 62.8-8.5s42.8 2.9 62.8 8.5c0 0 48.1-33.6 69-27 13.7 34.7 5.2 61.4 2.6 67.9 16 17.7 25.8 31.5 25.8 58.9 0 96.5-58.9 104.2-114.8 110.5 9.2 7.9 17 22.9 17 46Line truncated
|
||||
@@ -222,3 +222,12 @@ Output:
|
||||
"schema": 0
|
||||
}
|
||||
```
|
||||
|
||||
## See also
|
||||
|
||||
- [Binary formats](https://json.nlohmann.me/features/binary_formats/index.md) - overview and comparison of the supported binary formats
|
||||
- [Binary values](https://json.nlohmann.me/features/binary_values/index.md) - how binary values and subtypes map to BON8
|
||||
- [`to_bon8`](https://json.nlohmann.me/api/basic_json/to_bon8/index.md) - serialize a JSON value to BON8
|
||||
- [`from_bon8`](https://json.nlohmann.me/api/basic_json/from_bon8/index.md) - parse a JSON value from BON8
|
||||
- [`JSON_STRICT_BINARY_UTF8`](https://json.nlohmann.me/api/macros/json_strict_binary_utf8/index.md) - reject ill-formed UTF-8 in strings
|
||||
- [SAX interface](https://json.nlohmann.me/features/parsing/sax_interface/index.md) - decode binary formats without building a value
|
||||
@@ -252,3 +252,12 @@ The library maps BSON record types to JSON value types as follows:
|
||||
}
|
||||
|
||||
```
|
||||
|
||||
## See also
|
||||
|
||||
- [Binary formats](index.md) - overview and comparison of the supported binary formats
|
||||
- [Binary values](../binary_values.md) - how binary values and subtypes map to BSON
|
||||
- [`to_bson`](../../api/basic_json/to_bson.md) - serialize a JSON value to BSON
|
||||
- [`from_bson`](../../api/basic_json/from_bson.md) - parse a JSON value from BSON
|
||||
- [`JSON_STRICT_BINARY_UTF8`](../../api/macros/json_strict_binary_utf8.md) - reject ill-formed UTF-8 in strings
|
||||
- [SAX interface](../parsing/sax_interface.md) - decode binary formats without building a value
|
||||
@@ -1,4 +1,4 @@
|
||||
<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/binary_formats/bson/" rel="canonical"><link href=../bon8/ rel=prev><link href=../cbor/ rel=next><link rel=icon href=../../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>BSON - JSON for Modern C++</title><link rel=stylesheet href=../../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../../assets/stylesheets/palette.ab4e12ef.min.css><link rel="stylesheet" href="../../../assets/external/fonts.googleapis.com/css.61a430c9.css"><style>:root{--md-text-font:"Roboto";--md-code-font:"JetBrains Mono"}</style><link rel=stylesheet href=../../../css/custom.css><script>__md_scope=new URL("../../..",location),__md_hash=e=>[...e].reduce(((e,_)=>(e<<5)-e+_.charCodeAt(0)),0),__md_get=(e,_=localStorage,t=__md_scope)=>JSON.parse(_.getItem(t.pathname+"."+e)),__md_set=(e,_,t=localStorage,a=__md_scope)=>{try{t.setItem(a.pathname+"."+e,JSON.stringify(_))}catch(e){}}</script></head> <body dir=ltr data-md-color-scheme=default data-md-color-primary=indigo data-md-color-accent=indigo> <input class=md-toggle data-md-toggle=drawer type=checkbox id=__drawer autocomplete=off> <input class=md-toggle data-md-toggle=search type=checkbox id=__search autocomplete=off> <label class=md-overlay for=__drawer></label> <div data-md-component=skip> <a href=#bson class=md-skip> Skip to content </a> </div> <div data-md-component=announce> </div> <header class=md-header data-md-component=header> <nav class="md-header__inner md-grid" aria-label=Header> <a href=../../.. title="JSON for Modern C++" class="md-header__button md-logo" aria-label="JSON for Modern C++" data-md-component=logo> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 8a3 3 0 0 0 3-3 3 3 0 0 0-3-3 3 3 0 0 0-3 3 3 3 0 0 0 3 3m0 3.54C9.64 9.35 6.5 8 3 8v11c3.5 0 6.64 1.35 9 3.54 2.36-2.19 5.5-3.54 9-3.54V8c-3.5 0-6.64 1.35-9 3.54"/></svg> </a> <label class="md-header__button md-icon" for=__drawer> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M3 6h18v2H3zm0 5h18v2H3zm0 5h18v2H3z"/></svg> </label> <div class=md-header__title data-md-component=header-title> <div class=md-header__ellipsis> <div class=md-header__topic> <span class=md-ellipsis> JSON for Modern C++ </span> </div> <div class=md-header__topic data-md-component=header-topic> <span class=md-ellipsis> BSON </span> </div> </div> </div> <form class=md-header__option data-md-component=palette> <input class=md-option data-md-color-media="(prefers-color-scheme: light)" data-md-color-scheme=default data-md-color-primary=indigo data-md-color-accent=indigo aria-label="Switch to dark mode" type=radio name=__palette id=__palette_0> <label class="md-header__button md-icon" title="Switch to dark mode" for=__palette_1 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 8a4 4 0 0 0-4 4 4 4 0 0 0 4 4 4 4 0 0 0 4-4 4 4 0 0 0-4-4m0 10a6 6 0 0 1-6-6 6 6 0 0 1 6-6 6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> <input class=md-option data-md-color-media="(prefers-color-scheme: dark)" data-md-color-scheme=slate data-md-color-primary=indigo data-md-color-accent=indigo aria-label="Switch to light mode" type=radio name=__palette id=__palette_1> <label class="md-header__button md-icon" title="Switch to light mode" for=__palette_0 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 18c-.89 0-1.74-.2-2.5-.55C11.56 16.5 13 14.42 13 12s-1.44-4.5-3.5-5.45C10.26 6.2 11.11 6 12 6a6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> </form> <script>var palette=__md_get("__palette");if(palette&&palette.color){if("(prefers-color-scheme)"===palette.color.media){var media=matchMedia("(prefers-color-scheme: light)"),input=document.querySelector(media.matches?"[data-md-color-media='(prefers-color-scheme: light)']":"[data-md-color-media='(prefers-color-scheme: dark)']");palette.color.media=input.getAttribute("data-md-color-media"),palette.color.scheme=input.getAttribute("data-md-color-scheme"),palette.color.primary=input.getAttribute("data-md-color-primary"),palette.color.accent=input.getAttribute("data-md-color-accent")}for(var[key,value]of Object.entries(palette.color))document.body.setAttribute("data-md-color-"+key,value)}</script> <label class="md-header__button md-icon" for=__search> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M9.5 3A6.5 6.5 0 0 1 16 9.5c0 1.61-.59 3.09-1.56 4.23l.27.27h.79l5 5-1.5 1.5-5-5v-.79l-.27-.27A6.52 6.52 0 0 1 9.5 16 6.5 6.5 0 0 1 3 9.5 6.5 6.5 0 0 1 9.5 3m0 2C7 5 5 7 5 9.5S7 14 9.5 14 14 12 14 9.5 12 5 9.5 5"/></svg> </label> <div class=md-search dLine truncated
|
||||
<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=description content="JSON for Modern C++ is a header-only C++11 library that makes JSON a first-class data type."><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/binary_formats/bson/" rel="canonical"><link href=../bon8/ rel=prev><link href=../cbor/ rel=next><link rel=icon href=../../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>BSON - JSON for Modern C++</title><link rel=stylesheet href=../../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../../assets/stylesheets/palette.ab4e12ef.min.css><link rel=stylesheet href=../../../css/custom.css><link rel=stylesheet href=../../../css/brand.css><link rel=stylesheet href=../../../css/fig-sax.css><link rel=stylesheet href=../../../css/fig-jsonview.css><link rel=stylesheet href=../../../css/fig-exceptions.css><link rel=stylesheet href=../../../css/fig-pointer.css><link rel=stylesheet href=../../../css/fig-charts.css><link rel=stylesheet href=../../../css/fig-integration.css><link rel=stylesheet href=../../../css/fig-macros.css><script>__md_scope=new URL("../../..",location),__md_hash=e=>[...e].reduce(((e,_)=>(e<<5)-e+_.charCodeAt(0)),0),__md_get=(e,_=localStorage,t=__md_scope)=>JSON.parse(_.getItem(t.pathname+"."+e)),__md_set=(e,_,t=localStorage,a=__md_scope)=>{try{t.setItem(a.pathname+"."+e,JSON.stringify(_))}catch(e){}}</script></head> <body dir=ltr data-md-color-scheme=default data-md-color-primary=custom data-md-color-accent=custom> <input class=md-toggle data-md-toggle=drawer type=checkbox id=__drawer autocomplete=off> <input class=md-toggle data-md-toggle=search type=checkbox id=__search autocomplete=off> <label class=md-overlay for=__drawer></label> <div data-md-component=skip> <a href=#bson class=md-skip> Skip to content </a> </div> <div data-md-component=announce> </div> <header class=md-header data-md-component=header> <nav class="md-header__inner md-grid" aria-label=Header> <a href=../../.. title="JSON for Modern C++" class="md-header__button md-logo" aria-label="JSON for Modern C++" data-md-component=logo> <img src=../../../images/wordmark-header.svg alt=logo> </a> <label class="md-header__button md-icon" for=__drawer> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M3 6h18v2H3zm0 5h18v2H3zm0 5h18v2H3z"/></svg> </label> <div class=md-header__title data-md-component=header-title> <div class=md-header__ellipsis> <div class=md-header__topic> <span class=md-ellipsis> JSON for Modern C++ </span> </div> <div class=md-header__topic data-md-component=header-topic> <span class=md-ellipsis> BSON </span> </div> </div> </div> <form class=md-header__option data-md-component=palette> <input class=md-option data-md-color-media="(prefers-color-scheme: light)" data-md-color-scheme=default data-md-color-primary=custom data-md-color-accent=custom aria-label="Switch to dark mode" type=radio name=__palette id=__palette_0> <label class="md-header__button md-icon" title="Switch to dark mode" for=__palette_1 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 8a4 4 0 0 0-4 4 4 4 0 0 0 4 4 4 4 0 0 0 4-4 4 4 0 0 0-4-4m0 10a6 6 0 0 1-6-6 6 6 0 0 1 6-6 6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> <input class=md-option data-md-color-media="(prefers-color-scheme: dark)" data-md-color-scheme=slate data-md-color-primary=custom data-md-color-accent=custom aria-label="Switch to light mode" type=radio name=__palette id=__palette_1> <label class="md-header__button md-icon" title="Switch to light mode" for=__palette_0 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 18c-.89 0-1.74-.2-2.5-.55C11.56 16.5 13 14.42 13 12s-1.44-4.5-3.5-5.45C10.26 6.2 11.11 6 12 6a6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> </form> <script>var palette=__md_get("__palette");if(palette&&palette.color){if("(prefers-color-scheme)"===palette.color.media){var media=matchMedia("(prefers-color-scheme: light)"),input=document.querySelector(media.matches?"[data-md-color-media='(prefers-color-scheme: light)']":"[data-md-color-media='(prefers-color-scheme: dark)']");palette.color.media=input.getAttribute("data-md-color-media"),palette.color.scheme=input.getAttribute("data-md-color-scheme"),palette.color.primary=input.getAttribute("data-md-color-primary"),palette.color.accent=input.getAttribute("data-md-color-accent")}for(var[key,value]of Object.entries(palette.color))document.body.setAttribute("data-md-color-"+key,value)}</script> <label class="md-header__button md-icon" for=__search> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M9.5 3A6.5 6.5 0 0 1 16 9.5c0 1.61-.59 3.09-1.56 Line truncated
|
||||
<span class=w> </span><span class=nt>"id"</span><span class=p>:</span><span class=w> </span><span class=mi>300</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"name"</span><span class=p>:</span><span class=w> </span><span class=s2>"json"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"pi"</span><span class=p>:</span><span class=w> </span><span class=mf>3.14</span><span class=p>,</span>
|
||||
@@ -55,4 +55,4 @@
|
||||
<span class=w> </span><span class=nt>"compact"</span><span class=p>:</span><span class=w> </span><span class=kc>true</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"schema"</span><span class=p>:</span><span class=w> </span><span class=mi>0</span>
|
||||
<span class=p>}</span>
|
||||
</code></pre></div> </details> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 26, 2026 17:25:46 UTC">October 26, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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||||
</code></pre></div> </details> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">¶</a></h2> <ul> <li><a href=../ >Binary formats</a> - overview and comparison of the supported binary formats</li> <li><a href=../../binary_values/ >Binary values</a> - how binary values and subtypes map to <abbr title="Binary JSON">BSON</abbr></li> <li><a href=../../../api/basic_json/to_bson/ ><code>to_bson</code></a> - serialize a <abbr title="JavaScript Object Notation">JSON</abbr> value to <abbr title="Binary JSON">BSON</abbr></li> <li><a href=../../../api/basic_json/from_bson/ ><code>from_bson</code></a> - parse a <abbr title="JavaScript Object Notation">JSON</abbr> value from <abbr title="Binary JSON">BSON</abbr></li> <li><a href=../../../api/macros/json_strict_binary_utf8/ ><code>JSON_STRICT_BINARY_UTF8</code></a> - reject ill-formed <abbr title="Unicode Transformation Format">UTF</abbr>-8 in strings</li> <li><a href=../../parsing/sax_interface/ ><abbr title="Simple API for XML">SAX</abbr> interface</a> - decode binary formats without building a value</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../bon8/ class="md-footer__link md-footer__link--prev" aria-label="Previous: BON8"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> BON8 </div> </div> </a> <a href=../cbor/ class="md-footer__link md-footer__link--next" aria-label="Next: CBOR"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> CBOR </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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|
||||
@@ -203,3 +203,12 @@ Output:
|
||||
"schema": 0
|
||||
}
|
||||
```
|
||||
|
||||
## See also
|
||||
|
||||
- [Binary formats](https://json.nlohmann.me/features/binary_formats/index.md) - overview and comparison of the supported binary formats
|
||||
- [Binary values](https://json.nlohmann.me/features/binary_values/index.md) - how binary values and subtypes map to BSON
|
||||
- [`to_bson`](https://json.nlohmann.me/api/basic_json/to_bson/index.md) - serialize a JSON value to BSON
|
||||
- [`from_bson`](https://json.nlohmann.me/api/basic_json/from_bson/index.md) - parse a JSON value from BSON
|
||||
- [`JSON_STRICT_BINARY_UTF8`](https://json.nlohmann.me/api/macros/json_strict_binary_utf8/index.md) - reject ill-formed UTF-8 in strings
|
||||
- [SAX interface](https://json.nlohmann.me/features/parsing/sax_interface/index.md) - decode binary formats without building a value
|
||||
@@ -334,3 +334,13 @@ string of the same major type, as required by [RFC 8949, Section 3.2.3](https://
|
||||
}
|
||||
|
||||
```
|
||||
|
||||
## See also
|
||||
|
||||
- [Binary formats](index.md) - overview and comparison of the supported binary formats
|
||||
- [Binary values](../binary_values.md) - how binary values and subtypes map to CBOR
|
||||
- [`to_cbor`](../../api/basic_json/to_cbor.md) - serialize a JSON value to CBOR
|
||||
- [`from_cbor`](../../api/basic_json/from_cbor.md) - parse a JSON value from CBOR
|
||||
- [`JSON_STRICT_BINARY_UTF8`](../../api/macros/json_strict_binary_utf8.md) - reject ill-formed UTF-8 in strings
|
||||
- [SAX interface](../parsing/sax_interface.md) - decode binary formats without building a value
|
||||
- [`cbor_tag_handler_t`](../../api/basic_json/cbor_tag_handler_t.md) - how CBOR tags are handled when parsing
|
||||
@@ -1,4 +1,4 @@
|
||||
<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/binary_formats/cbor/" rel="canonical"><link href=../bson/ rel=prev><link href=../messagepack/ rel=next><link rel=icon href=../../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>CBOR - JSON for Modern C++</title><link rel=stylesheet href=../../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../../assets/stylesheets/palette.ab4e12ef.min.css><link rel="stylesheet" href="../../../assets/external/fonts.googleapis.com/css.61a430c9.css"><style>:root{--md-text-font:"Roboto";--md-code-font:"JetBrains Mono"}</style><link rel=stylesheet href=../../../css/custom.css><script>__md_scope=new URL("../../..",location),__md_hash=e=>[...e].reduce(((e,_)=>(e<<5)-e+_.charCodeAt(0)),0),__md_get=(e,_=localStorage,t=__md_scope)=>JSON.parse(_.getItem(t.pathname+"."+e)),__md_set=(e,_,t=localStorage,a=__md_scope)=>{try{t.setItem(a.pathname+"."+e,JSON.stringify(_))}catch(e){}}</script></head> <body dir=ltr data-md-color-scheme=default data-md-color-primary=indigo data-md-color-accent=indigo> <input class=md-toggle data-md-toggle=drawer type=checkbox id=__drawer autocomplete=off> <input class=md-toggle data-md-toggle=search type=checkbox id=__search autocomplete=off> <label class=md-overlay for=__drawer></label> <div data-md-component=skip> <a href=#cbor class=md-skip> Skip to content </a> </div> <div data-md-component=announce> </div> <header class=md-header data-md-component=header> <nav class="md-header__inner md-grid" aria-label=Header> <a href=../../.. title="JSON for Modern C++" class="md-header__button md-logo" aria-label="JSON for Modern C++" data-md-component=logo> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 8a3 3 0 0 0 3-3 3 3 0 0 0-3-3 3 3 0 0 0-3 3 3 3 0 0 0 3 3m0 3.54C9.64 9.35 6.5 8 3 8v11c3.5 0 6.64 1.35 9 3.54 2.36-2.19 5.5-3.54 9-3.54V8c-3.5 0-6.64 1.35-9 3.54"/></svg> </a> <label class="md-header__button md-icon" for=__drawer> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M3 6h18v2H3zm0 5h18v2H3zm0 5h18v2H3z"/></svg> </label> <div class=md-header__title data-md-component=header-title> <div class=md-header__ellipsis> <div class=md-header__topic> <span class=md-ellipsis> JSON for Modern C++ </span> </div> <div class=md-header__topic data-md-component=header-topic> <span class=md-ellipsis> CBOR </span> </div> </div> </div> <form class=md-header__option data-md-component=palette> <input class=md-option data-md-color-media="(prefers-color-scheme: light)" data-md-color-scheme=default data-md-color-primary=indigo data-md-color-accent=indigo aria-label="Switch to dark mode" type=radio name=__palette 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||||
<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=description content="JSON for Modern C++ is a header-only C++11 library that makes JSON a first-class data type."><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/binary_formats/cbor/" rel="canonical"><link href=../bson/ rel=prev><link href=../messagepack/ rel=next><link rel=icon href=../../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>CBOR - JSON for Modern C++</title><link rel=stylesheet href=../../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../../assets/stylesheets/palette.ab4e12ef.min.css><link rel=stylesheet href=../../../css/custom.css><link rel=stylesheet href=../../../css/brand.css><link rel=stylesheet href=../../../css/fig-sax.css><link rel=stylesheet href=../../../css/fig-jsonview.css><link rel=stylesheet href=../../../css/fig-exceptions.css><link rel=stylesheet href=../../../css/fig-pointer.css><link rel=stylesheet href=../../../css/fig-charts.css><link rel=stylesheet href=../../../css/fig-integration.css><link rel=stylesheet href=../../../css/fig-macros.css><script>__md_scope=new URL("../../..",location),__md_hash=e=>[...e].reduce(((e,_)=>(e<<5)-e+_.charCodeAt(0)),0),__md_get=(e,_=localStorage,t=__md_scope)=>JSON.parse(_.getItem(t.pathname+"."+e)),__md_set=(e,_,t=localStorage,a=__md_scope)=>{try{t.setItem(a.pathname+"."+e,JSON.stringify(_))}catch(e){}}</script></head> <body dir=ltr data-md-color-scheme=default data-md-color-primary=custom data-md-color-accent=custom> <input class=md-toggle data-md-toggle=drawer type=checkbox id=__drawer autocomplete=off> <input class=md-toggle data-md-toggle=search type=checkbox id=__search autocomplete=off> <label class=md-overlay for=__drawer></label> <div data-md-component=skip> <a href=#cbor class=md-skip> Skip to content </a> </div> <div data-md-component=announce> </div> <header class=md-header data-md-component=header> <nav class="md-header__inner md-grid" aria-label=Header> <a href=../../.. title="JSON for Modern C++" class="md-header__button md-logo" aria-label="JSON for Modern C++" data-md-component=logo> <img src=../../../images/wordmark-header.svg alt=logo> </a> <label class="md-header__button md-icon" for=__drawer> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M3 6h18v2H3zm0 5h18v2H3zm0 5h18v2H3z"/></svg> </label> <div class=md-header__title data-md-component=header-title> <div class=md-header__ellipsis> <div class=md-header__topic> <span class=md-ellipsis> JSON for Modern C++ </span> </div> <div class=md-header__topic data-md-component=header-topic> <span class=md-ellipsis> CBOR </span> </div> </div> </div> <form class=md-header__option data-md-component=palette> <input class=md-option data-md-color-media="(prefers-color-scheme: light)" data-md-color-scheme=default data-md-color-primary=custom data-md-color-accent=custom aria-label="Switch to dark mode" type=radio name=__palette id=__palette_0> <label class="md-header__button md-icon" title="Switch to dark mode" for=__palette_1 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 8a4 4 0 0 0-4 4 4 4 0 0 0 4 4 4 4 0 0 0 4-4 4 4 0 0 0-4-4m0 10a6 6 0 0 1-6-6 6 6 0 0 1 6-6 6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> <input class=md-option data-md-color-media="(prefers-color-scheme: dark)" data-md-color-scheme=slate data-md-color-primary=custom data-md-color-accent=custom aria-label="Switch to light mode" type=radio name=__palette id=__palette_1> <label class="md-header__button md-icon" title="Switch to light mode" for=__palette_0 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 18c-.89 0-1.74-.2-2.5-.55C11.56 16.5 13 14.42 13 12s-1.44-4.5-3.5-5.45C10.26 6.2 11.11 6 12 6a6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> </form> <script>var palette=__md_get("__palette");if(palette&&palette.color){if("(prefers-color-scheme)"===palette.color.media){var media=matchMedia("(prefers-color-scheme: light)"),input=document.querySelector(media.matches?"[data-md-color-media='(prefers-color-scheme: light)']":"[data-md-color-media='(prefers-color-scheme: dark)']");palette.color.media=input.getAttribute("data-md-color-media"),palette.color.scheme=input.getAttribute("data-md-color-scheme"),palette.color.primary=input.getAttribute("data-md-color-primary"),palette.color.accent=input.getAttribute("data-md-color-accent")}for(var[key,value]of Object.entries(palette.color))document.body.setAttribute("data-md-color-"+key,value)}</script> <label class="md-header__button md-icon" for=__search> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M9.5 3A6.5 6.5 0 0 1 16 9.5c0 1.61-.59 3.0Line truncated
|
||||
<span class=w> </span><span class=nt>"id"</span><span class=p>:</span><span class=w> </span><span class=mi>300</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"name"</span><span class=p>:</span><span class=w> </span><span class=s2>"json"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"pi"</span><span class=p>:</span><span class=w> </span><span class=mf>3.14</span><span class=p>,</span>
|
||||
@@ -55,4 +55,4 @@
|
||||
<span class=w> </span><span class=nt>"compact"</span><span class=p>:</span><span class=w> </span><span class=kc>true</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"schema"</span><span class=p>:</span><span class=w> </span><span class=mi>0</span>
|
||||
<span class=p>}</span>
|
||||
</code></pre></div> </details> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 26, 2026 17:25:44 UTC">October 26, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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|
||||
</code></pre></div> </details> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">¶</a></h2> <ul> <li><a href=../ >Binary formats</a> - overview and comparison of the supported binary formats</li> <li><a href=../../binary_values/ >Binary values</a> - how binary values and subtypes map to <abbr title="Concise Binary Object Representation">CBOR</abbr></li> <li><a href=../../../api/basic_json/to_cbor/ ><code>to_cbor</code></a> - serialize a <abbr title="JavaScript Object Notation">JSON</abbr> value to <abbr title="Concise Binary Object Representation">CBOR</abbr></li> <li><a href=../../../api/basic_json/from_cbor/ ><code>from_cbor</code></a> - parse a <abbr title="JavaScript Object Notation">JSON</abbr> value from <abbr title="Concise Binary Object Representation">CBOR</abbr></li> <li><a href=../../../api/macros/json_strict_binary_utf8/ ><code>JSON_STRICT_BINARY_UTF8</code></a> - reject ill-formed <abbr title="Unicode Transformation Format">UTF</abbr>-8 in strings</li> <li><a href=../../parsing/sax_interface/ ><abbr title="Simple API for XML">SAX</abbr> interface</a> - decode binary formats without building a value</li> <li><a href=../../../api/basic_json/cbor_tag_handler_t/ ><code>cbor_tag_handler_t</code></a> - how <abbr title="Concise Binary Object Representation">CBOR</abbr> tags are handled when parsing</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../bson/ class="md-footer__link md-footer__link--prev" aria-label="Previous: BSON"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> BSON </div> </div> </a> <a href=../messagepack/ class="md-footer__link md-footer__link--next" aria-label="Next: MessagePack"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> MessagePack </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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|
||||
@@ -284,3 +284,13 @@ Output:
|
||||
"schema": 0
|
||||
}
|
||||
```
|
||||
|
||||
## See also
|
||||
|
||||
- [Binary formats](https://json.nlohmann.me/features/binary_formats/index.md) - overview and comparison of the supported binary formats
|
||||
- [Binary values](https://json.nlohmann.me/features/binary_values/index.md) - how binary values and subtypes map to CBOR
|
||||
- [`to_cbor`](https://json.nlohmann.me/api/basic_json/to_cbor/index.md) - serialize a JSON value to CBOR
|
||||
- [`from_cbor`](https://json.nlohmann.me/api/basic_json/from_cbor/index.md) - parse a JSON value from CBOR
|
||||
- [`JSON_STRICT_BINARY_UTF8`](https://json.nlohmann.me/api/macros/json_strict_binary_utf8/index.md) - reject ill-formed UTF-8 in strings
|
||||
- [SAX interface](https://json.nlohmann.me/features/parsing/sax_interface/index.md) - decode binary formats without building a value
|
||||
- [`cbor_tag_handler_t`](https://json.nlohmann.me/api/basic_json/cbor_tag_handler_t/index.md) - how CBOR tags are handled when parsing
|
||||
@@ -1 +1 @@
|
||||
<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/binary_formats/" rel="canonical"><link href=../type_recipes/ rel=prev><link href=bjdata/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>Binary Formats - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.ab4e12ef.min.css><link rel="stylesheet" href="../../assets/external/fonts.googleapis.com/css.61a430c9.css"><style>:root{--md-text-font:"Roboto";--md-code-font:"JetBrains Mono"}</style><link rel=stylesheet href=../../css/custom.css><script>__md_scope=new 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15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> </form> <script>var palette=__md_get("__palette");if(palette&&palette.color){if("(prefers-color-scheme)"===palette.color.media){var media=matchMedia("(prefers-color-scheme: light)"),input=document.querySelector(media.matches?"[data-md-color-media='(prefers-color-scheme: light)']":"[data-md-color-media='(prefers-color-scheme: dark)']");palette.color.media=input.getAttribute("data-md-color-media"),palette.color.scheme=input.getAttribute("data-md-color-scheme"),palette.color.primary=input.getAttribute("data-md-color-primary"),palette.color.accent=input.getAttribute("data-md-color-accent")}for(var[key,value]of Object.entries(palette.color))document.body.setAttribute("data-md-color-"+key,value)}</script> <label class="md-header__button md-icon" for=__search> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M9.5 3A6.5 6.5 0 0 1 16 9.5c0 1.61-.59 3.09-1.56 4.23l.27.27h.79l5 5-1.5 1.5-5-5v-.79l-.27-.27A6.52 6.52 0 0 1 9.5 16 6.5 6.5 0 0 1 3 9.5 6.5 6.5 0 0 1 9.5 3m0 2C7 5 5 7 5 9.5S7 14 9.5 14 14 12 14 9.5 12 5 9.5 5"/></svg> </label> <div class=Line truncated
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<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=description content="JSON for Modern C++ is a header-only C++11 library that makes JSON a first-class data type."><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/binary_formats/" rel="canonical"><link href=../type_recipes/ rel=prev><link href=bjdata/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>Binary Formats - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.ab4e12ef.min.css><link rel=stylesheet href=../../css/custom.css><link rel=stylesheet href=../../css/brand.css><link rel=stylesheet href=../../css/fig-sax.css><link rel=stylesheet href=../../css/fig-jsonview.css><link rel=stylesheet href=../../css/fig-exceptions.css><link rel=stylesheet href=../../css/fig-pointer.css><link rel=stylesheet href=../../css/fig-charts.css><link rel=stylesheet href=../../css/fig-integration.css><link rel=stylesheet href=../../css/fig-macros.css><script>__md_scope=new URL("../..",location),__md_hash=e=>[...e].reduce(((e,_)=>(e<<5)-e+_.charCodeAt(0)),0),__md_get=(e,_=localStorage,t=__md_scope)=>JSON.parse(_.getItem(t.pathname+"."+e)),__md_set=(e,_,t=localStorage,a=__md_scope)=>{try{t.setItem(a.pathname+"."+e,JSON.stringify(_))}catch(e){}}</script></head> <body dir=ltr data-md-color-scheme=default data-md-color-primary=custom data-md-color-accent=custom> <input class=md-toggle data-md-toggle=drawer type=checkbox id=__drawer autocomplete=off> <input class=md-toggle data-md-toggle=search type=checkbox id=__search autocomplete=off> <label class=md-overlay for=__drawer></label> <div data-md-component=skip> <a href=#binary-formats class=md-skip> Skip to content </a> </div> <div data-md-component=announce> </div> <header class=md-header data-md-component=header> <nav class="md-header__inner md-grid" aria-label=Header> <a href=../.. title="JSON for Modern C++" class="md-header__button md-logo" aria-label="JSON for Modern C++" data-md-component=logo> <img src=../../images/wordmark-header.svg alt=logo> </a> <label class="md-header__button md-icon" for=__drawer> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M3 6h18v2H3zm0 5h18v2H3zm0 5h18v2H3z"/></svg> </label> <div class=md-header__title data-md-component=header-title> <div class=md-header__ellipsis> <div class=md-header__topic> <span class=md-ellipsis> JSON for Modern C++ </span> </div> <div class=md-header__topic data-md-component=header-topic> <span class=md-ellipsis> Binary Formats </span> </div> </div> </div> <form class=md-header__option data-md-component=palette> <input class=md-option data-md-color-media="(prefers-color-scheme: light)" data-md-color-scheme=default data-md-color-primary=custom data-md-color-accent=custom aria-label="Switch to dark mode" type=radio name=__palette id=__palette_0> <label class="md-header__button md-icon" title="Switch to dark mode" for=__palette_1 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 8a4 4 0 0 0-4 4 4 4 0 0 0 4 4 4 4 0 0 0 4-4 4 4 0 0 0-4-4m0 10a6 6 0 0 1-6-6 6 6 0 0 1 6-6 6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> <input class=md-option data-md-color-media="(prefers-color-scheme: dark)" data-md-color-scheme=slate data-md-color-primary=custom data-md-color-accent=custom aria-label="Switch to light mode" type=radio name=__palette id=__palette_1> <label class="md-header__button md-icon" title="Switch to light mode" for=__palette_0 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 18c-.89 0-1.74-.2-2.5-.55C11.56 16.5 13 14.42 13 12s-1.44-4.5-3.5-5.45C10.26 6.2 11.11 6 12 6a6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> </form> <script>var palette=__md_get("__palette");if(palette&&palette.color){if("(prefers-color-scheme)"===palette.color.media){var media=matchMedia("(prefers-color-scheme: light)"),input=document.querySelector(media.matches?"[data-md-color-media='(prefers-color-scheme: light)']":"[data-md-color-media='(prefers-color-scheme: dark)']");palette.color.media=input.getAttribute("data-md-color-media"),palette.color.scheme=input.getAttribute("data-md-color-scheme"),palette.color.primary=input.getAttribute("data-md-color-primary"),palette.color.accent=input.getAttribute("data-md-color-accent")}for(var[key,value]of Object.entries(palette.color))document.body.setAttribute("data-md-color-"+key,value)}</script> <label class="md-header__button md-icon" for=__search> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M9.5 3A6.5 6.5 0 0 1 16 9.5c0 1.61-.59 3.09-1.56 4.23l.27.27h.Line truncated
|
||||
@@ -52,11 +52,15 @@ See [binary values](https://json.nlohmann.me/features/binary_values/index.md) fo
|
||||
| UBJSON (size) | 58.6 % | 92.3 % | 86.8 % | 97.4 % |
|
||||
| UBJSON (size+type) | 55.9 % | 92.3 % | 85.0 % | 95.0 % |
|
||||
|
||||
Sizes compared to minified JSON value; the smallest size per file is printed in bold.
|
||||
Size of each test file in each format, in percent of the minified JSON value; the smallest size per file is printed in bold. The (size) and (size+type) rows are written with the `use_size` and `use_type` options of [`to_bjdata`](https://json.nlohmann.me/api/basic_json/to_bjdata/index.md) and [`to_ubjson`](https://json.nlohmann.me/api/basic_json/to_ubjson/index.md).
|
||||
|
||||
The table above as charts: size per base format relative to minified JSON (vertical line at 100 %); the smallest format per file is drawn in a stronger colour than the others. The size and size+type variants of BJData and UBJSON are omitted here; see the table.
|
||||
|
||||
## See also
|
||||
|
||||
- [`cbor_tag_handler_t`](https://json.nlohmann.me/api/basic_json/cbor_tag_handler_t/index.md) - how CBOR tags are handled when parsing
|
||||
- [`JSON_STRICT_BINARY_UTF8`](https://json.nlohmann.me/api/macros/json_strict_binary_utf8/index.md) - reject ill-formed UTF-8 in strings
|
||||
- [Binary values](https://json.nlohmann.me/features/binary_values/index.md) - binary values and subtypes
|
||||
- [**to_cbor**](https://json.nlohmann.me/api/basic_json/to_cbor/index.md) / [**from_cbor**](https://json.nlohmann.me/api/basic_json/from_cbor/index.md), [**to_msgpack**](https://json.nlohmann.me/api/basic_json/to_msgpack/index.md) / [**from_msgpack**](https://json.nlohmann.me/api/basic_json/from_msgpack/index.md), [**to_bson**](https://json.nlohmann.me/api/basic_json/to_bson/index.md) / [**from_bson**](https://json.nlohmann.me/api/basic_json/from_bson/index.md), [**to_ubjson**](https://json.nlohmann.me/api/basic_json/to_ubjson/index.md) / [**from_ubjson**](https://json.nlohmann.me/api/basic_json/from_ubjson/index.md), [**to_bjdata**](https://json.nlohmann.me/api/basic_json/to_bjdata/index.md) / [**from_bjdata**](https://json.nlohmann.me/api/basic_json/from_bjdata/index.md), [**to_bon8**](https://json.nlohmann.me/api/basic_json/to_bon8/index.md) / [**from_bon8**](https://json.nlohmann.me/api/basic_json/from_bon8/index.md) - the conversion functions
|
||||
- [Parsing untrusted input](https://json.nlohmann.me/features/parsing/untrusted_input/index.md)
|
||||
- [`to_cbor`](https://json.nlohmann.me/api/basic_json/to_cbor/index.md) - / [`from_cbor`](https://json.nlohmann.me/api/basic_json/from_cbor/index.md), [`to_msgpack`](https://json.nlohmann.me/api/basic_json/to_msgpack/index.md) / [`from_msgpack`](https://json.nlohmann.me/api/basic_json/from_msgpack/index.md), [`to_bson`](https://json.nlohmann.me/api/basic_json/to_bson/index.md) / [`from_bson`](https://json.nlohmann.me/api/basic_json/from_bson/index.md), [`to_ubjson`](https://json.nlohmann.me/api/basic_json/to_ubjson/index.md) / [`from_ubjson`](https://json.nlohmann.me/api/basic_json/from_ubjson/index.md), [`to_bjdata`](https://json.nlohmann.me/api/basic_json/to_bjdata/index.md) / [`from_bjdata`](https://json.nlohmann.me/api/basic_json/from_bjdata/index.md), [`to_bon8`](https://json.nlohmann.me/api/basic_json/to_bon8/index.md) / [`from_bon8`](https://json.nlohmann.me/api/basic_json/from_bon8/index.md) - the conversion functions
|
||||
- [Parsing untrusted input](https://json.nlohmann.me/features/parsing/untrusted_input/index.md) - harden an application that parses untrusted binary data
|
||||
- [SAX interface](https://json.nlohmann.me/features/parsing/sax_interface/index.md) - decode binary formats without building a value
|
||||
@@ -289,3 +289,12 @@ written as the reserved negative ext types (255 as -1), so `json::binary({1,2,3,
|
||||
}
|
||||
|
||||
```
|
||||
|
||||
## See also
|
||||
|
||||
- [Binary formats](index.md) - overview and comparison of the supported binary formats
|
||||
- [Binary values](../binary_values.md) - how binary values and subtypes map to MessagePack
|
||||
- [`to_msgpack`](../../api/basic_json/to_msgpack.md) - serialize a JSON value to MessagePack
|
||||
- [`from_msgpack`](../../api/basic_json/from_msgpack.md) - parse a JSON value from MessagePack
|
||||
- [`JSON_STRICT_BINARY_UTF8`](../../api/macros/json_strict_binary_utf8.md) - reject ill-formed UTF-8 in strings
|
||||
- [SAX interface](../parsing/sax_interface.md) - decode binary formats without building a value
|
||||
@@ -1,4 +1,4 @@
|
||||
<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/binary_formats/messagepack/" rel="canonical"><link href=../cbor/ rel=prev><link href=../ubjson/ rel=next><link rel=icon href=../../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>MessagePack - JSON for Modern C++</title><link rel=stylesheet href=../../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../../assets/stylesheets/palette.ab4e12ef.min.css><link rel="stylesheet" href="../../../assets/external/fonts.googleapis.com/css.61a430c9.css"><style>:root{--md-text-font:"Roboto";--md-code-font:"JetBrains Mono"}</style><link rel=stylesheet href=../../../css/custom.css><script>__md_scope=new 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xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 8a3 3 0 0 0 3-3 3 3 0 0 0-3-3 3 3 0 0 0-3 3 3 3 0 0 0 3 3m0 3.54C9.64 9.35 6.5 8 3 8v11c3.5 0 6.64 1.35 9 3.54 2.36-2.19 5.5-3.54 9-3.54V8c-3.5 0-6.64 1.35-9 3.54"/></svg> </a> <label class="md-header__button md-icon" for=__drawer> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M3 6h18v2H3zm0 5h18v2H3zm0 5h18v2H3z"/></svg> </label> <div class=md-header__title data-md-component=header-title> <div class=md-header__ellipsis> <div class=md-header__topic> <span class=md-ellipsis> JSON for Modern C++ </span> </div> <div class=md-header__topic data-md-component=header-topic> <span class=md-ellipsis> MessagePack </span> </div> </div> </div> <form class=md-header__option data-md-component=palette> <input class=md-option data-md-color-media="(prefers-color-scheme: light)" data-md-color-scheme=default data-md-color-primary=indigo data-md-color-accent=indigo aria-label="Switch to dark mode" type=radio name=__palette id=__palette_0> <label class="md-header__button md-icon" title="Switch to dark mode" for=__palette_1 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 8a4 4 0 0 0-4 4 4 4 0 0 0 4 4 4 4 0 0 0 4-4 4 4 0 0 0-4-4m0 10a6 6 0 0 1-6-6 6 6 0 0 1 6-6 6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> <input class=md-option data-md-color-media="(prefers-color-scheme: dark)" data-md-color-scheme=slate data-md-color-primary=indigo data-md-color-accent=indigo aria-label="Switch to light mode" type=radio name=__palette id=__palette_1> <label class="md-header__button md-icon" title="Switch to light mode" for=__palette_0 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 18c-.89 0-1.74-.2-2.5-.55C11.56 16.5 13 14.42 13 12s-1.44-4.5-3.5-5.45C10.26 6.2 11.11 6 12 6a6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> </form> <script>var palette=__md_get("__palette");if(palette&&palette.color){if("(prefers-color-scheme)"===palette.color.media){var media=matchMedia("(prefers-color-scheme: light)"),input=document.querySelector(media.matches?"[data-md-color-media='(prefers-color-scheme: light)']":"[data-md-color-media='(prefers-color-scheme: dark)']");palette.color.media=input.getAttribute("data-md-color-media"),palette.color.scheme=input.getAttribute("data-md-color-scheme"),palette.color.primary=input.getAttribute("data-md-color-primary"),palette.color.accent=input.getAttribute("data-md-color-accent")}for(var[key,value]of Object.entries(palette.color))document.body.setAttribute("data-md-color-"+key,value)}</script> <label class="md-header__button md-icon" for=__search> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M9.5 3A6.5 6.5 0 0 1 16 9.5c0 1.61-.59 3.09-1.56 4.23l.27.27h.79l5 5-1.5 1.5-5-5v-.79l-.27-.27A6.52 6.52 0 0 1 9.5 16 6.5 6.5 0 0 1 3 9.5 6.5 6.5 0 0 1 9.5 3m0 2C7 5 5 7 5 9.5S7 14 9.5 14 14 12 14 9.5 12 5 9.5 5"/></svg> <Line truncated
|
||||
<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=description content="JSON for Modern C++ is a header-only C++11 library that makes JSON a first-class data type."><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/binary_formats/messagepack/" rel="canonical"><link href=../cbor/ rel=prev><link href=../ubjson/ rel=next><link rel=icon href=../../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>MessagePack - JSON for Modern C++</title><link rel=stylesheet href=../../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../../assets/stylesheets/palette.ab4e12ef.min.css><link rel=stylesheet href=../../../css/custom.css><link rel=stylesheet href=../../../css/brand.css><link rel=stylesheet href=../../../css/fig-sax.css><link rel=stylesheet href=../../../css/fig-jsonview.css><link rel=stylesheet href=../../../css/fig-exceptions.css><link rel=stylesheet href=../../../css/fig-pointer.css><link rel=stylesheet href=../../../css/fig-charts.css><link rel=stylesheet href=../../../css/fig-integration.css><link rel=stylesheet href=../../../css/fig-macros.css><script>__md_scope=new URL("../../..",location),__md_hash=e=>[...e].reduce(((e,_)=>(e<<5)-e+_.charCodeAt(0)),0),__md_get=(e,_=localStorage,t=__md_scope)=>JSON.parse(_.getItem(t.pathname+"."+e)),__md_set=(e,_,t=localStorage,a=__md_scope)=>{try{t.setItem(a.pathname+"."+e,JSON.stringify(_))}catch(e){}}</script></head> <body dir=ltr data-md-color-scheme=default data-md-color-primary=custom data-md-color-accent=custom> <input class=md-toggle data-md-toggle=drawer type=checkbox id=__drawer autocomplete=off> <input class=md-toggle data-md-toggle=search type=checkbox id=__search autocomplete=off> <label class=md-overlay for=__drawer></label> <div data-md-component=skip> <a href=#messagepack class=md-skip> Skip to content </a> </div> <div data-md-component=announce> </div> <header class=md-header data-md-component=header> <nav class="md-header__inner md-grid" aria-label=Header> <a href=../../.. title="JSON for Modern C++" class="md-header__button md-logo" aria-label="JSON for Modern C++" data-md-component=logo> <img src=../../../images/wordmark-header.svg alt=logo> </a> <label class="md-header__button md-icon" for=__drawer> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M3 6h18v2H3zm0 5h18v2H3zm0 5h18v2H3z"/></svg> </label> <div class=md-header__title data-md-component=header-title> <div class=md-header__ellipsis> <div class=md-header__topic> <span class=md-ellipsis> JSON for Modern C++ </span> </div> <div class=md-header__topic data-md-component=header-topic> <span class=md-ellipsis> MessagePack </span> </div> </div> </div> <form class=md-header__option data-md-component=palette> <input class=md-option data-md-color-media="(prefers-color-scheme: light)" data-md-color-scheme=default data-md-color-primary=custom data-md-color-accent=custom aria-label="Switch to dark mode" type=radio name=__palette id=__palette_0> <label class="md-header__button md-icon" title="Switch to dark mode" for=__palette_1 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 8a4 4 0 0 0-4 4 4 4 0 0 0 4 4 4 4 0 0 0 4-4 4 4 0 0 0-4-4m0 10a6 6 0 0 1-6-6 6 6 0 0 1 6-6 6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> <input class=md-option data-md-color-media="(prefers-color-scheme: dark)" data-md-color-scheme=slate data-md-color-primary=custom data-md-color-accent=custom aria-label="Switch to light mode" type=radio name=__palette id=__palette_1> <label class="md-header__button md-icon" title="Switch to light mode" for=__palette_0 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 18c-.89 0-1.74-.2-2.5-.55C11.56 16.5 13 14.42 13 12s-1.44-4.5-3.5-5.45C10.26 6.2 11.11 6 12 6a6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> </form> <script>var palette=__md_get("__palette");if(palette&&palette.color){if("(prefers-color-scheme)"===palette.color.media){var media=matchMedia("(prefers-color-scheme: light)"),input=document.querySelector(media.matches?"[data-md-color-media='(prefers-color-scheme: light)']":"[data-md-color-media='(prefers-color-scheme: dark)']");palette.color.media=input.getAttribute("data-md-color-media"),palette.color.scheme=input.getAttribute("data-md-color-scheme"),palette.color.primary=input.getAttribute("data-md-color-primary"),palette.color.accent=input.getAttribute("data-md-color-accent")}for(var[key,value]of Object.entries(palette.color))document.body.setAttribute("data-md-color-"+key,value)}</script> <label class="md-header__button md-icon" for=__search> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M9.5 3A6.5 6.5 0 0 Line truncated
|
||||
<span class=w> </span><span class=nt>"id"</span><span class=p>:</span><span class=w> </span><span class=mi>300</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"name"</span><span class=p>:</span><span class=w> </span><span class=s2>"json"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"pi"</span><span class=p>:</span><span class=w> </span><span class=mf>3.14</span><span class=p>,</span>
|
||||
@@ -55,4 +55,4 @@
|
||||
<span class=w> </span><span class=nt>"compact"</span><span class=p>:</span><span class=w> </span><span class=kc>true</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"schema"</span><span class=p>:</span><span class=w> </span><span class=mi>0</span>
|
||||
<span class=p>}</span>
|
||||
</code></pre></div> </details> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 26, 2026 17:25:44 UTC">October 26, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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|
||||
</code></pre></div> </details> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">¶</a></h2> <ul> <li><a href=../ >Binary formats</a> - overview and comparison of the supported binary formats</li> <li><a href=../../binary_values/ >Binary values</a> - how binary values and subtypes map to MessagePack</li> <li><a href=../../../api/basic_json/to_msgpack/ ><code>to_msgpack</code></a> - serialize a <abbr title="JavaScript Object Notation">JSON</abbr> value to MessagePack</li> <li><a href=../../../api/basic_json/from_msgpack/ ><code>from_msgpack</code></a> - parse a <abbr title="JavaScript Object Notation">JSON</abbr> value from MessagePack</li> <li><a href=../../../api/macros/json_strict_binary_utf8/ ><code>JSON_STRICT_BINARY_UTF8</code></a> - reject ill-formed <abbr title="Unicode Transformation Format">UTF</abbr>-8 in strings</li> <li><a href=../../parsing/sax_interface/ ><abbr title="Simple API for XML">SAX</abbr> interface</a> - decode binary formats without building a value</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../cbor/ class="md-footer__link md-footer__link--prev" aria-label="Previous: CBOR"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> CBOR </div> </div> </a> <a href=../ubjson/ class="md-footer__link md-footer__link--next" aria-label="Next: UBJSON"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> UBJSON </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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|
||||
@@ -242,3 +242,12 @@ Output:
|
||||
"schema": 0
|
||||
}
|
||||
```
|
||||
|
||||
## See also
|
||||
|
||||
- [Binary formats](https://json.nlohmann.me/features/binary_formats/index.md) - overview and comparison of the supported binary formats
|
||||
- [Binary values](https://json.nlohmann.me/features/binary_values/index.md) - how binary values and subtypes map to MessagePack
|
||||
- [`to_msgpack`](https://json.nlohmann.me/api/basic_json/to_msgpack/index.md) - serialize a JSON value to MessagePack
|
||||
- [`from_msgpack`](https://json.nlohmann.me/api/basic_json/from_msgpack/index.md) - parse a JSON value from MessagePack
|
||||
- [`JSON_STRICT_BINARY_UTF8`](https://json.nlohmann.me/api/macros/json_strict_binary_utf8/index.md) - reject ill-formed UTF-8 in strings
|
||||
- [SAX interface](https://json.nlohmann.me/features/parsing/sax_interface/index.md) - decode binary formats without building a value
|
||||
@@ -313,3 +313,12 @@ The library maps UBJSON types to JSON value types as follows:
|
||||
}
|
||||
|
||||
```
|
||||
|
||||
## See also
|
||||
|
||||
- [Binary formats](index.md) - overview and comparison of the supported binary formats
|
||||
- [Binary values](../binary_values.md) - how binary values and subtypes map to UBJSON
|
||||
- [`to_ubjson`](../../api/basic_json/to_ubjson.md) - serialize a JSON value to UBJSON
|
||||
- [`from_ubjson`](../../api/basic_json/from_ubjson.md) - parse a JSON value from UBJSON
|
||||
- [`JSON_STRICT_BINARY_UTF8`](../../api/macros/json_strict_binary_utf8.md) - reject ill-formed UTF-8 in strings
|
||||
- [SAX interface](../parsing/sax_interface.md) - decode binary formats without building a value
|
||||
@@ -1,4 +1,4 @@
|
||||
<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/binary_formats/ubjson/" rel="canonical"><link href=../messagepack/ rel=prev><link href=../../binary_values/ rel=next><link rel=icon href=../../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>UBJSON - JSON for Modern C++</title><link rel=stylesheet href=../../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../../assets/stylesheets/palette.ab4e12ef.min.css><link rel="stylesheet" href="../../../assets/external/fonts.googleapis.com/css.61a430c9.css"><style>:root{--md-text-font:"Roboto";--md-code-font:"JetBrains Mono"}</style><link rel=stylesheet href=../../../css/custom.css><script>__md_scope=new 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<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=description content="JSON for Modern C++ is a header-only C++11 library that makes JSON a first-class data type."><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/binary_formats/ubjson/" rel="canonical"><link href=../messagepack/ rel=prev><link href=../../binary_values/ rel=next><link rel=icon href=../../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>UBJSON - JSON for Modern C++</title><link rel=stylesheet href=../../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../../assets/stylesheets/palette.ab4e12ef.min.css><link rel=stylesheet href=../../../css/custom.css><link rel=stylesheet href=../../../css/brand.css><link rel=stylesheet href=../../../css/fig-sax.css><link rel=stylesheet href=../../../css/fig-jsonview.css><link 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</div> <div data-md-component=announce> </div> <header class=md-header data-md-component=header> <nav class="md-header__inner md-grid" aria-label=Header> <a href=../../.. title="JSON for Modern C++" class="md-header__button md-logo" aria-label="JSON for Modern C++" data-md-component=logo> <img src=../../../images/wordmark-header.svg alt=logo> </a> <label class="md-header__button md-icon" for=__drawer> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M3 6h18v2H3zm0 5h18v2H3zm0 5h18v2H3z"/></svg> </label> <div class=md-header__title data-md-component=header-title> <div class=md-header__ellipsis> <div class=md-header__topic> <span class=md-ellipsis> JSON for Modern C++ </span> </div> <div class=md-header__topic data-md-component=header-topic> <span class=md-ellipsis> UBJSON </span> </div> </div> </div> <form class=md-header__option data-md-component=palette> <input class=md-option data-md-color-media="(prefers-color-scheme: light)" data-md-color-scheme=default 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12s-1.44-4.5-3.5-5.45C10.26 6.2 11.11 6 12 6a6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> </form> <script>var palette=__md_get("__palette");if(palette&&palette.color){if("(prefers-color-scheme)"===palette.color.media){var media=matchMedia("(prefers-color-scheme: light)"),input=document.querySelector(media.matches?"[data-md-color-media='(prefers-color-scheme: light)']":"[data-md-color-media='(prefers-color-scheme: dark)']");palette.color.media=input.getAttribute("data-md-color-media"),palette.color.scheme=input.getAttribute("data-md-color-scheme"),palette.color.primary=input.getAttribute("data-md-color-primary"),palette.color.accent=input.getAttribute("data-md-color-accent")}for(var[key,value]of Object.entries(palette.color))document.body.setAttribute("data-md-color-"+key,value)}</script> <label class="md-header__button md-icon" for=__search> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M9.5 3A6.5 6.5 0 0 1 1Line truncated
|
||||
<span class=w> </span><span class=nt>"id"</span><span class=p>:</span><span class=w> </span><span class=mi>300</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"name"</span><span class=p>:</span><span class=w> </span><span class=s2>"json"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"pi"</span><span class=p>:</span><span class=w> </span><span class=mf>3.14</span><span class=p>,</span>
|
||||
@@ -99,4 +99,4 @@
|
||||
<span class=w> </span><span class=nt>"compact"</span><span class=p>:</span><span class=w> </span><span class=kc>true</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"schema"</span><span class=p>:</span><span class=w> </span><span class=mi>0</span>
|
||||
<span class=p>}</span>
|
||||
</code></pre></div> </details> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 26, 2026 17:25:45 UTC">October 26, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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||||
</code></pre></div> </details> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">¶</a></h2> <ul> <li><a href=../ >Binary formats</a> - overview and comparison of the supported binary formats</li> <li><a href=../../binary_values/ >Binary values</a> - how binary values and subtypes map to <abbr title="Universal Binary JSON">UBJSON</abbr></li> <li><a href=../../../api/basic_json/to_ubjson/ ><code>to_ubjson</code></a> - serialize a <abbr title="JavaScript Object Notation">JSON</abbr> value to <abbr title="Universal Binary JSON">UBJSON</abbr></li> <li><a href=../../../api/basic_json/from_ubjson/ ><code>from_ubjson</code></a> - parse a <abbr title="JavaScript Object Notation">JSON</abbr> value from <abbr title="Universal Binary JSON">UBJSON</abbr></li> <li><a href=../../../api/macros/json_strict_binary_utf8/ ><code>JSON_STRICT_BINARY_UTF8</code></a> - reject ill-formed <abbr title="Unicode Transformation Format">UTF</abbr>-8 in strings</li> <li><a href=../../parsing/sax_interface/ ><abbr title="Simple API for XML">SAX</abbr> interface</a> - decode binary formats without building a value</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../messagepack/ class="md-footer__link md-footer__link--prev" aria-label="Previous: MessagePack"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> MessagePack </div> </div> </a> <a href=../../binary_values/ class="md-footer__link md-footer__link--next" aria-label="Next: Binary Values"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> Binary Values </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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|
||||
@@ -261,3 +261,12 @@ Output:
|
||||
"schema": 0
|
||||
}
|
||||
```
|
||||
|
||||
## See also
|
||||
|
||||
- [Binary formats](https://json.nlohmann.me/features/binary_formats/index.md) - overview and comparison of the supported binary formats
|
||||
- [Binary values](https://json.nlohmann.me/features/binary_values/index.md) - how binary values and subtypes map to UBJSON
|
||||
- [`to_ubjson`](https://json.nlohmann.me/api/basic_json/to_ubjson/index.md) - serialize a JSON value to UBJSON
|
||||
- [`from_ubjson`](https://json.nlohmann.me/api/basic_json/from_ubjson/index.md) - parse a JSON value from UBJSON
|
||||
- [`JSON_STRICT_BINARY_UTF8`](https://json.nlohmann.me/api/macros/json_strict_binary_utf8/index.md) - reject ill-formed UTF-8 in strings
|
||||
- [SAX interface](https://json.nlohmann.me/features/parsing/sax_interface/index.md) - decode binary formats without building a value
|
||||
@@ -25,6 +25,8 @@ class vector ["std::vector<uint8_t>"]
|
||||
vector <|-- binary_t
|
||||
```
|
||||
|
||||
<p class="nj-caption" markdown>Class diagram of `json::binary_t`: it derives from `std::vector<uint8_t>` and adds functions to set, clear, query, and test an optional subtype.</p>
|
||||
|
||||
By default, binary values are stored as `std::vector<std::uint8_t>`. This type can be changed by providing a template
|
||||
parameter to the `basic_json` type. To store binary subtypes, the storage type is extended and exposed as
|
||||
[`json::binary_t`](../api/basic_json/binary_t.md):
|
||||
@@ -430,3 +432,13 @@ as an array of uint8 values. The library implements this translation.
|
||||
"binary": [202, 254, 186, 190]
|
||||
}
|
||||
```
|
||||
|
||||
## See also
|
||||
|
||||
- [Binary formats](binary_formats/index.md) - the formats that can store binary values
|
||||
- [`binary_t`](../api/basic_json/binary_t.md) - type for binary values
|
||||
- [`byte_container_with_subtype`](../api/byte_container_with_subtype/index.md) - container for binary values with subtype
|
||||
- [`binary`](../api/basic_json/binary.md) - create a binary value
|
||||
- [`get_binary`](../api/basic_json/get_binary.md) - access the stored binary value
|
||||
- [`is_binary`](../api/basic_json/is_binary.md) - check whether a value is binary
|
||||
- [`cbor_tag_handler_t`](../api/basic_json/cbor_tag_handler_t.md) - store or ignore CBOR tags when parsing
|
||||
@@ -1,4 +1,4 @@
|
||||
<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/binary_values/" rel="canonical"><link href=../binary_formats/ubjson/ rel=prev><link href=../comments/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>Binary Values - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.ab4e12ef.min.css><link rel="stylesheet" href="../../assets/external/fonts.googleapis.com/css.61a430c9.css"><style>:root{--md-text-font:"Roboto";--md-code-font:"JetBrains Mono"}</style><link rel=stylesheet href=../../css/custom.css><script>__md_scope=new 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15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> </form> <script>var palette=__md_get("__palette");if(palette&&palette.color){if("(prefers-color-scheme)"===palette.color.media){var media=matchMedia("(prefers-color-scheme: light)"),input=document.querySelector(media.matches?"[data-md-color-media='(prefers-color-scheme: light)']":"[data-md-color-media='(prefers-color-scheme: dark)']");palette.color.media=input.getAttribute("data-md-color-media"),palette.color.scheme=input.getAttribute("data-md-color-scheme"),palette.color.primary=input.getAttribute("data-md-color-primary"),palette.color.accent=input.getAttribute("data-md-color-accent")}for(var[key,value]of Object.entries(palette.color))document.body.setAttribute("data-md-color-"+key,value)}</script> <label class="md-header__button md-icon" for=__search> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M9.5 3A6.5 6.5 0 0 1 16 9.5c0 1.61-.59 3.09-1.56 4.23l.27.27h.79l5 5-1.5 1.5-5-5v-.79l-.27-.27A6.52 6.52 0 0 1 9.5 16 6.5 6.5 0 0 1 3 9.5 6.5 6.5 0 0 1 9.5 3m0 2C7 5 5 7 5 9.5S7 14 9.5 14 14 12 14 9.5 12 5 9.5 5"/></svg> </label> <Line truncated
|
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<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=description content="JSON for Modern C++ is a header-only C++11 library that makes JSON a first-class data type."><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/binary_values/" rel="canonical"><link href=../binary_formats/ubjson/ rel=prev><link href=../comments/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>Binary Values - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.ab4e12ef.min.css><link rel=stylesheet href=../../css/custom.css><link rel=stylesheet href=../../css/brand.css><link rel=stylesheet href=../../css/fig-sax.css><link rel=stylesheet href=../../css/fig-jsonview.css><link rel=stylesheet 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data-md-component=announce> </div> <header class=md-header data-md-component=header> <nav class="md-header__inner md-grid" aria-label=Header> <a href=../.. title="JSON for Modern C++" class="md-header__button md-logo" aria-label="JSON for Modern C++" data-md-component=logo> <img src=../../images/wordmark-header.svg alt=logo> </a> <label class="md-header__button md-icon" for=__drawer> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M3 6h18v2H3zm0 5h18v2H3zm0 5h18v2H3z"/></svg> </label> <div class=md-header__title data-md-component=header-title> <div class=md-header__ellipsis> <div class=md-header__topic> <span class=md-ellipsis> JSON for Modern C++ </span> </div> <div class=md-header__topic data-md-component=header-topic> <span class=md-ellipsis> Binary Values </span> </div> </div> </div> <form class=md-header__option data-md-component=palette> <input class=md-option data-md-color-media="(prefers-color-scheme: light)" data-md-color-scheme=default data-md-color-primary=custom data-md-color-accent=custom aria-label="Switch to dark mode" type=radio name=__palette id=__palette_0> <label class="md-header__button md-icon" title="Switch to dark mode" for=__palette_1 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 8a4 4 0 0 0-4 4 4 4 0 0 0 4 4 4 4 0 0 0 4-4 4 4 0 0 0-4-4m0 10a6 6 0 0 1-6-6 6 6 0 0 1 6-6 6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> <input class=md-option data-md-color-media="(prefers-color-scheme: dark)" data-md-color-scheme=slate data-md-color-primary=custom data-md-color-accent=custom aria-label="Switch to light mode" type=radio name=__palette id=__palette_1> <label class="md-header__button md-icon" title="Switch to light mode" for=__palette_0 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 18c-.89 0-1.74-.2-2.5-.55C11.56 16.5 13 14.42 13 12s-1.44-4.5-3.5-5.45C10.26 6.2 11.11 6 12 6a6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> </form> <script>var palette=__md_get("__palette");if(palette&&palette.color){if("(prefers-color-scheme)"===palette.color.media){var media=matchMedia("(prefers-color-scheme: light)"),input=document.querySelector(media.matches?"[data-md-color-media='(prefers-color-scheme: light)']":"[data-md-color-media='(prefers-color-scheme: dark)']");palette.color.media=input.getAttribute("data-md-color-media"),palette.color.scheme=input.getAttribute("data-md-color-scheme"),palette.color.primary=input.getAttribute("data-md-color-primary"),palette.color.accent=input.getAttribute("data-md-color-accent")}for(var[key,value]of Object.entries(palette.color))document.body.setAttribute("data-md-color-"+key,value)}</script> <label class="md-header__button md-icon" for=__search> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M9.5 3A6.5 6.5 0 0 1 16 9.5c0 1.61-.59 3.09-1.56 4.2Line truncated
|
||||
|
||||
class binary_t ["json::binary_t"] {
|
||||
+void set_subtype(std::uint64_t subtype)
|
||||
@@ -9,7 +9,7 @@ class binary_t ["json::binary_t"] {
|
||||
|
||||
class vector ["std::vector<uint8_t>"]
|
||||
|
||||
vector <|-- binary_t</code></pre> <p>By default, binary values are stored as <code>std::vector<std::uint8_t></code>. This type can be changed by providing a template parameter to the <code>basic_json</code> type. To store binary subtypes, the storage type is extended and exposed as <a href=../../api/basic_json/binary_t/ ><code>json::binary_t</code></a>:</p> <div class=highlight><pre><span></span><code><span class=k>auto</span><span class=w> </span><span class=n>binary</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>binary_t</span><span class=p>({</span><span class=mh>0xCA</span><span class=p>,</span><span class=w> </span><span class=mh>0xFE</span><span class=p>,</span><span class=w> </span><span class=mh>0xBA</span><span class=p>,</span><span class=w> </span><span class=mh>0xBE</span><span class=p>});</span>
|
||||
vector <|-- binary_t</code></pre> <p class=nj-caption>Class diagram of <code>json::binary_t</code>: it derives from <code>std::vector<uint8_t></code> and adds functions to set, clear, query, and test an optional subtype.</p> <p>By default, binary values are stored as <code>std::vector<std::uint8_t></code>. This type can be changed by providing a template parameter to the <code>basic_json</code> type. To store binary subtypes, the storage type is extended and exposed as <a href=../../api/basic_json/binary_t/ ><code>json::binary_t</code></a>:</p> <div class=highlight><pre><span></span><code><span class=k>auto</span><span class=w> </span><span class=n>binary</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>binary_t</span><span class=p>({</span><span class=mh>0xCA</span><span class=p>,</span><span class=w> </span><span class=mh>0xFE</span><span class=p>,</span><span class=w> </span><span class=mh>0xBA</span><span class=p>,</span><span class=w> </span><span class=mh>0xBE</span><span class=p>});</span>
|
||||
<span class=k>auto</span><span class=w> </span><span class=n>binary_with_subtype</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>binary_t</span><span class=p>({</span><span class=mh>0xCA</span><span class=p>,</span><span class=w> </span><span class=mh>0xFE</span><span class=p>,</span><span class=w> </span><span class=mh>0xBA</span><span class=p>,</span><span class=w> </span><span class=mh>0xBE</span><span class=p>},</span><span class=w> </span><span class=mi>42</span><span class=p>);</span>
|
||||
</code></pre></div> <p>There are several convenience functions to check and set the subtype:</p> <div class=highlight><pre><span></span><code><span class=n>binary</span><span class=p>.</span><span class=n>has_subtype</span><span class=p>();</span><span class=w> </span><span class=c1>// returns false</span>
|
||||
<span class=n>binary_with_subtype</span><span class=p>.</span><span class=n>has_subtype</span><span class=p>();</span><span class=w> </span><span class=c1>// returns true</span>
|
||||
@@ -173,4 +173,4 @@ vector <|-- binary_t</code></pre> <p>By default, binary values are stored as
|
||||
</code></pre></div> <p>Note that subtype (42) is <strong>not</strong> serialized and that <abbr title="Universal Binary JSON">UBJSON</abbr> has <strong>no binary type</strong>, and deserializing <code>v</code> would yield the following value:</p> <div class=highlight><pre><span></span><code><span class=p>{</span>
|
||||
<span class=w> </span><span class=nt>"binary"</span><span class=p>:</span><span class=w> </span><span class=p>[</span><span class=mi>202</span><span class=p>,</span><span class=w> </span><span class=mi>254</span><span class=p>,</span><span class=w> </span><span class=mi>186</span><span class=p>,</span><span class=w> </span><span class=mi>190</span><span class=p>]</span>
|
||||
<span class=p>}</span>
|
||||
</code></pre></div> </details> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 26, 2026 17:25:46 UTC">October 26, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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|
||||
</code></pre></div> </details> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">¶</a></h2> <ul> <li><a href=../binary_formats/ >Binary formats</a> - the formats that can store binary values</li> <li><a href=../../api/basic_json/binary_t/ ><code>binary_t</code></a> - type for binary values</li> <li><a href=../../api/byte_container_with_subtype/ ><code>byte_container_with_subtype</code></a> - container for binary values with subtype</li> <li><a href=../../api/basic_json/binary/ ><code>binary</code></a> - create a binary value</li> <li><a href=../../api/basic_json/get_binary/ ><code>get_binary</code></a> - access the stored binary value</li> <li><a href=../../api/basic_json/is_binary/ ><code>is_binary</code></a> - check whether a value is binary</li> <li><a href=../../api/basic_json/cbor_tag_handler_t/ ><code>cbor_tag_handler_t</code></a> - store or ignore <abbr title="Concise Binary Object Representation">CBOR</abbr> tags when parsing</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../binary_formats/ubjson/ class="md-footer__link md-footer__link--prev" aria-label="Previous: UBJSON"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> UBJSON </div> </div> </a> <a href=../comments/ class="md-footer__link md-footer__link--next" aria-label="Next: Comments"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> Comments </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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|
||||
@@ -21,6 +21,8 @@ class vector ["std::vector<uint8_t>"]
|
||||
vector <|-- binary_t
|
||||
```
|
||||
|
||||
Class diagram of `json::binary_t`: it derives from `std::vector<uint8_t>` and adds functions to set, clear, query, and test an optional subtype.
|
||||
|
||||
By default, binary values are stored as `std::vector<std::uint8_t>`. This type can be changed by providing a template parameter to the `basic_json` type. To store binary subtypes, the storage type is extended and exposed as [`json::binary_t`](https://json.nlohmann.me/api/basic_json/binary_t/index.md):
|
||||
|
||||
```
|
||||
@@ -396,3 +398,13 @@ Note that subtype (42) is **not** serialized and that UBJSON has **no binary typ
|
||||
"binary": [202, 254, 186, 190]
|
||||
}
|
||||
```
|
||||
|
||||
## See also
|
||||
|
||||
- [Binary formats](https://json.nlohmann.me/features/binary_formats/index.md) - the formats that can store binary values
|
||||
- [`binary_t`](https://json.nlohmann.me/api/basic_json/binary_t/index.md) - type for binary values
|
||||
- [`byte_container_with_subtype`](https://json.nlohmann.me/api/byte_container_with_subtype/index.md) - container for binary values with subtype
|
||||
- [`binary`](https://json.nlohmann.me/api/basic_json/binary/index.md) - create a binary value
|
||||
- [`get_binary`](https://json.nlohmann.me/api/basic_json/get_binary/index.md) - access the stored binary value
|
||||
- [`is_binary`](https://json.nlohmann.me/api/basic_json/is_binary/index.md) - check whether a value is binary
|
||||
- [`cbor_tag_handler_t`](https://json.nlohmann.me/api/basic_json/cbor_tag_handler_t/index.md) - store or ignore CBOR tags when parsing
|
||||
@@ -11,7 +11,7 @@ This library does not support comments *by default*. It does so for three reason
|
||||
|
||||
3. It is dangerous for interoperability if some libraries add comment support while others do not. Please check [The Harmful Consequences of the Robustness Principle](https://tools.ietf.org/html/draft-iab-protocol-maintenance-01) on this.
|
||||
|
||||
However, you can set parameter `ignore_comments` to `#!cpp true` in the [`parse`](../api/basic_json/parse.md) function to ignore `//` or `/* */` comments. Comments will then be treated as whitespace. Combined with [`ignore_trailing_commas`](trailing_commas.md) (also a `parse` parameter), this covers what is commonly referred to as **JSONC** (JSON with Comments, as used e.g. by Visual Studio Code's `.jsonc` files) -- comments and trailing commas, nothing more. This is a different, smaller extension than [JSON5](https://json5.org), which additionally allows unquoted keys, single-quoted strings, and other syntax changes that this library does not support.
|
||||
However, you can set parameter `ignore_comments` to `#!cpp true` in the [`parse`](../api/basic_json/parse.md) function to ignore `//` or `/* */` comments. Comments will then be treated as whitespace. Combined with [`ignore_trailing_commas`](trailing_commas.md) (also a `parse` parameter), this covers what is commonly referred to as **JSONC** (JSON with Comments, as used e.g. by Visual Studio Code's `.jsonc` files) — comments and trailing commas, nothing more. This is a different, smaller extension than [JSON5](https://json5.org), which additionally allows unquoted keys, single-quoted strings, and other syntax changes that this library does not support.
|
||||
|
||||
For more information, see [JSON With Commas and Comments (JWCC)](https://nigeltao.github.io/blog/2021/json-with-commas-comments.html).
|
||||
|
||||
@@ -81,3 +81,10 @@ For more information, see [JSON With Commas and Comments (JWCC)](https://nigelta
|
||||
}
|
||||
|
||||
```
|
||||
|
||||
## See also
|
||||
|
||||
- [`parse`](../api/basic_json/parse.md) - deserialize from a compatible input
|
||||
- [Trailing Commas](trailing_commas.md) - accept trailing commas in arrays and objects
|
||||
- [Parsing](parsing/index.md) - overview of the parsing functions
|
||||
- [Parsing Untrusted Input](parsing/untrusted_input.md) - limits and checks for input you do not control
|
||||
@@ -1,4 +1,4 @@
|
||||
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0 24 24"><path d="M9.5 3A6.5 6.5 0 0 1 16 9.5c0 1.61-.59 3.09-1.56 4.23l.27.27h.79l5 5-1.5 1.5-Line truncated
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<span class=w> </span><span class=c1>// update in 2006: removed Pluto</span>
|
||||
<span class=w> </span><span class=nt>"planets"</span><span class=p>:</span><span class=w> </span><span class=p>[</span><span class=s2>"Mercury"</span><span class=p>,</span><span class=w> </span><span class=s2>"Venus"</span><span class=p>,</span><span class=w> </span><span class=s2>"Earth"</span><span class=p>,</span><span class=w> </span><span class=s2>"Mars"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=s2>"Jupiter"</span><span class=p>,</span><span class=w> </span><span class=s2>"Uranus"</span><span class=p>,</span><span class=w> </span><span class=s2>"Neptune"</span><span class=w> </span><span class=cm>/*, "Pluto" */</span><span class=p>]</span>
|
||||
@@ -45,4 +45,4 @@
|
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"Neptune"
|
||||
]
|
||||
}
|
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</code></pre></div> </div> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 2, 2026 09:32:15 UTC">October 2, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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||||
</code></pre></div> </div> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">¶</a></h2> <ul> <li><a href=../../api/basic_json/parse/ ><code>parse</code></a> - deserialize from a compatible input</li> <li><a href=../trailing_commas/ >Trailing Commas</a> - accept trailing commas in arrays and objects</li> <li><a href=../parsing/ >Parsing</a> - overview of the parsing functions</li> <li><a href=../parsing/untrusted_input/ >Parsing Untrusted Input</a> - limits and checks for input you do not control</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../binary_values/ class="md-footer__link md-footer__link--prev" aria-label="Previous: Binary Values"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> Binary Values </div> </div> </a> <a href=../conversions/ class="md-footer__link md-footer__link--next" aria-label="Next: Converting values"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> Converting values </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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|
||||
@@ -12,7 +12,7 @@ This library does not support comments *by default*. It does so for three reason
|
||||
|
||||
1. It is dangerous for interoperability if some libraries add comment support while others do not. Please check [The Harmful Consequences of the Robustness Principle](https://tools.ietf.org/html/draft-iab-protocol-maintenance-01) on this.
|
||||
|
||||
However, you can set parameter `ignore_comments` to `true` in the [`parse`](https://json.nlohmann.me/api/basic_json/parse/index.md) function to ignore `//` or `/* */` comments. Comments will then be treated as whitespace. Combined with [`ignore_trailing_commas`](https://json.nlohmann.me/features/trailing_commas/index.md) (also a `parse` parameter), this covers what is commonly referred to as **JSONC** (JSON with Comments, as used e.g. by Visual Studio Code's `.jsonc` files) -- comments and trailing commas, nothing more. This is a different, smaller extension than [JSON5](https://json5.org), which additionally allows unquoted keys, single-quoted strings, and other syntax changes that this library does not support.
|
||||
However, you can set parameter `ignore_comments` to `true` in the [`parse`](https://json.nlohmann.me/api/basic_json/parse/index.md) function to ignore `//` or `/* */` comments. Comments will then be treated as whitespace. Combined with [`ignore_trailing_commas`](https://json.nlohmann.me/features/trailing_commas/index.md) (also a `parse` parameter), this covers what is commonly referred to as **JSONC** (JSON with Comments, as used e.g. by Visual Studio Code's `.jsonc` files) — comments and trailing commas, nothing more. This is a different, smaller extension than [JSON5](https://json5.org), which additionally allows unquoted keys, single-quoted strings, and other syntax changes that this library does not support.
|
||||
|
||||
For more information, see [JSON With Commas and Comments (JWCC)](https://nigeltao.github.io/blog/2021/json-with-commas-comments.html).
|
||||
|
||||
@@ -79,3 +79,10 @@ Output:
|
||||
]
|
||||
}
|
||||
```
|
||||
|
||||
## See also
|
||||
|
||||
- [`parse`](https://json.nlohmann.me/api/basic_json/parse/index.md) - deserialize from a compatible input
|
||||
- [Trailing Commas](https://json.nlohmann.me/features/trailing_commas/index.md) - accept trailing commas in arrays and objects
|
||||
- [Parsing](https://json.nlohmann.me/features/parsing/index.md) - overview of the parsing functions
|
||||
- [Parsing Untrusted Input](https://json.nlohmann.me/features/parsing/untrusted_input/index.md) - limits and checks for input you do not control
|
||||
+10
-5
@@ -247,7 +247,7 @@ which forces the explicit `get` form and can catch unintended conversions at com
|
||||
`std::optional<T>` from delegating to `get<T>()`, to avoid a constructor ambiguity with
|
||||
`std::optional<T>`'s own converting constructor from `basic_json`. As a result,
|
||||
`static_cast<std::optional<T>>(json_value)` goes through `std::optional<T>`'s own converting
|
||||
constructor rather than through `get<std::optional<T>>()`, which can behave differently -- for example,
|
||||
constructor rather than through `get<std::optional<T>>()`, which can behave differently — for example,
|
||||
with a custom `adl_serializer<std::optional<T>>` specialization. Prefer `get<std::optional<T>>()`/`get_to()`
|
||||
over `static_cast` for optional types.
|
||||
|
||||
@@ -263,7 +263,7 @@ which forces the explicit `get` form and can catch unintended conversions at com
|
||||
- The JSON array has **too many** elements: the surplus is **silently discarded**, and no exception is
|
||||
thrown.
|
||||
- The JSON array has **too few** elements: `at` throws
|
||||
[`out_of_range.401`](../home/exceptions.md#jsonexceptionout_of_range401) for the first missing index --
|
||||
[`out_of_range.401`](../home/exceptions.md#jsonexceptionout_of_range401) for the first missing index —
|
||||
an out-of-range error, not a [`type_error`](../home/exceptions.md#type-errors), even though the cause
|
||||
is a shape mismatch.
|
||||
|
||||
@@ -281,7 +281,7 @@ which forces the explicit `get` form and can catch unintended conversions at com
|
||||
|
||||
## Omitting a field when serializing `std::optional`
|
||||
|
||||
By default, `to_json` for `std::optional<T>` writes either the value or `#!json null` -- there is no built-in way
|
||||
By default, `to_json` for `std::optional<T>` writes either the value or `#!json null` — there is no built-in way
|
||||
to make a field disappear from the serialized object entirely when the `std::optional` is `std::nullopt`. Because
|
||||
a specialization of `adl_serializer<std::optional<T>>` only controls how the *value* is converted (it cannot
|
||||
prevent the containing object's `to_json` from inserting the key in the first place), omission has to be
|
||||
@@ -313,7 +313,7 @@ json j = numbers; // [1,2,3]
|
||||
!!! info "Constructing from a C++20 range view"
|
||||
|
||||
A `json` array can also be constructed directly from a C++20 range view (`std::ranges::view`), such as the result
|
||||
of `std::views::filter` or `std::views::transform` -- no intermediate container is needed:
|
||||
of `std::views::filter` or `std::views::transform` — no intermediate container is needed:
|
||||
|
||||
```cpp
|
||||
std::vector<int> nums{1, 2, 37, 42, 21};
|
||||
@@ -336,7 +336,12 @@ The conversions above are built in for standard types. To make the same syntax w
|
||||
|
||||
- [`get`](../api/basic_json/get.md) - get a copy converted to a given type
|
||||
- [`get_to`](../api/basic_json/get_to.md) - convert into an existing variable
|
||||
- [`get_ref`](../api/basic_json/get_ref.md) / [`get_ptr`](../api/basic_json/get_ptr.md) - access the stored value without copying
|
||||
- [`get_ref`](../api/basic_json/get_ref.md) - / [`get_ptr`](../api/basic_json/get_ptr.md) - access the stored value without copying
|
||||
- [Arbitrary types conversions](arbitrary_types.md) - support your own types
|
||||
- [`JSON_USE_IMPLICIT_CONVERSIONS`](../api/macros/json_use_implicit_conversions.md) - toggle implicit conversions
|
||||
- [Zero-copy JSON views](json_view.md) - get values out of a document or a view
|
||||
- [`adl_serializer`](../api/adl_serializer/index.md) - the default serializer behind the conversions
|
||||
- [`type`](../api/basic_json/type.md) - query the type of a value
|
||||
- [`JSON_DISABLE_TUPLE_REFERENCE_CONVERSION`](../api/macros/json_disable_tuple_reference_conversion.md) - toggle conversion to tuples of references
|
||||
- [`JSON_HAS_RANGE_VIEW_CONVERSION`](../api/macros/json_has_range_view_conversion.md) - toggle conversion from ranges
|
||||
|
||||
@@ -1,4 +1,4 @@
|
||||
<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/conversions/" rel="canonical"><link href=../comments/ rel=prev><link href=../creating_values/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>Converting values - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.ab4e12ef.min.css><link rel="stylesheet" href="../../assets/external/fonts.googleapis.com/css.61a430c9.css"><style>:root{--md-text-font:"Roboto";--md-code-font:"JetBrains Mono"}</style><link rel=stylesheet href=../../css/custom.css><script>__md_scope=new 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<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=description content="JSON for Modern C++ is a header-only C++11 library that makes JSON a first-class data type."><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/conversions/" rel="canonical"><link href=../comments/ rel=prev><link href=../creating_values/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>Converting values - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.ab4e12ef.min.css><link rel=stylesheet href=../../css/custom.css><link rel=stylesheet href=../../css/brand.css><link rel=stylesheet href=../../css/fig-sax.css><link rel=stylesheet href=../../css/fig-jsonview.css><link rel=stylesheet 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data-md-component=announce> </div> <header class=md-header data-md-component=header> <nav class="md-header__inner md-grid" aria-label=Header> <a href=../.. title="JSON for Modern C++" class="md-header__button md-logo" aria-label="JSON for Modern C++" data-md-component=logo> <img src=../../images/wordmark-header.svg alt=logo> </a> <label class="md-header__button md-icon" for=__drawer> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M3 6h18v2H3zm0 5h18v2H3zm0 5h18v2H3z"/></svg> </label> <div class=md-header__title data-md-component=header-title> <div class=md-header__ellipsis> <div class=md-header__topic> <span class=md-ellipsis> JSON for Modern C++ </span> </div> <div class=md-header__topic data-md-component=header-topic> <span class=md-ellipsis> Converting values </span> </div> </div> </div> <form class=md-header__option data-md-component=palette> <input class=md-option data-md-color-media="(prefers-color-scheme: light)" data-md-color-scheme=default data-md-color-primary=custom data-md-color-accent=custom aria-label="Switch to dark mode" type=radio name=__palette id=__palette_0> <label class="md-header__button md-icon" title="Switch to dark mode" for=__palette_1 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 8a4 4 0 0 0-4 4 4 4 0 0 0 4 4 4 4 0 0 0 4-4 4 4 0 0 0-4-4m0 10a6 6 0 0 1-6-6 6 6 0 0 1 6-6 6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> <input class=md-option data-md-color-media="(prefers-color-scheme: dark)" data-md-color-scheme=slate data-md-color-primary=custom data-md-color-accent=custom aria-label="Switch to light mode" type=radio name=__palette id=__palette_1> <label class="md-header__button md-icon" title="Switch to light mode" for=__palette_0 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 18c-.89 0-1.74-.2-2.5-.55C11.56 16.5 13 14.42 13 12s-1.44-4.5-3.5-5.45C10.26 6.2 11.11 6 12 6a6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> </form> <script>var palette=__md_get("__palette");if(palette&&palette.color){if("(prefers-color-scheme)"===palette.color.media){var media=matchMedia("(prefers-color-scheme: light)"),input=document.querySelector(media.matches?"[data-md-color-media='(prefers-color-scheme: light)']":"[data-md-color-media='(prefers-color-scheme: dark)']");palette.color.media=input.getAttribute("data-md-color-media"),palette.color.scheme=input.getAttribute("data-md-color-scheme"),palette.color.primary=input.getAttribute("data-md-color-primary"),palette.color.accent=input.getAttribute("data-md-color-accent")}for(var[key,value]of Object.entries(palette.color))document.body.setAttribute("data-md-color-"+key,value)}</script> <label class="md-header__button md-icon" for=__search> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M9.5 3A6.5 6.5 0 0 1 16 9.5c0 1.61-.59 3.09-1.56Line truncated
|
||||
|
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<span class=k>auto</span><span class=w> </span><span class=n>name</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>j</span><span class=p>[</span><span class=s>"name"</span><span class=p>].</span><span class=n>get</span><span class=o><</span><span class=n>std</span><span class=o>::</span><span class=n>string</span><span class=o>></span><span class=p>();</span><span class=w> </span><span class=c1>// "Mary"</span>
|
||||
<span class=k>auto</span><span class=w> </span><span class=n>age</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>j</span><span class=p>[</span><span class=s>"age"</span><span class=p>].</span><span class=n>get</span><span class=o><</span><span class=kt>int</span><span class=o>></span><span class=p>();</span><span class=w> </span><span class=c1>// 42</span>
|
||||
@@ -97,14 +97,14 @@
|
||||
<span class=n>std</span><span class=o>::</span><span class=n>optional</span><span class=o><</span><span class=n>std</span><span class=o>::</span><span class=n>string</span><span class=o>></span><span class=w> </span><span class=n>opt</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>j_null</span><span class=p>;</span><span class=w> </span><span class=c1>// ❌ throws type_error 302</span>
|
||||
</code></pre></div> <p>This is due to C++ language rules: <code>std::optional<T></code> has its own converting constructor that is chosen over <code>basic_json::operator T()</code> when both are viable. Use <code>get<std::optional<T>>()</code> or <code>get_to()</code> instead:</p> <div class=highlight><pre><span></span><code><span class=k>auto</span><span class=w> </span><span class=n>opt</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>j_null</span><span class=p>.</span><span class=n>get</span><span class=o><</span><span class=n>std</span><span class=o>::</span><span class=n>optional</span><span class=o><</span><span class=n>std</span><span class=o>::</span><span class=n>string</span><span class=o>>></span><span class=p>();</span><span class=w> </span><span class=c1>// ✅ std::nullopt</span>
|
||||
<span class=n>j_null</span><span class=p>.</span><span class=n>get_to</span><span class=p>(</span><span class=n>opt</span><span class=p>);</span><span class=w> </span><span class=c1>// ✅ std::nullopt</span>
|
||||
</code></pre></div> </div> <div class="admonition warning"> <p class=admonition-title><code>static_cast</code> and <code>get<std::optional<T>>()</code> are not guaranteed equivalent</p> <p><code>operator ValueType()</code> (used by <code>static_cast</code> and implicit conversions) intentionally excludes <code>std::optional<T></code> from delegating to <code>get<T>()</code>, to avoid a constructor ambiguity with <code>std::optional<T></code>'s own converting constructor from <code>basic_json</code>. As a result, <code>static_cast<std::optional<T>>(json_value)</code> goes through <code>std::optional<T></code>'s own converting constructor rather than through <code>get<std::optional<T>>()</code>, which can behave differently -- for example, with a custom <code>adl_serializer<std::optional<T>></code> specialization. Prefer <code>get<std::optional<T>>()</code>/<code>get_to()</code> over <code>static_cast</code> for optional types.</p> </div> <div class="admonition warning"> <p class=admonition-title>Converting to a fixed-size destination does not check the array size</p> <p>Some destination types have a size that is fixed by their C++ type rather than by the <abbr title="JavaScript Object Notation">JSON</abbr> value: <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>pair</span><span class=o><</span><span class=n>A</span><span class=p>,</span><span class=w> </span><span class=n>B</span><span class=o>></span></code>, <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>tuple</span><span class=o><</span><span class=n>Ts</span><span class=p>...</span><span class=o>></span></code>, <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>array</span><span class=o><</span><span class=n>T</span><span class=p>,</span><span class=w> </span><span class=n>N</span><span class=o>></span></code>, C arrays <code class=highlight><span class=n>T</span><span class=p>[</span><span class=n>N</span><span class=p>]</span></code>, and <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>map</span></code>/<code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>unordered_map</span></code> with a non-string key type (which is read from an array of two-element arrays). All of them read exactly as many elements as they need via <a href=../../api/basic_json/at/ ><code>at</code></a> and <strong>never compare the <abbr title="JavaScript Object Notation">JSON</abbr> array's size to that number</strong>. The two mismatch directions therefore behave differently:</p> <ul> <li>The <abbr title="JavaScript Object Notation">JSON</abbr> array has <strong>too many</strong> elements: the surplus is <strong>silently discarded</strong>, and no exception is thrown.</li> <li>The <abbr title="JavaScript Object Notation">JSON</abbr> array has <strong>too few</strong> elements: <code>at</code> throws <a href=../../home/exceptions/#jsonexceptionout_of_range401><code>out_of_range.401</code></a> for the first missing index -- an out-of-range error, not a <a href=../../home/exceptions/#type-errors><code>type_error</code></a>, even though the cause is a shape mismatch.</li> </ul> <div class=highlight><pre><span></span><code><span class=n>json</span><span class=w> </span><span class=n>j</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=p>{</span><span class=mi>1</span><span class=p>,</span><span class=w> </span><span class=mi>2</span><span class=p>,</span><span class=w> </span><span class=mi>3</span><span class=p>,</span><span class=w> </span><span class=mi>4</span><span class=p>,</span><span class=w> </span><span class=mi>5</span><span class=p>};</span>
|
||||
</code></pre></div> </div> <div class="admonition warning"> <p class=admonition-title><code>static_cast</code> and <code>get<std::optional<T>>()</code> are not guaranteed equivalent</p> <p><code>operator ValueType()</code> (used by <code>static_cast</code> and implicit conversions) intentionally excludes <code>std::optional<T></code> from delegating to <code>get<T>()</code>, to avoid a constructor ambiguity with <code>std::optional<T></code>'s own converting constructor from <code>basic_json</code>. As a result, <code>static_cast<std::optional<T>>(json_value)</code> goes through <code>std::optional<T></code>'s own converting constructor rather than through <code>get<std::optional<T>>()</code>, which can behave differently — for example, with a custom <code>adl_serializer<std::optional<T>></code> specialization. Prefer <code>get<std::optional<T>>()</code>/<code>get_to()</code> over <code>static_cast</code> for optional types.</p> </div> <div class="admonition warning"> <p class=admonition-title>Converting to a fixed-size destination does not check the array size</p> <p>Some destination types have a size that is fixed by their C++ type rather than by the <abbr title="JavaScript Object Notation">JSON</abbr> value: <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>pair</span><span class=o><</span><span class=n>A</span><span class=p>,</span><span class=w> </span><span class=n>B</span><span class=o>></span></code>, <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>tuple</span><span class=o><</span><span class=n>Ts</span><span class=p>...</span><span class=o>></span></code>, <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>array</span><span class=o><</span><span class=n>T</span><span class=p>,</span><span class=w> </span><span class=n>N</span><span class=o>></span></code>, C arrays <code class=highlight><span class=n>T</span><span class=p>[</span><span class=n>N</span><span class=p>]</span></code>, and <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>map</span></code>/<code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>unordered_map</span></code> with a non-string key type (which is read from an array of two-element arrays). All of them read exactly as many elements as they need via <a href=../../api/basic_json/at/ ><code>at</code></a> and <strong>never compare the <abbr title="JavaScript Object Notation">JSON</abbr> array's size to that number</strong>. The two mismatch directions therefore behave differently:</p> <ul> <li>The <abbr title="JavaScript Object Notation">JSON</abbr> array has <strong>too many</strong> elements: the surplus is <strong>silently discarded</strong>, and no exception is thrown.</li> <li>The <abbr title="JavaScript Object Notation">JSON</abbr> array has <strong>too few</strong> elements: <code>at</code> throws <a href=../../home/exceptions/#jsonexceptionout_of_range401><code>out_of_range.401</code></a> for the first missing index — an out-of-range error, not a <a href=../../home/exceptions/#type-errors><code>type_error</code></a>, even though the cause is a shape mismatch.</li> </ul> <div class=highlight><pre><span></span><code><span class=n>json</span><span class=w> </span><span class=n>j</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=p>{</span><span class=mi>1</span><span class=p>,</span><span class=w> </span><span class=mi>2</span><span class=p>,</span><span class=w> </span><span class=mi>3</span><span class=p>,</span><span class=w> </span><span class=mi>4</span><span class=p>,</span><span class=w> </span><span class=mi>5</span><span class=p>};</span>
|
||||
|
||||
<span class=k>auto</span><span class=w> </span><span class=n>a</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>j</span><span class=p>.</span><span class=n>get</span><span class=o><</span><span class=n>std</span><span class=o>::</span><span class=n>array</span><span class=o><</span><span class=kt>int</span><span class=p>,</span><span class=w> </span><span class=mi>3</span><span class=o>>></span><span class=p>();</span><span class=w> </span><span class=c1>// {1, 2, 3} -- elements 4 and 5 silently dropped</span>
|
||||
<span class=k>auto</span><span class=w> </span><span class=n>p</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>j</span><span class=p>.</span><span class=n>get</span><span class=o><</span><span class=n>std</span><span class=o>::</span><span class=n>pair</span><span class=o><</span><span class=kt>int</span><span class=p>,</span><span class=w> </span><span class=kt>int</span><span class=o>>></span><span class=p>();</span><span class=w> </span><span class=c1>// (1, 2) -- elements 3, 4, and 5 silently dropped</span>
|
||||
|
||||
<span class=n>json</span><span class=w> </span><span class=n>k</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=p>{</span><span class=mi>1</span><span class=p>};</span>
|
||||
<span class=k>auto</span><span class=w> </span><span class=n>q</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>k</span><span class=p>.</span><span class=n>get</span><span class=o><</span><span class=n>std</span><span class=o>::</span><span class=n>pair</span><span class=o><</span><span class=kt>int</span><span class=p>,</span><span class=w> </span><span class=kt>int</span><span class=o>>></span><span class=p>();</span><span class=w> </span><span class=c1>// ❌ throws out_of_range.401</span>
|
||||
</code></pre></div> <p>If a size mismatch is an error in your application, check the size yourself before converting.</p> </div> <h2 id=omitting-a-field-when-serializing-stdoptional>Omitting a field when serializing <code>std::optional</code><a class=headerlink href=#omitting-a-field-when-serializing-stdoptional title="Permanent link">¶</a></h2> <p>By default, <code>to_json</code> for <code>std::optional<T></code> writes either the value or <code class=highlight><span class=kc>null</span></code> -- there is no built-in way to make a field disappear from the serialized object entirely when the <code>std::optional</code> is <code>std::nullopt</code>. Because a specialization of <code>adl_serializer<std::optional<T>></code> only controls how the <em>value</em> is converted (it cannot prevent the containing object's <code>to_json</code> from inserting the key in the first place), omission has to be implemented in the <em>containing</em> type's <code>to_json</code>:</p> <div class=highlight><pre><span></span><code><span class=k>struct</span><span class=w> </span><span class=nc>person</span><span class=w> </span><span class=p>{</span>
|
||||
</code></pre></div> <p>If a size mismatch is an error in your application, check the size yourself before converting.</p> </div> <h2 id=omitting-a-field-when-serializing-stdoptional>Omitting a field when serializing <code>std::optional</code><a class=headerlink href=#omitting-a-field-when-serializing-stdoptional title="Permanent link">¶</a></h2> <p>By default, <code>to_json</code> for <code>std::optional<T></code> writes either the value or <code class=highlight><span class=kc>null</span></code> — there is no built-in way to make a field disappear from the serialized object entirely when the <code>std::optional</code> is <code>std::nullopt</code>. Because a specialization of <code>adl_serializer<std::optional<T>></code> only controls how the <em>value</em> is converted (it cannot prevent the containing object's <code>to_json</code> from inserting the key in the first place), omission has to be implemented in the <em>containing</em> type's <code>to_json</code>:</p> <div class=highlight><pre><span></span><code><span class=k>struct</span><span class=w> </span><span class=nc>person</span><span class=w> </span><span class=p>{</span>
|
||||
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>string</span><span class=w> </span><span class=n>name</span><span class=p>;</span>
|
||||
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>optional</span><span class=o><</span><span class=kt>int</span><span class=o>></span><span class=w> </span><span class=n>age</span><span class=p>;</span>
|
||||
<span class=p>};</span>
|
||||
@@ -117,7 +117,7 @@
|
||||
<span class=p>}</span>
|
||||
</code></pre></div> <h2 id=putting-values-in>Putting values in<a class=headerlink href=#putting-values-in title="Permanent link">¶</a></h2> <p>The reverse direction works the same way: assigning or constructing a <code>json</code> from a C++ value converts it to <abbr title="JavaScript Object Notation">JSON</abbr>.</p> <div class=highlight><pre><span></span><code><span class=n>std</span><span class=o>::</span><span class=n>vector</span><span class=o><</span><span class=kt>int</span><span class=o>></span><span class=w> </span><span class=n>numbers</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=p>{</span><span class=mi>1</span><span class=p>,</span><span class=w> </span><span class=mi>2</span><span class=p>,</span><span class=w> </span><span class=mi>3</span><span class=p>};</span>
|
||||
<span class=n>json</span><span class=w> </span><span class=n>j</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>numbers</span><span class=p>;</span><span class=w> </span><span class=c1>// [1,2,3]</span>
|
||||
</code></pre></div> <div class="admonition info"> <p class=admonition-title>Constructing from a C++20 range view</p> <p>A <code>json</code> array can also be constructed directly from a C++20 range view (<code>std::ranges::view</code>), such as the result of <code>std::views::filter</code> or <code>std::views::transform</code> -- no intermediate container is needed:</p> <div class=highlight><pre><span></span><code><span class=n>std</span><span class=o>::</span><span class=n>vector</span><span class=o><</span><span class=kt>int</span><span class=o>></span><span class=w> </span><span class=n>nums</span><span class=p>{</span><span class=mi>1</span><span class=p>,</span><span class=w> </span><span class=mi>2</span><span class=p>,</span><span class=w> </span><span class=mi>37</span><span class=p>,</span><span class=w> </span><span class=mi>42</span><span class=p>,</span><span class=w> </span><span class=mi>21</span><span class=p>};</span>
|
||||
</code></pre></div> <div class="admonition info"> <p class=admonition-title>Constructing from a C++20 range view</p> <p>A <code>json</code> array can also be constructed directly from a C++20 range view (<code>std::ranges::view</code>), such as the result of <code>std::views::filter</code> or <code>std::views::transform</code> — no intermediate container is needed:</p> <div class=highlight><pre><span></span><code><span class=n>std</span><span class=o>::</span><span class=n>vector</span><span class=o><</span><span class=kt>int</span><span class=o>></span><span class=w> </span><span class=n>nums</span><span class=p>{</span><span class=mi>1</span><span class=p>,</span><span class=w> </span><span class=mi>2</span><span class=p>,</span><span class=w> </span><span class=mi>37</span><span class=p>,</span><span class=w> </span><span class=mi>42</span><span class=p>,</span><span class=w> </span><span class=mi>21</span><span class=p>};</span>
|
||||
<span class=k>auto</span><span class=w> </span><span class=n>filtered</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>nums</span><span class=w> </span><span class=o>|</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>views</span><span class=o>::</span><span class=n>filter</span><span class=p>([](</span><span class=kt>int</span><span class=w> </span><span class=n>i</span><span class=p>)</span><span class=w> </span><span class=p>{</span><span class=w> </span><span class=k>return</span><span class=w> </span><span class=n>i</span><span class=w> </span><span class=o>></span><span class=w> </span><span class=mi>10</span><span class=p>;</span><span class=w> </span><span class=p>});</span>
|
||||
<span class=n>json</span><span class=w> </span><span class=nf>j</span><span class=p>(</span><span class=n>filtered</span><span class=p>);</span><span class=w> </span><span class=c1>// [37,42,21]</span>
|
||||
</code></pre></div> <p>This requires <a href=../../api/macros/json_has_ranges/ ><code>JSON_HAS_RANGES</code></a> to be enabled and is unavailable on <abbr title="Minimalist GNU for Windows">MinGW</abbr> due to incomplete C++20 ranges support there; see <a href=../../api/macros/json_has_range_view_conversion/ ><code>JSON_HAS_RANGE_VIEW_CONVERSION</code></a>.</p> </div> <h2 id=your-own-types>Your own types<a class=headerlink href=#your-own-types title="Permanent link">¶</a></h2> <p>The conversions above are built in for standard types. To make the same syntax work for <strong>your own</strong> types, provide <code>to_json</code>/<code>from_json</code> functions (or use one of the convenience macros). This is described in detail on the <a href=../arbitrary_types/ >arbitrary types conversions</a> page. Enums can be mapped to strings as described in <a href=../enum_conversion/ >specializing enum conversion</a>.</p> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">¶</a></h2> <ul> <li><a href=../../api/basic_json/get/ ><code>get</code></a> - get a copy converted to a given type</li> <li><a href=../../api/basic_json/get_to/ ><code>get_to</code></a> - convert into an existing variable</li> <li><a href=../../api/basic_json/get_ref/ ><code>get_ref</code></a> / <a href=../../api/basic_json/get_ptr/ ><code>get_ptr</code></a> - access the stored value without copying</li> <li><a href=../arbitrary_types/ >Arbitrary types conversions</a> - support your own types</li> <li><a href=../../api/macros/json_use_implicit_conversions/ ><code>JSON_USE_IMPLICIT_CONVERSIONS</code></a> - toggle implicit conversions</li> <li><a href=../json_view/ >Zero-copy <abbr title="JavaScript Object Notation">JSON</abbr> views</a> - get values out of a document or a view</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 25, 2026 11:07:13 UTC">October 25, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2025 Fonticons, Inc.--><path d="M173.9 397.4c0 2-2.3 3.6-5.2 3.6-3.3.3-5.6-1.3-5.6-3.6 0-2 2.3-3.6 5.2-3.6 3-.3 5.6 1.3 5.6 3.6m-31.1-4.5c-.7 2 1.3 4.3 4.3 4.9 2.6 1 5.6 0 6.2-2s-1.3-4.3-4.3-5.2c-2.6-.7-5.5.3-6.2 2.3m44.2-1.7c-2.9.7-4.9 2.6-4.6 4.9.3 2 2.9 3.3 5.9 2.6 2.9-.7 4.9-2.6 4.6-4.6-.3-1.9-3-3.2-5.9-2.9M252.8 8C114.1 8 8 113.3 8 252c0 110.9 69.8 205.8 169.5 239.2 12.8 2.3 17.3-5.6 17.3-12.1 0-6.2-.3-40.4-.3-61.4 0 0-70 15-84.7-29.8 0 0-11.4-29.1-27.8-36.6 0 0-22.9-15.7 1.6-15.4 0 0 24.9 2 38.6 25.8 21.9 38.6 58.6 27.5 72.9 20.9 2.3-16 8.8-27.1 16-33.7-55.9-6.2-112.3-14.3-112.3-110.5 0-27.5 7.6-41.3 23.6-58.9-2.6-6.5-11.1-33.3 2.6-67.9 20.9-6.5 69 27 69 27 20-5.6 41.5-8.5 62.8-8.5s42.8 2.9 62.8 8.5c0 0 48.1-33.6 69-27 13.7 34.7 5.2 61.4 2.6 67.9 16 17.7 25.8 31.5 25.8 58.9 0 96.5-58.9 104.2-114.8 110.5 9.2 7.9 17 22.9 17 46.4 0 33.7-.3 75.4-.3 83.6 0 6.5 4.6 14.4 17.3 12.1C436.2 457.8 504 362.9 504 252 504 113.3 391.5 8 252.8 8M105.2 352.9c-1.3 1-1 3.3.7 5.2 1.6 1.6 3.9 2.3 5.2 1 1.3-1 1-3.3-.7-5.2-1.6-1.6-3.9-2.3-5.2-1m-1Line truncated
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||||
</code></pre></div> <p>This requires <a href=../../api/macros/json_has_ranges/ ><code>JSON_HAS_RANGES</code></a> to be enabled and is unavailable on <abbr title="Minimalist GNU for Windows">MinGW</abbr> due to incomplete C++20 ranges support there; see <a href=../../api/macros/json_has_range_view_conversion/ ><code>JSON_HAS_RANGE_VIEW_CONVERSION</code></a>.</p> </div> <h2 id=your-own-types>Your own types<a class=headerlink href=#your-own-types title="Permanent link">¶</a></h2> <p>The conversions above are built in for standard types. To make the same syntax work for <strong>your own</strong> types, provide <code>to_json</code>/<code>from_json</code> functions (or use one of the convenience macros). This is described in detail on the <a href=../arbitrary_types/ >arbitrary types conversions</a> page. Enums can be mapped to strings as described in <a href=../enum_conversion/ >specializing enum conversion</a>.</p> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">¶</a></h2> <ul> <li><a href=../../api/basic_json/get/ ><code>get</code></a> - get a copy converted to a given type</li> <li><a href=../../api/basic_json/get_to/ ><code>get_to</code></a> - convert into an existing variable</li> <li><a href=../../api/basic_json/get_ref/ ><code>get_ref</code></a> - / <a href=../../api/basic_json/get_ptr/ ><code>get_ptr</code></a> - access the stored value without copying</li> <li><a href=../arbitrary_types/ >Arbitrary types conversions</a> - support your own types</li> <li><a href=../../api/macros/json_use_implicit_conversions/ ><code>JSON_USE_IMPLICIT_CONVERSIONS</code></a> - toggle implicit conversions</li> <li><a href=../json_view/ >Zero-copy <abbr title="JavaScript Object Notation">JSON</abbr> views</a> - get values out of a document or a view</li> <li><a href=../../api/adl_serializer/ ><code>adl_serializer</code></a> - the default serializer behind the conversions</li> <li><a href=../../api/basic_json/type/ ><code>type</code></a> - query the type of a value</li> <li><a href=../../api/macros/json_disable_tuple_reference_conversion/ ><code>JSON_DISABLE_TUPLE_REFERENCE_CONVERSION</code></a> - toggle conversion to tuples of references</li> <li><a href=../../api/macros/json_has_range_view_conversion/ ><code>JSON_HAS_RANGE_VIEW_CONVERSION</code></a> - toggle conversion from ranges</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../comments/ class="md-footer__link md-footer__link--prev" aria-label="Previous: Comments"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> Comments </div> </div> </a> <a href=../creating_values/ class="md-footer__link md-footer__link--next" aria-label="Next: Creating JSON values"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> Creating JSON values </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &Line truncated
|
||||
@@ -197,14 +197,14 @@ j_null.get_to(opt); // ✅ std::nullopt
|
||||
|
||||
`static_cast` and `get<std::optional<T>>()` are not guaranteed equivalent
|
||||
|
||||
`operator ValueType()` (used by `static_cast` and implicit conversions) intentionally excludes `std::optional<T>` from delegating to `get<T>()`, to avoid a constructor ambiguity with `std::optional<T>`'s own converting constructor from `basic_json`. As a result, `static_cast<std::optional<T>>(json_value)` goes through `std::optional<T>`'s own converting constructor rather than through `get<std::optional<T>>()`, which can behave differently -- for example, with a custom `adl_serializer<std::optional<T>>` specialization. Prefer `get<std::optional<T>>()`/`get_to()` over `static_cast` for optional types.
|
||||
`operator ValueType()` (used by `static_cast` and implicit conversions) intentionally excludes `std::optional<T>` from delegating to `get<T>()`, to avoid a constructor ambiguity with `std::optional<T>`'s own converting constructor from `basic_json`. As a result, `static_cast<std::optional<T>>(json_value)` goes through `std::optional<T>`'s own converting constructor rather than through `get<std::optional<T>>()`, which can behave differently — for example, with a custom `adl_serializer<std::optional<T>>` specialization. Prefer `get<std::optional<T>>()`/`get_to()` over `static_cast` for optional types.
|
||||
|
||||
Converting to a fixed-size destination does not check the array size
|
||||
|
||||
Some destination types have a size that is fixed by their C++ type rather than by the JSON value: `std::pair<A, B>`, `std::tuple<Ts...>`, `std::array<T, N>`, C arrays `T[N]`, and `std::map`/`std::unordered_map` with a non-string key type (which is read from an array of two-element arrays). All of them read exactly as many elements as they need via [`at`](https://json.nlohmann.me/api/basic_json/at/index.md) and **never compare the JSON array's size to that number**. The two mismatch directions therefore behave differently:
|
||||
|
||||
- The JSON array has **too many** elements: the surplus is **silently discarded**, and no exception is thrown.
|
||||
- The JSON array has **too few** elements: `at` throws [`out_of_range.401`](https://json.nlohmann.me/home/exceptions/#jsonexceptionout_of_range401) for the first missing index -- an out-of-range error, not a [`type_error`](https://json.nlohmann.me/home/exceptions/#type-errors), even though the cause is a shape mismatch.
|
||||
- The JSON array has **too few** elements: `at` throws [`out_of_range.401`](https://json.nlohmann.me/home/exceptions/#jsonexceptionout_of_range401) for the first missing index — an out-of-range error, not a [`type_error`](https://json.nlohmann.me/home/exceptions/#type-errors), even though the cause is a shape mismatch.
|
||||
|
||||
```
|
||||
json j = {1, 2, 3, 4, 5};
|
||||
@@ -220,7 +220,7 @@ If a size mismatch is an error in your application, check the size yourself befo
|
||||
|
||||
## Omitting a field when serializing `std::optional`
|
||||
|
||||
By default, `to_json` for `std::optional<T>` writes either the value or `null` -- there is no built-in way to make a field disappear from the serialized object entirely when the `std::optional` is `std::nullopt`. Because a specialization of `adl_serializer<std::optional<T>>` only controls how the *value* is converted (it cannot prevent the containing object's `to_json` from inserting the key in the first place), omission has to be implemented in the *containing* type's `to_json`:
|
||||
By default, `to_json` for `std::optional<T>` writes either the value or `null` — there is no built-in way to make a field disappear from the serialized object entirely when the `std::optional` is `std::nullopt`. Because a specialization of `adl_serializer<std::optional<T>>` only controls how the *value* is converted (it cannot prevent the containing object's `to_json` from inserting the key in the first place), omission has to be implemented in the *containing* type's `to_json`:
|
||||
|
||||
```
|
||||
struct person {
|
||||
@@ -247,7 +247,7 @@ json j = numbers; // [1,2,3]
|
||||
|
||||
Constructing from a C++20 range view
|
||||
|
||||
A `json` array can also be constructed directly from a C++20 range view (`std::ranges::view`), such as the result of `std::views::filter` or `std::views::transform` -- no intermediate container is needed:
|
||||
A `json` array can also be constructed directly from a C++20 range view (`std::ranges::view`), such as the result of `std::views::filter` or `std::views::transform` — no intermediate container is needed:
|
||||
|
||||
```
|
||||
std::vector<int> nums{1, 2, 37, 42, 21};
|
||||
@@ -265,7 +265,11 @@ The conversions above are built in for standard types. To make the same syntax w
|
||||
|
||||
- [`get`](https://json.nlohmann.me/api/basic_json/get/index.md) - get a copy converted to a given type
|
||||
- [`get_to`](https://json.nlohmann.me/api/basic_json/get_to/index.md) - convert into an existing variable
|
||||
- [`get_ref`](https://json.nlohmann.me/api/basic_json/get_ref/index.md) / [`get_ptr`](https://json.nlohmann.me/api/basic_json/get_ptr/index.md) - access the stored value without copying
|
||||
- [`get_ref`](https://json.nlohmann.me/api/basic_json/get_ref/index.md) - / [`get_ptr`](https://json.nlohmann.me/api/basic_json/get_ptr/index.md) - access the stored value without copying
|
||||
- [Arbitrary types conversions](https://json.nlohmann.me/features/arbitrary_types/index.md) - support your own types
|
||||
- [`JSON_USE_IMPLICIT_CONVERSIONS`](https://json.nlohmann.me/api/macros/json_use_implicit_conversions/index.md) - toggle implicit conversions
|
||||
- [Zero-copy JSON views](https://json.nlohmann.me/features/json_view/index.md) - get values out of a document or a view
|
||||
- [`adl_serializer`](https://json.nlohmann.me/api/adl_serializer/index.md) - the default serializer behind the conversions
|
||||
- [`type`](https://json.nlohmann.me/api/basic_json/type/index.md) - query the type of a value
|
||||
- [`JSON_DISABLE_TUPLE_REFERENCE_CONVERSION`](https://json.nlohmann.me/api/macros/json_disable_tuple_reference_conversion/index.md) - toggle conversion to tuples of references
|
||||
- [`JSON_HAS_RANGE_VIEW_CONVERSION`](https://json.nlohmann.me/api/macros/json_has_range_view_conversion/index.md) - toggle conversion from ranges
|
||||
@@ -135,8 +135,13 @@ string `#!json "42"`.
|
||||
## See also
|
||||
|
||||
- [`basic_json` constructors](../api/basic_json/basic_json.md) - all ways to construct a value
|
||||
- [`array`](../api/basic_json/array.md) / [`object`](../api/basic_json/object.md) - force array or object type
|
||||
- [`array`](../api/basic_json/array.md) - / [`object`](../api/basic_json/object.md) - force array or object type
|
||||
- [`operator""_json`](../api/operator_literal_json.md) - the `_json` literal
|
||||
- [Converting values](conversions.md) - which C++ types can be used
|
||||
- [Parsing](parsing/index.md) - create a value from JSON text
|
||||
- [Zero-copy JSON views](json_view.md) - create a document and a view from JSON text instead of a value
|
||||
- [`basic_json`](../api/basic_json/index.md) - the class template and its member functions
|
||||
- [Types](types/index.md) - the JSON value types
|
||||
- [`JSON_BRACE_INIT_COPY_SEMANTICS`](../api/macros/json_brace_init_copy_semantics.md) - change how a single braced value is interpreted
|
||||
- [`operator=`](../api/basic_json/operator=.md) - assign a new value
|
||||
|
||||
@@ -1,4 +1,4 @@
|
||||
<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/creating_values/" rel="canonical"><link href=../conversions/ rel=prev><link href=../element_access/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>Creating JSON values - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.ab4e12ef.min.css><link rel="stylesheet" href="../../assets/external/fonts.googleapis.com/css.61a430c9.css"><style>:root{--md-text-font:"Roboto";--md-code-font:"JetBrains Mono"}</style><link rel=stylesheet href=../../css/custom.css><script>__md_scope=new 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<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=description content="JSON for Modern C++ is a header-only C++11 library that makes JSON a first-class data type."><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/creating_values/" rel="canonical"><link href=../conversions/ rel=prev><link href=../element_access/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>Creating JSON values - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.ab4e12ef.min.css><link rel=stylesheet href=../../css/custom.css><link rel=stylesheet href=../../css/brand.css><link rel=stylesheet href=../../css/fig-sax.css><link rel=stylesheet href=../../css/fig-jsonview.css><link rel=stylesheet 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<span class=n>json</span><span class=w> </span><span class=n>j_float</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=mf>3.141</span><span class=p>;</span>
|
||||
<span class=n>json</span><span class=w> </span><span class=n>j_string</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=s>"Hello"</span><span class=p>;</span>
|
||||
<span class=n>json</span><span class=w> </span><span class=n>j_boolean</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=nb>true</span><span class=p>;</span>
|
||||
@@ -42,4 +42,4 @@
|
||||
<span class=w> </span><span class=nt>"answer"</span><span class=p>:</span><span class=w> </span><span class=mi>42</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"hello"</span><span class=p>:</span><span class=w> </span><span class=s2>"world"</span>
|
||||
<span class=p>}</span>
|
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</code></pre></div> </details> <p>Note this <strong>parses</strong> the string, so <code class=highlight><span class=s>"42"</span><span class=n>_json</span></code> is the number <code class=highlight><span class=mi>42</span></code>, whereas <code class=highlight><span class=n>json</span><span class=p>(</span><span class=s>"42"</span><span class=p>)</span></code> is the <abbr title="JavaScript Object Notation">JSON</abbr> string <code class=highlight><span class=s2>"42"</span></code>.</p> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">¶</a></h2> <ul> <li><a href=../../api/basic_json/basic_json/ ><code>basic_json</code> constructors</a> - all ways to construct a value</li> <li><a href=../../api/basic_json/array/ ><code>array</code></a> / <a href=../../api/basic_json/object/ ><code>object</code></a> - force array or object type</li> <li><a href=../../api/operator_literal_json/ ><code>operator""_json</code></a> - the <code>_json</code> literal</li> <li><a href=../conversions/ >Converting values</a> - which C++ types can be used</li> <li><a href=../parsing/ >Parsing</a> - create a value from <abbr title="JavaScript Object Notation">JSON</abbr> text</li> <li><a href=../json_view/ >Zero-copy <abbr title="JavaScript Object Notation">JSON</abbr> views</a> - create a document and a view from <abbr title="JavaScript Object Notation">JSON</abbr> text instead of a value</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 25, 2026 11:07:13 UTC">October 25, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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</code></pre></div> </details> <p>Note this <strong>parses</strong> the string, so <code class=highlight><span class=s>"42"</span><span class=n>_json</span></code> is the number <code class=highlight><span class=mi>42</span></code>, whereas <code class=highlight><span class=n>json</span><span class=p>(</span><span class=s>"42"</span><span class=p>)</span></code> is the <abbr title="JavaScript Object Notation">JSON</abbr> string <code class=highlight><span class=s2>"42"</span></code>.</p> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">¶</a></h2> <ul> <li><a href=../../api/basic_json/basic_json/ ><code>basic_json</code> constructors</a> - all ways to construct a value</li> <li><a href=../../api/basic_json/array/ ><code>array</code></a> - / <a href=../../api/basic_json/object/ ><code>object</code></a> - force array or object type</li> <li><a href=../../api/operator_literal_json/ ><code>operator""_json</code></a> - the <code>_json</code> literal</li> <li><a href=../conversions/ >Converting values</a> - which C++ types can be used</li> <li><a href=../parsing/ >Parsing</a> - create a value from <abbr title="JavaScript Object Notation">JSON</abbr> text</li> <li><a href=../json_view/ >Zero-copy <abbr title="JavaScript Object Notation">JSON</abbr> views</a> - create a document and a view from <abbr title="JavaScript Object Notation">JSON</abbr> text instead of a value</li> <li><a href=../../api/basic_json/ ><code>basic_json</code></a> - the class template and its member functions</li> <li><a href=../types/ >Types</a> - the <abbr title="JavaScript Object Notation">JSON</abbr> value types</li> <li><a href=../../api/macros/json_brace_init_copy_semantics/ ><code>JSON_BRACE_INIT_COPY_SEMANTICS</code></a> - change how a single braced value is interpreted</li> <li><a href=../../api/basic_json/operator%3D/ ><code>operator=</code></a> - assign a new value</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../conversions/ class="md-footer__link md-footer__link--prev" aria-label="Previous: Converting values"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> Converting values </div> </div> </a> <a href=../element_access/ class="md-footer__link md-footer__link--next" aria-label="Next: Element Access"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> Element Access </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2025 Line truncated
|
||||
@@ -119,8 +119,12 @@ Note this **parses** the string, so `"42"_json` is the number `42`, whereas `jso
|
||||
## See also
|
||||
|
||||
- [`basic_json` constructors](https://json.nlohmann.me/api/basic_json/basic_json/index.md) - all ways to construct a value
|
||||
- [`array`](https://json.nlohmann.me/api/basic_json/array/index.md) / [`object`](https://json.nlohmann.me/api/basic_json/object/index.md) - force array or object type
|
||||
- [`array`](https://json.nlohmann.me/api/basic_json/array/index.md) - / [`object`](https://json.nlohmann.me/api/basic_json/object/index.md) - force array or object type
|
||||
- [`operator""_json`](https://json.nlohmann.me/api/operator_literal_json/index.md) - the `_json` literal
|
||||
- [Converting values](https://json.nlohmann.me/features/conversions/index.md) - which C++ types can be used
|
||||
- [Parsing](https://json.nlohmann.me/features/parsing/index.md) - create a value from JSON text
|
||||
- [Zero-copy JSON views](https://json.nlohmann.me/features/json_view/index.md) - create a document and a view from JSON text instead of a value
|
||||
- [`basic_json`](https://json.nlohmann.me/api/basic_json/index.md) - the class template and its member functions
|
||||
- [Types](https://json.nlohmann.me/features/types/index.md) - the JSON value types
|
||||
- [`JSON_BRACE_INIT_COPY_SEMANTICS`](https://json.nlohmann.me/api/macros/json_brace_init_copy_semantics/index.md) - change how a single braced value is interpreted
|
||||
- [`operator=`](https://json.nlohmann.me/api/basic_json/operator%3D/index.md) - assign a new value
|
||||
+26
-12
@@ -12,16 +12,23 @@ Testing whether a key or index exists before accessing it is also possible, with
|
||||
[`contains`](../../api/basic_json/contains.md) or [`find`](../../api/basic_json/find.md) (which returns an iterator to
|
||||
the value, or `end()` if it is not found).
|
||||
|
||||
How do I get a value out? Start at the top and follow the first question that applies:
|
||||
|
||||
```mermaid
|
||||
flowchart TD
|
||||
A["accessing a value"] --> B{"must it exist?"}
|
||||
B -->|"yes, missing is an error"| C["at() -- throws"]
|
||||
B -->|"yes, but checking is my job"| D["operator[] -- unchecked"]
|
||||
B -->|"no, a fallback is fine"| E["value() -- default value"]
|
||||
A --> F{"just testing first?"}
|
||||
F -->|"yes"| G["contains() / find()"]
|
||||
A["getting a value out"] --> P{{"nested path such as /server/port?"}}
|
||||
P -->|"yes"| JP["JSON Pointer: at(ptr), value(ptr, default), contains(ptr)"]
|
||||
P -->|"no, one key or index"| T{{"only testing whether it exists?"}}
|
||||
T -->|"yes"| CF["contains() / find()"]
|
||||
T -->|"no"| M{{"may it be missing?"}}
|
||||
M -->|"yes, a fallback is fine"| V["value(key, default)"]
|
||||
M -->|"yes, and missing is an error"| AT["at(): throws out_of_range"]
|
||||
M -->|"no, it exists and its type is known"| OP["operator[] and get#lt;T#gt;()"]
|
||||
OP -.- W["non-const operator[] inserts null for a missing key"]
|
||||
```
|
||||
|
||||
<p class="nj-caption" markdown>Decision tree for reading a value: the labels on the arrows are the answers to the questions, and the boxes name the functions to use. The dashed line marks a side effect of the non-const `operator[]`.</p>
|
||||
|
||||
## Does this key exist?
|
||||
|
||||
Use [`contains`](../../api/basic_json/contains.md) to test for a key; it does not modify the value and does not throw:
|
||||
@@ -63,9 +70,16 @@ if (auto it = j.find("name"); it != j.end())
|
||||
|
||||
## See also
|
||||
|
||||
- [**operator[]**](../../api/basic_json/operator%5B%5D.md) - unchecked access
|
||||
- [**at**](../../api/basic_json/at.md) - checked access
|
||||
- [**value**](../../api/basic_json/value.md) - access with default value
|
||||
- [**contains**](../../api/basic_json/contains.md) and [**find**](../../api/basic_json/find.md) - test for existence
|
||||
- [**front**](../../api/basic_json/front.md) and [**back**](../../api/basic_json/back.md) - first and last element
|
||||
- [**items**](../../api/basic_json/items.md) - iterate over keys and values
|
||||
- [`operator[]`](../../api/basic_json/operator%5B%5D.md) - unchecked access
|
||||
- [`at`](../../api/basic_json/at.md) - checked access
|
||||
- [`value`](../../api/basic_json/value.md) - access with default value
|
||||
- [`contains`](../../api/basic_json/contains.md) - and [`find`](../../api/basic_json/find.md) - test for existence
|
||||
- [`front`](../../api/basic_json/front.md) - and [`back`](../../api/basic_json/back.md) - first and last element
|
||||
- [`items`](../../api/basic_json/items.md) - iterate over keys and values
|
||||
- [Checked access: at](checked_access.md) - access with range checking
|
||||
- [Unchecked access: operator[]](unchecked_access.md) - access without range checking
|
||||
- [Access with default value: value](default_value.md) - fall back to a default instead of throwing
|
||||
- [Iterators](../iterators.md) - visit all elements of arrays and objects
|
||||
- [JSON Pointer](../json_pointer.md) - address nested values with a single string
|
||||
- [Modifying values](../modifying_values.md) - add, merge and remove elements
|
||||
- [`basic_json`](../../api/basic_json/index.md) - all member functions
|
||||
@@ -89,3 +89,13 @@ non-existing, an exception is thrown.
|
||||
| access to valid array index | reference to existing value is returned | const reference to existing value is returned |
|
||||
| access to non-existing object key | `basic_json::out_of_range` exception is thrown | `basic_json::out_of_range` exception is thrown |
|
||||
| access to invalid array index | `basic_json::out_of_range` exception is thrown | `basic_json::out_of_range` exception is thrown |
|
||||
|
||||
<p class="nj-table-caption" markdown>Result of [`at`](../../api/basic_json/at.md) in each scenario, for a non-const and a const value.</p>
|
||||
|
||||
## See also
|
||||
|
||||
- [Element Access](index.md) - overview of all ways to access elements
|
||||
- [Unchecked access: operator[]](unchecked_access.md) - access without range checking
|
||||
- [Access with default value: value](default_value.md) - fall back to a default instead of throwing
|
||||
- [`at`](../../api/basic_json/at.md) - checked access by reference
|
||||
- [JSON Pointer](../json_pointer.md) - access nested values with `at` and a pointer
|
||||
@@ -1,4 +1,4 @@
|
||||
<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/element_access/checked_access/" rel="canonical"><link href=../unchecked_access/ rel=prev><link href=../default_value/ rel=next><link rel=icon href=../../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>Checked access: at - JSON for Modern C++</title><link rel=stylesheet href=../../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../../assets/stylesheets/palette.ab4e12ef.min.css><link rel="stylesheet" href="../../../assets/external/fonts.googleapis.com/css.61a430c9.css"><style>:root{--md-text-font:"Roboto";--md-code-font:"JetBrains Mono"}</style><link rel=stylesheet href=../../../css/custom.css><script>__md_scope=new URL("../../..",location),__md_hash=e=>[...e].reduce(((e,_)=>(e<<5)-e+_.charCodeAt(0)),0),__md_get=(e,_=localStorage,t=__md_scope)=>JSON.parse(_.getItem(t.pathname+"."+e)),__md_set=(e,_,t=localStorage,a=__md_scope)=>{try{t.setItem(a.pathname+"."+e,JSON.stringify(_))}catch(e){}}</script></head> <body dir=ltr data-md-color-scheme=default data-md-color-primary=indigo data-md-color-accent=indigo> <input class=md-toggle data-md-toggle=drawer type=checkbox id=__drawer autocomplete=off> <input class=md-toggle data-md-toggle=search type=checkbox id=__search autocomplete=off> <label class=md-overlay for=__drawer></label> <div data-md-component=skip> <a href=#checked-access-at class=md-skip> Skip to content </a> </div> <div data-md-component=announce> </div> <header class=md-header data-md-component=header> <nav class="md-header__inner md-grid" aria-label=Header> <a href=../../.. title="JSON for Modern C++" class="md-header__button md-logo" aria-label="JSON for Modern C++" data-md-component=logo> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 8a3 3 0 0 0 3-3 3 3 0 0 0-3-3 3 3 0 0 0-3 3 3 3 0 0 0 3 3m0 3.54C9.64 9.35 6.5 8 3 8v11c3.5 0 6.64 1.35 9 3.54 2.36-2.19 5.5-3.54 9-3.54V8c-3.5 0-6.64 1.35-9 3.54"/></svg> </a> <label class="md-header__button md-icon" for=__drawer> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M3 6h18v2H3zm0 5h18v2H3zm0 5h18v2H3z"/></svg> </label> <div class=md-header__title data-md-component=header-title> <div class=md-header__ellipsis> <div class=md-header__topic> <span class=md-ellipsis> JSON for Modern C++ </span> </div> <div class=md-header__topic data-md-component=header-topic> <span class=md-ellipsis> Checked access: at </span> </div> </div> </div> <form class=md-header__option data-md-component=palette> <input class=md-option data-md-color-media="(prefers-color-scheme: light)" data-md-color-scheme=default data-md-color-primary=indigo data-md-color-accent=indigo aria-label="Switch to dark mode" type=radio name=__palette id=__palette_0> <label class="md-header__button md-icon" title="Switch to dark mode" for=__palette_1 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 8a4 4 0 0 0-4 4 4 4 0 0 0 4 4 4 4 0 0 0 4-4 4 4 0 0 0-4-4m0 10a6 6 0 0 1-6-6 6 6 0 0 1 6-6 6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> <input class=md-option data-md-color-media="(prefers-color-scheme: dark)" data-md-color-scheme=slate data-md-color-primary=indigo data-md-color-accent=indigo aria-label="Switch to light mode" type=radio name=__palette id=__palette_1> <label class="md-header__button md-icon" title="Switch to light mode" for=__palette_0 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 18c-.89 0-1.74-.2-2.5-.55C11.56 16.5 13 14.42 13 12s-1.44-4.5-3.5-5.45C10.26 6.2 11.11 6 12 6a6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> </form> <script>var palette=__md_get("__palette");if(palette&&palette.color){if("(prefers-color-scheme)"===palette.color.media){var media=matchMedia("(prefers-color-scheme: light)"),input=document.querySelector(media.matches?"[data-md-color-media='(prefers-color-scheme: light)']":"[data-md-color-media='(prefers-color-scheme: dark)']");palette.color.media=input.getAttribute("data-md-color-media"),palette.color.scheme=input.getAttribute("data-md-color-scheme"),palette.color.primary=input.getAttribute("data-md-color-primary"),palette.color.accent=input.getAttribute("data-md-color-accent")}for(var[key,value]of Object.entries(palette.color))document.body.setAttribute("data-md-color-"+key,value)}</script> <label class="md-header__button md-icon" for=__search> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M9.5 3A6.5 6.5 0 0 1 16 9.5c0 1.61-.59 3.09-1.56 4.23l.27.27h.79l5 5-1.5 1.5-5-5v-.79l-.27-.27A6.52 6.52 0 0 1 9.5 16 6.5 6.5 0 0 1 3 9.5 6.5 6.5 0 0 1 9.5 3m0 2C7 5 5 7 5 9.5S7 14Line truncated
|
||||
<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=description content="JSON for Modern C++ is a header-only C++11 library that makes JSON a first-class data type."><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/element_access/checked_access/" rel="canonical"><link href=../unchecked_access/ rel=prev><link href=../default_value/ rel=next><link rel=icon href=../../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>Checked access: at - JSON for Modern C++</title><link rel=stylesheet href=../../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../../assets/stylesheets/palette.ab4e12ef.min.css><link rel=stylesheet href=../../../css/custom.css><link rel=stylesheet href=../../../css/brand.css><link rel=stylesheet href=../../../css/fig-sax.css><link rel=stylesheet href=../../../css/fig-jsonview.css><link rel=stylesheet href=../../../css/fig-exceptions.css><link rel=stylesheet href=../../../css/fig-pointer.css><link rel=stylesheet href=../../../css/fig-charts.css><link rel=stylesheet href=../../../css/fig-integration.css><link rel=stylesheet href=../../../css/fig-macros.css><script>__md_scope=new URL("../../..",location),__md_hash=e=>[...e].reduce(((e,_)=>(e<<5)-e+_.charCodeAt(0)),0),__md_get=(e,_=localStorage,t=__md_scope)=>JSON.parse(_.getItem(t.pathname+"."+e)),__md_set=(e,_,t=localStorage,a=__md_scope)=>{try{t.setItem(a.pathname+"."+e,JSON.stringify(_))}catch(e){}}</script></head> <body dir=ltr data-md-color-scheme=default data-md-color-primary=custom data-md-color-accent=custom> <input class=md-toggle data-md-toggle=drawer type=checkbox id=__drawer autocomplete=off> <input class=md-toggle data-md-toggle=search type=checkbox id=__search autocomplete=off> <label class=md-overlay for=__drawer></label> <div data-md-component=skip> <a href=#checked-access-at class=md-skip> Skip to content </a> </div> <div data-md-component=announce> </div> <header class=md-header data-md-component=header> <nav class="md-header__inner md-grid" aria-label=Header> <a href=../../.. title="JSON for Modern C++" class="md-header__button md-logo" aria-label="JSON for Modern C++" data-md-component=logo> <img src=../../../images/wordmark-header.svg alt=logo> </a> <label class="md-header__button md-icon" for=__drawer> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M3 6h18v2H3zm0 5h18v2H3zm0 5h18v2H3z"/></svg> </label> <div class=md-header__title data-md-component=header-title> <div class=md-header__ellipsis> <div class=md-header__topic> <span class=md-ellipsis> JSON for Modern C++ </span> </div> <div class=md-header__topic data-md-component=header-topic> <span class=md-ellipsis> Checked access: at </span> </div> </div> </div> <form class=md-header__option data-md-component=palette> <input class=md-option data-md-color-media="(prefers-color-scheme: light)" data-md-color-scheme=default data-md-color-primary=custom data-md-color-accent=custom aria-label="Switch to dark mode" type=radio name=__palette id=__palette_0> <label class="md-header__button md-icon" title="Switch to dark mode" for=__palette_1 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 8a4 4 0 0 0-4 4 4 4 0 0 0 4 4 4 4 0 0 0 4-4 4 4 0 0 0-4-4m0 10a6 6 0 0 1-6-6 6 6 0 0 1 6-6 6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> <input class=md-option data-md-color-media="(prefers-color-scheme: dark)" data-md-color-scheme=slate data-md-color-primary=custom data-md-color-accent=custom aria-label="Switch to light mode" type=radio name=__palette id=__palette_1> <label class="md-header__button md-icon" title="Switch to light mode" for=__palette_0 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 18c-.89 0-1.74-.2-2.5-.55C11.56 16.5 13 14.42 13 12s-1.44-4.5-3.5-5.45C10.26 6.2 11.11 6 12 6a6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> </form> <script>var palette=__md_get("__palette");if(palette&&palette.color){if("(prefers-color-scheme)"===palette.color.media){var media=matchMedia("(prefers-color-scheme: light)"),input=document.querySelector(media.matches?"[data-md-color-media='(prefers-color-scheme: light)']":"[data-md-color-media='(prefers-color-scheme: dark)']");palette.color.media=input.getAttribute("data-md-color-media"),palette.color.scheme=input.getAttribute("data-md-color-scheme"),palette.color.primary=input.getAttribute("data-md-color-primary"),palette.color.accent=input.getAttribute("data-md-color-accent")}for(var[key,value]of Object.entries(palette.color))document.body.setAttribute("data-md-color-"+key,value)}</script> <label class="md-header__button md-icon" for=__search> <svg xmlns=http://www.w3.org/2000/svg viewboLine truncated
|
||||
<span class=w> </span><span class=nt>"name"</span><span class=p>:</span><span class=w> </span><span class=s2>"Mary Smith"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"age"</span><span class=p>:</span><span class=w> </span><span class=mi>42</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"hobbies"</span><span class=p>:</span><span class=w> </span><span class=p>[</span><span class=s2>"hiking"</span><span class=p>,</span><span class=w> </span><span class=s2>"reading"</span><span class=p>]</span>
|
||||
@@ -12,4 +12,4 @@
|
||||
</code></pre></div> </details> <p>When accessing an invalid index (i.e., an index greater than or equal to the array size) or the passed object key is non-existing, an exception is thrown.</p> <details class=example> <summary>Example: access via invalid index or missing key</summary> <div class=highlight><pre><span></span><code><span class=n>j</span><span class=p>.</span><span class=n>at</span><span class=p>(</span><span class=s>"hobbies"</span><span class=p>).</span><span class=n>at</span><span class=p>(</span><span class=mi>3</span><span class=p>)</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=s>"cooking"</span><span class=p>;</span>
|
||||
</code></pre></div> <p>This code produces the following exception:</p> <div class=highlight><pre><span></span><code>[json.exception.out_of_range.401] array index 3 is out of range
|
||||
</code></pre></div> <p>When <a href=../../../home/exceptions/#extended-diagnostic-messages>extended diagnostic messages</a> are enabled by defining <a href=../../../api/macros/json_diagnostics/ ><code>JSON_DIAGNOSTICS</code></a>, the exception further gives information where the key or index is missing or out of range.</p> <div class=highlight><pre><span></span><code>[json.exception.out_of_range.401] (/hobbies) array index 3 is out of range
|
||||
</code></pre></div> </details> <h2 id=notes>Notes<a class=headerlink href=#notes title="Permanent link">¶</a></h2> <div class="admonition failure"> <p class=admonition-title>Exceptions</p> <ul> <li><a href=../../../api/basic_json/at/ ><code>at</code></a> can only be used with objects (with a string argument) or with arrays (with a numeric argument). For other types, a <a href=../../../home/exceptions/#jsonexceptiontype_error304><code>basic_json::type_error</code></a> is thrown.</li> <li><a href=../../../home/exceptions/#out-of-range><code>basic_json::out_of_range</code> exception</a> exceptions are thrown if the provided key is not found in an object or the provided index is invalid.</li> </ul> </div> <h2 id=summary>Summary<a class=headerlink href=#summary title="Permanent link">¶</a></h2> <table> <thead> <tr> <th>scenario</th> <th>non-const value</th> <th>const value</th> </tr> </thead> <tbody> <tr> <td>access to existing object key</td> <td>reference to existing value is returned</td> <td>const reference to existing value is returned</td> </tr> <tr> <td>access to valid array index</td> <td>reference to existing value is returned</td> <td>const reference to existing value is returned</td> </tr> <tr> <td>access to non-existing object key</td> <td><code>basic_json::out_of_range</code> exception is thrown</td> <td><code>basic_json::out_of_range</code> exception is thrown</td> </tr> <tr> <td>access to invalid array index</td> <td><code>basic_json::out_of_range</code> exception is thrown</td> <td><code>basic_json::out_of_range</code> exception is thrown</td> </tr> </tbody> </table> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 2, 2026 09:32:15 UTC">October 2, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2025 Fonticons, Inc.--><path d="M173.9 397.4c0 2-2.3 3.6-5.2 3.6-3.3.3-5.6-1.3-5.6-3.6 0-2 2.3-3.6 5.2-3.6 3-.3 5.6 1.3 5.6 3.6m-31.1-4.5c-.7 2 1.3 4.3 4.3 4.9 2.6 1 5.6 0 6.2-2s-1.3-4.3-4.3-5.2c-2.6-.7-5.5.3-6.2 2.3m44.2-1.7c-2.9.7-4.9 2.6-4.6 4.9.3 2 2.9 3.3 5.9 2.6 2.9-.7 4.9-2.6 4.6-4.6-.3-1.9-3-3.2-5.9-2.9M252.8 8C114.1 8 8 113.3 8 252c0 110.9 69.8 205.8 169.5 239.2 12.8 2.3 17.3-5.6 17.3-12.1 0-6.2-.3-40.4-.3-61.4 0 0-70 15-84.7-29.8 0 0-11.4-29.1-27.8-36.6 0 0-22.9-15.7 1.6-15.4 0 0 24.9 2 38.6 25.8 21.9 38.6 58.6 27.5 72.9 20.9 2.3-16 8.8-27.1 16-33.7-55.9-6.2-112.3-14.3-112.3-110.5 0-27.5 7.6-41.3 23.6-58.9-2.6-6.5-11.1-33.3 2.6-67.9 20.9-6.5 69 27 69 27 20-5.6 41.5-8.5 62.8-8.5s42.8 2.9 62.8 8.5c0 0 48.1-33.6 69-27 13.7 34.7 5.2 61.4 2.6 67.9 16 17.7 25.8 31.5 25.8 58.9 0 96.5-58.9 104.2-114.8 110.5 9.2 7.9 17 22.9 17 46.4 0 33.7-.3 75.4-.3 83.6 0 6.5 4.6 14.4 17.3 12.1C436.2 457.8 504 362.9 504 252 504 113.3 391.5 8 252.8 8M105.2 352.9c-1.3 1-1 3.3.7 5.2 1.6 1.6 3.9 2.3 5.2 1 1.3-1 1-3.3-.7-5.2-1.6-1.6-3.9-2.3-5.2-1m-10.8-8.1c-.7 1.3.3 2.9 2.3 3.9 1.6 1 3.6.7 4.3-.7.7-1.3-.3-2.9-2.3-3.9-2-.6-3.6-.3-4.3.7m32.4 35.6c-1.6 1.3-1 4.3 1.3 6.2 2.3 2.3 5.2 2.6 6.5 1 1.3-1.3.7-4.3-1.3-6.2-2.2-2.3-5.2-2.6-6.5-1m-11.4-14.7c-1.Line truncated
|
||||
</code></pre></div> </details> <h2 id=notes>Notes<a class=headerlink href=#notes title="Permanent link">¶</a></h2> <div class="admonition failure"> <p class=admonition-title>Exceptions</p> <ul> <li><a href=../../../api/basic_json/at/ ><code>at</code></a> can only be used with objects (with a string argument) or with arrays (with a numeric argument). For other types, a <a href=../../../home/exceptions/#jsonexceptiontype_error304><code>basic_json::type_error</code></a> is thrown.</li> <li><a href=../../../home/exceptions/#out-of-range><code>basic_json::out_of_range</code> exception</a> exceptions are thrown if the provided key is not found in an object or the provided index is invalid.</li> </ul> </div> <h2 id=summary>Summary<a class=headerlink href=#summary title="Permanent link">¶</a></h2> <table> <thead> <tr> <th>scenario</th> <th>non-const value</th> <th>const value</th> </tr> </thead> <tbody> <tr> <td>access to existing object key</td> <td>reference to existing value is returned</td> <td>const reference to existing value is returned</td> </tr> <tr> <td>access to valid array index</td> <td>reference to existing value is returned</td> <td>const reference to existing value is returned</td> </tr> <tr> <td>access to non-existing object key</td> <td><code>basic_json::out_of_range</code> exception is thrown</td> <td><code>basic_json::out_of_range</code> exception is thrown</td> </tr> <tr> <td>access to invalid array index</td> <td><code>basic_json::out_of_range</code> exception is thrown</td> <td><code>basic_json::out_of_range</code> exception is thrown</td> </tr> </tbody> </table> <p class=nj-table-caption>Result of <a href=../../../api/basic_json/at/ ><code>at</code></a> in each scenario, for a non-const and a const value.</p> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">¶</a></h2> <ul> <li><a href=../ >Element Access</a> - overview of all ways to access elements</li> <li><a href=../unchecked_access/ >Unchecked access: operator[]</a> - access without range checking</li> <li><a href=../default_value/ >Access with default value: value</a> - fall back to a default instead of throwing</li> <li><a href=../../../api/basic_json/at/ ><code>at</code></a> - checked access by reference</li> <li><a href=../../json_pointer/ ><abbr title="JavaScript Object Notation">JSON</abbr> Pointer</a> - access nested values with <code>at</code> and a pointer</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../unchecked_access/ class="md-footer__link md-footer__link--prev" aria-label="Previous: Unchecked access: operator[]"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> Unchecked access: operator[] </div> </div> </a> <a href=../default_value/ class="md-footer__link md-footer__link--next" aria-label="Next: Access with default value: value"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> Access with default value: value </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11z"/></svg> </div> </a> </nav> <div clasLine truncated
|
||||
@@ -80,3 +80,13 @@ Exceptions
|
||||
| access to valid array index | reference to existing value is returned | const reference to existing value is returned |
|
||||
| access to non-existing object key | `basic_json::out_of_range` exception is thrown | `basic_json::out_of_range` exception is thrown |
|
||||
| access to invalid array index | `basic_json::out_of_range` exception is thrown | `basic_json::out_of_range` exception is thrown |
|
||||
|
||||
Result of [`at`](https://json.nlohmann.me/api/basic_json/at/index.md) in each scenario, for a non-const and a const value.
|
||||
|
||||
## See also
|
||||
|
||||
- [Element Access](https://json.nlohmann.me/features/element_access/index.md) - overview of all ways to access elements
|
||||
- [Unchecked access: operator[]](https://json.nlohmann.me/features/element_access/unchecked_access/index.md) - access without range checking
|
||||
- [Access with default value: value](https://json.nlohmann.me/features/element_access/default_value/index.md) - fall back to a default instead of throwing
|
||||
- [`at`](https://json.nlohmann.me/api/basic_json/at/index.md) - checked access by reference
|
||||
- [JSON Pointer](https://json.nlohmann.me/features/json_pointer/index.md) - access nested values with `at` and a pointer
|
||||
@@ -145,8 +145,10 @@ containing `.` need a different separator.
|
||||
|
||||
## See also
|
||||
|
||||
- [`value`](../../api/basic_json/value.md) for access with default value
|
||||
- documentation on [checked access](checked_access.md)
|
||||
- documentation on [JSON Pointer](../json_pointer.md)
|
||||
- [`contains`](../../api/basic_json/contains.md) to check whether a key or JSON Pointer exists
|
||||
- [`json_pointer::operator/=`](../../api/json_pointer/operator_slasheq.md) to build a pointer token by token
|
||||
- [`value`](../../api/basic_json/value.md) - for access with default value
|
||||
- [Checked access: at](checked_access.md) - access with bounds checking
|
||||
- [JSON Pointer](../json_pointer.md) - addressing values by path
|
||||
- [`contains`](../../api/basic_json/contains.md) - to check whether a key or JSON Pointer exists
|
||||
- [`json_pointer::operator/=`](../../api/json_pointer/operator_slasheq.md) - to build a pointer token by token
|
||||
- [Unchecked access: operator[]](unchecked_access.md) - access without range checking
|
||||
- [Element Access](index.md) - overview of all ways to access elements
|
||||
@@ -1,4 +1,4 @@
|
||||
<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/element_access/default_value/" rel="canonical"><link href=../checked_access/ rel=prev><link href=../../iterators/ rel=next><link rel=icon href=../../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>Access with default value: value - JSON for Modern C++</title><link rel=stylesheet href=../../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../../assets/stylesheets/palette.ab4e12ef.min.css><link rel="stylesheet" href="../../../assets/external/fonts.googleapis.com/css.61a430c9.css"><style>:root{--md-text-font:"Roboto";--md-code-font:"JetBrains Mono"}</style><link rel=stylesheet href=../../../css/custom.css><script>__md_scope=new 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<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=description content="JSON for Modern C++ is a header-only C++11 library that makes JSON a first-class data type."><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/element_access/default_value/" rel="canonical"><link href=../checked_access/ rel=prev><link href=../../iterators/ rel=next><link rel=icon href=../../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>Access with default value: value - JSON for Modern C++</title><link rel=stylesheet href=../../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../../assets/stylesheets/palette.ab4e12ef.min.css><link rel=stylesheet href=../../../css/custom.css><link rel=stylesheet href=../../../css/brand.css><link rel=stylesheet href=../../../css/fig-sax.css><link rel=stylesheet 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href=#access-with-default-value-value class=md-skip> Skip to content </a> </div> <div data-md-component=announce> </div> <header class=md-header data-md-component=header> <nav class="md-header__inner md-grid" aria-label=Header> <a href=../../.. title="JSON for Modern C++" class="md-header__button md-logo" aria-label="JSON for Modern C++" data-md-component=logo> <img src=../../../images/wordmark-header.svg alt=logo> </a> <label class="md-header__button md-icon" for=__drawer> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M3 6h18v2H3zm0 5h18v2H3zm0 5h18v2H3z"/></svg> </label> <div class=md-header__title data-md-component=header-title> <div class=md-header__ellipsis> <div class=md-header__topic> <span class=md-ellipsis> JSON for Modern C++ </span> </div> <div class=md-header__topic data-md-component=header-topic> <span class=md-ellipsis> Access with default value: value </span> </div> </div> </div> <form class=md-header__option data-md-component=palette> <input class=md-option data-md-color-media="(prefers-color-scheme: light)" data-md-color-scheme=default data-md-color-primary=custom data-md-color-accent=custom aria-label="Switch to dark mode" type=radio name=__palette id=__palette_0> <label class="md-header__button md-icon" title="Switch to dark mode" for=__palette_1 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 8a4 4 0 0 0-4 4 4 4 0 0 0 4 4 4 4 0 0 0 4-4 4 4 0 0 0-4-4m0 10a6 6 0 0 1-6-6 6 6 0 0 1 6-6 6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> <input class=md-option data-md-color-media="(prefers-color-scheme: dark)" data-md-color-scheme=slate data-md-color-primary=custom data-md-color-accent=custom aria-label="Switch to light mode" type=radio name=__palette id=__palette_1> <label class="md-header__button md-icon" title="Switch to light mode" for=__palette_0 hidden> <svg xmlns=http://www.w3.org/2000/svg 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class="md-header__button md-icon" for=__search> <svg xLine truncated
|
||||
<span class=w> </span><span class=nt>"logOutput"</span><span class=p>:</span><span class=w> </span><span class=s2>"result.log"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"append"</span><span class=p>:</span><span class=w> </span><span class=kc>true</span>
|
||||
<span class=p>}</span>
|
||||
@@ -48,4 +48,4 @@
|
||||
<span class=err>de</span><span class=kc>fault</span><span class=w> </span><span class=err>value</span><span class=w> </span><span class=err>(i</span><span class=kc>nt</span><span class=err>)</span><span class=p>:</span><span class=w> </span><span class=mi>-1</span>
|
||||
<span class=err>de</span><span class=kc>fault</span><span class=w> </span><span class=err>value</span><span class=w> </span><span class=err>(ui</span><span class=kc>nt</span><span class=mi>64</span><span class=err>_</span><span class=kc>t</span><span class=err>)</span><span class=p>:</span><span class=w> </span><span class=mi>18446744073709551615</span>
|
||||
<span class=err>explici</span><span class=kc>t</span><span class=w> </span><span class=err>re</span><span class=kc>turn</span><span class=w> </span><span class=err>value</span><span class=w> </span><span class=kc>t</span><span class=err>ype</span><span class=p>:</span><span class=w> </span><span class=mi>18446744073709551615</span>
|
||||
</code></pre></div> </div> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">¶</a></h2> <ul> <li><a href=../../../api/basic_json/value/ ><code>value</code></a> for access with default value</li> <li>documentation on <a href=../checked_access/ >checked access</a></li> <li>documentation on <a href=../../json_pointer/ ><abbr title="JavaScript Object Notation">JSON</abbr> Pointer</a></li> <li><a href=../../../api/basic_json/contains/ ><code>contains</code></a> to check whether a key or <abbr title="JavaScript Object Notation">JSON</abbr> Pointer exists</li> <li><a href=../../../api/json_pointer/operator_slasheq/ ><code>json_pointer::operator/=</code></a> to build a pointer token by token</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 11, 2026 06:22:10 UTC">October 11, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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||||
</code></pre></div> </div> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">¶</a></h2> <ul> <li><a href=../../../api/basic_json/value/ ><code>value</code></a> - for access with default value</li> <li><a href=../checked_access/ >Checked access: at</a> - access with bounds checking</li> <li><a href=../../json_pointer/ ><abbr title="JavaScript Object Notation">JSON</abbr> Pointer</a> - addressing values by path</li> <li><a href=../../../api/basic_json/contains/ ><code>contains</code></a> - to check whether a key or <abbr title="JavaScript Object Notation">JSON</abbr> Pointer exists</li> <li><a href=../../../api/json_pointer/operator_slasheq/ ><code>json_pointer::operator/=</code></a> - to build a pointer token by token</li> <li><a href=../unchecked_access/ >Unchecked access: operator[]</a> - access without range checking</li> <li><a href=../ >Element Access</a> - overview of all ways to access elements</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../checked_access/ class="md-footer__link md-footer__link--prev" aria-label="Previous: Checked access: at"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> Checked access: at </div> </div> </a> <a href=../../iterators/ class="md-footer__link md-footer__link--next" aria-label="Next: Iterators"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> Iterators </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2025 Fonticons, Inc.--><path d="M173.9 397.4c0 2-2.3 3.6-5.2 3.6-3.3.3-5.6-1.3-5.6-3.6 0-2 2.3-3.6 5.2-3.6 3-.3 5.6 1.3 5.6 3.6m-31.1-4.5c-.7 2 1.3 4.3 4.3 4.9 2.6 1 5.6 0 6.2-2s-1.3-4.3-4.3-5.2c-2.6-.7-5.5.3-6.2 2.3m44.2-1.7c-2.9.7-4.9 2.6-4.6 4.9.3 2 2.9 3.3 5.9 2.6 2.9-.7 4.9-2.6 4.6-4.6-.3-1.9-3-3.2-5.9-2.9M252.8 8C114.1 8 8 113.3 8 252c0 110.9 69.8 205.8 169.5 239.2 12.8 2.3 17.3-5.6 17.3-12.1 0-6.2-.3-40.4-.3-61.4 0 0-70 15-84.7-29.8 0 0-11.4-29.1-27.8-36.6 0 0-22.9-15.7 1.6-15.4 0 0 24.9 2 38.6 25.8 21.9 38.6 58.6 27.5 72.9 20.9 2.3-16 8.8-27.1 16-33.7-55.9-6.2-112.3-14.3-112.3-110.5 0-27.5 7.6-41.3 23.6-58.9-2.6-6.5-11.1-33.3 2.6-67.9 20.9-6.5 69 27 69 27 20-5.6 41.5-8.5 62.8-8.5s42.8 2.9 62.8 8.5c0 0 48.1-33.6 69-27 13.7 34.7 5.2 61.4 2.6 67.9 16 17.7 25.8 31.5 25.8 58.9 0 96.5-58.9 104.2-114.8 110.5 9.2 7.9 17 22.9 17 46.4 0 33.7-.3 75.4-.3 83.6 0 6.5 4.6 14.4 17.3 12.1C436.2 457.8 504 362.9 504 252 504 113.3 391.5 8 252.8 8M105.2 352.9c-1.3 1-1 3Line truncated
|
||||
@@ -124,8 +124,10 @@ explicit return value type: 18446744073709551615
|
||||
|
||||
## See also
|
||||
|
||||
- [`value`](https://json.nlohmann.me/api/basic_json/value/index.md) for access with default value
|
||||
- documentation on [checked access](https://json.nlohmann.me/features/element_access/checked_access/index.md)
|
||||
- documentation on [JSON Pointer](https://json.nlohmann.me/features/json_pointer/index.md)
|
||||
- [`contains`](https://json.nlohmann.me/api/basic_json/contains/index.md) to check whether a key or JSON Pointer exists
|
||||
- [`json_pointer::operator/=`](https://json.nlohmann.me/api/json_pointer/operator_slasheq/index.md) to build a pointer token by token
|
||||
- [`value`](https://json.nlohmann.me/api/basic_json/value/index.md) - for access with default value
|
||||
- [Checked access: at](https://json.nlohmann.me/features/element_access/checked_access/index.md) - access with bounds checking
|
||||
- [JSON Pointer](https://json.nlohmann.me/features/json_pointer/index.md) - addressing values by path
|
||||
- [`contains`](https://json.nlohmann.me/api/basic_json/contains/index.md) - to check whether a key or JSON Pointer exists
|
||||
- [`json_pointer::operator/=`](https://json.nlohmann.me/api/json_pointer/operator_slasheq/index.md) - to build a pointer token by token
|
||||
- [Unchecked access: operator[]](https://json.nlohmann.me/features/element_access/unchecked_access/index.md) - access without range checking
|
||||
- [Element Access](https://json.nlohmann.me/features/element_access/index.md) - overview of all ways to access elements
|
||||
@@ -1,10 +1,13 @@
|
||||
<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/element_access/" rel="canonical"><link href=../creating_values/ rel=prev><link href=unchecked_access/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>Element Access - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.ab4e12ef.min.css><link rel="stylesheet" href="../../assets/external/fonts.googleapis.com/css.61a430c9.css"><style>:root{--md-text-font:"Roboto";--md-code-font:"JetBrains Mono"}</style><link rel=stylesheet href=../../css/custom.css><script>__md_scope=new URL("../..",location),__md_hash=e=>[...e].reduce(((e,_)=>(e<<5)-e+_.charCodeAt(0)),0),__md_get=(e,_=localStorage,t=__md_scope)=>JSON.parse(_.getItem(t.pathname+"."+e)),__md_set=(e,_,t=localStorage,a=__md_scope)=>{try{t.setItem(a.pathname+"."+e,JSON.stringify(_))}catch(e){}}</script></head> <body dir=ltr data-md-color-scheme=default data-md-color-primary=indigo data-md-color-accent=indigo> <input class=md-toggle data-md-toggle=drawer type=checkbox id=__drawer autocomplete=off> <input class=md-toggle data-md-toggle=search type=checkbox id=__search autocomplete=off> <label class=md-overlay for=__drawer></label> <div data-md-component=skip> <a href=#element-access class=md-skip> Skip to content </a> </div> <div data-md-component=announce> </div> <header class=md-header data-md-component=header> <nav class="md-header__inner md-grid" aria-label=Header> <a href=../.. title="JSON for Modern C++" class="md-header__button md-logo" aria-label="JSON for Modern C++" data-md-component=logo> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 8a3 3 0 0 0 3-3 3 3 0 0 0-3-3 3 3 0 0 0-3 3 3 3 0 0 0 3 3m0 3.54C9.64 9.35 6.5 8 3 8v11c3.5 0 6.64 1.35 9 3.54 2.36-2.19 5.5-3.54 9-3.54V8c-3.5 0-6.64 1.35-9 3.54"/></svg> </a> <label class="md-header__button md-icon" for=__drawer> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M3 6h18v2H3zm0 5h18v2H3zm0 5h18v2H3z"/></svg> </label> <div class=md-header__title data-md-component=header-title> <div class=md-header__ellipsis> <div class=md-header__topic> <span class=md-ellipsis> JSON for Modern C++ </span> </div> <div class=md-header__topic data-md-component=header-topic> <span class=md-ellipsis> Element Access </span> </div> </div> </div> <form class=md-header__option data-md-component=palette> <input class=md-option data-md-color-media="(prefers-color-scheme: light)" data-md-color-scheme=default data-md-color-primary=indigo data-md-color-accent=indigo aria-label="Switch to dark mode" type=radio name=__palette id=__palette_0> <label class="md-header__button md-icon" title="Switch to dark mode" for=__palette_1 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 8a4 4 0 0 0-4 4 4 4 0 0 0 4 4 4 4 0 0 0 4-4 4 4 0 0 0-4-4m0 10a6 6 0 0 1-6-6 6 6 0 0 1 6-6 6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> <input class=md-option data-md-color-media="(prefers-color-scheme: dark)" data-md-color-scheme=slate data-md-color-primary=indigo data-md-color-accent=indigo aria-label="Switch to light mode" type=radio name=__palette id=__palette_1> <label class="md-header__button md-icon" title="Switch to light mode" for=__palette_0 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 18c-.89 0-1.74-.2-2.5-.55C11.56 16.5 13 14.42 13 12s-1.44-4.5-3.5-5.45C10.26 6.2 11.11 6 12 6a6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> </form> <script>var palette=__md_get("__palette");if(palette&&palette.color){if("(prefers-color-scheme)"===palette.color.media){var media=matchMedia("(prefers-color-scheme: light)"),input=document.querySelector(media.matches?"[data-md-color-media='(prefers-color-scheme: light)']":"[data-md-color-media='(prefers-color-scheme: dark)']");palette.color.media=input.getAttribute("data-md-color-media"),palette.color.scheme=input.getAttribute("data-md-color-scheme"),palette.color.primary=input.getAttribute("data-md-color-primary"),palette.color.accent=input.getAttribute("data-md-color-accent")}for(var[key,value]of Object.entries(palette.color))document.body.setAttribute("data-md-color-"+key,value)}</script> <label class="md-header__button md-icon" for=__search> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M9.5 3A6.5 6.5 0 0 1 16 9.5c0 1.61-.59 3.09-1.56 4.23l.27.27h.79l5 5-1.5 1.5-5-5v-.79l-.27-.27A6.52 6.52 0 0 1 9.5 16 6.5 6.5 0 0 1 3 9.5 6.5 6.5 0 0 1 9.5 3m0 2C7 5 5 7 5 9.5S7 14 9.5 14 14 12 14 9.5 12 5 9.5 5"/></svg> </labelLine truncated
|
||||
A["accessing a value"] --> B{"must it exist?"}
|
||||
B -->|"yes, missing is an error"| C["at() -- throws"]
|
||||
B -->|"yes, but checking is my job"| D["operator[] -- unchecked"]
|
||||
B -->|"no, a fallback is fine"| E["value() -- default value"]
|
||||
A --> F{"just testing first?"}
|
||||
F -->|"yes"| G["contains() / find()"]</code></pre> <h2 id=does-this-key-exist>Does this key exist?<a class=headerlink href=#does-this-key-exist title="Permanent link">¶</a></h2> <p>Use <a href=../../api/basic_json/contains/ ><code>contains</code></a> to test for a key; it does not modify the value and does not throw:</p> <div class=highlight><pre><span></span><code><span class=n>json</span><span class=w> </span><span class=n>j</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=p>{{</span><span class=s>"name"</span><span class=p>,</span><span class=w> </span><span class=s>"Mary"</span><span class=p>},</span><span class=w> </span><span class=p>{</span><span class=s>"address"</span><span class=p>,</span><span class=w> </span><span class=p>{{</span><span class=s>"city"</span><span class=p>,</span><span class=w> </span><span class=s>"Berlin"</span><span class=p>}}}};</span>
|
||||
<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=description content="JSON for Modern C++ is a header-only C++11 library that makes JSON a first-class data type."><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/element_access/" rel="canonical"><link href=../creating_values/ rel=prev><link href=unchecked_access/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>Element Access - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.ab4e12ef.min.css><link rel=stylesheet href=../../css/custom.css><link rel=stylesheet href=../../css/brand.css><link rel=stylesheet href=../../css/fig-sax.css><link rel=stylesheet href=../../css/fig-jsonview.css><link rel=stylesheet href=../../css/fig-exceptions.css><link rel=stylesheet href=../../css/fig-pointer.css><link rel=stylesheet href=../../css/fig-charts.css><link rel=stylesheet href=../../css/fig-integration.css><link rel=stylesheet href=../../css/fig-macros.css><script>__md_scope=new URL("../..",location),__md_hash=e=>[...e].reduce(((e,_)=>(e<<5)-e+_.charCodeAt(0)),0),__md_get=(e,_=localStorage,t=__md_scope)=>JSON.parse(_.getItem(t.pathname+"."+e)),__md_set=(e,_,t=localStorage,a=__md_scope)=>{try{t.setItem(a.pathname+"."+e,JSON.stringify(_))}catch(e){}}</script></head> <body dir=ltr data-md-color-scheme=default data-md-color-primary=custom data-md-color-accent=custom> <input class=md-toggle data-md-toggle=drawer type=checkbox id=__drawer autocomplete=off> <input class=md-toggle data-md-toggle=search type=checkbox id=__search autocomplete=off> <label class=md-overlay for=__drawer></label> <div data-md-component=skip> <a href=#element-access class=md-skip> Skip to content </a> </div> <div data-md-component=announce> </div> <header class=md-header data-md-component=header> <nav class="md-header__inner md-grid" aria-label=Header> <a href=../.. title="JSON for Modern C++" class="md-header__button md-logo" aria-label="JSON for Modern C++" data-md-component=logo> <img src=../../images/wordmark-header.svg alt=logo> </a> <label class="md-header__button md-icon" for=__drawer> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M3 6h18v2H3zm0 5h18v2H3zm0 5h18v2H3z"/></svg> </label> <div class=md-header__title data-md-component=header-title> <div class=md-header__ellipsis> <div class=md-header__topic> <span class=md-ellipsis> JSON for Modern C++ </span> </div> <div class=md-header__topic data-md-component=header-topic> <span class=md-ellipsis> Element Access </span> </div> </div> </div> <form class=md-header__option data-md-component=palette> <input class=md-option data-md-color-media="(prefers-color-scheme: light)" data-md-color-scheme=default data-md-color-primary=custom data-md-color-accent=custom aria-label="Switch to dark mode" type=radio name=__palette id=__palette_0> <label class="md-header__button md-icon" title="Switch to dark mode" for=__palette_1 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 8a4 4 0 0 0-4 4 4 4 0 0 0 4 4 4 4 0 0 0 4-4 4 4 0 0 0-4-4m0 10a6 6 0 0 1-6-6 6 6 0 0 1 6-6 6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> <input class=md-option data-md-color-media="(prefers-color-scheme: dark)" data-md-color-scheme=slate data-md-color-primary=custom data-md-color-accent=custom aria-label="Switch to light mode" type=radio name=__palette id=__palette_1> <label class="md-header__button md-icon" title="Switch to light mode" for=__palette_0 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 18c-.89 0-1.74-.2-2.5-.55C11.56 16.5 13 14.42 13 12s-1.44-4.5-3.5-5.45C10.26 6.2 11.11 6 12 6a6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> </form> <script>var palette=__md_get("__palette");if(palette&&palette.color){if("(prefers-color-scheme)"===palette.color.media){var media=matchMedia("(prefers-color-scheme: light)"),input=document.querySelector(media.matches?"[data-md-color-media='(prefers-color-scheme: light)']":"[data-md-color-media='(prefers-color-scheme: dark)']");palette.color.media=input.getAttribute("data-md-color-media"),palette.color.scheme=input.getAttribute("data-md-color-scheme"),palette.color.primary=input.getAttribute("data-md-color-primary"),palette.color.accent=input.getAttribute("data-md-color-accent")}for(var[key,value]of Object.entries(palette.color))document.body.setAttribute("data-md-color-"+key,value)}</script> <label class="md-header__button md-icon" for=__search> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M9.5 3A6.5 6.5 0 0 1 16 9.5c0 1.61-.59 3.09-1.56 Line truncated
|
||||
A["getting a value out"] --> P{{"nested path such as /server/port?"}}
|
||||
P -->|"yes"| JP["JSON Pointer: at(ptr), value(ptr, default), contains(ptr)"]
|
||||
P -->|"no, one key or index"| T{{"only testing whether it exists?"}}
|
||||
T -->|"yes"| CF["contains() / find()"]
|
||||
T -->|"no"| M{{"may it be missing?"}}
|
||||
M -->|"yes, a fallback is fine"| V["value(key, default)"]
|
||||
M -->|"yes, and missing is an error"| AT["at(): throws out_of_range"]
|
||||
M -->|"no, it exists and its type is known"| OP["operator[] and get#lt;T#gt;()"]
|
||||
OP -.- W["non-const operator[] inserts null for a missing key"]</code></pre> <p class=nj-caption>Decision tree for reading a value: the labels on the arrows are the answers to the questions, and the boxes name the functions to use. The dashed line marks a side effect of the non-const <code>operator[]</code>.</p> <h2 id=does-this-key-exist>Does this key exist?<a class=headerlink href=#does-this-key-exist title="Permanent link">¶</a></h2> <p>Use <a href=../../api/basic_json/contains/ ><code>contains</code></a> to test for a key; it does not modify the value and does not throw:</p> <div class=highlight><pre><span></span><code><span class=n>json</span><span class=w> </span><span class=n>j</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=p>{{</span><span class=s>"name"</span><span class=p>,</span><span class=w> </span><span class=s>"Mary"</span><span class=p>},</span><span class=w> </span><span class=p>{</span><span class=s>"address"</span><span class=p>,</span><span class=w> </span><span class=p>{{</span><span class=s>"city"</span><span class=p>,</span><span class=w> </span><span class=s>"Berlin"</span><span class=p>}}}};</span>
|
||||
|
||||
<span class=n>j</span><span class=p>.</span><span class=n>contains</span><span class=p>(</span><span class=s>"name"</span><span class=p>);</span><span class=w> </span><span class=c1>// true</span>
|
||||
<span class=n>j</span><span class=p>.</span><span class=n>contains</span><span class=p>(</span><span class=s>"age"</span><span class=p>);</span><span class=w> </span><span class=c1>// false</span>
|
||||
@@ -17,4 +20,4 @@
|
||||
<span class=p>}</span>
|
||||
</code></pre></div> <p><code>contains</code> also returns <code>true</code> for a key whose value is <code>null</code>; use <code>j.contains("k") && !j["k"].is_null()</code> if a <code>null</code> should count as missing. To read a value with a fallback instead, see <a href=default_value/ ><code>value</code></a>.</p> <div class="admonition warning"> <p class=admonition-title><code>operator[]</code> inserts missing keys</p> <p>Like <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>map</span><span class=o>::</span><span class=k>operator</span><span class=p>[]</span></code>, the non-const <a href=unchecked_access/ ><code>operator[]</code></a> adds a <code>null</code> value if the key is missing. Testing a key with it therefore <em>changes the object</em>:</p> <div class=highlight><pre><span></span><code><span class=n>json</span><span class=w> </span><span class=n>j</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=p>{{</span><span class=s>"foo"</span><span class=p>,</span><span class=w> </span><span class=mi>1</span><span class=p>}};</span>
|
||||
<span class=kt>bool</span><span class=w> </span><span class=n>absent</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>j</span><span class=p>[</span><span class=s>"bar"</span><span class=p>].</span><span class=n>is_null</span><span class=p>();</span><span class=w> </span><span class=c1>// true, but j is now {"bar": null, "foo": 1}</span>
|
||||
</code></pre></div> <p>Use <code>contains</code>/<code>find</code> to test, and <a href=checked_access/ ><code>at</code></a> to access a value that must exist (it throws an <a href=../../home/exceptions/#out-of-range><code>out_of_range</code></a> exception otherwise).</p> </div> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">¶</a></h2> <ul> <li><a href=../../api/basic_json/operator%5B%5D/ ><strong>operator[]</strong></a> - unchecked access</li> <li><a href=../../api/basic_json/at/ ><strong>at</strong></a> - checked access</li> <li><a href=../../api/basic_json/value/ ><strong>value</strong></a> - access with default value</li> <li><a href=../../api/basic_json/contains/ ><strong>contains</strong></a> and <a href=../../api/basic_json/find/ ><strong>find</strong></a> - test for existence</li> <li><a href=../../api/basic_json/front/ ><strong>front</strong></a> and <a href=../../api/basic_json/back/ ><strong>back</strong></a> - first and last element</li> <li><a href=../../api/basic_json/items/ ><strong>items</strong></a> - iterate over keys and values</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 25, 2026 11:07:13 UTC">October 25, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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</code></pre></div> <p>Use <code>contains</code>/<code>find</code> to test, and <a href=checked_access/ ><code>at</code></a> to access a value that must exist (it throws an <a href=../../home/exceptions/#out-of-range><code>out_of_range</code></a> exception otherwise).</p> </div> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">¶</a></h2> <ul> <li><a href=../../api/basic_json/operator%5B%5D/ ><code>operator[]</code></a> - unchecked access</li> <li><a href=../../api/basic_json/at/ ><code>at</code></a> - checked access</li> <li><a href=../../api/basic_json/value/ ><code>value</code></a> - access with default value</li> <li><a href=../../api/basic_json/contains/ ><code>contains</code></a> - and <a href=../../api/basic_json/find/ ><code>find</code></a> - test for existence</li> <li><a href=../../api/basic_json/front/ ><code>front</code></a> - and <a href=../../api/basic_json/back/ ><code>back</code></a> - first and last element</li> <li><a href=../../api/basic_json/items/ ><code>items</code></a> - iterate over keys and values</li> <li><a href=checked_access/ >Checked access: at</a> - access with range checking</li> <li><a href=unchecked_access/ >Unchecked access: operator[]</a> - access without range checking</li> <li><a href=default_value/ >Access with default value: value</a> - fall back to a default instead of throwing</li> <li><a href=../iterators/ >Iterators</a> - visit all elements of arrays and objects</li> <li><a href=../json_pointer/ ><abbr title="JavaScript Object Notation">JSON</abbr> Pointer</a> - address nested values with a single string</li> <li><a href=../modifying_values/ >Modifying values</a> - add, merge and remove elements</li> <li><a href=../../api/basic_json/ ><code>basic_json</code></a> - all member functions</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../creating_values/ class="md-footer__link md-footer__link--prev" aria-label="Previous: Creating JSON values"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> Creating JSON values </div> </div> </a> <a href=unchecked_access/ class="md-footer__link md-footer__link--next" aria-label="Next: Unchecked access: operator[]"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> Unchecked access: operator[] </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2025 Fonticons, Inc.--><path d="M173.9 397.4c0 2-2.3 3.6-5.2 3.6-3.3.3-5.6-1.3-5.6-Line truncated
|
||||
@@ -10,16 +10,23 @@ There are many ways elements in a JSON value can be accessed:
|
||||
|
||||
Testing whether a key or index exists before accessing it is also possible, with [`contains`](https://json.nlohmann.me/api/basic_json/contains/index.md) or [`find`](https://json.nlohmann.me/api/basic_json/find/index.md) (which returns an iterator to the value, or `end()` if it is not found).
|
||||
|
||||
How do I get a value out? Start at the top and follow the first question that applies:
|
||||
|
||||
```
|
||||
flowchart TD
|
||||
A["accessing a value"] --> B{"must it exist?"}
|
||||
B -->|"yes, missing is an error"| C["at() -- throws"]
|
||||
B -->|"yes, but checking is my job"| D["operator[] -- unchecked"]
|
||||
B -->|"no, a fallback is fine"| E["value() -- default value"]
|
||||
A --> F{"just testing first?"}
|
||||
F -->|"yes"| G["contains() / find()"]
|
||||
A["getting a value out"] --> P{{"nested path such as /server/port?"}}
|
||||
P -->|"yes"| JP["JSON Pointer: at(ptr), value(ptr, default), contains(ptr)"]
|
||||
P -->|"no, one key or index"| T{{"only testing whether it exists?"}}
|
||||
T -->|"yes"| CF["contains() / find()"]
|
||||
T -->|"no"| M{{"may it be missing?"}}
|
||||
M -->|"yes, a fallback is fine"| V["value(key, default)"]
|
||||
M -->|"yes, and missing is an error"| AT["at(): throws out_of_range"]
|
||||
M -->|"no, it exists and its type is known"| OP["operator[] and get#lt;T#gt;()"]
|
||||
OP -.- W["non-const operator[] inserts null for a missing key"]
|
||||
```
|
||||
|
||||
Decision tree for reading a value: the labels on the arrows are the answers to the questions, and the boxes name the functions to use. The dashed line marks a side effect of the non-const `operator[]`.
|
||||
|
||||
## Does this key exist?
|
||||
|
||||
Use [`contains`](https://json.nlohmann.me/api/basic_json/contains/index.md) to test for a key; it does not modify the value and does not throw:
|
||||
@@ -58,9 +65,16 @@ Use `contains`/`find` to test, and [`at`](https://json.nlohmann.me/features/elem
|
||||
|
||||
## See also
|
||||
|
||||
- [**operator[]**](https://json.nlohmann.me/api/basic_json/operator%5B%5D/index.md) - unchecked access
|
||||
- [**at**](https://json.nlohmann.me/api/basic_json/at/index.md) - checked access
|
||||
- [**value**](https://json.nlohmann.me/api/basic_json/value/index.md) - access with default value
|
||||
- [**contains**](https://json.nlohmann.me/api/basic_json/contains/index.md) and [**find**](https://json.nlohmann.me/api/basic_json/find/index.md) - test for existence
|
||||
- [**front**](https://json.nlohmann.me/api/basic_json/front/index.md) and [**back**](https://json.nlohmann.me/api/basic_json/back/index.md) - first and last element
|
||||
- [**items**](https://json.nlohmann.me/api/basic_json/items/index.md) - iterate over keys and values
|
||||
- [`operator[]`](https://json.nlohmann.me/api/basic_json/operator%5B%5D/index.md) - unchecked access
|
||||
- [`at`](https://json.nlohmann.me/api/basic_json/at/index.md) - checked access
|
||||
- [`value`](https://json.nlohmann.me/api/basic_json/value/index.md) - access with default value
|
||||
- [`contains`](https://json.nlohmann.me/api/basic_json/contains/index.md) - and [`find`](https://json.nlohmann.me/api/basic_json/find/index.md) - test for existence
|
||||
- [`front`](https://json.nlohmann.me/api/basic_json/front/index.md) - and [`back`](https://json.nlohmann.me/api/basic_json/back/index.md) - first and last element
|
||||
- [`items`](https://json.nlohmann.me/api/basic_json/items/index.md) - iterate over keys and values
|
||||
- [Checked access: at](https://json.nlohmann.me/features/element_access/checked_access/index.md) - access with range checking
|
||||
- [Unchecked access: operator[]](https://json.nlohmann.me/features/element_access/unchecked_access/index.md) - access without range checking
|
||||
- [Access with default value: value](https://json.nlohmann.me/features/element_access/default_value/index.md) - fall back to a default instead of throwing
|
||||
- [Iterators](https://json.nlohmann.me/features/iterators/index.md) - visit all elements of arrays and objects
|
||||
- [JSON Pointer](https://json.nlohmann.me/features/json_pointer/index.md) - address nested values with a single string
|
||||
- [Modifying values](https://json.nlohmann.me/features/modifying_values/index.md) - add, merge and remove elements
|
||||
- [`basic_json`](https://json.nlohmann.me/api/basic_json/index.md) - all member functions
|
||||
@@ -125,3 +125,13 @@ for (int i = 0; i < 1000; ++i) {
|
||||
| access to valid array index | reference to existing value is returned | const reference to existing value is returned |
|
||||
| access to non-existing object key | reference to newly inserted `#!json null` value is returned | **undefined behavior**; [runtime assertion](../assertions.md) in debug mode |
|
||||
| access to invalid array index | reference to newly inserted `#!json null` value is returned; any index between previous maximal index and passed index are filled with `#!json null` | **undefined behavior**; [runtime assertion](../assertions.md) in debug mode |
|
||||
|
||||
<p class="nj-table-caption" markdown>Result of [`operator[]`](../../api/basic_json/operator%5B%5D.md) in each scenario, for a non-const and a const value.</p>
|
||||
|
||||
## See also
|
||||
|
||||
- [Element Access](index.md) - overview of all ways to access elements
|
||||
- [Checked access: at](checked_access.md) - access with range checking
|
||||
- [Access with default value: value](default_value.md) - fall back to a default instead of throwing
|
||||
- [`operator[]`](../../api/basic_json/operator%5B%5D.md) - unchecked access by reference
|
||||
- [Runtime assertions](../assertions.md) - assertions that guard `operator[]` misuse
|
||||
@@ -1,4 +1,4 @@
|
||||
<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/element_access/unchecked_access/" rel="canonical"><link href=../ rel=prev><link href=../checked_access/ rel=next><link rel=icon href=../../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>Unchecked access: operator[] - JSON for Modern C++</title><link rel=stylesheet href=../../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../../assets/stylesheets/palette.ab4e12ef.min.css><link rel="stylesheet" href="../../../assets/external/fonts.googleapis.com/css.61a430c9.css"><style>:root{--md-text-font:"Roboto";--md-code-font:"JetBrains Mono"}</style><link rel=stylesheet href=../../../css/custom.css><script>__md_scope=new 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md-icon" for=__search> <svg xmlns=http://www.w3.org/2Line truncated
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<span class=w> </span><span class=nt>"name"</span><span class=p>:</span><span class=w> </span><span class=s2>"Mary Smith"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"age"</span><span class=p>:</span><span class=w> </span><span class=mi>42</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"hobbies"</span><span class=p>:</span><span class=w> </span><span class=p>[</span><span class=s2>"hiking"</span><span class=p>,</span><span class=w> </span><span class=s2>"reading"</span><span class=p>]</span>
|
||||
@@ -24,4 +24,4 @@
|
||||
<span class=k>for</span><span class=w> </span><span class=p>(</span><span class=kt>int</span><span class=w> </span><span class=n>i</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=mi>0</span><span class=p>;</span><span class=w> </span><span class=n>i</span><span class=w> </span><span class=o><</span><span class=w> </span><span class=mi>1000</span><span class=p>;</span><span class=w> </span><span class=o>++</span><span class=n>i</span><span class=p>)</span><span class=w> </span><span class=p>{</span>
|
||||
<span class=w> </span><span class=n>j</span><span class=p>.</span><span class=n>push_back</span><span class=p>(</span><span class=n>i</span><span class=p>);</span>
|
||||
<span class=p>}</span>
|
||||
</code></pre></div> <h2 id=summary>Summary<a class=headerlink href=#summary title="Permanent link">¶</a></h2> <table> <thead> <tr> <th>scenario</th> <th>non-const value</th> <th>const value</th> </tr> </thead> <tbody> <tr> <td>access to existing object key</td> <td>reference to existing value is returned</td> <td>const reference to existing value is returned</td> </tr> <tr> <td>access to valid array index</td> <td>reference to existing value is returned</td> <td>const reference to existing value is returned</td> </tr> <tr> <td>access to non-existing object key</td> <td>reference to newly inserted <code class=highlight><span class=kc>null</span></code> value is returned</td> <td><strong>undefined behavior</strong>; <a href=../../assertions/ >runtime assertion</a> in debug mode</td> </tr> <tr> <td>access to invalid array index</td> <td>reference to newly inserted <code class=highlight><span class=kc>null</span></code> value is returned; any index between previous maximal index and passed index are filled with <code class=highlight><span class=kc>null</span></code></td> <td><strong>undefined behavior</strong>; <a href=../../assertions/ >runtime assertion</a> in debug mode</td> </tr> </tbody> </table> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 2, 2026 09:32:15 UTC">October 2, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2025 Fonticons, Inc.--><path d="M173.9 397.4c0 2-2.3 3.6-5.2 3.6-3.3.3-5.6-1.3-5.6-3.6 0-2 2.3-3.6 5.2-3.6 3-.3 5.6 1.3 5.6 3.6m-31.1-4.5c-.7 2 1.3 4.3 4.3 4.9 2.6 1 5.6 0 6.2-2s-1.3-4.3-4.3-5.2c-2.6-.7-5.5.3-6.2 2.3m44.2-1.7c-2.9.7-4.9 2.6-4.6 4.9.3 2 2.9 3.3 5.9 2.6 2.9-.7 4.9-2.6 4.6-4.6-.3-1.9-3-3.2-5.9-2.9M252.8 8C114.1 8 8 113.3 8 252c0 110.9 69.8 205.8 169.5 239.2 12.8 2.3 17.3-5.6 17.3-12.1 0-6.2-.3-40.4-.3-61.4 0 0-70 15-84.7-29.8 0 0-11.4-29.1-27.8-36.6 0 0-22.9-15.7 1.6-15.4 0 0 24.9 2 38.6 25.8 21.9 38.6 58.6 27.5 72.9 20.9 2.3-16 8.8-27.1 16-33.7-55.9-6.2-112.3-14.3-112.3-110.5 0-27.5 7.6-41.3 23.6-58.9-2.6-6.5-11.1-33.3 2.6-67.9 20.9-6.5 69 27 69 27 20-5.6 41.5-8.5 62.8-8.5s42.8 2.9 62.8 8.5c0 0 48.1-33.6 69-27 13.7 34.7 5.2 61.4 2.6 67.9 16 17.7 25.8 31.5 25.8 58.9 0 96.5-58.9 104.2-114.8 110.5 9.2 7.9 17 22.9 17 46.4 0 33.7-.3 75.4-.3 83.6 0 6.5 4.6 14.4 17.3 12.1C436.2 457.8 504 362.9 504 252 504 113.3 391.5 8 252.8 8M105.2 352.9c-1.3 1-1 3.3.7 5.2 1.6 1.6 3.9 2.3 5.2 1 1.3-1 1-3.3-.7-5.2-1.6-1.6-3.9-2.3-5.2-1m-10.8-8.1c-.7 1.3.3 2.9 2.3 3.9 1.6 1 3.6.7 4.3-.7.7-1.3-.3-2.9-2.3-3.9-2-.6-3.6-.3-4.3.7m32.4 35.6c-1.6 1.3-1 4.3 1.3 6.2 2.3 2.3 5.2 2.6 6.5 1 1.3-1.3.7-4.3-1.3-6.2-2.2-2.3-5.2-2.6-6.5-1m-11.4-14.7c-1.6 1-1.6 3.6 0 5.9s4.3 3.3 5.6 2.3c1.6-1.3 1.6-3.9 0-6.2-1.4-2.3-4-3.3-5.6-2"/></svg> </a> <a href="https://www.linkedin.com/in/nielslohmann/" target="_blank" rel="noopener" title="www.linkedin.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 448 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/licenLine truncated
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||||
</code></pre></div> <h2 id=summary>Summary<a class=headerlink href=#summary title="Permanent link">¶</a></h2> <table> <thead> <tr> <th>scenario</th> <th>non-const value</th> <th>const value</th> </tr> </thead> <tbody> <tr> <td>access to existing object key</td> <td>reference to existing value is returned</td> <td>const reference to existing value is returned</td> </tr> <tr> <td>access to valid array index</td> <td>reference to existing value is returned</td> <td>const reference to existing value is returned</td> </tr> <tr> <td>access to non-existing object key</td> <td>reference to newly inserted <code class=highlight><span class=kc>null</span></code> value is returned</td> <td><strong>undefined behavior</strong>; <a href=../../assertions/ >runtime assertion</a> in debug mode</td> </tr> <tr> <td>access to invalid array index</td> <td>reference to newly inserted <code class=highlight><span class=kc>null</span></code> value is returned; any index between previous maximal index and passed index are filled with <code class=highlight><span class=kc>null</span></code></td> <td><strong>undefined behavior</strong>; <a href=../../assertions/ >runtime assertion</a> in debug mode</td> </tr> </tbody> </table> <p class=nj-table-caption>Result of <a href=../../../api/basic_json/operator%5B%5D/ ><code>operator[]</code></a> in each scenario, for a non-const and a const value.</p> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">¶</a></h2> <ul> <li><a href=../ >Element Access</a> - overview of all ways to access elements</li> <li><a href=../checked_access/ >Checked access: at</a> - access with range checking</li> <li><a href=../default_value/ >Access with default value: value</a> - fall back to a default instead of throwing</li> <li><a href=../../../api/basic_json/operator%5B%5D/ ><code>operator[]</code></a> - unchecked access by reference</li> <li><a href=../../assertions/ >Runtime assertions</a> - assertions that guard <code>operator[]</code> misuse</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../ class="md-footer__link md-footer__link--prev" aria-label="Previous: Element Access"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> Element Access </div> </div> </a> <a href=../checked_access/ class="md-footer__link md-footer__link--next" aria-label="Next: Checked access: at"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> Checked access: at </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - https://fontawesome.com License - https://fontawesomLine truncated
|
||||
@@ -112,3 +112,13 @@ for (int i = 0; i < 1000; ++i) {
|
||||
| access to valid array index | reference to existing value is returned | const reference to existing value is returned |
|
||||
| access to non-existing object key | reference to newly inserted `null` value is returned | **undefined behavior**; [runtime assertion](https://json.nlohmann.me/features/assertions/index.md) in debug mode |
|
||||
| access to invalid array index | reference to newly inserted `null` value is returned; any index between previous maximal index and passed index are filled with `null` | **undefined behavior**; [runtime assertion](https://json.nlohmann.me/features/assertions/index.md) in debug mode |
|
||||
|
||||
Result of [`operator[]`](https://json.nlohmann.me/api/basic_json/operator%5B%5D/index.md) in each scenario, for a non-const and a const value.
|
||||
|
||||
## See also
|
||||
|
||||
- [Element Access](https://json.nlohmann.me/features/element_access/index.md) - overview of all ways to access elements
|
||||
- [Checked access: at](https://json.nlohmann.me/features/element_access/checked_access/index.md) - access with range checking
|
||||
- [Access with default value: value](https://json.nlohmann.me/features/element_access/default_value/index.md) - fall back to a default instead of throwing
|
||||
- [`operator[]`](https://json.nlohmann.me/api/basic_json/operator%5B%5D/index.md) - unchecked access by reference
|
||||
- [Runtime assertions](https://json.nlohmann.me/features/assertions/index.md) - assertions that guard `operator[]` misuse
|
||||
@@ -225,3 +225,12 @@ Other Important points:
|
||||
deserialization failed: [json.exception.out_of_range.410] enum value out of range for Color: "what"
|
||||
|
||||
```
|
||||
|
||||
## See also
|
||||
|
||||
- [Arbitrary Type Conversions](arbitrary_types.md) - convert your own types
|
||||
- [Converting values](conversions.md) - the general overview of `get`/`get_to`
|
||||
- [`NLOHMANN_JSON_SERIALIZE_ENUM`](../api/macros/nlohmann_json_serialize_enum.md) - define the enum-to-JSON mapping
|
||||
- [`NLOHMANN_JSON_SERIALIZE_ENUM_STRICT`](../api/macros/nlohmann_json_serialize_enum_strict.md) - throw on unmapped values
|
||||
- [`JSON_DISABLE_ENUM_SERIALIZATION`](../api/macros/json_disable_enum_serialization.md) - disable the default enum conversion
|
||||
- [`JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS`](../api/macros/json_use_objects_for_enum_keyed_maps.md) - serialize maps with enum keys as objects
|
||||
@@ -1,4 +1,4 @@
|
||||
<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/enum_conversion/" rel="canonical"><link href=../serialization/ rel=prev><link href=../macros/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>Specializing enum conversion - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.ab4e12ef.min.css><link rel="stylesheet" href="../../assets/external/fonts.googleapis.com/css.61a430c9.css"><style>:root{--md-text-font:"Roboto";--md-code-font:"JetBrains Mono"}</style><link rel=stylesheet href=../../css/custom.css><script>__md_scope=new 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6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> </form> <script>var palette=__md_get("__palette");if(palette&&palette.color){if("(prefers-color-scheme)"===palette.color.media){var media=matchMedia("(prefers-color-scheme: light)"),input=document.querySelector(media.matches?"[data-md-color-media='(prefers-color-scheme: light)']":"[data-md-color-media='(prefers-color-scheme: dark)']");palette.color.media=input.getAttribute("data-md-color-media"),palette.color.scheme=input.getAttribute("data-md-color-scheme"),palette.color.primary=input.getAttribute("data-md-color-primary"),palette.color.accent=input.getAttribute("data-md-color-accent")}for(var[key,value]of Object.entries(palette.color))document.body.setAttribute("data-md-color-"+key,value)}</script> <label class="md-header__button md-icon" for=__search> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M9.5 3A6.5 6.5 0 0 1 16 9.5c0 1.61-.59 3.09-1.56 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<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=description content="JSON for Modern C++ is a header-only C++11 library that makes JSON a first-class data type."><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/enum_conversion/" rel="canonical"><link href=../serialization/ rel=prev><link href=../macros/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>Specializing enum conversion - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.ab4e12ef.min.css><link rel=stylesheet href=../../css/custom.css><link rel=stylesheet href=../../css/brand.css><link rel=stylesheet href=../../css/fig-sax.css><link rel=stylesheet href=../../css/fig-jsonview.css><link rel=stylesheet 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12s-1.44-4.5-3.5-5.45C10.26 6.2 11.11 6 12 6a6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> </form> <script>var palette=__md_get("__palette");if(palette&&palette.color){if("(prefers-color-scheme)"===palette.color.media){var media=matchMedia("(prefers-color-scheme: light)"),input=document.querySelector(media.matches?"[data-md-color-media='(prefers-color-scheme: light)']":"[data-md-color-media='(prefers-color-scheme: dark)']");palette.color.media=input.getAttribute("data-md-color-media"),palette.color.scheme=input.getAttribute("data-md-color-scheme"),palette.color.primary=input.getAttribute("data-md-color-primary"),palette.color.accent=input.getAttribute("data-md-color-accent")}for(var[key,value]of Object.entries(palette.color))document.body.setAttribute("data-md-color-"+key,value)}</script> <label class="md-header__button md-icon" for=__search> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M9.5 3A6.5 6.5 Line truncated
|
||||
<span class=k>enum</span><span class=w> </span><span class=nc>TaskState</span><span class=w> </span><span class=p>{</span>
|
||||
<span class=w> </span><span class=n>TS_STOPPED</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=n>TS_RUNNING</span><span class=p>,</span>
|
||||
@@ -146,4 +146,4 @@
|
||||
<span class=p>}</span>
|
||||
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code><span class=err>deserializa</span><span class=kc>t</span><span class=err>io</span><span class=kc>n</span><span class=w> </span><span class=kc>fa</span><span class=err>iled</span><span class=p>:</span><span class=w> </span><span class=p>[</span><span class=err>jso</span><span class=kc>n</span><span class=err>.excep</span><span class=kc>t</span><span class=err>io</span><span class=kc>n</span><span class=err>.ou</span><span class=kc>t</span><span class=err>_o</span><span class=kc>f</span><span class=err>_ra</span><span class=kc>n</span><span class=err>ge.</span><span class=mi>410</span><span class=p>]</span><span class=w> </span><span class=err>e</span><span class=kc>nu</span><span class=err>m</span><span class=w> </span><span class=err>value</span><span class=w> </span><span class=err>ou</span><span class=kc>t</span><span class=w> </span><span class=err>o</span><span class=kc>f</span><span class=w> </span><span class=err>ra</span><span class=kc>n</span><span class=err>ge</span><span class=w> </span><span class=kc>f</span><span class=err>or</span><span class=w> </span><span class=err>Color</span>
|
||||
<span class=err>deserializa</span><span class=kc>t</span><span class=err>io</span><span class=kc>n</span><span class=w> </span><span class=kc>fa</span><span class=err>iled</span><span class=p>:</span><span class=w> </span><span class=p>[</span><span class=err>jso</span><span class=kc>n</span><span class=err>.excep</span><span class=kc>t</span><span class=err>io</span><span class=kc>n</span><span class=err>.ou</span><span class=kc>t</span><span class=err>_o</span><span class=kc>f</span><span class=err>_ra</span><span class=kc>n</span><span class=err>ge.</span><span class=mi>410</span><span class=p>]</span><span class=w> </span><span class=err>e</span><span class=kc>nu</span><span class=err>m</span><span class=w> </span><span class=err>value</span><span class=w> </span><span class=err>ou</span><span class=kc>t</span><span class=w> </span><span class=err>o</span><span class=kc>f</span><span class=w> </span><span class=err>ra</span><span class=kc>n</span><span class=err>ge</span><span class=w> </span><span class=kc>f</span><span class=err>or</span><span class=w> </span><span class=err>Color</span><span class=p>:</span><span class=w> </span><span class=s2>"what"</span>
|
||||
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</code></pre></div> </details> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">¶</a></h2> <ul> <li><a href=../arbitrary_types/ >Arbitrary Type Conversions</a> - convert your own types</li> <li><a href=../conversions/ >Converting values</a> - the general overview of <code>get</code>/<code>get_to</code></li> <li><a href=../../api/macros/nlohmann_json_serialize_enum/ ><code>NLOHMANN_JSON_SERIALIZE_ENUM</code></a> - define the enum-to-<abbr title="JavaScript Object Notation">JSON</abbr> mapping</li> <li><a href=../../api/macros/nlohmann_json_serialize_enum_strict/ ><code>NLOHMANN_JSON_SERIALIZE_ENUM_STRICT</code></a> - throw on unmapped values</li> <li><a href=../../api/macros/json_disable_enum_serialization/ ><code>JSON_DISABLE_ENUM_SERIALIZATION</code></a> - disable the default enum conversion</li> <li><a href=../../api/macros/json_use_objects_for_enum_keyed_maps/ ><code>JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS</code></a> - serialize maps with enum keys as objects</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../serialization/ class="md-footer__link md-footer__link--prev" aria-label="Previous: Serialization"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> Serialization </div> </div> </a> <a href=../macros/ class="md-footer__link md-footer__link--next" aria-label="Next: Supported Macros"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> Supported Macros </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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|
||||
@@ -205,3 +205,12 @@ Output:
|
||||
deserialization failed: [json.exception.out_of_range.410] enum value out of range for Color
|
||||
deserialization failed: [json.exception.out_of_range.410] enum value out of range for Color: "what"
|
||||
```
|
||||
|
||||
## See also
|
||||
|
||||
- [Arbitrary Type Conversions](https://json.nlohmann.me/features/arbitrary_types/index.md) - convert your own types
|
||||
- [Converting values](https://json.nlohmann.me/features/conversions/index.md) - the general overview of `get`/`get_to`
|
||||
- [`NLOHMANN_JSON_SERIALIZE_ENUM`](https://json.nlohmann.me/api/macros/nlohmann_json_serialize_enum/index.md) - define the enum-to-JSON mapping
|
||||
- [`NLOHMANN_JSON_SERIALIZE_ENUM_STRICT`](https://json.nlohmann.me/api/macros/nlohmann_json_serialize_enum_strict/index.md) - throw on unmapped values
|
||||
- [`JSON_DISABLE_ENUM_SERIALIZATION`](https://json.nlohmann.me/api/macros/json_disable_enum_serialization/index.md) - disable the default enum conversion
|
||||
- [`JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS`](https://json.nlohmann.me/api/macros/json_use_objects_for_enum_keyed_maps/index.md) - serialize maps with enum keys as objects
|
||||
+1
-1
@@ -1 +1 @@
|
||||
<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/" rel="canonical"><link href=../home/debugging/ rel=prev><link href=arbitrary_types/ rel=next><link rel=icon href=../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>Features - JSON for Modern C++</title><link rel=stylesheet href=../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../assets/stylesheets/palette.ab4e12ef.min.css><link rel="stylesheet" href="../assets/external/fonts.googleapis.com/css.61a430c9.css"><style>:root{--md-text-font:"Roboto";--md-code-font:"JetBrains Mono"}</style><link rel=stylesheet href=../css/custom.css><script>__md_scope=new 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<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=description content="JSON for Modern C++ is a header-only C++11 library that makes JSON a first-class data type."><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/" rel="canonical"><link href=../home/debugging/ rel=prev><link href=arbitrary_types/ rel=next><link rel=icon href=../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>Features - JSON for Modern C++</title><link rel=stylesheet href=../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../assets/stylesheets/palette.ab4e12ef.min.css><link rel=stylesheet href=../css/custom.css><link rel=stylesheet href=../css/brand.css><link rel=stylesheet href=../css/fig-sax.css><link rel=stylesheet href=../css/fig-jsonview.css><link rel=stylesheet href=../css/fig-exceptions.css><link rel=stylesheet 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class="md-header__inner md-grid" aria-label=Header> <a href=.. title="JSON for Modern C++" class="md-header__button md-logo" aria-label="JSON for Modern C++" data-md-component=logo> <img src=../images/wordmark-header.svg alt=logo> </a> <label class="md-header__button md-icon" for=__drawer> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M3 6h18v2H3zm0 5h18v2H3zm0 5h18v2H3z"/></svg> </label> <div class=md-header__title data-md-component=header-title> <div class=md-header__ellipsis> <div class=md-header__topic> <span class=md-ellipsis> JSON for Modern C++ </span> </div> <div class=md-header__topic data-md-component=header-topic> <span class=md-ellipsis> Features </span> </div> </div> </div> <form class=md-header__option data-md-component=palette> <input class=md-option data-md-color-media="(prefers-color-scheme: light)" data-md-color-scheme=default data-md-color-primary=custom data-md-color-accent=custom aria-label="Switch to dark mode" type=radio name=__palette 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6.5 6.5 0 0Line truncated
|
||||
+14
-2
@@ -5,7 +5,7 @@ This section describes the features of the library in detail. If you are new to
|
||||
## Creating and reading values
|
||||
|
||||
- [Creating JSON values](https://json.nlohmann.me/features/creating_values/index.md) — build values from literals, initializer lists, and STL containers, and understand the `{}` vs. `[]` ambiguity.
|
||||
- [Parsing](https://json.nlohmann.me/features/parsing/index.md) — read a JSON value from a string, file, or stream, including [JSON Lines](https://json.nlohmann.me/features/parsing/json_lines/index.md), [callbacks](https://json.nlohmann.me/features/parsing/parser_callbacks/index.md), the [SAX interface](https://json.nlohmann.me/features/parsing/sax_interface/index.md), [error handling](https://json.nlohmann.me/features/parsing/parse_exceptions/index.md), and [parsing untrusted input](https://json.nlohmann.me/features/parsing/untrusted_input/index.md).
|
||||
- [Parsing](https://json.nlohmann.me/features/parsing/index.md) — read a JSON value from a string, file, or stream, including [JSON Lines](https://json.nlohmann.me/features/parsing/json_lines/index.md), [callbacks](https://json.nlohmann.me/features/parsing/parser_callbacks/index.md), the [SAX interface](https://json.nlohmann.me/features/parsing/sax_interface/index.md), [error handling](https://json.nlohmann.me/features/parsing/parse_exceptions/index.md), [large and partial input](https://json.nlohmann.me/features/parsing/large_and_partial_input/index.md), and [parsing untrusted input](https://json.nlohmann.me/features/parsing/untrusted_input/index.md).
|
||||
- [Zero-copy JSON views](https://json.nlohmann.me/features/json_view/index.md) — read a JSON text through a flat index instead of building a `json` tree; strings and numbers stay in the input and are only decoded when needed.
|
||||
- [Editable documents](https://json.nlohmann.me/features/json_view/#editing-a-document) — modify a parsed document without building a `json` tree.
|
||||
- [Images](https://json.nlohmann.me/features/json_view/#images) — save a parsed document and load it again without parsing it.
|
||||
@@ -23,6 +23,7 @@ This section describes the features of the library in detail. If you are new to
|
||||
|
||||
- [Converting values](https://json.nlohmann.me/features/conversions/index.md) — get values out with [`get`](https://json.nlohmann.me/api/basic_json/get/index.md)/[`get_to`](https://json.nlohmann.me/api/basic_json/get_to/index.md), and understand implicit conversions.
|
||||
- [Arbitrary types conversions](https://json.nlohmann.me/features/arbitrary_types/index.md) — teach the library about your own structs and classes.
|
||||
- [Recipes for common types](https://json.nlohmann.me/features/type_recipes/index.md) — convert smart pointers, `std::variant`, `std::chrono` types, and class hierarchies, which the library deliberately leaves to you.
|
||||
- [Specializing enum conversion](https://json.nlohmann.me/features/enum_conversion/index.md) — map enums to strings instead of integers.
|
||||
|
||||
## Serializing values
|
||||
@@ -37,6 +38,7 @@ This section describes the features of the library in detail. If you are new to
|
||||
- [Template parameter requirements](https://json.nlohmann.me/features/types/template_parameters/index.md) — what a type passed as one of `basic_json`'s template parameters has to provide.
|
||||
- [Object order](https://json.nlohmann.me/features/object_order/index.md) — keep insertion order with [`ordered_json`](https://json.nlohmann.me/api/ordered_json/index.md).
|
||||
- [Performance](https://json.nlohmann.me/features/performance/index.md) — practical advice on parsing, memory use, serialization, and compile times.
|
||||
- [Working without exceptions](https://json.nlohmann.me/features/no_exceptions/index.md) — what to use when exceptions are unwanted or unavailable.
|
||||
- [Runtime assertions](https://json.nlohmann.me/features/assertions/index.md), [supported macros](https://json.nlohmann.me/features/macros/index.md), the [`nlohmann` namespace](https://json.nlohmann.me/features/namespace/index.md), and [C++ modules](https://json.nlohmann.me/features/modules/index.md) — build-time and runtime configuration.
|
||||
|
||||
Looking for a specific function?
|
||||
@@ -48,5 +50,15 @@ This section gives conceptual overviews. For the precise signature, parameters,
|
||||
- [API documentation](https://json.nlohmann.me/api/basic_json/index.md) - reference of all classes and functions
|
||||
- [FAQ](https://json.nlohmann.me/home/faq/index.md) - answers to common questions
|
||||
- [Exceptions](https://json.nlohmann.me/home/exceptions/index.md) - all exceptions and their error codes
|
||||
- [Design goals](https://json.nlohmann.me/home/design_goals/index.md)
|
||||
- [Design goals](https://json.nlohmann.me/home/design_goals/index.md) - the goals behind the library
|
||||
- [Integration](https://json.nlohmann.me/integration/index.md) - how to add the library to a project
|
||||
- [Macros](https://json.nlohmann.me/api/macros/index.md) - reference of all macros
|
||||
- [`json_pointer`](https://json.nlohmann.me/api/json_pointer/index.md) - the JSON Pointer class
|
||||
- [`adl_serializer`](https://json.nlohmann.me/api/adl_serializer/index.md) - customization point for type conversions
|
||||
- [`json_sax`](https://json.nlohmann.me/api/json_sax/index.md) - interface of the SAX parser events
|
||||
- [`json_document`](https://json.nlohmann.me/api/json_document/index.md) - an immutable document with a flat index
|
||||
- [`json_view`](https://json.nlohmann.me/api/json_view/index.md) - a non-owning view of a value in a document
|
||||
- [`byte_container_with_subtype`](https://json.nlohmann.me/api/byte_container_with_subtype/index.md) - binary values with a subtype
|
||||
- [`ordered_json`](https://json.nlohmann.me/api/ordered_json/index.md) - a JSON type preserving insertion order
|
||||
- [Supported Macros](https://json.nlohmann.me/features/macros/index.md) - all supported macros
|
||||
- [`nlohmann` Namespace](https://json.nlohmann.me/features/namespace/index.md) - the library namespace
|
||||
+102
-2
@@ -9,7 +9,39 @@ As for other containers, [`begin()`](../api/basic_json/begin.md) returns an iter
|
||||
placeholder and cannot be dereferenced. In case of null values, empty arrays, or empty objects, `begin()` will return
|
||||
`end()`.
|
||||
|
||||
|
||||
<div class="nj-fig-wrap" markdown>
|
||||
<svg class="nj-fig nj-fig--range" viewBox="0 0 600 150" role="img" aria-labelledby="fig-be-title" xmlns="http://www.w3.org/2000/svg">
|
||||
<title id="fig-be-title">A row of fifteen elements followed by a dashed past-the-last position. begin() points to the first element and end() points to the past-the-last position.</title>
|
||||
<defs><marker id="fig-be-arrow" viewBox="0 0 10 10" refX="9" refY="5" markerWidth="7" markerHeight="7" orient="auto"><path class="fg-head" d="M0 1 L10 5 L0 9 z"/></marker></defs>
|
||||
<rect class="fg-cell" x="30" y="62" width="30" height="30" rx="5"/>
|
||||
<rect class="fg-cell" x="64" y="62" width="30" height="30" rx="5"/>
|
||||
<rect class="fg-cell" x="98" y="62" width="30" height="30" rx="5"/>
|
||||
<rect class="fg-cell" x="132" y="62" width="30" height="30" rx="5"/>
|
||||
<rect class="fg-cell" x="166" y="62" width="30" height="30" rx="5"/>
|
||||
<rect class="fg-cell" x="200" y="62" width="30" height="30" rx="5"/>
|
||||
<rect class="fg-cell" x="234" y="62" width="30" height="30" rx="5"/>
|
||||
<rect class="fg-cell" x="268" y="62" width="30" height="30" rx="5"/>
|
||||
<rect class="fg-cell" x="302" y="62" width="30" height="30" rx="5"/>
|
||||
<rect class="fg-cell" x="336" y="62" width="30" height="30" rx="5"/>
|
||||
<rect class="fg-cell" x="370" y="62" width="30" height="30" rx="5"/>
|
||||
<rect class="fg-cell" x="404" y="62" width="30" height="30" rx="5"/>
|
||||
<rect class="fg-cell" x="438" y="62" width="30" height="30" rx="5"/>
|
||||
<rect class="fg-cell" x="472" y="62" width="30" height="30" rx="5"/>
|
||||
<rect class="fg-cell" x="506" y="62" width="30" height="30" rx="5"/>
|
||||
<rect class="fg-cell fg-cell--end" x="540" y="62" width="30" height="30" rx="5"/>
|
||||
<rect class="fg-pill" x="15" y="8" width="60" height="24" rx="6"/>
|
||||
<text class="fg-pilltext" x="45" y="24">begin</text>
|
||||
<line class="fg-arrow" x1="45" y1="32" x2="45" y2="60" marker-end="url(#fig-be-arrow)"/>
|
||||
<rect class="fg-pill" x="531" y="8" width="48" height="24" rx="6"/>
|
||||
<text class="fg-pilltext" x="555" y="24">end</text>
|
||||
<line class="fg-arrow" x1="555" y1="32" x2="555" y2="60" marker-end="url(#fig-be-arrow)"/>
|
||||
<text class="fg-note fg-note--r" x="537" y="132">Past-the-last element</text>
|
||||
<line class="fg-arrow" x1="555" y1="124" x2="555" y2="96" marker-end="url(#fig-be-arrow)"/>
|
||||
</svg>
|
||||
|
||||
</div>
|
||||
|
||||
<p class="nj-caption" markdown>The range of `begin()` and `end()`: `begin()` refers to the first element, `end()` to the position after the last element, which must not be dereferenced.</p>
|
||||
|
||||
### Iteration order for objects
|
||||
|
||||
@@ -112,7 +144,65 @@ for (auto& [key, val] : j_object.items())
|
||||
[`rbegin()`](../api/basic_json/rbegin.md) and [`rend()`](../api/basic_json/rend.md) return iterators in the reverse
|
||||
sequence.
|
||||
|
||||
|
||||
<div class="nj-fig-wrap" markdown>
|
||||
<svg class="nj-fig nj-fig--range" viewBox="0 0 600 236" role="img" aria-labelledby="fig-rbe-title" xmlns="http://www.w3.org/2000/svg">
|
||||
<title id="fig-rbe-title">Two rows of sixteen cells. The upper row shows the reversed sequence: rend() points to a dashed position before the first element and rbegin() to the last element. The lower row shows the forward sequence: begin() points to the first element and end() to a dashed position after the last element. A reverse iterator stores an iterator to the next element, so rend() corresponds to begin() and rbegin() to end().</title>
|
||||
<defs><marker id="fig-rbe-arrow" viewBox="0 0 10 10" refX="9" refY="5" markerWidth="7" markerHeight="7" orient="auto"><path class="fg-head" d="M0 1 L10 5 L0 9 z"/></marker></defs>
|
||||
<rect class="fg-cell fg-cell--end" x="30" y="62" width="30" height="30" rx="5"/>
|
||||
<rect class="fg-cell" x="64" y="62" width="30" height="30" rx="5"/>
|
||||
<rect class="fg-cell" x="98" y="62" width="30" height="30" rx="5"/>
|
||||
<rect class="fg-cell" x="132" y="62" width="30" height="30" rx="5"/>
|
||||
<rect class="fg-cell" x="166" y="62" width="30" height="30" rx="5"/>
|
||||
<rect class="fg-cell" x="200" y="62" width="30" height="30" rx="5"/>
|
||||
<rect class="fg-cell" x="234" y="62" width="30" height="30" rx="5"/>
|
||||
<rect class="fg-cell" x="268" y="62" width="30" height="30" rx="5"/>
|
||||
<rect class="fg-cell" x="302" y="62" width="30" height="30" rx="5"/>
|
||||
<rect class="fg-cell" x="336" y="62" width="30" height="30" rx="5"/>
|
||||
<rect class="fg-cell" x="370" y="62" width="30" height="30" rx="5"/>
|
||||
<rect class="fg-cell" x="404" y="62" width="30" height="30" rx="5"/>
|
||||
<rect class="fg-cell" x="438" y="62" width="30" height="30" rx="5"/>
|
||||
<rect class="fg-cell" x="472" y="62" width="30" height="30" rx="5"/>
|
||||
<rect class="fg-cell" x="506" y="62" width="30" height="30" rx="5"/>
|
||||
<rect class="fg-cell" x="540" y="62" width="30" height="30" rx="5"/>
|
||||
<rect class="fg-cell" x="30" y="144" width="30" height="30" rx="5"/>
|
||||
<rect class="fg-cell" x="64" y="144" width="30" height="30" rx="5"/>
|
||||
<rect class="fg-cell" x="98" y="144" width="30" height="30" rx="5"/>
|
||||
<rect class="fg-cell" x="132" y="144" width="30" height="30" rx="5"/>
|
||||
<rect class="fg-cell" x="166" y="144" width="30" height="30" rx="5"/>
|
||||
<rect class="fg-cell" x="200" y="144" width="30" height="30" rx="5"/>
|
||||
<rect class="fg-cell" x="234" y="144" width="30" height="30" rx="5"/>
|
||||
<rect class="fg-cell" x="268" y="144" width="30" height="30" rx="5"/>
|
||||
<rect class="fg-cell" x="302" y="144" width="30" height="30" rx="5"/>
|
||||
<rect class="fg-cell" x="336" y="144" width="30" height="30" rx="5"/>
|
||||
<rect class="fg-cell" x="370" y="144" width="30" height="30" rx="5"/>
|
||||
<rect class="fg-cell" x="404" y="144" width="30" height="30" rx="5"/>
|
||||
<rect class="fg-cell" x="438" y="144" width="30" height="30" rx="5"/>
|
||||
<rect class="fg-cell" x="472" y="144" width="30" height="30" rx="5"/>
|
||||
<rect class="fg-cell" x="506" y="144" width="30" height="30" rx="5"/>
|
||||
<rect class="fg-cell fg-cell--end" x="540" y="144" width="30" height="30" rx="5"/>
|
||||
<rect class="fg-pill" x="19" y="8" width="52" height="24" rx="6"/>
|
||||
<text class="fg-pilltext" x="45" y="24">rend</text>
|
||||
<line class="fg-arrow" x1="45" y1="32" x2="45" y2="60" marker-end="url(#fig-rbe-arrow)"/>
|
||||
<rect class="fg-pill" x="521" y="8" width="68" height="24" rx="6"/>
|
||||
<text class="fg-pilltext" x="555" y="24">rbegin</text>
|
||||
<line class="fg-arrow" x1="555" y1="32" x2="555" y2="60" marker-end="url(#fig-rbe-arrow)"/>
|
||||
<text class="fg-note fg-note--l" x="81" y="24">Reverse past-the-last element</text>
|
||||
<line class="fg-arrow fg-arrow--dash" x1="45" y1="94" x2="45" y2="142"/>
|
||||
<line class="fg-arrow fg-arrow--dash" x1="555" y1="94" x2="555" y2="142"/>
|
||||
<text class="fg-note" x="300" y="123">A reverse iterator stores an iterator to the next element</text>
|
||||
<text class="fg-note" x="300" y="141" dy="0" style="display:none"></text>
|
||||
<rect class="fg-pill" x="15" y="204" width="60" height="24" rx="6"/>
|
||||
<text class="fg-pilltext" x="45" y="220">begin</text>
|
||||
<line class="fg-arrow" x1="45" y1="202" x2="45" y2="178" marker-end="url(#fig-rbe-arrow)"/>
|
||||
<rect class="fg-pill" x="531" y="204" width="48" height="24" rx="6"/>
|
||||
<text class="fg-pilltext" x="555" y="220">end</text>
|
||||
<line class="fg-arrow" x1="555" y1="202" x2="555" y2="178" marker-end="url(#fig-rbe-arrow)"/>
|
||||
<text class="fg-note fg-note--r" x="515" y="221">Past-the-last element</text>
|
||||
</svg>
|
||||
|
||||
</div>
|
||||
|
||||
<p class="nj-caption" markdown>The range of `rbegin()` and `rend()` over the same elements: `rbegin()` refers to the last element and `rend()` to the position before the first one. Each reverse iterator stores an iterator to the element after the one it refers to.</p>
|
||||
|
||||
??? example "Example: reverse iteration with `rbegin()` and `rend()`"
|
||||
|
||||
@@ -159,3 +249,13 @@ Note that "value" means a JSON value in this setting, not values stored in the u
|
||||
| Operations | invalidated iterators |
|
||||
|------------|-----------------------|
|
||||
| `clear` | all |
|
||||
|
||||
## See also
|
||||
|
||||
- [`begin`](../api/basic_json/begin.md) - / [`end`](../api/basic_json/end.md) - iterators to the first and one past the last element
|
||||
- [`rbegin`](../api/basic_json/rbegin.md) - / [`rend`](../api/basic_json/rend.md) - reverse iterators
|
||||
- [`cbegin`](../api/basic_json/cbegin.md) - / [`cend`](../api/basic_json/cend.md) - const iterators
|
||||
- [`items`](../api/basic_json/items.md) - iterate over keys and values
|
||||
- [Object Order](object_order.md) - iteration order of object keys
|
||||
- [Element Access](element_access/index.md) - access single elements
|
||||
- [Modifying values](modifying_values.md) - add and remove elements (including iterator invalidation rules)
|
||||
@@ -1,4 +1,4 @@
|
||||
<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/iterators/" rel="canonical"><link href=../element_access/default_value/ rel=prev><link href=../merge_patch/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>Iterators - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.ab4e12ef.min.css><link rel="stylesheet" href="../../assets/external/fonts.googleapis.com/css.61a430c9.css"><style>:root{--md-text-font:"Roboto";--md-code-font:"JetBrains Mono"}</style><link rel=stylesheet href=../../css/custom.css><script>__md_scope=new 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15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> </form> <script>var palette=__md_get("__palette");if(palette&&palette.color){if("(prefers-color-scheme)"===palette.color.media){var media=matchMedia("(prefers-color-scheme: light)"),input=document.querySelector(media.matches?"[data-md-color-media='(prefers-color-scheme: light)']":"[data-md-color-media='(prefers-color-scheme: dark)']");palette.color.media=input.getAttribute("data-md-color-media"),palette.color.scheme=input.getAttribute("data-md-color-scheme"),palette.color.primary=input.getAttribute("data-md-color-primary"),palette.color.accent=input.getAttribute("data-md-color-accent")}for(var[key,value]of Object.entries(palette.color))document.body.setAttribute("data-md-color-"+key,value)}</script> <label class="md-header__button md-icon" for=__search> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M9.5 3A6.5 6.5 0 0 1 16 9.5c0 1.61-.59 3.09-1.56 4.23l.27.27h.79l5 5-1.5 1.5-5-5v-.79l-.27-.27A6.52 6.52 0 0 1 9.5 16 6.5 6.5 0 0 1 3 9.5 6.5 6.5 0 0 1 9.5 3m0 2C7 5 5 7 5 9.5S7 14 9.5 14 14 12 14 9.5 12 5 9.5 5"/></svg> </label> <div clLine truncated
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<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=description content="JSON for Modern C++ is a header-only C++11 library that makes JSON a first-class data type."><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/iterators/" rel="canonical"><link href=../element_access/default_value/ rel=prev><link href=../merge_patch/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>Iterators - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.ab4e12ef.min.css><link rel=stylesheet href=../../css/custom.css><link rel=stylesheet href=../../css/brand.css><link rel=stylesheet href=../../css/fig-sax.css><link rel=stylesheet href=../../css/fig-jsonview.css><link rel=stylesheet 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data-md-component=announce> </div> <header class=md-header data-md-component=header> <nav class="md-header__inner md-grid" aria-label=Header> <a href=../.. title="JSON for Modern C++" class="md-header__button md-logo" aria-label="JSON for Modern C++" data-md-component=logo> <img src=../../images/wordmark-header.svg alt=logo> </a> <label class="md-header__button md-icon" for=__drawer> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M3 6h18v2H3zm0 5h18v2H3zm0 5h18v2H3z"/></svg> </label> <div class=md-header__title data-md-component=header-title> <div class=md-header__ellipsis> <div class=md-header__topic> <span class=md-ellipsis> JSON for Modern C++ </span> </div> <div class=md-header__topic data-md-component=header-topic> <span class=md-ellipsis> Iterators </span> </div> </div> </div> <form class=md-header__option data-md-component=palette> <input class=md-option data-md-color-media="(prefers-color-scheme: light)" data-md-color-scheme=default data-md-color-primary=custom data-md-color-accent=custom aria-label="Switch to dark mode" type=radio name=__palette id=__palette_0> <label class="md-header__button md-icon" title="Switch to dark mode" for=__palette_1 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 8a4 4 0 0 0-4 4 4 4 0 0 0 4 4 4 4 0 0 0 4-4 4 4 0 0 0-4-4m0 10a6 6 0 0 1-6-6 6 6 0 0 1 6-6 6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> <input class=md-option data-md-color-media="(prefers-color-scheme: dark)" data-md-color-scheme=slate data-md-color-primary=custom data-md-color-accent=custom aria-label="Switch to light mode" type=radio name=__palette id=__palette_1> <label class="md-header__button md-icon" title="Switch to light mode" for=__palette_0 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 18c-.89 0-1.74-.2-2.5-.55C11.56 16.5 13 14.42 13 12s-1.44-4.5-3.5-5.45C10.26 6.2 11.11 6 12 6a6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> </form> <script>var palette=__md_get("__palette");if(palette&&palette.color){if("(prefers-color-scheme)"===palette.color.media){var media=matchMedia("(prefers-color-scheme: light)"),input=document.querySelector(media.matches?"[data-md-color-media='(prefers-color-scheme: light)']":"[data-md-color-media='(prefers-color-scheme: dark)']");palette.color.media=input.getAttribute("data-md-color-media"),palette.color.scheme=input.getAttribute("data-md-color-scheme"),palette.color.primary=input.getAttribute("data-md-color-primary"),palette.color.accent=input.getAttribute("data-md-color-accent")}for(var[key,value]of Object.entries(palette.color))document.body.setAttribute("data-md-color-"+key,value)}</script> <label class="md-header__button md-icon" for=__search> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M9.5 3A6.5 6.5 0 0 1 16 9.5c0 1.61-.59 3.09-1.56 4.23l.27.Line truncated
|
||||
<span class=n>json</span><span class=w> </span><span class=n>j</span><span class=p>;</span>
|
||||
<span class=n>j</span><span class=p>[</span><span class=s>"one"</span><span class=p>]</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=mi>1</span><span class=p>;</span>
|
||||
<span class=n>j</span><span class=p>[</span><span class=s>"two"</span><span class=p>]</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=mi>2</span><span class=p>;</span>
|
||||
@@ -37,7 +37,7 @@
|
||||
<span class=p>{</span>
|
||||
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=s>"key: "</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=n>key</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=s>", value:"</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=n>val</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=sc>'\n'</span><span class=p>;</span>
|
||||
<span class=p>}</span>
|
||||
</code></pre></div> <div class="admonition note"> <p class=admonition-title>Note</p> <p>When iterating over an array, <code>key()</code> will return the index of the element as string. For primitive types (e.g., numbers), <code>key()</code> returns an empty string.</p> </div> <div class="admonition warning"> <p class=admonition-title>Warning</p> <p>Using <code>items()</code> on temporary objects is dangerous. Make sure the object's lifetime exceeds the iteration. See <a href="https://github.com/nlohmann/json/issues/2040">#2040</a> for more information.</p> </div> <h3 id=reverse-iteration-order>Reverse iteration order<a class=headerlink href=#reverse-iteration-order title="Permanent link">¶</a></h3> <p><a href=../../api/basic_json/rbegin/ ><code>rbegin()</code></a> and <a href=../../api/basic_json/rend/ ><code>rend()</code></a> return iterators in the reverse sequence.</p> <p><img alt="Illustration from cppreference.com" src=../../images/range-rbegin-rend.svg></p> <details class=example> <summary>Example: reverse iteration with <code>rbegin()</code> and <code>rend()</code></summary> <div class=highlight><pre><span></span><code><span class=n>json</span><span class=w> </span><span class=n>j</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=p>{</span><span class=mi>1</span><span class=p>,</span><span class=w> </span><span class=mi>2</span><span class=p>,</span><span class=w> </span><span class=mi>3</span><span class=p>,</span><span class=w> </span><span class=mi>4</span><span class=p>};</span>
|
||||
</code></pre></div> <div class="admonition note"> <p class=admonition-title>Note</p> <p>When iterating over an array, <code>key()</code> will return the index of the element as string. For primitive types (e.g., numbers), <code>key()</code> returns an empty string.</p> </div> <div class="admonition warning"> <p class=admonition-title>Warning</p> <p>Using <code>items()</code> on temporary objects is dangerous. Make sure the object's lifetime exceeds the iteration. See <a href="https://github.com/nlohmann/json/issues/2040">#2040</a> for more information.</p> </div> <h3 id=reverse-iteration-order>Reverse iteration order<a class=headerlink href=#reverse-iteration-order title="Permanent link">¶</a></h3> <p><a href=../../api/basic_json/rbegin/ ><code>rbegin()</code></a> and <a href=../../api/basic_json/rend/ ><code>rend()</code></a> return iterators in the reverse sequence.</p> <div class=nj-fig-wrap> <p><svg class="nj-fig nj-fig--range" viewbox="0 0 600 236" role=img aria-labelledby=fig-rbe-title xmlns=http://www.w3.org/2000/svg> <title id=fig-rbe-title>Two rows of sixteen cells. The upper row shows the reversed sequence: rend() points to a dashed position before the first element and rbegin() to the last element. The lower row shows the forward sequence: begin() points to the first element and end() to a dashed position after the last element. A reverse iterator stores an iterator to the next element, so rend() corresponds to begin() and rbegin() to end().</title> <defs><marker id=fig-rbe-arrow viewbox="0 0 10 10" refx=9 refy=5 markerwidth=7 markerheight=7 orient=auto><path class=fg-head d="M0 1 L10 5 L0 9 z"/></marker></defs> <rect x=30 y=62 width=30 height=30 rx=5 class="fg-cell fg-cell--end"/> <rect class=fg-cell x=64 y=62 width=30 height=30 rx=5 /> <rect class=fg-cell x=98 y=62 width=30 height=30 rx=5 /> <rect class=fg-cell x=132 y=62 width=30 height=30 rx=5 /> <rect class=fg-cell x=166 y=62 width=30 height=30 rx=5 /> <rect class=fg-cell x=200 y=62 width=30 height=30 rx=5 /> <rect class=fg-cell x=234 y=62 width=30 height=30 rx=5 /> <rect class=fg-cell x=268 y=62 width=30 height=30 rx=5 /> <rect class=fg-cell x=302 y=62 width=30 height=30 rx=5 /> <rect class=fg-cell x=336 y=62 width=30 height=30 rx=5 /> <rect class=fg-cell x=370 y=62 width=30 height=30 rx=5 /> <rect class=fg-cell x=404 y=62 width=30 height=30 rx=5 /> <rect class=fg-cell x=438 y=62 width=30 height=30 rx=5 /> <rect class=fg-cell x=472 y=62 width=30 height=30 rx=5 /> <rect class=fg-cell x=506 y=62 width=30 height=30 rx=5 /> <rect class=fg-cell x=540 y=62 width=30 height=30 rx=5 /> <rect class=fg-cell x=30 y=144 width=30 height=30 rx=5 /> <rect class=fg-cell x=64 y=144 width=30 height=30 rx=5 /> <rect class=fg-cell x=98 y=144 width=30 height=30 rx=5 /> <rect class=fg-cell x=132 y=144 width=30 height=30 rx=5 /> <rect class=fg-cell x=166 y=144 width=30 height=30 rx=5 /> <rect class=fg-cell x=200 y=144 width=30 height=30 rx=5 /> <rect class=fg-cell x=234 y=144 width=30 height=30 rx=5 /> <rect class=fg-cell x=268 y=144 width=30 height=30 rx=5 /> <rect class=fg-cell x=302 y=144 width=30 height=30 rx=5 /> <rect class=fg-cell x=336 y=144 width=30 height=30 rx=5 /> <rect class=fg-cell x=370 y=144 width=30 height=30 rx=5 /> <rect class=fg-cell x=404 y=144 width=30 height=30 rx=5 /> <rect class=fg-cell x=438 y=144 width=30 height=30 rx=5 /> <rect class=fg-cell x=472 y=144 width=30 height=30 rx=5 /> <rect class=fg-cell x=506 y=144 width=30 height=30 rx=5 /> <rect x=540 y=144 width=30 height=30 rx=5 class="fg-cell fg-cell--end"/> <rect class=fg-pill x=19 y=8 width=52 height=24 rx=6 /> <text class=fg-pilltext x=45 y=24>rend</text> <line class=fg-arrow x1=45 y1=32 x2=45 y2=60 marker-end=url(#fig-rbe-arrow) /> <rect class=fg-pill x=521 y=8 width=68 height=24 rx=6 /> <text class=fg-pilltext x=555 y=24>rbegin</text> <line class=fg-arrow x1=555 y1=32 x2=555 y2=60 marker-end=url(#fig-rbe-arrow) /> <text class="fg-note fg-note--l" x=81 y=24>Reverse past-the-last element</text> <line x1=45 y1=94 x2=45 y2=142 class="fg-arrow fg-arrow--dash"/> <line x1=555 y1=94 x2=555 y2=142 class="fg-arrow fg-arrow--dash"/> <text class=fg-note x=300 y=123>A reverse iterator stores an iterator to the next element</text> <text class=fg-note x=300 y=141 dy=0 style=display:none></text> <rect class=fg-pill x=15 y=204 width=60 height=24 rx=6 /> <text class=fg-pilltext x=45 y=220>begin</text> <line class=fg-arrow x1=45 y1=202 x2=45 y2=178 marker-end=url(#fig-rbe-arrow) /> <rect class=fg-pill x=531 y=204 width=48 height=24 rx=6 /> <text class=fg-pilltext x=555 y=220>end</text> <line class=fg-arrow x1=555 y1=202 x2=555 y2=178 marker-end=url(#fig-rbe-arrow) /> <text class="fg-note fg-note--r" x=515 y=221>Past-the-last element</text> </svg></p> </div> <p class=nj-caption>The range of <code>rbegin()</code> and <code>rend()</code> over the same elements: <code>rbegin()</code> refers to the last element and <code>rend()</code> to the position before the first one. Each reverse iterator storLine truncated
|
||||
|
||||
<span class=k>for</span><span class=w> </span><span class=p>(</span><span class=k>auto</span><span class=w> </span><span class=n>it</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>j</span><span class=p>.</span><span class=n>rbegin</span><span class=p>();</span><span class=w> </span><span class=n>it</span><span class=w> </span><span class=o>!=</span><span class=w> </span><span class=n>j</span><span class=p>.</span><span class=n>rend</span><span class=p>();</span><span class=w> </span><span class=o>++</span><span class=n>it</span><span class=p>)</span>
|
||||
<span class=p>{</span>
|
||||
@@ -53,4 +53,4 @@
|
||||
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=o>*</span><span class=n>it</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>endl</span><span class=p>;</span>
|
||||
<span class=p>}</span>
|
||||
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code><span class=s2>"Hello, world"</span>
|
||||
</code></pre></div> </details> <h2 id=iterator-invalidation>Iterator invalidation<a class=headerlink href=#iterator-invalidation title="Permanent link">¶</a></h2> <table> <thead> <tr> <th>Operations</th> <th>invalidated iterators</th> </tr> </thead> <tbody> <tr> <td><code>clear</code></td> <td>all</td> </tr> </tbody> </table> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 2, 2026 09:32:15 UTC">October 2, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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</code></pre></div> </details> <h2 id=iterator-invalidation>Iterator invalidation<a class=headerlink href=#iterator-invalidation title="Permanent link">¶</a></h2> <table> <thead> <tr> <th>Operations</th> <th>invalidated iterators</th> </tr> </thead> <tbody> <tr> <td><code>clear</code></td> <td>all</td> </tr> </tbody> </table> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">¶</a></h2> <ul> <li><a href=../../api/basic_json/begin/ ><code>begin</code></a> - / <a href=../../api/basic_json/end/ ><code>end</code></a> - iterators to the first and one past the last element</li> <li><a href=../../api/basic_json/rbegin/ ><code>rbegin</code></a> - / <a href=../../api/basic_json/rend/ ><code>rend</code></a> - reverse iterators</li> <li><a href=../../api/basic_json/cbegin/ ><code>cbegin</code></a> - / <a href=../../api/basic_json/cend/ ><code>cend</code></a> - const iterators</li> <li><a href=../../api/basic_json/items/ ><code>items</code></a> - iterate over keys and values</li> <li><a href=../object_order/ >Object Order</a> - iteration order of object keys</li> <li><a href=../element_access/ >Element Access</a> - access single elements</li> <li><a href=../modifying_values/ >Modifying values</a> - add and remove elements (including iterator invalidation rules)</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../element_access/default_value/ class="md-footer__link md-footer__link--prev" aria-label="Previous: Access with default value: value"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> Access with default value: value </div> </div> </a> <a href=../merge_patch/ class="md-footer__link md-footer__link--next" aria-label="Next: JSON Merge Patch"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> JSON Merge Patch </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2025 Fonticons, Inc.--><path d="M173.9 397.4c0 2-2.3 3.6-5.2 3.6-3.3.3-5.6-1.3-5.6-3.6 0-2 2.3-3.6 5.2-3.6 3-.3 5.6 1.3 5.6 3.6m-31.1-4.5c-.7 2 1.3 4.3 4.3 4.9 2.6 1 5.6 0 6.2-2s-1.3-4.3-4.3-5.2c-2.6-.7-5.5.3-6.2 2.3m44.2-1.7c-2.9.7-4.9 2.6-4.6 4.9.3 2 2.9 3.3 5.9 2.6 2.9-.7 4.9-2.6 4.6-4.6-.3-1.9-3-3.2-5.9-2.9M252.8 8C114.1 8 8 113.3 8 252c0 110.9 69.8 205.8 169.5 239.2 12.8 2.3 17.3-5.6 17.3-12.1 0-6.2-.3-40.4-.3-61.4 0 0-70 15-84.7-29.8 0 0-11.4-29.1-27.8-36.6 0 0-22.9-15.7 1.6-15.4 0 0 24.9 2 38.6 25.8 21.9 38.6 58.6 27.5 72.9 20.9 2.3-16 8.8-27.Line truncated
|
||||
@@ -6,6 +6,8 @@ A `basic_json` value is a container and allows access via iterators. Depending o
|
||||
|
||||
As for other containers, [`begin()`](https://json.nlohmann.me/api/basic_json/begin/index.md) returns an iterator to the first value and [`end()`](https://json.nlohmann.me/api/basic_json/end/index.md) returns an iterator to the value following the last value. The latter iterator is a placeholder and cannot be dereferenced. In case of null values, empty arrays, or empty objects, `begin()` will return `end()`.
|
||||
|
||||
The range of `begin()` and `end()`: `begin()` refers to the first element, `end()` to the position after the last element, which must not be dereferenced.
|
||||
|
||||
### Iteration order for objects
|
||||
|
||||
When iterating over objects, values are ordered with respect to the `object_comparator_t` type which defaults to `std::less`. See the [types documentation](https://json.nlohmann.me/features/types/#key-order) for more information.
|
||||
@@ -104,6 +106,8 @@ Using `items()` on temporary objects is dangerous. Make sure the object's lifeti
|
||||
|
||||
[`rbegin()`](https://json.nlohmann.me/api/basic_json/rbegin/index.md) and [`rend()`](https://json.nlohmann.me/api/basic_json/rend/index.md) return iterators in the reverse sequence.
|
||||
|
||||
The range of `rbegin()` and `rend()` over the same elements: `rbegin()` refers to the last element and `rend()` to the position before the first one. Each reverse iterator stores an iterator to the element after the one it refers to.
|
||||
|
||||
Example: reverse iteration with `rbegin()` and `rend()`
|
||||
|
||||
```
|
||||
@@ -149,3 +153,13 @@ Output:
|
||||
| Operations | invalidated iterators |
|
||||
| ---------- | --------------------- |
|
||||
| `clear` | all |
|
||||
|
||||
## See also
|
||||
|
||||
- [`begin`](https://json.nlohmann.me/api/basic_json/begin/index.md) - / [`end`](https://json.nlohmann.me/api/basic_json/end/index.md) - iterators to the first and one past the last element
|
||||
- [`rbegin`](https://json.nlohmann.me/api/basic_json/rbegin/index.md) - / [`rend`](https://json.nlohmann.me/api/basic_json/rend/index.md) - reverse iterators
|
||||
- [`cbegin`](https://json.nlohmann.me/api/basic_json/cbegin/index.md) - / [`cend`](https://json.nlohmann.me/api/basic_json/cend/index.md) - const iterators
|
||||
- [`items`](https://json.nlohmann.me/api/basic_json/items/index.md) - iterate over keys and values
|
||||
- [Object Order](https://json.nlohmann.me/features/object_order/index.md) - iteration order of object keys
|
||||
- [Element Access](https://json.nlohmann.me/features/element_access/index.md) - access single elements
|
||||
- [Modifying values](https://json.nlohmann.me/features/modifying_values/index.md) - add and remove elements (including iterator invalidation rules)
|
||||
@@ -7,6 +7,24 @@ of operations to apply to a JSON document. Operations address locations in the d
|
||||
[JSON Pointer](json_pointer.md) paths. With the [`patch`](../api/basic_json/patch.md) function, a JSON Patch is applied
|
||||
to the current JSON value by executing all operations from the patch, yielding the patched document as a new value.
|
||||
|
||||
The following steps through one operation of each kind. The first three operations are those of the example below; the
|
||||
other three continue on its result.
|
||||
|
||||
<div class="nj-patch" data-nj-patch markdown>
|
||||
|
||||
| Operation | Before | After |
|
||||
|-------------------------------------------------------------|------------------------------------------------|-------------------------------------------------|
|
||||
| `{"op": "replace", "path": "/baz", "value": "boo"}` | `{"baz": "qux", "foo": "bar"}` | `{"baz": "boo", "foo": "bar"}` |
|
||||
| `{"op": "add", "path": "/hello", "value": ["world"]}` | `{"baz": "boo", "foo": "bar"}` | `{"baz": "boo", "foo": "bar", "hello": ["world"]}` |
|
||||
| `{"op": "remove", "path": "/foo"}` | `{"baz": "boo", "foo": "bar", "hello": ["world"]}` | `{"baz": "boo", "hello": ["world"]}` |
|
||||
| `{"op": "move", "from": "/baz", "path": "/hello/1"}` | `{"baz": "boo", "hello": ["world"]}` | `{"hello": ["world", "boo"]}` |
|
||||
| `{"op": "copy", "from": "/hello/0", "path": "/greeting"}` | `{"hello": ["world", "boo"]}` | `{"greeting": "world", "hello": ["world", "boo"]}` |
|
||||
| `{"op": "test", "path": "/greeting", "value": "world"}` | `{"greeting": "world", "hello": ["world", "boo"]}` | unchanged (a failed test throws) |
|
||||
|
||||
</div>
|
||||
|
||||
<p class="nj-caption" markdown>Stepper through the six operations: *Next* applies the next operation, *Back* undoes it, and *Reset* returns to the original document. The current operation is highlighted in the patch list, and the lines it changes are marked in the document.</p>
|
||||
|
||||
!!! tip "Applying a patch without copying"
|
||||
|
||||
[`patch`](../api/basic_json/patch.md) leaves the original value unchanged and returns the patched result as a copy.
|
||||
@@ -89,6 +107,8 @@ flowchart LR
|
||||
P -.->|"applied to source, yields"| T
|
||||
```
|
||||
|
||||
<p class="nj-caption" markdown>How `diff` and `patch` relate: `diff(source, target)` yields a patch, and applying that patch to the source yields the target.</p>
|
||||
|
||||
!!! success "Invariant"
|
||||
|
||||
For two JSON values *source* and *target*, the following code yields true:
|
||||
@@ -191,3 +211,6 @@ flowchart LR
|
||||
- [`patch`](../api/basic_json/patch.md) - apply a JSON Patch, returning the result as a copy
|
||||
- [`patch_inplace`](../api/basic_json/patch_inplace.md) - apply a JSON Patch without copying
|
||||
- [`diff`](../api/basic_json/diff.md) - compute a JSON Patch from two values
|
||||
- [`json_pointer`](../api/json_pointer/index.md) - type to represent JSON Pointers
|
||||
- [`merge_diff`](../api/basic_json/merge_diff.md) - compute a JSON Merge Patch from two values
|
||||
- [Modifying values](modifying_values.md) - simple in-place modifications
|
||||
@@ -1,4 +1,4 @@
|
||||
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0 24 24"><path d="M9.5 3A6.5 6.5 0 0 1 16 9.5c0 1.61-.59 3.09Line truncated
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<span class=cp>#include</span><span class=w> </span><span class=cpf><iomanip></span>
|
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<span class=cp>#include</span><span class=w> </span><span class=cpf><nlohmann/json.hpp></span>
|
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@@ -45,7 +45,7 @@
|
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</code></pre></div> </details> <h2 id=diff>Diff<a class=headerlink href=#diff title="Permanent link">¶</a></h2> <p>The library can also calculate a <abbr title="JavaScript Object Notation">JSON</abbr> patch (i.e., a <strong>diff</strong>) given two <abbr title="JavaScript Object Notation">JSON</abbr> values with the <a href=../../api/basic_json/diff/ ><code>diff</code></a> function.</p> <pre class=mermaid><code>flowchart LR
|
||||
S["source"] -->|"diff(source, target)"| P["patch"]
|
||||
S -->|"source.patch(patch)"| T["target"]
|
||||
P -.->|"applied to source, yields"| T</code></pre> <div class="admonition success"> <p class=admonition-title>Invariant</p> <p>For two <abbr title="JavaScript Object Notation">JSON</abbr> values <em>source</em> and <em>target</em>, the following code yields true:</p> <div class=highlight><pre><span></span><code><span class=n>source</span><span class=p>.</span><span class=n>patch</span><span class=p>(</span><span class=n>diff</span><span class=p>(</span><span class=n>source</span><span class=p>,</span><span class=w> </span><span class=n>target</span><span class=p>))</span><span class=w> </span><span class=o>==</span><span class=w> </span><span class=n>target</span><span class=p>;</span>
|
||||
P -.->|"applied to source, yields"| T</code></pre> <p class=nj-caption>How <code>diff</code> and <code>patch</code> relate: <code>diff(source, target)</code> yields a patch, and applying that patch to the source yields the target.</p> <div class="admonition success"> <p class=admonition-title>Invariant</p> <p>For two <abbr title="JavaScript Object Notation">JSON</abbr> values <em>source</em> and <em>target</em>, the following code yields true:</p> <div class=highlight><pre><span></span><code><span class=n>source</span><span class=p>.</span><span class=n>patch</span><span class=p>(</span><span class=n>diff</span><span class=p>(</span><span class=n>source</span><span class=p>,</span><span class=w> </span><span class=n>target</span><span class=p>))</span><span class=w> </span><span class=o>==</span><span class=w> </span><span class=n>target</span><span class=p>;</span>
|
||||
</code></pre></div> <p>The equality holds up to <a href=../../api/basic_json/operator_eq/ ><code>operator==</code></a>, with these exceptions:</p> <ul> <li><code>NaN</code> and <a href=../../api/basic_json/is_discarded/ >discarded</a> values are not equal to themselves, so the invariant cannot hold for them. <code>diff</code> emits a <code>replace</code> operation for such a value, and a <code>NaN</code> in a patch is serialized as <code class=highlight><span class=kc>null</span></code>.</li> <li>Numbers that compare equal produce no operation even if their types differ: <code>diff(1, 1.0)</code>, <code>diff(1u, 1)</code>, and <code>diff(0.0, -0.0)</code> return <code class=highlight><span class=p>[]</span></code>, so the patched value keeps the number type of <em>source</em>.</li> <li>A <a href=../binary_values/ >binary value</a> in a patch is serialized as an object (<code class=highlight><span class=p>{</span><span class=nt>"bytes"</span><span class=p>:[</span><span class=err>...</span><span class=p>],</span><span class=nt>"subtype"</span><span class=p>:</span><span class=kc>null</span><span class=p>}</span></code>), so a patch that was dumped and parsed again no longer holds a binary value.</li> </ul> </div> <details class=example> <summary>Example: create a <abbr title="JavaScript Object Notation">JSON</abbr> Patch from the difference of two values</summary> <p>The following code shows how a <abbr title="JavaScript Object Notation">JSON</abbr> patch is created as a diff for two <abbr title="JavaScript Object Notation">JSON</abbr> values.</p> <div class=highlight><pre><span></span><code><span class=cp>#include</span><span class=w> </span><span class=cpf><iostream></span>
|
||||
<span class=cp>#include</span><span class=w> </span><span class=cpf><iomanip></span>
|
||||
<span class=cp>#include</span><span class=w> </span><span class=cpf><nlohmann/json.hpp></span>
|
||||
@@ -108,4 +108,4 @@
|
||||
<span class=w> </span><span class=s2>"world"</span>
|
||||
<span class=w> </span><span class=p>]</span>
|
||||
<span class=p>}</span>
|
||||
</code></pre></div> </details> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">¶</a></h2> <ul> <li><a href=../json_pointer/ ><abbr title="JavaScript Object Notation">JSON</abbr> Pointer</a> - the addressing scheme used for patch paths</li> <li><a href=../merge_patch/ ><abbr title="JavaScript Object Notation">JSON</abbr> Merge Patch</a> - a simpler, less expressive alternative patch format</li> <li><a href=../../api/basic_json/patch/ ><code>patch</code></a> - apply a <abbr title="JavaScript Object Notation">JSON</abbr> Patch, returning the result as a copy</li> <li><a href=../../api/basic_json/patch_inplace/ ><code>patch_inplace</code></a> - apply a <abbr title="JavaScript Object Notation">JSON</abbr> Patch without copying</li> <li><a href=../../api/basic_json/diff/ ><code>diff</code></a> - compute a <abbr title="JavaScript Object Notation">JSON</abbr> Patch from two values</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 26, 2026 16:12:05 UTC">October 26, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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||||
</code></pre></div> </details> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">¶</a></h2> <ul> <li><a href=../json_pointer/ ><abbr title="JavaScript Object Notation">JSON</abbr> Pointer</a> - the addressing scheme used for patch paths</li> <li><a href=../merge_patch/ ><abbr title="JavaScript Object Notation">JSON</abbr> Merge Patch</a> - a simpler, less expressive alternative patch format</li> <li><a href=../../api/basic_json/patch/ ><code>patch</code></a> - apply a <abbr title="JavaScript Object Notation">JSON</abbr> Patch, returning the result as a copy</li> <li><a href=../../api/basic_json/patch_inplace/ ><code>patch_inplace</code></a> - apply a <abbr title="JavaScript Object Notation">JSON</abbr> Patch without copying</li> <li><a href=../../api/basic_json/diff/ ><code>diff</code></a> - compute a <abbr title="JavaScript Object Notation">JSON</abbr> Patch from two values</li> <li><a href=../../api/json_pointer/ ><code>json_pointer</code></a> - type to represent <abbr title="JavaScript Object Notation">JSON</abbr> Pointers</li> <li><a href=../../api/basic_json/merge_diff/ ><code>merge_diff</code></a> - compute a <abbr title="JavaScript Object Notation">JSON</abbr> Merge Patch from two values</li> <li><a href=../modifying_values/ >Modifying values</a> - simple in-place modifications</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../merge_patch/ class="md-footer__link md-footer__link--prev" aria-label="Previous: JSON Merge Patch"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> JSON Merge Patch </div> </div> </a> <a href=../json_pointer/ class="md-footer__link md-footer__link--next" aria-label="Next: JSON Pointer"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> JSON Pointer </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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|
||||
@@ -4,6 +4,19 @@
|
||||
|
||||
JSON Patch ([RFC 6902](https://tools.ietf.org/html/rfc6902)) defines a JSON document structure for expressing a sequence of operations to apply to a JSON document. Operations address locations in the document using [JSON Pointer](https://json.nlohmann.me/features/json_pointer/index.md) paths. With the [`patch`](https://json.nlohmann.me/api/basic_json/patch/index.md) function, a JSON Patch is applied to the current JSON value by executing all operations from the patch, yielding the patched document as a new value.
|
||||
|
||||
The following steps through one operation of each kind. The first three operations are those of the example below; the other three continue on its result.
|
||||
|
||||
| Operation | Before | After |
|
||||
| --------------------------------------------------------- | -------------------------------------------------- | -------------------------------------------------- |
|
||||
| `{"op": "replace", "path": "/baz", "value": "boo"}` | `{"baz": "qux", "foo": "bar"}` | `{"baz": "boo", "foo": "bar"}` |
|
||||
| `{"op": "add", "path": "/hello", "value": ["world"]}` | `{"baz": "boo", "foo": "bar"}` | `{"baz": "boo", "foo": "bar", "hello": ["world"]}` |
|
||||
| `{"op": "remove", "path": "/foo"}` | `{"baz": "boo", "foo": "bar", "hello": ["world"]}` | `{"baz": "boo", "hello": ["world"]}` |
|
||||
| `{"op": "move", "from": "/baz", "path": "/hello/1"}` | `{"baz": "boo", "hello": ["world"]}` | `{"hello": ["world", "boo"]}` |
|
||||
| `{"op": "copy", "from": "/hello/0", "path": "/greeting"}` | `{"hello": ["world", "boo"]}` | `{"greeting": "world", "hello": ["world", "boo"]}` |
|
||||
| `{"op": "test", "path": "/greeting", "value": "world"}` | `{"greeting": "world", "hello": ["world", "boo"]}` | unchanged (a failed test throws) |
|
||||
|
||||
Stepper through the six operations: *Next* applies the next operation, *Back* undoes it, and *Reset* returns to the original document. The current operation is highlighted in the patch list, and the lines it changes are marked in the document.
|
||||
|
||||
Applying a patch without copying
|
||||
|
||||
[`patch`](https://json.nlohmann.me/api/basic_json/patch/index.md) leaves the original value unchanged and returns the patched result as a copy. If the document is large and the original value is no longer needed, [`patch_inplace`](https://json.nlohmann.me/api/basic_json/patch_inplace/index.md) applies the same operations in place instead.
|
||||
@@ -79,6 +92,8 @@ flowchart LR
|
||||
P -.->|"applied to source, yields"| T
|
||||
```
|
||||
|
||||
How `diff` and `patch` relate: `diff(source, target)` yields a patch, and applying that patch to the source yields the target.
|
||||
|
||||
Invariant
|
||||
|
||||
For two JSON values *source* and *target*, the following code yields true:
|
||||
@@ -174,3 +189,6 @@ Output:
|
||||
- [`patch`](https://json.nlohmann.me/api/basic_json/patch/index.md) - apply a JSON Patch, returning the result as a copy
|
||||
- [`patch_inplace`](https://json.nlohmann.me/api/basic_json/patch_inplace/index.md) - apply a JSON Patch without copying
|
||||
- [`diff`](https://json.nlohmann.me/api/basic_json/diff/index.md) - compute a JSON Patch from two values
|
||||
- [`json_pointer`](https://json.nlohmann.me/api/json_pointer/index.md) - type to represent JSON Pointers
|
||||
- [`merge_diff`](https://json.nlohmann.me/api/basic_json/merge_diff/index.md) - compute a JSON Merge Patch from two values
|
||||
- [Modifying values](https://json.nlohmann.me/features/modifying_values/index.md) - simple in-place modifications
|
||||
+101
-5
@@ -18,6 +18,40 @@ Consider the following JSON document
|
||||
}
|
||||
```
|
||||
|
||||
Select a pointer (or a node of the tree) to walk it through this document; each segment of the pointer goes one level
|
||||
deeper. The table below lists the same pointers.
|
||||
|
||||
<div class="nj-pointer-walk" data-nj-pointer-walk data-doc="{"array":["A","B","C"],"nested":{"one":1,"two":2,"three":[true,false]}}" role="group" aria-label="Pointer walker for the example document">
|
||||
<p class="nj-pointer-static">Highlighted: the path of the pointer <code>/nested/three/1</code>, which identifies <code>false</code>.</p>
|
||||
<ul class="nj-pointer-tree">
|
||||
<li data-p="" class="is-on"><span class="nj-pointer-n"><span class="nj-pointer-k">root</span> <span class="nj-pointer-t">object</span></span>
|
||||
<ul>
|
||||
<li data-p="/array" class="is-pass"><span class="nj-pointer-n"><span class="nj-pointer-k">"array"</span> <span class="nj-pointer-t">array</span></span>
|
||||
<ul>
|
||||
<li data-p="/array/0"><span class="nj-pointer-n"><span class="nj-pointer-k">0</span>: <span class="nj-pointer-v nj-pointer-v--s">"A"</span></span></li>
|
||||
<li data-p="/array/1"><span class="nj-pointer-n"><span class="nj-pointer-k">1</span>: <span class="nj-pointer-v nj-pointer-v--s">"B"</span></span></li>
|
||||
<li data-p="/array/2"><span class="nj-pointer-n"><span class="nj-pointer-k">2</span>: <span class="nj-pointer-v nj-pointer-v--s">"C"</span></span></li>
|
||||
</ul>
|
||||
</li>
|
||||
<li data-p="/nested" class="is-on"><span class="nj-pointer-n"><span class="nj-pointer-k">"nested"</span> <span class="nj-pointer-t">object</span></span>
|
||||
<ul>
|
||||
<li data-p="/nested/one" class="is-pass"><span class="nj-pointer-n"><span class="nj-pointer-k">"one"</span>: <span class="nj-pointer-v nj-pointer-v--n">1</span></span></li>
|
||||
<li data-p="/nested/two" class="is-pass"><span class="nj-pointer-n"><span class="nj-pointer-k">"two"</span>: <span class="nj-pointer-v nj-pointer-v--n">2</span></span></li>
|
||||
<li data-p="/nested/three" class="is-on"><span class="nj-pointer-n"><span class="nj-pointer-k">"three"</span> <span class="nj-pointer-t">array</span></span>
|
||||
<ul>
|
||||
<li data-p="/nested/three/0" class="is-pass"><span class="nj-pointer-n"><span class="nj-pointer-k">0</span>: <span class="nj-pointer-v nj-pointer-v--b">true</span></span></li>
|
||||
<li data-p="/nested/three/1" class="is-on is-target"><span class="nj-pointer-n"><span class="nj-pointer-k">1</span>: <span class="nj-pointer-v nj-pointer-v--b">false</span></span></li>
|
||||
</ul>
|
||||
</li>
|
||||
</ul>
|
||||
</li>
|
||||
</ul>
|
||||
</li>
|
||||
</ul>
|
||||
</div>
|
||||
|
||||
<p class="nj-caption" markdown>The highlighted nodes are the path from the root to the selected level, and the line below the tree shows the value the pointer resolves to. Select a segment in the pointer bar to stop at a shallower level, or a node of the tree to select its pointer.</p>
|
||||
|
||||
Then every value inside the JSON document can be identified as follows:
|
||||
|
||||
| JSON Pointer | JSON value |
|
||||
@@ -138,12 +172,74 @@ The reverse function, [`unflatten`](../api/basic_json/unflatten.md) recreates th
|
||||
auto j_original = j_flat.unflatten();
|
||||
```
|
||||
|
||||
Each leaf of the document becomes one key of the flat object, and `unflatten()` reverses the mapping:
|
||||
|
||||
<div class="nj-fig-wrap" markdown>
|
||||
<svg class="nj-fig nj-pointer-fl" viewBox="0 0 760 442" role="img" aria-labelledby="fig-flat-title" xmlns="http://www.w3.org/2000/svg">
|
||||
<title id="fig-flat-title">flatten() turns the nested example document into a flat object, and unflatten() turns it back. Left, the document as a tree: array with the elements A, B, C, and nested with one = 1, two = 2, and three with the elements true and false. Right, the flat object with one key per leaf: /array/0 = A, /array/1 = B, /array/2 = C, /nested/one = 1, /nested/two = 2, /nested/three/0 = true, /nested/three/1 = false. Each leaf is joined to its key by a line.</title>
|
||||
<defs><marker id="fig-flat-arrow" viewBox="0 0 10 10" refX="9" refY="5" markerWidth="7" markerHeight="7" orient="auto"><path class="fg-head" d="M0 1 L10 5 L0 9 z"/></marker></defs>
|
||||
<text class="fg-h" x="20" y="26">nested value</text>
|
||||
<text class="fg-h" x="470" y="26">flat object: key = J​SON Pointer</text>
|
||||
<line class="fg-arrow" x1="250" y1="52" x2="450" y2="52" marker-end="url(#fig-flat-arrow)"/>
|
||||
<text class="nj-pointer-fl-op" x="350" y="44">flatten()</text>
|
||||
<line class="fg-arrow" x1="450" y1="420" x2="250" y2="420" marker-end="url(#fig-flat-arrow)"/>
|
||||
<text class="nj-pointer-fl-op" x="350" y="412">unflatten()</text>
|
||||
<path class="nj-pointer-fl-edge" d="M25 99 V115 H42"/>
|
||||
<path class="nj-pointer-fl-edge" d="M51 127 V143 H68"/>
|
||||
<path class="nj-pointer-fl-edge" d="M51 127 V171 H68"/>
|
||||
<path class="nj-pointer-fl-edge" d="M51 127 V199 H68"/>
|
||||
<path class="nj-pointer-fl-edge" d="M25 99 V227 H42"/>
|
||||
<path class="nj-pointer-fl-edge" d="M51 239 V255 H68"/>
|
||||
<path class="nj-pointer-fl-edge" d="M51 239 V283 H68"/>
|
||||
<path class="nj-pointer-fl-edge" d="M51 239 V311 H68"/>
|
||||
<path class="nj-pointer-fl-edge" d="M77 323 V339 H94"/>
|
||||
<path class="nj-pointer-fl-edge" d="M77 323 V367 H94"/>
|
||||
<path class="nj-pointer-fl-link nj-pointer-fl--s" d="M139.6 143 C259.6 143 340 171 460 171"/>
|
||||
<circle class="nj-pointer-fl-dot nj-pointer-fl--s" cx="139.6" cy="143" r="3.5"/>
|
||||
<text class="fg-code" x="470" y="176" xml:space="preserve"><tspan class="nj-pointer-fl-key">"/array/0"</tspan>: <tspan class="nj-pointer-fl-v nj-pointer-fl--s">"A"</tspan></text>
|
||||
<path class="nj-pointer-fl-link nj-pointer-fl--s" d="M139.6 171 C259.6 171 340 199 460 199"/>
|
||||
<circle class="nj-pointer-fl-dot nj-pointer-fl--s" cx="139.6" cy="171" r="3.5"/>
|
||||
<text class="fg-code" x="470" y="204" xml:space="preserve"><tspan class="nj-pointer-fl-key">"/array/1"</tspan>: <tspan class="nj-pointer-fl-v nj-pointer-fl--s">"B"</tspan></text>
|
||||
<path class="nj-pointer-fl-link nj-pointer-fl--s" d="M139.6 199 C259.6 199 340 227 460 227"/>
|
||||
<circle class="nj-pointer-fl-dot nj-pointer-fl--s" cx="139.6" cy="199" r="3.5"/>
|
||||
<text class="fg-code" x="470" y="232" xml:space="preserve"><tspan class="nj-pointer-fl-key">"/array/2"</tspan>: <tspan class="nj-pointer-fl-v nj-pointer-fl--s">"C"</tspan></text>
|
||||
<path class="nj-pointer-fl-link nj-pointer-fl--n" d="M158.8 255 C278.8 255 340 255 460 255"/>
|
||||
<circle class="nj-pointer-fl-dot nj-pointer-fl--n" cx="158.8" cy="255" r="3.5"/>
|
||||
<text class="fg-code" x="470" y="260" xml:space="preserve"><tspan class="nj-pointer-fl-key">"/nested/one"</tspan>: <tspan class="nj-pointer-fl-v nj-pointer-fl--n">1</tspan></text>
|
||||
<path class="nj-pointer-fl-link nj-pointer-fl--n" d="M158.8 283 C278.8 283 340 283 460 283"/>
|
||||
<circle class="nj-pointer-fl-dot nj-pointer-fl--n" cx="158.8" cy="283" r="3.5"/>
|
||||
<text class="fg-code" x="470" y="288" xml:space="preserve"><tspan class="nj-pointer-fl-key">"/nested/two"</tspan>: <tspan class="nj-pointer-fl-v nj-pointer-fl--n">2</tspan></text>
|
||||
<path class="nj-pointer-fl-link nj-pointer-fl--b" d="M175.2 339 C295.2 339 340 311 460 311"/>
|
||||
<circle class="nj-pointer-fl-dot nj-pointer-fl--b" cx="175.2" cy="339" r="3.5"/>
|
||||
<text class="fg-code" x="470" y="316" xml:space="preserve"><tspan class="nj-pointer-fl-key">"/nested/three/0"</tspan>: <tspan class="nj-pointer-fl-v nj-pointer-fl--b">true</tspan></text>
|
||||
<path class="nj-pointer-fl-link nj-pointer-fl--b" d="M184.8 367 C304.8 367 340 339 460 339"/>
|
||||
<circle class="nj-pointer-fl-dot nj-pointer-fl--b" cx="184.8" cy="367" r="3.5"/>
|
||||
<text class="fg-code" x="470" y="344" xml:space="preserve"><tspan class="nj-pointer-fl-key">"/nested/three/1"</tspan>: <tspan class="nj-pointer-fl-v nj-pointer-fl--b">false</tspan></text>
|
||||
<text class="fg-h" x="20" y="92">root</text>
|
||||
<text class="fg-code nj-pointer-fl-inner" x="46" y="120">"array"</text>
|
||||
<text class="fg-code" x="72" y="148" xml:space="preserve"><tspan class="nj-pointer-fl-k">0</tspan>: <tspan class="nj-pointer-fl-v nj-pointer-fl--s">"A"</tspan></text>
|
||||
<text class="fg-code" x="72" y="176" xml:space="preserve"><tspan class="nj-pointer-fl-k">1</tspan>: <tspan class="nj-pointer-fl-v nj-pointer-fl--s">"B"</tspan></text>
|
||||
<text class="fg-code" x="72" y="204" xml:space="preserve"><tspan class="nj-pointer-fl-k">2</tspan>: <tspan class="nj-pointer-fl-v nj-pointer-fl--s">"C"</tspan></text>
|
||||
<text class="fg-code nj-pointer-fl-inner" x="46" y="232">"nested"</text>
|
||||
<text class="fg-code" x="72" y="260" xml:space="preserve"><tspan class="nj-pointer-fl-k">"one"</tspan>: <tspan class="nj-pointer-fl-v nj-pointer-fl--n">1</tspan></text>
|
||||
<text class="fg-code" x="72" y="288" xml:space="preserve"><tspan class="nj-pointer-fl-k">"two"</tspan>: <tspan class="nj-pointer-fl-v nj-pointer-fl--n">2</tspan></text>
|
||||
<text class="fg-code nj-pointer-fl-inner" x="72" y="316">"three"</text>
|
||||
<text class="fg-code" x="98" y="344" xml:space="preserve"><tspan class="nj-pointer-fl-k">0</tspan>: <tspan class="nj-pointer-fl-v nj-pointer-fl--b">true</tspan></text>
|
||||
<text class="fg-code" x="98" y="372" xml:space="preserve"><tspan class="nj-pointer-fl-k">1</tspan>: <tspan class="nj-pointer-fl-v nj-pointer-fl--b">false</tspan></text>
|
||||
</svg>
|
||||
|
||||
</div>
|
||||
|
||||
<p class="nj-caption" markdown>Left: the example document as a tree; right: the flat object it becomes. `flatten()` converts from left to right, `unflatten()` from right to left.</p>
|
||||
|
||||
## See also
|
||||
|
||||
- Class [`json_pointer`](../api/json_pointer/index.md)
|
||||
- Functions [`value`](../api/basic_json/value.md), [`contains`](../api/basic_json/contains.md), and
|
||||
[`at`](../api/basic_json/at.md) accept JSON Pointers; see [Nested values](element_access/default_value.md#nested-values)
|
||||
- Function [`flatten`](../api/basic_json/flatten.md)
|
||||
- Function [`unflatten`](../api/basic_json/unflatten.md)
|
||||
- [`json_pointer`](../api/json_pointer/index.md) - the JSON Pointer class
|
||||
- [Nested values](element_access/default_value.md#nested-values) - [`value`](../api/basic_json/value.md),
|
||||
[`contains`](../api/basic_json/contains.md), and [`at`](../api/basic_json/at.md) accept JSON Pointers
|
||||
- [`flatten`](../api/basic_json/flatten.md) - convert a value to an object of JSON Pointers and primitive values
|
||||
- [`unflatten`](../api/basic_json/unflatten.md) - restore a value from a flattened object
|
||||
- [JSON Patch](json_patch.md) - paths inside a patch are JSON Pointers
|
||||
- [JSON Merge Patch](merge_patch.md) - an alternative patch format that does not use JSON Pointer
|
||||
- [`operator""_json_pointer`](../api/operator_literal_json_pointer.md) - user-defined literal to create a JSON pointer
|
||||
- [Element Access](element_access/index.md) - other ways to access elements
|
||||
@@ -1,4 +1,4 @@
|
||||
<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/json_pointer/" rel="canonical"><link href=../json_patch/ rel=prev><link href=../modifying_values/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>JSON Pointer - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.ab4e12ef.min.css><link rel="stylesheet" href="../../assets/external/fonts.googleapis.com/css.61a430c9.css"><style>:root{--md-text-font:"Roboto";--md-code-font:"JetBrains Mono"}</style><link rel=stylesheet href=../../css/custom.css><script>__md_scope=new 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xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 8a3 3 0 0 0 3-3 3 3 0 0 0-3-3 3 3 0 0 0-3 3 3 3 0 0 0 3 3m0 3.54C9.64 9.35 6.5 8 3 8v11c3.5 0 6.64 1.35 9 3.54 2.36-2.19 5.5-3.54 9-3.54V8c-3.5 0-6.64 1.35-9 3.54"/></svg> </a> <label class="md-header__button md-icon" for=__drawer> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M3 6h18v2H3zm0 5h18v2H3zm0 5h18v2H3z"/></svg> </label> <div class=md-header__title data-md-component=header-title> <div class=md-header__ellipsis> <div class=md-header__topic> <span class=md-ellipsis> JSON for Modern C++ </span> </div> <div class=md-header__topic data-md-component=header-topic> <span class=md-ellipsis> JSON Pointer </span> </div> </div> </div> <form class=md-header__option data-md-component=palette> <input class=md-option data-md-color-media="(prefers-color-scheme: light)" data-md-color-scheme=default data-md-color-primary=indigo data-md-color-accent=indigo aria-label="Switch to dark mode" type=radio name=__palette id=__palette_0> <label class="md-header__button md-icon" title="Switch to dark mode" for=__palette_1 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 8a4 4 0 0 0-4 4 4 4 0 0 0 4 4 4 4 0 0 0 4-4 4 4 0 0 0-4-4m0 10a6 6 0 0 1-6-6 6 6 0 0 1 6-6 6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> <input class=md-option data-md-color-media="(prefers-color-scheme: dark)" data-md-color-scheme=slate data-md-color-primary=indigo data-md-color-accent=indigo aria-label="Switch to light mode" type=radio name=__palette id=__palette_1> <label class="md-header__button md-icon" title="Switch to light mode" for=__palette_0 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 18c-.89 0-1.74-.2-2.5-.55C11.56 16.5 13 14.42 13 12s-1.44-4.5-3.5-5.45C10.26 6.2 11.11 6 12 6a6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> </form> <script>var palette=__md_get("__palette");if(palette&&palette.color){if("(prefers-color-scheme)"===palette.color.media){var media=matchMedia("(prefers-color-scheme: light)"),input=document.querySelector(media.matches?"[data-md-color-media='(prefers-color-scheme: light)']":"[data-md-color-media='(prefers-color-scheme: dark)']");palette.color.media=input.getAttribute("data-md-color-media"),palette.color.scheme=input.getAttribute("data-md-color-scheme"),palette.color.primary=input.getAttribute("data-md-color-primary"),palette.color.accent=input.getAttribute("data-md-color-accent")}for(var[key,value]of Object.entries(palette.color))document.body.setAttribute("data-md-color-"+key,value)}</script> <label class="md-header__button md-icon" for=__search> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M9.5 3A6.5 6.5 0 0 1 16 9.5c0 1.61-.59 3.09-1.56 4.23l.27.27h.79l5 5-1.5 1.5-5-5v-.79l-.27-.27A6.52 6.52 0 0 1 9.5 16 6.5 6.5 0 0 1 3 9.5 6.5 6.5 0 0 1 9.5 3m0 2C7 5 5 7 5 9.5S7 14 9.5 14 14 12 14 9.5 12 5 9.5 5"/></svg> </label> <div claLine truncated
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<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=description content="JSON for Modern C++ is a header-only C++11 library that makes JSON a first-class data type."><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/json_pointer/" rel="canonical"><link href=../json_patch/ rel=prev><link href=../modifying_values/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>JSON Pointer - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.ab4e12ef.min.css><link rel=stylesheet href=../../css/custom.css><link rel=stylesheet href=../../css/brand.css><link rel=stylesheet href=../../css/fig-sax.css><link rel=stylesheet href=../../css/fig-jsonview.css><link rel=stylesheet 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data-md-component=announce> </div> <header class=md-header data-md-component=header> <nav class="md-header__inner md-grid" aria-label=Header> <a href=../.. title="JSON for Modern C++" class="md-header__button md-logo" aria-label="JSON for Modern C++" data-md-component=logo> <img src=../../images/wordmark-header.svg alt=logo> </a> <label class="md-header__button md-icon" for=__drawer> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M3 6h18v2H3zm0 5h18v2H3zm0 5h18v2H3z"/></svg> </label> <div class=md-header__title data-md-component=header-title> <div class=md-header__ellipsis> <div class=md-header__topic> <span class=md-ellipsis> JSON for Modern C++ </span> </div> <div class=md-header__topic data-md-component=header-topic> <span class=md-ellipsis> JSON Pointer </span> </div> </div> </div> <form class=md-header__option data-md-component=palette> <input class=md-option data-md-color-media="(prefers-color-scheme: light)" data-md-color-scheme=default data-md-color-primary=custom data-md-color-accent=custom aria-label="Switch to dark mode" type=radio name=__palette id=__palette_0> <label class="md-header__button md-icon" title="Switch to dark mode" for=__palette_1 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 8a4 4 0 0 0-4 4 4 4 0 0 0 4 4 4 4 0 0 0 4-4 4 4 0 0 0-4-4m0 10a6 6 0 0 1-6-6 6 6 0 0 1 6-6 6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> <input class=md-option data-md-color-media="(prefers-color-scheme: dark)" data-md-color-scheme=slate data-md-color-primary=custom data-md-color-accent=custom aria-label="Switch to light mode" type=radio name=__palette id=__palette_1> <label class="md-header__button md-icon" title="Switch to light mode" for=__palette_0 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 18c-.89 0-1.74-.2-2.5-.55C11.56 16.5 13 14.42 13 12s-1.44-4.5-3.5-5.45C10.26 6.2 11.11 6 12 6a6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> </form> <script>var palette=__md_get("__palette");if(palette&&palette.color){if("(prefers-color-scheme)"===palette.color.media){var media=matchMedia("(prefers-color-scheme: light)"),input=document.querySelector(media.matches?"[data-md-color-media='(prefers-color-scheme: light)']":"[data-md-color-media='(prefers-color-scheme: dark)']");palette.color.media=input.getAttribute("data-md-color-media"),palette.color.scheme=input.getAttribute("data-md-color-scheme"),palette.color.primary=input.getAttribute("data-md-color-primary"),palette.color.accent=input.getAttribute("data-md-color-accent")}for(var[key,value]of Object.entries(palette.color))document.body.setAttribute("data-md-color-"+key,value)}</script> <label class="md-header__button md-icon" for=__search> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M9.5 3A6.5 6.5 0 0 1 16 9.5c0 1.61-.59 3.09-1.56 4.23l.27.2Line truncated
|
||||
<span class=w> </span><span class=nt>"array"</span><span class=p>:</span><span class=w> </span><span class=p>[</span><span class=s2>"A"</span><span class=p>,</span><span class=w> </span><span class=s2>"B"</span><span class=p>,</span><span class=w> </span><span class=s2>"C"</span><span class=p>],</span>
|
||||
<span class=w> </span><span class=nt>"nested"</span><span class=p>:</span><span class=w> </span><span class=p>{</span>
|
||||
<span class=w> </span><span class=nt>"one"</span><span class=p>:</span><span class=w> </span><span class=mi>1</span><span class=p>,</span>
|
||||
@@ -6,7 +6,7 @@
|
||||
<span class=w> </span><span class=nt>"three"</span><span class=p>:</span><span class=w> </span><span class=p>[</span><span class=kc>true</span><span class=p>,</span><span class=w> </span><span class=kc>false</span><span class=p>]</span>
|
||||
<span class=w> </span><span class=p>}</span>
|
||||
<span class=p>}</span>
|
||||
</code></pre></div> <p>Then every value inside the <abbr title="JavaScript Object Notation">JSON</abbr> document can be identified as follows:</p> <table> <thead> <tr> <th><abbr title="JavaScript Object Notation">JSON</abbr> Pointer</th> <th><abbr title="JavaScript Object Notation">JSON</abbr> value</th> </tr> </thead> <tbody> <tr> <td>``</td> <td><code class=highlight><span class=p>{</span><span class=nt>"array"</span><span class=p>:[</span><span class=s2>"A"</span><span class=p>,</span><span class=s2>"B"</span><span class=p>,</span><span class=s2>"C"</span><span class=p>],</span><span class=nt>"nested"</span><span class=p>:{</span><span class=nt>"one"</span><span class=p>:</span><span class=mi>1</span><span class=p>,</span><span class=nt>"two"</span><span class=p>:</span><span class=mi>2</span><span class=p>,</span><span class=nt>"three"</span><span class=p>:[</span><span class=kc>true</span><span class=p>,</span><span class=kc>false</span><span class=p>]}}</span></code></td> </tr> <tr> <td><code>/array</code></td> <td><code class=highlight><span class=p>[</span><span class=s2>"A"</span><span class=p>,</span><span class=s2>"B"</span><span class=p>,</span><span class=s2>"C"</span><span class=p>]</span></code></td> </tr> <tr> <td><code>/array/0</code></td> <td><code class=highlight><span class=err>A</span></code></td> </tr> <tr> <td><code>/array/1</code></td> <td><code class=highlight><span class=err>B</span></code></td> </tr> <tr> <td><code>/array/2</code></td> <td><code class=highlight><span class=err>C</span></code></td> </tr> <tr> <td><code>/nested</code></td> <td><code class=highlight><span class=p>{</span><span class=nt>"one"</span><span class=p>:</span><span class=mi>1</span><span class=p>,</span><span class=nt>"two"</span><span class=p>:</span><span class=mi>2</span><span class=p>,</span><span class=nt>"three"</span><span class=p>:[</span><span class=kc>true</span><span class=p>,</span><span class=kc>false</span><span class=p>]}</span></code></td> </tr> <tr> <td><code>/nested/one</code></td> <td><code class=highlight><span class=mi>1</span></code></td> </tr> <tr> <td><code>/nested/two</code></td> <td><code class=highlight><span class=mi>2</span></code></td> </tr> <tr> <td><code>/nested/three</code></td> <td><code class=highlight><span class=p>[</span><span class=kc>true</span><span class=p>,</span><span class=kc>false</span><span class=p>]</span></code></td> </tr> <tr> <td><code>/nested/three/0</code></td> <td><code class=highlight><span class=kc>true</span></code></td> </tr> <tr> <td><code>/nested/three/1</code></td> <td><code class=highlight><span class=kc>false</span></code></td> </tr> </tbody> </table> <p>Note <code>/</code> does not identify the root (i.e., the whole document), but an object entry with empty key <code>""</code>. See <a href="https://tools.ietf.org/html/rfc6901"><abbr title="Request for Comments">RFC</abbr> 6901</a> for more information.</p> <p><a href="https://tools.ietf.org/html/rfc6901#section-4"><abbr title="Request for Comments">RFC</abbr> 6901, section 4</a> states that evaluation fails if an object has non-unique member names. A <code>basic_json</code> object holds one value per key (see <a href=../../api/basic_json/object_t/ ><code>object_t</code></a> for how duplicate keys are handled while parsing), so evaluating a <abbr title="JavaScript Object Notation">JSON</abbr> Pointer never encounters duplicates.</p> <h2 id=json-pointer-creation><abbr title="JavaScript Object Notation">JSON</abbr> Pointer creation<a class=headerlink href=#json-pointer-creation title="Permanent link">¶</a></h2> <p><abbr title="JavaScript Object Notation">JSON</abbr> Pointers can be created from a string:</p> <div class=highlight><pre><span></span><code><span class=n>json</span><span class=o>::</span><span class=n>json_pointer</span><span class=w> </span><span class=n>p</span><span class=p>(</span><span class=s>"/nested/one"</span><span class=p>);</span>
|
||||
</code></pre></div> <p>Select a pointer (or a node of the tree) to walk it through this document; each segment of the pointer goes one level deeper. The table below lists the same pointers.</p> <div class=nj-pointer-walk data-nj-pointer-walk data-doc='{"array":["A","B","C"],"nested":{"one":1,"two":2,"three":[true,false]}}' role=group aria-label="Pointer walker for the example document"> <p class=nj-pointer-static>Highlighted: the path of the pointer <code>/nested/three/1</code>, which identifies <code>false</code>.</p> <ul class=nj-pointer-tree> <li data-p class=is-on><span class=nj-pointer-n><span class=nj-pointer-k>root</span> <span class=nj-pointer-t>object</span></span> <ul> <li data-p=/array class=is-pass><span class=nj-pointer-n><span class=nj-pointer-k>"array"</span> <span class=nj-pointer-t>array</span></span> <ul> <li data-p=/array/0><span class=nj-pointer-n><span class=nj-pointer-k>0</span>: <span class="nj-pointer-v nj-pointer-v--s">"A"</span></span></li> <li data-p=/array/1><span class=nj-pointer-n><span class=nj-pointer-k>1</span>: <span class="nj-pointer-v nj-pointer-v--s">"B"</span></span></li> <li data-p=/array/2><span class=nj-pointer-n><span class=nj-pointer-k>2</span>: <span class="nj-pointer-v nj-pointer-v--s">"C"</span></span></li> </ul> </li> <li data-p=/nested class=is-on><span class=nj-pointer-n><span class=nj-pointer-k>"nested"</span> <span class=nj-pointer-t>object</span></span> <ul> <li data-p=/nested/one class=is-pass><span class=nj-pointer-n><span class=nj-pointer-k>"one"</span>: <span class="nj-pointer-v nj-pointer-v--n">1</span></span></li> <li data-p=/nested/two class=is-pass><span class=nj-pointer-n><span class=nj-pointer-k>"two"</span>: <span class="nj-pointer-v nj-pointer-v--n">2</span></span></li> <li data-p=/nested/three class=is-on><span class=nj-pointer-n><span class=nj-pointer-k>"three"</span> <span class=nj-pointer-t>array</span></span> <ul> <li data-p=/nested/three/0 class=is-pass><span class=nj-pointer-n><span class=nj-pointer-k>0</span>: <span class="nj-pointer-v nj-pointer-v--b">true</span></span></li> <li data-p=/nested/three/1 class="is-on is-target"><span class=nj-pointer-n><span class=nj-pointer-k>1</span>: <span class="nj-pointer-v nj-pointer-v--b">false</span></span></li> </ul> </li> </ul> </li> </ul> </li> </ul> </div> <p class=nj-caption>The highlighted nodes are the path from the root to the selected level, and the line below the tree shows the value the pointer resolves to. Select a segment in the pointer bar to stop at a shallower level, or a node of the tree to select its pointer.</p> <p>Then every value inside the <abbr title="JavaScript Object Notation">JSON</abbr> document can be identified as follows:</p> <table> <thead> <tr> <th><abbr title="JavaScript Object Notation">JSON</abbr> Pointer</th> <th><abbr title="JavaScript Object Notation">JSON</abbr> value</th> </tr> </thead> <tbody> <tr> <td>``</td> <td><code class=highlight><span class=p>{</span><span class=nt>"array"</span><span class=p>:[</span><span class=s2>"A"</span><span class=p>,</span><span class=s2>"B"</span><span class=p>,</span><span class=s2>"C"</span><span class=p>],</span><span class=nt>"nested"</span><span class=p>:{</span><span class=nt>"one"</span><span class=p>:</span><span class=mi>1</span><span class=p>,</span><span class=nt>"two"</span><span class=p>:</span><span class=mi>2</span><span class=p>,</span><span class=nt>"three"</span><span class=p>:[</span><span class=kc>true</span><span class=p>,</span><span class=kc>false</span><span class=p>]}}</span></code></td> </tr> <tr> <td><code>/array</code></td> <td><code class=highlight><span class=p>[</span><span class=s2>"A"</span><span class=p>,</span><span class=s2>"B"</span><span class=p>,</span><span class=s2>"C"</span><span class=p>]</span></code></td> </tr> <tr> <td><code>/array/0</code></td> <td><code class=highlight><span class=err>A</span></code></td> </tr> <tr> <td><code>/array/1</code></td> <td><code class=highlight><span class=err>B</span></code></td> </tr> <tr> <td><code>/array/2</code></td> <td><code class=highlight><span class=err>C</span></code></td> </tr> <tr> <td><code>/nested</code></td> <td><code class=highlight><span class=p>{</span><span class=nt>"one"</span><span class=p>:</span><span class=mi>1</span><span class=p>,</span><span class=nt>"two"</span><span class=p>:</span><span class=mi>2</span><span class=p>,</span><span class=nt>"three"</span><span class=p>:[</span><span class=kc>true</span><span class=p>,</span><span class=kc>false</span><span class=p>]}</span></code></td> </tr> <tr> <td><code>/nested/one</code></td> <td><code class=highlight><span class=mi>1</span></code></td> </tr> <tr> <td><code>/nested/two</code></td> <td><code class=highlight><span class=mi>2</span></code></td> </tr> <tr> <td><code>/nested/three</code></td> <td><code class=highlight><span class=p>[</span><span class=kc>true</span><span class=p>,</span><spanLine truncated
|
||||
</code></pre></div> <p>Furthermore, a user-defined string literal can be used to achieve the same result:</p> <div class=highlight><pre><span></span><code><span class=k>auto</span><span class=w> </span><span class=n>p</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=s>"/nested/one"</span><span class=n>_json_pointer</span><span class=p>;</span>
|
||||
</code></pre></div> <p>The escaping rules of <a href="https://tools.ietf.org/html/rfc6901"><abbr title="Request for Comments">RFC</abbr> 6901</a> are implemented. See the <a href=../../api/json_pointer/json_pointer/ >constructor documentation</a> for more information.</p> <div class="admonition note"> <p class=admonition-title>URI fragment representation</p> <p><a href="https://tools.ietf.org/html/rfc6901#section-6"><abbr title="Request for Comments">RFC</abbr> 6901, section 6</a> also defines a URI fragment identifier representation (<code>#/a%20b</code>). It is not supported: <code class=highlight><span class=n>json</span><span class=o>::</span><span class=n>json_pointer</span><span class=p>(</span><span class=s>"#/a"</span><span class=p>)</span></code> throws <a href=../../home/exceptions/#jsonexceptionparse_error107><code>parse_error.107</code></a>. Strip the leading <code>#</code> and percent-decode the remainder before creating the <abbr title="JavaScript Object Notation">JSON</abbr> pointer.</p> </div> <h2 id=value-access>Value access<a class=headerlink href=#value-access title="Permanent link">¶</a></h2> <p><abbr title="JavaScript Object Notation">JSON</abbr> Pointers can be used in the <a href=../../api/basic_json/at/ ><code>at</code></a>, <a href=../../api/basic_json/operator%5B%5D/ ><code>operator[]</code></a>, and <a href=../../api/basic_json/value/ ><code>value</code></a> functions just like object keys or array indices.</p> <div class=highlight><pre><span></span><code><span class=c1>// the JSON value from above</span>
|
||||
<span class=k>auto</span><span class=w> </span><span class=n>j</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>parse</span><span class=p>(</span><span class=sa>R</span><span class=s>"</span><span class=dl>(</span><span class=s>{</span>
|
||||
@@ -45,4 +45,4 @@
|
||||
<span class=w> </span><span class=nt>"/nested/three/1"</span><span class=p>:</span><span class=w> </span><span class=kc>false</span>
|
||||
<span class=p>}</span>
|
||||
</code></pre></div> <p>The reverse function, <a href=../../api/basic_json/unflatten/ ><code>unflatten</code></a> recreates the original value.</p> <div class=highlight><pre><span></span><code><span class=k>auto</span><span class=w> </span><span class=n>j_original</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>j_flat</span><span class=p>.</span><span class=n>unflatten</span><span class=p>();</span>
|
||||
</code></pre></div> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">¶</a></h2> <ul> <li>Class <a href=../../api/json_pointer/ ><code>json_pointer</code></a></li> <li>Functions <a href=../../api/basic_json/value/ ><code>value</code></a>, <a href=../../api/basic_json/contains/ ><code>contains</code></a>, and <a href=../../api/basic_json/at/ ><code>at</code></a> accept <abbr title="JavaScript Object Notation">JSON</abbr> Pointers; see <a href=../element_access/default_value/#nested-values>Nested values</a></li> <li>Function <a href=../../api/basic_json/flatten/ ><code>flatten</code></a></li> <li>Function <a href=../../api/basic_json/unflatten/ ><code>unflatten</code></a></li> <li><a href=../json_patch/ ><abbr title="JavaScript Object Notation">JSON</abbr> Patch</a> - paths inside a patch are <abbr title="JavaScript Object Notation">JSON</abbr> Pointers</li> <li><a href=../merge_patch/ ><abbr title="JavaScript Object Notation">JSON</abbr> Merge Patch</a> - an alternative patch format that does not use <abbr title="JavaScript Object Notation">JSON</abbr> Pointer</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 26, 2026 16:11:46 UTC">October 26, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2025 Fonticons, Inc.Line truncated
|
||||
</code></pre></div> <p>Each leaf of the document becomes one key of the flat object, and <code>unflatten()</code> reverses the mapping:</p> <div class=nj-fig-wrap> <p><svg class="nj-fig nj-pointer-fl" viewbox="0 0 760 442" role=img aria-labelledby=fig-flat-title xmlns=http://www.w3.org/2000/svg> <title id=fig-flat-title>flatten() turns the nested example document into a flat object, and unflatten() turns it back. Left, the document as a tree: array with the elements A, B, C, and nested with one = 1, two = 2, and three with the elements true and false. Right, the flat object with one key per leaf: /array/0 = A, /array/1 = B, /array/2 = C, /nested/one = 1, /nested/two = 2, /nested/three/0 = true, /nested/three/1 = false. Each leaf is joined to its key by a line.</title> <defs><marker id=fig-flat-arrow viewbox="0 0 10 10" refx=9 refy=5 markerwidth=7 markerheight=7 orient=auto><path class=fg-head d="M0 1 L10 5 L0 9 z"/></marker></defs> <text class=fg-h x=20 y=26>nested value</text> <text class=fg-h x=470 y=26>flat object: key = J​SON Pointer</text> <line class=fg-arrow x1=250 y1=52 x2=450 y2=52 marker-end=url(#fig-flat-arrow) /> <text class=nj-pointer-fl-op x=350 y=44>flatten()</text> <line class=fg-arrow x1=450 y1=420 x2=250 y2=420 marker-end=url(#fig-flat-arrow) /> <text class=nj-pointer-fl-op x=350 y=412>unflatten()</text> <path class=nj-pointer-fl-edge d="M25 99 V115 H42"/> <path class=nj-pointer-fl-edge d="M51 127 V143 H68"/> <path class=nj-pointer-fl-edge d="M51 127 V171 H68"/> <path class=nj-pointer-fl-edge d="M51 127 V199 H68"/> <path class=nj-pointer-fl-edge d="M25 99 V227 H42"/> <path class=nj-pointer-fl-edge d="M51 239 V255 H68"/> <path class=nj-pointer-fl-edge d="M51 239 V283 H68"/> <path class=nj-pointer-fl-edge d="M51 239 V311 H68"/> <path class=nj-pointer-fl-edge d="M77 323 V339 H94"/> <path class=nj-pointer-fl-edge d="M77 323 V367 H94"/> <path class="nj-pointer-fl-link nj-pointer-fl--s" d="M139.6 143 C259.6 143 340 171 460 171"/> <circle cx=139.6 cy=143 r=3.5 class="nj-pointer-fl-dot nj-pointer-fl--s"/> <text class=fg-code x=470 y=176 xml:space=preserve><tspan class=nj-pointer-fl-key>"/array/0"</tspan>: <tspan class="nj-pointer-fl-v nj-pointer-fl--s">"A"</tspan></text> <path class="nj-pointer-fl-link nj-pointer-fl--s" d="M139.6 171 C259.6 171 340 199 460 199"/> <circle cx=139.6 cy=171 r=3.5 class="nj-pointer-fl-dot nj-pointer-fl--s"/> <text class=fg-code x=470 y=204 xml:space=preserve><tspan class=nj-pointer-fl-key>"/array/1"</tspan>: <tspan class="nj-pointer-fl-v nj-pointer-fl--s">"B"</tspan></text> <path class="nj-pointer-fl-link nj-pointer-fl--s" d="M139.6 199 C259.6 199 340 227 460 227"/> <circle cx=139.6 cy=199 r=3.5 class="nj-pointer-fl-dot nj-pointer-fl--s"/> <text class=fg-code x=470 y=232 xml:space=preserve><tspan class=nj-pointer-fl-key>"/array/2"</tspan>: <tspan class="nj-pointer-fl-v nj-pointer-fl--s">"C"</tspan></text> <path class="nj-pointer-fl-link nj-pointer-fl--n" d="M158.8 255 C278.8 255 340 255 460 255"/> <circle cx=158.8 cy=255 r=3.5 class="nj-pointer-fl-dot nj-pointer-fl--n"/> <text class=fg-code x=470 y=260 xml:space=preserve><tspan class=nj-pointer-fl-key>"/nested/one"</tspan>: <tspan class="nj-pointer-fl-v nj-pointer-fl--n">1</tspan></text> <path class="nj-pointer-fl-link nj-pointer-fl--n" d="M158.8 283 C278.8 283 340 283 460 283"/> <circle cx=158.8 cy=283 r=3.5 class="nj-pointer-fl-dot nj-pointer-fl--n"/> <text class=fg-code x=470 y=288 xml:space=preserve><tspan class=nj-pointer-fl-key>"/nested/two"</tspan>: <tspan class="nj-pointer-fl-v nj-pointer-fl--n">2</tspan></text> <path class="nj-pointer-fl-link nj-pointer-fl--b" d="M175.2 339 C295.2 339 340 311 460 311"/> <circle cx=175.2 cy=339 r=3.5 class="nj-pointer-fl-dot nj-pointer-fl--b"/> <text class=fg-code x=470 y=316 xml:space=preserve><tspan class=nj-pointer-fl-key>"/nested/three/0"</tspan>: <tspan class="nj-pointer-fl-v nj-pointer-fl--b">true</tspan></text> <path class="nj-pointer-fl-link nj-pointer-fl--b" d="M184.8 367 C304.8 367 340 339 460 339"/> <circle cx=184.8 cy=367 r=3.5 class="nj-pointer-fl-dot nj-pointer-fl--b"/> <text class=fg-code x=470 y=344 xml:space=preserve><tspan class=nj-pointer-fl-key>"/nested/three/1"</tspan>: <tspan class="nj-pointer-fl-v nj-pointer-fl--b">false</tspan></text> <text class=fg-h x=20 y=92>root</text> <text class="fg-code nj-pointer-fl-inner" x=46 y=120>"array"</text> <text class=fg-code x=72 y=148 xml:space=preserve><tspan class=nj-pointer-fl-k>0</tspan>: <tspan class="nj-pointer-fl-v nj-pointer-fl--s">"A"</tspan></text> <text class=fg-code x=72 y=176 xml:space=preserve><tspan class=nj-pointer-fl-k>1</tspan>: <tspan class="nj-pointer-fl-v nj-pointer-fl--s">"B"</tspan></text> <text class=fg-code x=72 y=204 xml:space=preserve><tspan class=nj-pointer-fl-k>2</tspan>: <tspan class="nj-pointer-fl-v nj-pointer-fl--s">"C"</tspan></text> <text class="fg-code nj-pointer-fl-inner" x=46 y=232>"nested"</text> <text class=fg-code x=72 y=260 xml:space=preserve><tspan class=nj-pointer-fl-k>Line truncated
|
||||
@@ -17,6 +17,24 @@ Consider the following JSON document
|
||||
}
|
||||
```
|
||||
|
||||
Select a pointer (or a node of the tree) to walk it through this document; each segment of the pointer goes one level deeper. The table below lists the same pointers.
|
||||
|
||||
Highlighted: the path of the pointer `/nested/three/1`, which identifies `false`.
|
||||
|
||||
- root object
|
||||
- "array" array
|
||||
- 0: "A"
|
||||
- 1: "B"
|
||||
- 2: "C"
|
||||
- "nested" object
|
||||
- "one": 1
|
||||
- "two": 2
|
||||
- "three" array
|
||||
- 0: true
|
||||
- 1: false
|
||||
|
||||
The highlighted nodes are the path from the root to the selected level, and the line below the tree shows the value the pointer resolves to. Select a segment in the pointer bar to stop at a shallower level, or a node of the tree to select its pointer.
|
||||
|
||||
Then every value inside the JSON document can be identified as follows:
|
||||
|
||||
| JSON Pointer | JSON value |
|
||||
@@ -124,11 +142,17 @@ The reverse function, [`unflatten`](https://json.nlohmann.me/api/basic_json/unfl
|
||||
auto j_original = j_flat.unflatten();
|
||||
```
|
||||
|
||||
Each leaf of the document becomes one key of the flat object, and `unflatten()` reverses the mapping:
|
||||
|
||||
Left: the example document as a tree; right: the flat object it becomes. `flatten()` converts from left to right, `unflatten()` from right to left.
|
||||
|
||||
## See also
|
||||
|
||||
- Class [`json_pointer`](https://json.nlohmann.me/api/json_pointer/index.md)
|
||||
- Functions [`value`](https://json.nlohmann.me/api/basic_json/value/index.md), [`contains`](https://json.nlohmann.me/api/basic_json/contains/index.md), and [`at`](https://json.nlohmann.me/api/basic_json/at/index.md) accept JSON Pointers; see [Nested values](https://json.nlohmann.me/features/element_access/default_value/#nested-values)
|
||||
- Function [`flatten`](https://json.nlohmann.me/api/basic_json/flatten/index.md)
|
||||
- Function [`unflatten`](https://json.nlohmann.me/api/basic_json/unflatten/index.md)
|
||||
- [`json_pointer`](https://json.nlohmann.me/api/json_pointer/index.md) - the JSON Pointer class
|
||||
- [Nested values](https://json.nlohmann.me/features/element_access/default_value/#nested-values) - [`value`](https://json.nlohmann.me/api/basic_json/value/index.md), [`contains`](https://json.nlohmann.me/api/basic_json/contains/index.md), and [`at`](https://json.nlohmann.me/api/basic_json/at/index.md) accept JSON Pointers
|
||||
- [`flatten`](https://json.nlohmann.me/api/basic_json/flatten/index.md) - convert a value to an object of JSON Pointers and primitive values
|
||||
- [`unflatten`](https://json.nlohmann.me/api/basic_json/unflatten/index.md) - restore a value from a flattened object
|
||||
- [JSON Patch](https://json.nlohmann.me/features/json_patch/index.md) - paths inside a patch are JSON Pointers
|
||||
- [JSON Merge Patch](https://json.nlohmann.me/features/merge_patch/index.md) - an alternative patch format that does not use JSON Pointer
|
||||
- [`operator""_json_pointer`](https://json.nlohmann.me/api/operator_literal_json_pointer/index.md) - user-defined literal to create a JSON pointer
|
||||
- [Element Access](https://json.nlohmann.me/features/element_access/index.md) - other ways to access elements
|
||||
+749
-32
@@ -21,18 +21,208 @@ referenced in place in the source text: a float is converted when it is read, an
|
||||
sequences are decoded, into one shared buffer owned by the document.
|
||||
|
||||
[`basic_json_view`](../api/basic_json_view/index.md) is a small, trivially copyable handle (two pointers) into that
|
||||
index. It gives you the read-only, type-inspection part of the `basic_json` interface --
|
||||
index. It gives you the read-only, type-inspection part of the `basic_json` interface —
|
||||
[`type()`](../api/basic_json_view/type.md) and the `is_*()` predicates,
|
||||
[`size()`](../api/basic_json_view/size.md)/[`empty()`](../api/basic_json_view/empty.md) -- as well as element access
|
||||
[`size()`](../api/basic_json_view/size.md)/[`empty()`](../api/basic_json_view/empty.md) — as well as element access
|
||||
([`operator[]`](../api/basic_json_view/operator%5B%5D.md), [`at`](../api/basic_json_view/at.md),
|
||||
[`front`](../api/basic_json_view/front.md)/[`back`](../api/basic_json_view/back.md)), lookup
|
||||
([`find`](../api/basic_json_view/find.md), [`contains`](../api/basic_json_view/contains.md),
|
||||
[`count`](../api/basic_json_view/count.md)), and iteration
|
||||
([`begin`](../api/basic_json_view/begin.md)/[`end`](../api/basic_json_view/end.md),
|
||||
[`items`](../api/basic_json_view/items.md)) -- without ever allocating a `basic_json` value. When you do need an
|
||||
[`items`](../api/basic_json_view/items.md)) — without ever allocating a `basic_json` value. When you do need an
|
||||
actual `basic_json` value for a subtree, [`materialize()`](../api/basic_json_view/materialize.md) builds exactly the
|
||||
one [`parse()`](../api/basic_json/parse.md) would have produced for it.
|
||||
|
||||
The same small text, parsed both ways:
|
||||
|
||||
<div class="nj-fig-wrap" markdown>
|
||||
<svg class="nj-fig nj-jsonview " viewBox="0 0 840 692" role="img" aria-labelledby="fig-jvl-title fig-jvl-title-d" xmlns="http://www.w3.org/2000/svg">
|
||||
<title id="fig-jvl-title">The same J​SON text parsed by basic_json::parse() and by json_document::parse()</title>
|
||||
<desc id="fig-jvl-title-d">The text {"port": 6379, "price": 19.90, "tags": ["eu", "a\"b"]}. Top: basic_json::parse() builds a tree: the root json value points to a std::map whose three nodes are separate heap allocations; the price is stored as the double 19.9; the tags array is a std::vector of two json values, each pointing to its own std::string. Bottom: json_document::parse() keeps the 54 bytes of source text and builds a flat index of nine 16-byte nodes in document order: object, key port, unsigned integer 6379 (the converted value is in the node), key price, float, key tags, array, string eu, and an escaped string. Keys, the float, and the plain string point back into the source text; the escaped string points into the document's buffer of decoded strings. A json_view is two pointers: one to the document and one to a node, here the array.</desc>
|
||||
<defs>
|
||||
<marker id="jvl-line" viewBox="0 0 10 10" refX="9" refY="5" markerWidth="6.5" markerHeight="6.5" orient="auto-start-reverse"><path class="nj-jsonview-c-line nj-jsonview-head" d="M0 1 L10 5 L0 9 z"/></marker>
|
||||
<marker id="jvl-str" viewBox="0 0 10 10" refX="9" refY="5" markerWidth="6.5" markerHeight="6.5" orient="auto-start-reverse"><path class="nj-jsonview-c-str nj-jsonview-head" d="M0 1 L10 5 L0 9 z"/></marker>
|
||||
<marker id="jvl-key" viewBox="0 0 10 10" refX="9" refY="5" markerWidth="6.5" markerHeight="6.5" orient="auto-start-reverse"><path class="nj-jsonview-c-key nj-jsonview-head" d="M0 1 L10 5 L0 9 z"/></marker>
|
||||
<marker id="jvl-num" viewBox="0 0 10 10" refX="9" refY="5" markerWidth="6.5" markerHeight="6.5" orient="auto-start-reverse"><path class="nj-jsonview-c-num nj-jsonview-head" d="M0 1 L10 5 L0 9 z"/></marker>
|
||||
<marker id="jvl-dec" viewBox="0 0 10 10" refX="9" refY="5" markerWidth="6.5" markerHeight="6.5" orient="auto-start-reverse"><path class="nj-jsonview-c-dec nj-jsonview-head" d="M0 1 L10 5 L0 9 z"/></marker>
|
||||
<marker id="jvl-own" viewBox="0 0 10 10" refX="9" refY="5" markerWidth="6.5" markerHeight="6.5" orient="auto-start-reverse"><path class="nj-jsonview-c-own nj-jsonview-head" d="M0 1 L10 5 L0 9 z"/></marker>
|
||||
<marker id="jvl-struct" viewBox="0 0 10 10" refX="9" refY="5" markerWidth="6.5" markerHeight="6.5" orient="auto-start-reverse"><path class="nj-jsonview-c-struct nj-jsonview-head" d="M0 1 L10 5 L0 9 z"/></marker>
|
||||
<marker id="jvl-bad" viewBox="0 0 10 10" refX="9" refY="5" markerWidth="6.5" markerHeight="6.5" orient="auto-start-reverse"><path class="nj-jsonview-c-bad nj-jsonview-head" d="M0 1 L10 5 L0 9 z"/></marker>
|
||||
</defs>
|
||||
<text class="nj-jsonview-h" x="16" y="26" xml:space="preserve">basic_json::parse(): a tree, every array, object, and string in its own heap allocation</text>
|
||||
<rect class="nj-jsonview-box" x="16" y="98" width="60" height="34" rx="5"/>
|
||||
<text class="nj-jsonview-mono" x="46" y="120" text-anchor="middle" xml:space="preserve">json</text>
|
||||
<path class="nj-jsonview-arr nj-jsonview-c-line" d="M76 115 L106 115" marker-end="url(#jvl-line)"/>
|
||||
<rect class="nj-jsonview-heap" x="108" y="98" width="84" height="34" rx="5"/>
|
||||
<text class="nj-jsonview-mono" x="150" y="120" text-anchor="middle" xml:space="preserve">std::map</text>
|
||||
<path class="nj-jsonview-arr nj-jsonview-c-line" d="M192 115 C192 115 226 67 226 67" marker-end="url(#jvl-line)"/>
|
||||
<rect class="nj-jsonview-heap" x="228" y="50" width="180" height="34" rx="5"/>
|
||||
<text class="nj-jsonview-mono nj-jsonview-c-key" x="240" y="72" xml:space="preserve">"port"</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-c-num" x="396" y="72" text-anchor="end" xml:space="preserve">6379</text>
|
||||
<path class="nj-jsonview-arr nj-jsonview-c-line" d="M192 115 C192 115 226 115 226 115" marker-end="url(#jvl-line)"/>
|
||||
<rect class="nj-jsonview-heap" x="228" y="98" width="180" height="34" rx="5"/>
|
||||
<text class="nj-jsonview-mono nj-jsonview-c-key" x="240" y="120" xml:space="preserve">"price"</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-c-num" x="396" y="120" text-anchor="end" xml:space="preserve">19.9</text>
|
||||
<text class="nj-jsonview-note" x="418" y="120" xml:space="preserve">double: 19.90 is gone</text>
|
||||
<path class="nj-jsonview-arr nj-jsonview-c-line" d="M192 115 C192 115 226 163 226 163" marker-end="url(#jvl-line)"/>
|
||||
<rect class="nj-jsonview-heap" x="228" y="146" width="180" height="34" rx="5"/>
|
||||
<text class="nj-jsonview-mono nj-jsonview-c-key" x="240" y="168" xml:space="preserve">"tags"</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-c-line" x="396" y="168" text-anchor="end" xml:space="preserve">•</text>
|
||||
<path class="nj-jsonview-arr nj-jsonview-c-line" d="M408 163 L442 163" marker-end="url(#jvl-line)"/>
|
||||
<rect class="nj-jsonview-heap" x="444" y="146" width="104" height="34" rx="5"/>
|
||||
<text class="nj-jsonview-mono" x="496" y="168" text-anchor="middle" xml:space="preserve">std::vector</text>
|
||||
<path class="nj-jsonview-arr nj-jsonview-c-line" d="M548 163 L576 163" marker-end="url(#jvl-line)"/>
|
||||
<rect class="nj-jsonview-heap" x="578" y="146" width="92" height="34" rx="5"/>
|
||||
<path class="nj-jsonview-sep" d="M624 146 V180"/>
|
||||
<text class="nj-jsonview-mono" x="601" y="168" text-anchor="middle" xml:space="preserve">json</text>
|
||||
<text class="nj-jsonview-mono" x="647" y="168" text-anchor="middle" xml:space="preserve">json</text>
|
||||
<path class="nj-jsonview-arr nj-jsonview-c-line" d="M601 146 C601 126 706 150 706 130" marker-end="url(#jvl-line)"/>
|
||||
<path class="nj-jsonview-arr nj-jsonview-c-line" d="M647 180 C647 200 706 176 706 196" marker-end="url(#jvl-line)"/>
|
||||
<rect class="nj-jsonview-heap" x="708" y="113" width="126" height="34" rx="5"/>
|
||||
<text class="nj-jsonview-mono nj-jsonview-sm nj-jsonview-c-str" x="771" y="135" text-anchor="middle" xml:space="preserve">std::string eu</text>
|
||||
<rect class="nj-jsonview-heap" x="708" y="179" width="126" height="34" rx="5"/>
|
||||
<text class="nj-jsonview-mono nj-jsonview-sm nj-jsonview-c-str" x="771" y="201" text-anchor="middle" xml:space="preserve">std::string a"b</text>
|
||||
<rect class="nj-jsonview-heap" x="16" y="186" width="18" height="14" rx="3"/>
|
||||
<text class="nj-jsonview-note" x="42" y="198" xml:space="preserve">a separate heap allocation</text>
|
||||
<path class="nj-jsonview-sep" d="M16 234 H824"/>
|
||||
<text class="nj-jsonview-h" x="16" y="262" xml:space="preserve">json_document::parse(): the source text stays where it is; one flat index of 16-byte nodes</text>
|
||||
<text class="nj-jsonview-note" x="16" y="290" xml:space="preserve">source text, 54 bytes (borrowed, or owned by the document)</text>
|
||||
<rect class="nj-jsonview-ribbon" x="42" y="300" width="756" height="26" rx="4"/>
|
||||
<rect class="nj-jsonview-span nj-jsonview-c-key" x="71" y="302" width="54" height="22" rx="3"/>
|
||||
<rect class="nj-jsonview-span nj-jsonview-c-num" x="169" y="302" width="54" height="22" rx="3"/>
|
||||
<rect class="nj-jsonview-span nj-jsonview-c-key" x="267" y="302" width="68" height="22" rx="3"/>
|
||||
<rect class="nj-jsonview-span nj-jsonview-c-num" x="379" y="302" width="68" height="22" rx="3"/>
|
||||
<rect class="nj-jsonview-span nj-jsonview-c-key" x="491" y="302" width="54" height="22" rx="3"/>
|
||||
<rect class="nj-jsonview-span nj-jsonview-c-str" x="617" y="302" width="26" height="22" rx="3"/>
|
||||
<rect class="nj-jsonview-span nj-jsonview-c-dec" x="701" y="302" width="54" height="22" rx="3"/>
|
||||
<text class="nj-jsonview-mono nj-jsonview-byte" x="49" y="317.5" text-anchor="middle" xml:space="preserve">{</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-byte" x="63" y="317.5" text-anchor="middle" xml:space="preserve">"</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-byte" x="77" y="317.5" text-anchor="middle" xml:space="preserve">p</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-byte" x="91" y="317.5" text-anchor="middle" xml:space="preserve">o</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-byte" x="105" y="317.5" text-anchor="middle" xml:space="preserve">r</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-byte" x="119" y="317.5" text-anchor="middle" xml:space="preserve">t</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-byte" x="133" y="317.5" text-anchor="middle" xml:space="preserve">"</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-byte" x="147" y="317.5" text-anchor="middle" xml:space="preserve">:</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-byte" x="175" y="317.5" text-anchor="middle" xml:space="preserve">6</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-byte" x="189" y="317.5" text-anchor="middle" xml:space="preserve">3</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-byte" x="203" y="317.5" text-anchor="middle" xml:space="preserve">7</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-byte" x="217" y="317.5" text-anchor="middle" xml:space="preserve">9</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-byte" x="231" y="317.5" text-anchor="middle" xml:space="preserve">,</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-byte" x="259" y="317.5" text-anchor="middle" xml:space="preserve">"</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-byte" x="273" y="317.5" text-anchor="middle" xml:space="preserve">p</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-byte" x="287" y="317.5" text-anchor="middle" xml:space="preserve">r</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-byte" x="301" y="317.5" text-anchor="middle" xml:space="preserve">i</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-byte" x="315" y="317.5" text-anchor="middle" xml:space="preserve">c</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-byte" x="329" y="317.5" text-anchor="middle" xml:space="preserve">e</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-byte" x="343" y="317.5" text-anchor="middle" xml:space="preserve">"</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-byte" x="357" y="317.5" text-anchor="middle" xml:space="preserve">:</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-byte" x="385" y="317.5" text-anchor="middle" xml:space="preserve">1</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-byte" x="399" y="317.5" text-anchor="middle" xml:space="preserve">9</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-byte" x="413" y="317.5" text-anchor="middle" xml:space="preserve">.</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-byte" x="427" y="317.5" text-anchor="middle" xml:space="preserve">9</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-byte" x="441" y="317.5" text-anchor="middle" xml:space="preserve">0</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-byte" x="455" y="317.5" text-anchor="middle" xml:space="preserve">,</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-byte" x="483" y="317.5" text-anchor="middle" xml:space="preserve">"</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-byte" x="497" y="317.5" text-anchor="middle" xml:space="preserve">t</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-byte" x="511" y="317.5" text-anchor="middle" xml:space="preserve">a</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-byte" x="525" y="317.5" text-anchor="middle" xml:space="preserve">g</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-byte" x="539" y="317.5" text-anchor="middle" xml:space="preserve">s</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-byte" x="553" y="317.5" text-anchor="middle" xml:space="preserve">"</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-byte" x="567" y="317.5" text-anchor="middle" xml:space="preserve">:</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-byte" x="595" y="317.5" text-anchor="middle" xml:space="preserve">[</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-byte" x="609" y="317.5" text-anchor="middle" xml:space="preserve">"</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-byte" x="623" y="317.5" text-anchor="middle" xml:space="preserve">e</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-byte" x="637" y="317.5" text-anchor="middle" xml:space="preserve">u</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-byte" x="651" y="317.5" text-anchor="middle" xml:space="preserve">"</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-byte" x="665" y="317.5" text-anchor="middle" xml:space="preserve">,</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-byte" x="693" y="317.5" text-anchor="middle" xml:space="preserve">"</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-byte" x="707" y="317.5" text-anchor="middle" xml:space="preserve">a</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-byte" x="721" y="317.5" text-anchor="middle" xml:space="preserve">\</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-byte" x="735" y="317.5" text-anchor="middle" xml:space="preserve">"</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-byte" x="749" y="317.5" text-anchor="middle" xml:space="preserve">b</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-byte" x="763" y="317.5" text-anchor="middle" xml:space="preserve">"</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-byte" x="777" y="317.5" text-anchor="middle" xml:space="preserve">]</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-byte" x="791" y="317.5" text-anchor="middle" xml:space="preserve">}</text>
|
||||
<text class="nj-jsonview-tiny" x="49" y="340" text-anchor="middle" xml:space="preserve">0</text>
|
||||
<text class="nj-jsonview-tiny" x="189" y="340" text-anchor="middle" xml:space="preserve">10</text>
|
||||
<text class="nj-jsonview-tiny" x="329" y="340" text-anchor="middle" xml:space="preserve">20</text>
|
||||
<text class="nj-jsonview-tiny" x="469" y="340" text-anchor="middle" xml:space="preserve">30</text>
|
||||
<text class="nj-jsonview-tiny" x="609" y="340" text-anchor="middle" xml:space="preserve">40</text>
|
||||
<text class="nj-jsonview-tiny" x="749" y="340" text-anchor="middle" xml:space="preserve">50</text>
|
||||
<rect class="nj-jsonview-frame" x="16" y="366" width="808" height="254" rx="10"/>
|
||||
<text class="nj-jsonview-note nj-jsonview-strong" x="30" y="602" xml:space="preserve">json_document: owns the node index and the buffer of decoded strings</text>
|
||||
<text class="nj-jsonview-note" x="30" y="500" xml:space="preserve">node index: 9 nodes × 16 bytes = 144 bytes, in document order</text>
|
||||
<rect class="nj-jsonview-node nj-jsonview-c-struct" x="36" y="406" width="84" height="68" rx="5"/>
|
||||
<text class="nj-jsonview-tiny" x="42" y="420" xml:space="preserve">0</text>
|
||||
<text class="nj-jsonview-kind nj-jsonview-c-struct" x="78" y="434" text-anchor="middle" xml:space="preserve">object</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-xs" x="78" y="451" text-anchor="middle" xml:space="preserve">len 3</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-xs" x="78" y="465" text-anchor="middle" xml:space="preserve">next 9</text>
|
||||
<rect class="nj-jsonview-node nj-jsonview-c-key" x="124" y="406" width="84" height="68" rx="5"/>
|
||||
<text class="nj-jsonview-tiny" x="130" y="420" xml:space="preserve">1</text>
|
||||
<text class="nj-jsonview-kind nj-jsonview-c-key" x="166" y="434" text-anchor="middle" xml:space="preserve">key</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-xs" x="166" y="451" text-anchor="middle" xml:space="preserve">off 2</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-xs" x="166" y="465" text-anchor="middle" xml:space="preserve">len 4</text>
|
||||
<path class="nj-jsonview-arr nj-jsonview-c-key" d="M166 406 C166 367 98 367 98 328" marker-end="url(#jvl-key)"/>
|
||||
<rect class="nj-jsonview-node nj-jsonview-c-num" x="212" y="406" width="84" height="68" rx="5"/>
|
||||
<text class="nj-jsonview-tiny" x="218" y="420" xml:space="preserve">2</text>
|
||||
<text class="nj-jsonview-kind nj-jsonview-c-num" x="254" y="434" text-anchor="middle" xml:space="preserve">uint</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-xs" x="254" y="451" text-anchor="middle" xml:space="preserve">6379</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-xs" x="254" y="465" text-anchor="middle" xml:space="preserve">(converted)</text>
|
||||
<rect class="nj-jsonview-node nj-jsonview-c-key" x="300" y="406" width="84" height="68" rx="5"/>
|
||||
<text class="nj-jsonview-tiny" x="306" y="420" xml:space="preserve">3</text>
|
||||
<text class="nj-jsonview-kind nj-jsonview-c-key" x="342" y="434" text-anchor="middle" xml:space="preserve">key</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-xs" x="342" y="451" text-anchor="middle" xml:space="preserve">off 16</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-xs" x="342" y="465" text-anchor="middle" xml:space="preserve">len 5</text>
|
||||
<path class="nj-jsonview-arr nj-jsonview-c-key" d="M342 406 C342 367 301 367 301 328" marker-end="url(#jvl-key)"/>
|
||||
<rect class="nj-jsonview-node nj-jsonview-c-num" x="388" y="406" width="84" height="68" rx="5"/>
|
||||
<text class="nj-jsonview-tiny" x="394" y="420" xml:space="preserve">4</text>
|
||||
<text class="nj-jsonview-kind nj-jsonview-c-num" x="430" y="434" text-anchor="middle" xml:space="preserve">float</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-xs" x="430" y="451" text-anchor="middle" xml:space="preserve">off 24</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-xs" x="430" y="465" text-anchor="middle" xml:space="preserve">len 5</text>
|
||||
<path class="nj-jsonview-arr nj-jsonview-c-num" d="M430 406 C430 367 413 367 413 328" marker-end="url(#jvl-num)"/>
|
||||
<rect class="nj-jsonview-node nj-jsonview-c-key" x="476" y="406" width="84" height="68" rx="5"/>
|
||||
<text class="nj-jsonview-tiny" x="482" y="420" xml:space="preserve">5</text>
|
||||
<text class="nj-jsonview-kind nj-jsonview-c-key" x="518" y="434" text-anchor="middle" xml:space="preserve">key</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-xs" x="518" y="451" text-anchor="middle" xml:space="preserve">off 32</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-xs" x="518" y="465" text-anchor="middle" xml:space="preserve">len 4</text>
|
||||
<path class="nj-jsonview-arr nj-jsonview-c-key" d="M518 406 C518 367 518 367 518 328" marker-end="url(#jvl-key)"/>
|
||||
<rect class="nj-jsonview-node nj-jsonview-c-struct" x="564" y="406" width="84" height="68" rx="5"/>
|
||||
<text class="nj-jsonview-tiny" x="570" y="420" xml:space="preserve">6</text>
|
||||
<text class="nj-jsonview-kind nj-jsonview-c-struct" x="606" y="434" text-anchor="middle" xml:space="preserve">array</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-xs" x="606" y="451" text-anchor="middle" xml:space="preserve">len 2</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-xs" x="606" y="465" text-anchor="middle" xml:space="preserve">next 3</text>
|
||||
<rect class="nj-jsonview-node nj-jsonview-c-str" x="652" y="406" width="84" height="68" rx="5"/>
|
||||
<text class="nj-jsonview-tiny" x="658" y="420" xml:space="preserve">7</text>
|
||||
<text class="nj-jsonview-kind nj-jsonview-c-str" x="694" y="434" text-anchor="middle" xml:space="preserve">string</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-xs" x="694" y="451" text-anchor="middle" xml:space="preserve">off 41</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-xs" x="694" y="465" text-anchor="middle" xml:space="preserve">len 2</text>
|
||||
<path class="nj-jsonview-arr nj-jsonview-c-str" d="M694 406 C694 367 630 367 630 328" marker-end="url(#jvl-str)"/>
|
||||
<rect class="nj-jsonview-node nj-jsonview-c-dec" x="740" y="406" width="84" height="68" rx="5"/>
|
||||
<text class="nj-jsonview-tiny" x="746" y="420" xml:space="preserve">8</text>
|
||||
<text class="nj-jsonview-kind nj-jsonview-c-dec" x="782" y="434" text-anchor="middle" xml:space="preserve">string</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-xs" x="782" y="451" text-anchor="middle" xml:space="preserve">escaped</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-xs" x="782" y="465" text-anchor="middle" xml:space="preserve">len 3</text>
|
||||
<text class="nj-jsonview-note" x="773" y="568" text-anchor="middle" xml:space="preserve">decoded strings</text>
|
||||
<rect class="nj-jsonview-node nj-jsonview-c-dec" x="740" y="518" width="66" height="30" rx="4"/>
|
||||
<text class="nj-jsonview-mono" x="751" y="538" text-anchor="middle" xml:space="preserve">a</text>
|
||||
<text class="nj-jsonview-mono" x="773" y="538" text-anchor="middle" xml:space="preserve">"</text>
|
||||
<text class="nj-jsonview-mono" x="795" y="538" text-anchor="middle" xml:space="preserve">b</text>
|
||||
<path class="nj-jsonview-arr nj-jsonview-c-dec" d="M782 474 C782 496 773 496 773 518" marker-end="url(#jvl-dec)"/>
|
||||
<text class="nj-jsonview-mono nj-jsonview-sm" x="470" y="670" text-anchor="end" xml:space="preserve">json_view v = doc.root()["tags"]</text>
|
||||
<rect class="nj-jsonview-node nj-jsonview-c-struct" x="480" y="648" width="140" height="34" rx="6"/>
|
||||
<path class="nj-jsonview-sep" d="M550 648 V682"/>
|
||||
<text class="nj-jsonview-mono" x="515" y="670" text-anchor="middle" xml:space="preserve">doc*</text>
|
||||
<text class="nj-jsonview-mono" x="585" y="670" text-anchor="middle" xml:space="preserve">node*</text>
|
||||
<path class="nj-jsonview-arr nj-jsonview-c-struct" d="M515 648 L515 622" marker-end="url(#jvl-struct)"/>
|
||||
<path class="nj-jsonview-arr nj-jsonview-c-struct" d="M585 648 C585 561 606 561 606 474" marker-end="url(#jvl-struct)"/>
|
||||
<text class="nj-jsonview-note" x="30" y="524" xml:space="preserve">integers are converted while parsing and kept in their node;</text>
|
||||
<text class="nj-jsonview-note" x="30" y="542" xml:space="preserve">floats and strings point back into the text</text>
|
||||
</svg>
|
||||
|
||||
</div>
|
||||
|
||||
<p class="nj-caption" markdown>Top: the tree of separate heap allocations that `basic_json::parse()` builds. Bottom: `json_document::parse()` keeps the source text and builds one flat index of 16-byte nodes that point back into it, plus a buffer of decoded strings.</p>
|
||||
|
||||
## How to use it
|
||||
|
||||
Include `<nlohmann/json_view.hpp>` in addition to (or instead of) `<nlohmann/json.hpp>`. Parse into a
|
||||
@@ -80,7 +270,7 @@ A document either **borrows** the text it was parsed from, or **owns** its own c
|
||||
[`owns_source()`](../api/basic_json_document/owns_source.md) to find out which happened.
|
||||
[`parse()`](../api/basic_json_document/parse.md) decides this from the value category and type of its argument (an
|
||||
lvalue `#!cpp std::string` is borrowed; an rvalue `#!cpp std::string` is moved in, owned without a copy; a stream is
|
||||
read into an owned buffer; and so on -- see [`parse`'s Notes](../api/basic_json_document/parse.md#notes) for the full
|
||||
read into an owned buffer; and so on — see [`parse`'s Notes](../api/basic_json_document/parse.md#notes) for the full
|
||||
table). [`parse_copy()`](../api/basic_json_document/parse_copy.md) always owns a copy, regardless of the input.
|
||||
|
||||
!!! warning "A borrowed document depends on your buffer"
|
||||
@@ -102,10 +292,77 @@ Moving the `basic_json_document<>` itself is fine and does **not** invalidate it
|
||||
allocation that keeps its address across the move. Take a fresh view from [`root()`](../api/basic_json_document/root.md)
|
||||
whenever any of the other conditions above was not met.
|
||||
|
||||
<div class="nj-fig-wrap" markdown>
|
||||
<svg class="nj-fig nj-jsonview " viewBox="0 0 840 572" role="img" aria-labelledby="fig-jvt-title fig-jvt-title-d" xmlns="http://www.w3.org/2000/svg">
|
||||
<title id="fig-jvt-title">When a json_view becomes invalid</title>
|
||||
<desc id="fig-jvt-title-d">Four timelines. First: a document borrows a std::string; when the string is destroyed, the document and every view taken from it dangle. Second: doc.read() re-parses the document; views taken before are invalid, a fresh view from root() is valid. Third: doc.shrink_to_fit() has the same effect. Fourth: moving the document does not invalidate its views; destroying the moved-to document does.</desc>
|
||||
<defs>
|
||||
<marker id="jvt-bad" viewBox="0 0 10 10" refX="9" refY="5" markerWidth="6.5" markerHeight="6.5" orient="auto-start-reverse"><path class="nj-jsonview-c-bad nj-jsonview-head" d="M0 1 L10 5 L0 9 z"/></marker>
|
||||
</defs>
|
||||
<rect class="nj-jsonview-bar nj-jsonview-c-struct" x="210" y="12" width="34" height="12" rx="3"/>
|
||||
<text class="nj-jsonview-note" x="252" y="23" xml:space="preserve">alive / valid</text>
|
||||
<rect class="nj-jsonview-bad" x="370" y="12" width="34" height="12" rx="3"/>
|
||||
<text class="nj-jsonview-note" x="412" y="23" xml:space="preserve">dangling: using it is undefined behavior</text>
|
||||
<path class="nj-jsonview-arr nj-jsonview-c-line" d="M210 44 L820 44"/>
|
||||
<text class="nj-jsonview-tiny" x="820" y="38" text-anchor="end" xml:space="preserve">time →</text>
|
||||
<text class="nj-jsonview-h nj-jsonview-sm2" x="16" y="74" xml:space="preserve">The borrowed text is destroyed</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-sm" x="198" y="99" text-anchor="end" xml:space="preserve">std::string text</text>
|
||||
<rect class="nj-jsonview-bar nj-jsonview-c-struct" x="210" y="87" width="260" height="16" rx="3"/>
|
||||
<text class="nj-jsonview-mono nj-jsonview-sm" x="198" y="127" text-anchor="end" xml:space="preserve">json_document doc</text>
|
||||
<rect class="nj-jsonview-bar nj-jsonview-c-struct" x="270" y="115" width="200" height="16" rx="3"/>
|
||||
<rect class="nj-jsonview-bad" x="470" y="115" width="350" height="16" rx="3"/>
|
||||
<text class="nj-jsonview-mono nj-jsonview-sm" x="198" y="155" text-anchor="end" xml:space="preserve">json_view v</text>
|
||||
<rect class="nj-jsonview-bar nj-jsonview-c-struct" x="330" y="143" width="140" height="16" rx="3"/>
|
||||
<rect class="nj-jsonview-bad" x="470" y="143" width="350" height="16" rx="3"/>
|
||||
<path class="nj-jsonview-evt" d="M470 82 V168"/>
|
||||
<text class="nj-jsonview-tiny nj-jsonview-c-bad" x="474" y="80" xml:space="preserve">text destroyed</text>
|
||||
<text class="nj-jsonview-note nj-jsonview-onbad" x="476" y="155" xml:space="preserve">every view, and the document itself</text>
|
||||
<text class="nj-jsonview-h nj-jsonview-sm2" x="16" y="202" xml:space="preserve">The document is re-parsed</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-sm" x="198" y="227" text-anchor="end" xml:space="preserve">json_document doc</text>
|
||||
<rect class="nj-jsonview-bar nj-jsonview-c-struct" x="210" y="215" width="610" height="16" rx="3"/>
|
||||
<text class="nj-jsonview-mono nj-jsonview-sm" x="198" y="255" text-anchor="end" xml:space="preserve">json_view v</text>
|
||||
<rect class="nj-jsonview-bar nj-jsonview-c-struct" x="270" y="243" width="200" height="16" rx="3"/>
|
||||
<rect class="nj-jsonview-bad" x="470" y="243" width="350" height="16" rx="3"/>
|
||||
<text class="nj-jsonview-mono nj-jsonview-sm" x="198" y="283" text-anchor="end" xml:space="preserve">v = doc.root()</text>
|
||||
<rect class="nj-jsonview-bar nj-jsonview-c-struct" x="530" y="271" width="290" height="16" rx="3"/>
|
||||
<path class="nj-jsonview-evt" d="M470 210 V296"/>
|
||||
<text class="nj-jsonview-tiny nj-jsonview-c-bad" x="474" y="208" xml:space="preserve">doc.read(other)</text>
|
||||
<text class="nj-jsonview-note nj-jsonview-onbad" x="476" y="255" xml:space="preserve">views taken before</text>
|
||||
<text class="nj-jsonview-note nj-jsonview-onok" x="536" y="283" xml:space="preserve">a fresh view is valid again</text>
|
||||
<text class="nj-jsonview-h nj-jsonview-sm2" x="16" y="330" xml:space="preserve">The document is shrunk</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-sm" x="198" y="355" text-anchor="end" xml:space="preserve">json_document doc</text>
|
||||
<rect class="nj-jsonview-bar nj-jsonview-c-struct" x="210" y="343" width="610" height="16" rx="3"/>
|
||||
<text class="nj-jsonview-mono nj-jsonview-sm" x="198" y="383" text-anchor="end" xml:space="preserve">json_view v</text>
|
||||
<rect class="nj-jsonview-bar nj-jsonview-c-struct" x="270" y="371" width="200" height="16" rx="3"/>
|
||||
<rect class="nj-jsonview-bad" x="470" y="371" width="350" height="16" rx="3"/>
|
||||
<text class="nj-jsonview-mono nj-jsonview-sm" x="198" y="411" text-anchor="end" xml:space="preserve">v = doc.root()</text>
|
||||
<rect class="nj-jsonview-bar nj-jsonview-c-struct" x="530" y="399" width="290" height="16" rx="3"/>
|
||||
<path class="nj-jsonview-evt" d="M470 338 V424"/>
|
||||
<text class="nj-jsonview-tiny nj-jsonview-c-bad" x="474" y="336" xml:space="preserve">doc.shrink_to_fit()</text>
|
||||
<text class="nj-jsonview-note nj-jsonview-onbad" x="476" y="383" xml:space="preserve">views taken before</text>
|
||||
<text class="nj-jsonview-note nj-jsonview-onok" x="536" y="411" xml:space="preserve">a fresh view is valid again</text>
|
||||
<text class="nj-jsonview-h nj-jsonview-sm2" x="16" y="458" xml:space="preserve">The document is moved, then destroyed</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-sm" x="198" y="483" text-anchor="end" xml:space="preserve">json_document doc</text>
|
||||
<rect class="nj-jsonview-bar nj-jsonview-c-struct" x="210" y="471" width="210" height="16" rx="3"/>
|
||||
<text class="nj-jsonview-mono nj-jsonview-sm" x="198" y="511" text-anchor="end" xml:space="preserve">moved = std::move(doc)</text>
|
||||
<rect class="nj-jsonview-bar nj-jsonview-c-struct" x="420" y="499" width="260" height="16" rx="3"/>
|
||||
<text class="nj-jsonview-mono nj-jsonview-sm" x="198" y="539" text-anchor="end" xml:space="preserve">json_view v</text>
|
||||
<rect class="nj-jsonview-bar nj-jsonview-c-struct" x="270" y="527" width="410" height="16" rx="3"/>
|
||||
<rect class="nj-jsonview-bad" x="680" y="527" width="140" height="16" rx="3"/>
|
||||
<path class="nj-jsonview-evt nj-jsonview-evt--ok" d="M420 466 V552"/>
|
||||
<text class="nj-jsonview-tiny" x="424" y="464" xml:space="preserve">std::move: views stay valid</text>
|
||||
<path class="nj-jsonview-evt" d="M680 466 V552"/>
|
||||
<text class="nj-jsonview-tiny nj-jsonview-c-bad" x="684" y="464" xml:space="preserve">moved is destroyed</text>
|
||||
</svg>
|
||||
|
||||
</div>
|
||||
|
||||
<p class="nj-caption" markdown>Four timelines of a document and its views, each bar showing when an object is alive or valid and when it dangles. Destroying the borrowed text, re-parsing, or shrinking the document invalidates the views taken before; moving the document does not.</p>
|
||||
|
||||
A view can dangle, exactly like a `#!cpp std::string_view`: nothing stops you from using it after its document is
|
||||
gone. This is deliberate. A view is two pointers and trivially copyable; making it safe (a `#!cpp std::shared_ptr` in
|
||||
the document and a `#!cpp std::weak_ptr` in each view) would make it larger, add reference counting to every copy,
|
||||
and a `lock()` to every access -- exactly the costs the view exists to avoid. What is caught is the obvious case:
|
||||
and a `lock()` to every access — exactly the costs the view exists to avoid. What is caught is the obvious case:
|
||||
because a view dies with its document, [`root()`](../api/basic_json_document/root.md) is not callable on a temporary
|
||||
document: `#!cpp auto v = json_document::parse(text).root();` does not compile. Give the document a name first.
|
||||
|
||||
@@ -164,7 +421,7 @@ document: `#!cpp auto v = json_document::parse(text).root();` does not compile.
|
||||
[`parse()`](../api/basic_json_document/parse.md) accept and reject exactly the same inputs as
|
||||
[`basic_json::accept()`](../api/basic_json/accept.md)/[`basic_json::parse()`](../api/basic_json/parse.md), with the
|
||||
same `ignore_comments` and `ignore_trailing_commas` options.
|
||||
- **Errors.** A failing parse throws the same exception -- the same id, message, and position -- because on failure
|
||||
- **Errors.** A failing parse throws the same exception — the same id, message, and position — because on failure
|
||||
the library's own parser is run on the same bytes to produce the diagnostic. Valid input never pays for this, and
|
||||
the replay builds no values: it costs about a lexing pass up to the error.
|
||||
`#!cpp allow_exceptions == false` skips the replay and gives a
|
||||
@@ -210,7 +467,7 @@ document: `#!cpp auto v = json_document::parse(text).root();` does not compile.
|
||||
element access and lookup functions never carry the JSON Pointer path `JSON_DIAGNOSTICS` would otherwise add: the
|
||||
view has no `basic_json` value to point at, so the exception is created without one, regardless of how
|
||||
`BasicJsonType` was built.
|
||||
- **Ordering comparisons are not provided** by `basic_json_view` -- there is no `#!cpp operator<`.
|
||||
- **Ordering comparisons are not provided** by `basic_json_view` — there is no `#!cpp operator<`.
|
||||
[`operator==`](../api/basic_json_view/operator_eq.md) and [`operator!=`](../api/basic_json_view/operator_ne.md) are
|
||||
provided, though: two views, or a view and a `BasicJsonType` value, compare equal exactly when
|
||||
[`materialize()`](../api/basic_json_view/materialize.md) or [`parse()`](../api/basic_json/parse.md) would produce
|
||||
@@ -282,15 +539,15 @@ and other types the library converts (`#!cpp std::vector`, `#!cpp std::map`, `#!
|
||||
Two conversions never copy at all:
|
||||
|
||||
- [`get_string()`](../api/basic_json_view/get_string.md) (equivalently, `#!cpp get<string_view_t>()`) returns a
|
||||
string as a `string_view_t` pointing into the document's [`source()`](../api/basic_json_document/source.md) text --
|
||||
or, for a string that contains escape sequences, into the document's own buffer of decoded strings -- instead of
|
||||
string as a `string_view_t` pointing into the document's [`source()`](../api/basic_json_document/source.md) text —
|
||||
or, for a string that contains escape sequences, into the document's own buffer of decoded strings — instead of
|
||||
allocating a new `#!cpp std::string`.
|
||||
- [`number_token()`](../api/basic_json_view/number_token.md) returns a number exactly as it was written in the
|
||||
source, e.g. `#!cpp "1.50"`, `#!cpp "1E2"`, or an integer with more digits than any number type holds, instead of
|
||||
rounding it into a `#!cpp double`/`#!cpp int64_t` the way `#!cpp get<T>()` (and
|
||||
[`basic_json::parse()`](../api/basic_json/parse.md)) would.
|
||||
|
||||
Both results are only valid as long as the view -- and, for a string with no escapes, the borrowed source text -- is.
|
||||
Both results are only valid as long as the view — and, for a string with no escapes, the borrowed source text — is.
|
||||
|
||||
## Writing generic code
|
||||
|
||||
@@ -383,14 +640,14 @@ Two things differ when a `from_json()` reads the view:
|
||||
|
||||
[`dump()`](../api/basic_json_view/dump.md) serializes a view directly from the flat index, without ever building a
|
||||
`basic_json` value. An object's members are written in document order, not sorted by key, and *every* occurrence of a
|
||||
repeated key is written, not only the last one -- the same two ways [iteration](#what-is-different) already differs
|
||||
repeated key is written, not only the last one — the same two ways [iteration](#what-is-different) already differs
|
||||
from a [`materialize()`](../api/basic_json_view/materialize.md)d value, see [Duplicate keys](#duplicate-keys).
|
||||
`#!cpp materialize().dump()` gives a different result in both respects for a `json_view`.
|
||||
|
||||
By default, numbers are written the way [`basic_json::dump()`](../api/basic_json/dump.md) would.
|
||||
[`number_format::source`](../api/basic_json_view/number_format.md) instead copies every number exactly as it was
|
||||
written in the source text -- a price like `#!cpp 19.90`, a long order or account ID with more digits than any number
|
||||
type holds, or a high-precision coordinate -- something `basic_json` cannot do at all, since parsing already reduces
|
||||
written in the source text — a price like `#!cpp 19.90`, a long order or account ID with more digits than any number
|
||||
type holds, or a high-precision coordinate — something `basic_json` cannot do at all, since parsing already reduces
|
||||
a number to its parsed `#!cpp double`/`#!cpp int64_t` value.
|
||||
|
||||
[`operator<<`](../api/basic_json_view/operator_ltlt.md) writes a view to a stream the way `basic_json`'s does, using
|
||||
@@ -399,14 +656,14 @@ the stream's `width`/`fill` for indentation.
|
||||
## Editing a document
|
||||
|
||||
Everything above is read-only: a `json_document`/`json_view` lets you look at a parsed text without copying it, but
|
||||
not change it. [`basic_json_document<BasicJsonType, view_mode::editable>`](../api/basic_json_document/index.md) -- more conveniently
|
||||
not change it. [`basic_json_document<BasicJsonType, view_mode::editable>`](../api/basic_json_document/index.md) — more conveniently
|
||||
spelled [`json_editable_document`](../api/json_editable_document.md) or
|
||||
[`ordered_json_editable_document`](../api/ordered_json_editable_document.md) -- also lets you
|
||||
[`ordered_json_editable_document`](../api/ordered_json_editable_document.md) — also lets you
|
||||
[`set`](../api/basic_json_document/set.md) a value, [`push_back`](../api/basic_json_document/push_back.md) onto or
|
||||
[`insert`](../api/basic_json_document/insert.md) into an array, and [`erase`](../api/basic_json_document/erase.md)
|
||||
an object member or an array element, still without ever building a `basic_json` tree for parts you do not touch.
|
||||
|
||||
The [mode](../api/view_mode.md) defaults to `#!cpp view_mode::read_only`, so `json_document`/`ordered_json_document` are unaffected -- they carry
|
||||
The [mode](../api/view_mode.md) defaults to `#!cpp view_mode::read_only`, so `json_document`/`ordered_json_document` are unaffected — they carry
|
||||
none of the bookkeeping edits need, and calling `set`/`push_back`/`insert`/`erase` on one is a compile error, not a
|
||||
runtime one.
|
||||
|
||||
@@ -416,15 +673,35 @@ The `#!cpp 19.90` price from [above](#writing-a-view-back) is exactly the kind o
|
||||
or `ordered_json` value in place lossy. Say you parse a configuration file, patch one field, and write it back:
|
||||
|
||||
- **`json`** re-sorts every key on the way in (`object_t` is a `#!cpp std::map`) and rewrites every number to its
|
||||
shortest round-trip form on the way out -- a one-field patch turns into a diff that reorders the whole file and
|
||||
shortest round-trip form on the way out — a one-field patch turns into a diff that reorders the whole file and
|
||||
rewrites `#!cpp 19.90` to `#!cpp 19.9`.
|
||||
- **`ordered_json`** keeps the key order, but still rewrites every number the same way: parsing has already reduced
|
||||
it to a `#!cpp double`/`#!cpp int64_t`, and there is no way back to how it was spelled in the source text.
|
||||
|
||||
The patch of the example below (set `"host"`, add `"retries"`) written back by each of them:
|
||||
|
||||
<div class="nj-jsonview-cmp" role="group" aria-label="The same patch written back by json, ordered_json, and json_editable_document">
|
||||
<div><p class="nj-jsonview-cmp-h"><code>json</code></p><p class="nj-jsonview-cmp-v">keys sorted, numbers rewritten</p><pre>{<span class="nj-jsonview-bad">"host"</span>:"db2",
|
||||
"name":"cache",
|
||||
"port":6379,
|
||||
"price":<span class="nj-jsonview-bad">19.9</span>,
|
||||
"retries":3}</pre></div>
|
||||
<div><p class="nj-jsonview-cmp-h"><code>ordered_json</code></p><p class="nj-jsonview-cmp-v">key order kept, numbers rewritten</p><pre>{"name":"cache",
|
||||
"host":"db2",
|
||||
"port":6379,
|
||||
"price":<span class="nj-jsonview-bad">19.9</span>,
|
||||
"retries":3}</pre></div>
|
||||
<div class="nj-jsonview-cmp--best"><p class="nj-jsonview-cmp-h"><code>json_editable_document</code></p><p class="nj-jsonview-cmp-v">order and spelling kept</p><pre>{"name":"cache",
|
||||
"host":"db2",
|
||||
"port":6379,
|
||||
"price":<span class="nj-jsonview-ok">19.90</span>,
|
||||
"retries":3}</pre></div>
|
||||
</div>
|
||||
|
||||
An editable document keeps both. [`dump()`](../api/basic_json_view/dump.md) of an edited document writes members in
|
||||
document order -- a member [`set`](../api/basic_json_document/set.md) added goes at the end, exactly where it was
|
||||
document order — a member [`set`](../api/basic_json_document/set.md) added goes at the end, exactly where it was
|
||||
inserted, and an [`erase`](../api/basic_json_document/erase.md)d member simply leaves a gap: everything around it
|
||||
keeps its place -- and [`number_format::source`](../api/basic_json_view/number_format.md) keeps the exact spelling
|
||||
keeps its place — and [`number_format::source`](../api/basic_json_view/number_format.md) keeps the exact spelling
|
||||
of every number an edit did not itself touch; a number an edit *did* touch is written the way
|
||||
[`BasicJsonType::dump()`](../api/basic_json/dump.md) would write it, since there is no source spelling for a brand
|
||||
new value.
|
||||
@@ -477,14 +754,14 @@ new value.
|
||||
|
||||
### What stays valid, and what an edit costs
|
||||
|
||||
The source text itself is **never written**, and the parsed index never moves -- a value keeps the node it was
|
||||
The source text itself is **never written**, and the parsed index never moves — a value keeps the node it was
|
||||
parsed into for as long as it is not itself replaced. So every [view](../api/basic_json_view/index.md) taken before
|
||||
an edit, including a previously obtained [`root()`](../api/basic_json_document/root.md), stays valid and, if it
|
||||
still refers to the edited value, sees the edit; a view of a value a later edit drops or replaces just keeps showing
|
||||
what it last held. New values go to storage the document allocates and owns on demand. The one thing an edit does
|
||||
invalidate is the **iterators** taken over an edited array or object: the first time one of its elements is set,
|
||||
appended to, inserted into, or erased, its elements move from the parsed, fixed layout to a growable block of links
|
||||
so that [`push_back`](../api/basic_json_document/push_back.md) can later grow it in amortized constant time --
|
||||
invalidate is the **iterators** taken over an edited array or object: the first time an element is added to it,
|
||||
inserted into it, erased from it, or replaced by a value that spans a different number of nodes, its elements move from the parsed, fixed layout to a growable block of links
|
||||
so that [`push_back`](../api/basic_json_document/push_back.md) can later grow it in amortized constant time —
|
||||
existing elements are not touched, but an iterator that was walking the old layout no longer matches. A string
|
||||
obtained with
|
||||
[`get_string()`](../api/basic_json_view/get_string.md) is unaffected either way and stays valid across further
|
||||
@@ -493,16 +770,443 @@ edits. See [`basic_json_document`'s Edits](../api/basic_json_document/index.md#e
|
||||
throws (the *basic* guarantee, not the strong one `dump()` and the read-only functions provide). How edits are kept in
|
||||
the index is described in the [architecture overview](../home/architecture.md#node-index-of-json-views).
|
||||
|
||||
Step through the edits of a small document (the node fields follow the [architecture overview](../home/architecture.md#node-index-of-json-views)):
|
||||
|
||||
<div class="nj-jsonview-steps" data-nj-jsonview-steps>
|
||||
<figure class="nj-jsonview-step" data-label="After parsing">
|
||||
<div class="nj-jsonview-scroll">
|
||||
<svg class="nj-fig nj-jsonview nj-jsonview-editfig" viewBox="0 0 840 372" role="img" aria-labelledby="fig-jve0-title fig-jve0-title-d" xmlns="http://www.w3.org/2000/svg">
|
||||
<title id="fig-jve0-title">Step 1 of 5: parsed</title>
|
||||
<desc id="fig-jve0-title-d">After parse: nine nodes, nothing owned yet. The dump with number_format::source is {"port":6379,"price":19.90,"tags":["eu","a\"b"]}.</desc>
|
||||
<defs>
|
||||
<marker id="jve0-line" viewBox="0 0 10 10" refX="9" refY="5" markerWidth="6.5" markerHeight="6.5" orient="auto-start-reverse"><path class="nj-jsonview-c-line nj-jsonview-head" d="M0 1 L10 5 L0 9 z"/></marker>
|
||||
<marker id="jve0-own" viewBox="0 0 10 10" refX="9" refY="5" markerWidth="6.5" markerHeight="6.5" orient="auto-start-reverse"><path class="nj-jsonview-c-own nj-jsonview-head" d="M0 1 L10 5 L0 9 z"/></marker>
|
||||
<marker id="jve0-struct" viewBox="0 0 10 10" refX="9" refY="5" markerWidth="6.5" markerHeight="6.5" orient="auto-start-reverse"><path class="nj-jsonview-c-struct nj-jsonview-head" d="M0 1 L10 5 L0 9 z"/></marker>
|
||||
<marker id="jve0-bad" viewBox="0 0 10 10" refX="9" refY="5" markerWidth="6.5" markerHeight="6.5" orient="auto-start-reverse"><path class="nj-jsonview-c-bad nj-jsonview-head" d="M0 1 L10 5 L0 9 z"/></marker>
|
||||
</defs>
|
||||
<text class="nj-jsonview-note nj-jsonview-strong" x="16" y="22" xml:space="preserve">parsed index: fixed, never moves</text>
|
||||
<text class="nj-jsonview-note" x="824" y="22" text-anchor="end" xml:space="preserve">the source text is never written</text>
|
||||
<rect class="nj-jsonview-node nj-jsonview-c-struct" x="36" y="34" width="84" height="50" rx="5"/>
|
||||
<text class="nj-jsonview-tiny" x="42" y="47" xml:space="preserve">0</text>
|
||||
<text class="nj-jsonview-kind nj-jsonview-xs nj-jsonview-c-struct" x="78" y="61" text-anchor="middle" xml:space="preserve">object</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-sm" x="78" y="77" text-anchor="middle" xml:space="preserve">{ } 3</text>
|
||||
<rect class="nj-jsonview-node nj-jsonview-c-key" x="124" y="34" width="84" height="50" rx="5"/>
|
||||
<text class="nj-jsonview-tiny" x="130" y="47" xml:space="preserve">1</text>
|
||||
<text class="nj-jsonview-kind nj-jsonview-xs nj-jsonview-c-key" x="166" y="61" text-anchor="middle" xml:space="preserve">key</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-sm" x="166" y="77" text-anchor="middle" xml:space="preserve">port</text>
|
||||
<rect class="nj-jsonview-node nj-jsonview-c-num" x="212" y="34" width="84" height="50" rx="5"/>
|
||||
<text class="nj-jsonview-tiny" x="218" y="47" xml:space="preserve">2</text>
|
||||
<text class="nj-jsonview-kind nj-jsonview-xs nj-jsonview-c-num" x="254" y="61" text-anchor="middle" xml:space="preserve">uint</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-sm" x="254" y="77" text-anchor="middle" xml:space="preserve">6379</text>
|
||||
<rect class="nj-jsonview-node nj-jsonview-c-key" x="300" y="34" width="84" height="50" rx="5"/>
|
||||
<text class="nj-jsonview-tiny" x="306" y="47" xml:space="preserve">3</text>
|
||||
<text class="nj-jsonview-kind nj-jsonview-xs nj-jsonview-c-key" x="342" y="61" text-anchor="middle" xml:space="preserve">key</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-sm" x="342" y="77" text-anchor="middle" xml:space="preserve">price</text>
|
||||
<rect class="nj-jsonview-node nj-jsonview-c-num" x="388" y="34" width="84" height="50" rx="5"/>
|
||||
<text class="nj-jsonview-tiny" x="394" y="47" xml:space="preserve">4</text>
|
||||
<text class="nj-jsonview-kind nj-jsonview-xs nj-jsonview-c-num" x="430" y="61" text-anchor="middle" xml:space="preserve">float</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-sm" x="430" y="77" text-anchor="middle" xml:space="preserve">19.90</text>
|
||||
<rect class="nj-jsonview-node nj-jsonview-c-key" x="476" y="34" width="84" height="50" rx="5"/>
|
||||
<text class="nj-jsonview-tiny" x="482" y="47" xml:space="preserve">5</text>
|
||||
<text class="nj-jsonview-kind nj-jsonview-xs nj-jsonview-c-key" x="518" y="61" text-anchor="middle" xml:space="preserve">key</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-sm" x="518" y="77" text-anchor="middle" xml:space="preserve">tags</text>
|
||||
<rect class="nj-jsonview-node nj-jsonview-c-struct" x="564" y="34" width="84" height="50" rx="5"/>
|
||||
<text class="nj-jsonview-tiny" x="570" y="47" xml:space="preserve">6</text>
|
||||
<text class="nj-jsonview-kind nj-jsonview-xs nj-jsonview-c-struct" x="606" y="61" text-anchor="middle" xml:space="preserve">array</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-sm" x="606" y="77" text-anchor="middle" xml:space="preserve">[ ] 2</text>
|
||||
<rect class="nj-jsonview-node nj-jsonview-c-str" x="652" y="34" width="84" height="50" rx="5"/>
|
||||
<text class="nj-jsonview-tiny" x="658" y="47" xml:space="preserve">7</text>
|
||||
<text class="nj-jsonview-kind nj-jsonview-xs nj-jsonview-c-str" x="694" y="61" text-anchor="middle" xml:space="preserve">string</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-sm" x="694" y="77" text-anchor="middle" xml:space="preserve">"eu"</text>
|
||||
<rect class="nj-jsonview-node nj-jsonview-c-dec" x="740" y="34" width="84" height="50" rx="5"/>
|
||||
<text class="nj-jsonview-tiny" x="746" y="47" xml:space="preserve">8</text>
|
||||
<text class="nj-jsonview-kind nj-jsonview-xs nj-jsonview-c-dec" x="782" y="61" text-anchor="middle" xml:space="preserve">string</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-sm" x="782" y="77" text-anchor="middle" xml:space="preserve">a"b</text>
|
||||
<text class="nj-jsonview-note nj-jsonview-strong nj-jsonview-c-own" x="16" y="288" xml:space="preserve">owned storage: allocated on demand, never moves</text>
|
||||
<rect class="nj-jsonview-store" x="10" y="192" width="820" height="54" rx="8"/>
|
||||
<text class="nj-jsonview-note" x="26" y="224" xml:space="preserve">empty: a read-only json_document has none of this</text>
|
||||
<path class="nj-jsonview-sep" d="M16 300 H824"/>
|
||||
<text class="nj-jsonview-mono nj-jsonview-xs" x="16" y="318" xml:space="preserve">view v = root()["tags"] (taken after parse):</text>
|
||||
<text class="nj-jsonview-note nj-jsonview-c-struct nj-jsonview-strong" x="320" y="318" xml:space="preserve">valid, sees every edit</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-xs" x="16" y="338" xml:space="preserve">iterator over tags (taken after parse):</text>
|
||||
<text class="nj-jsonview-note nj-jsonview-c-struct nj-jsonview-strong" x="320" y="338" xml:space="preserve">valid</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-xs nj-jsonview-muted" x="16" y="360" xml:space="preserve">dump(): {"port":6379,"price":19.90,"tags":["eu","a\"b"]}</text>
|
||||
</svg>
|
||||
|
||||
</div>
|
||||
<figcaption>Step 1 of 5. After parsing: nine nodes in the parsed index, no owned storage yet.</figcaption>
|
||||
</figure>
|
||||
<figure class="nj-jsonview-step" data-label="set an existing member">
|
||||
<div class="nj-jsonview-scroll">
|
||||
<svg class="nj-fig nj-jsonview nj-jsonview-editfig" viewBox="0 0 840 372" role="img" aria-labelledby="fig-jve1-title fig-jve1-title-d" xmlns="http://www.w3.org/2000/svg">
|
||||
<title id="fig-jve1-title">Step 2 of 5: doc.set(doc.root(), "port", 6380)</title>
|
||||
<desc id="fig-jve1-title-d">set: the node of the integer is overwritten in place. The dump with number_format::source is {"port":6380,"price":19.90,"tags":["eu","a\"b"]}.</desc>
|
||||
<defs>
|
||||
<marker id="jve1-line" viewBox="0 0 10 10" refX="9" refY="5" markerWidth="6.5" markerHeight="6.5" orient="auto-start-reverse"><path class="nj-jsonview-c-line nj-jsonview-head" d="M0 1 L10 5 L0 9 z"/></marker>
|
||||
<marker id="jve1-own" viewBox="0 0 10 10" refX="9" refY="5" markerWidth="6.5" markerHeight="6.5" orient="auto-start-reverse"><path class="nj-jsonview-c-own nj-jsonview-head" d="M0 1 L10 5 L0 9 z"/></marker>
|
||||
<marker id="jve1-struct" viewBox="0 0 10 10" refX="9" refY="5" markerWidth="6.5" markerHeight="6.5" orient="auto-start-reverse"><path class="nj-jsonview-c-struct nj-jsonview-head" d="M0 1 L10 5 L0 9 z"/></marker>
|
||||
<marker id="jve1-bad" viewBox="0 0 10 10" refX="9" refY="5" markerWidth="6.5" markerHeight="6.5" orient="auto-start-reverse"><path class="nj-jsonview-c-bad nj-jsonview-head" d="M0 1 L10 5 L0 9 z"/></marker>
|
||||
</defs>
|
||||
<text class="nj-jsonview-note nj-jsonview-strong" x="16" y="22" xml:space="preserve">parsed index: fixed, never moves</text>
|
||||
<text class="nj-jsonview-note" x="824" y="22" text-anchor="end" xml:space="preserve">the source text is never written</text>
|
||||
<rect class="nj-jsonview-node nj-jsonview-c-struct" x="36" y="34" width="84" height="50" rx="5"/>
|
||||
<text class="nj-jsonview-tiny" x="42" y="47" xml:space="preserve">0</text>
|
||||
<text class="nj-jsonview-kind nj-jsonview-xs nj-jsonview-c-struct" x="78" y="61" text-anchor="middle" xml:space="preserve">object</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-sm" x="78" y="77" text-anchor="middle" xml:space="preserve">{ } 3</text>
|
||||
<rect class="nj-jsonview-node nj-jsonview-c-key" x="124" y="34" width="84" height="50" rx="5"/>
|
||||
<text class="nj-jsonview-tiny" x="130" y="47" xml:space="preserve">1</text>
|
||||
<text class="nj-jsonview-kind nj-jsonview-xs nj-jsonview-c-key" x="166" y="61" text-anchor="middle" xml:space="preserve">key</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-sm" x="166" y="77" text-anchor="middle" xml:space="preserve">port</text>
|
||||
<rect class="nj-jsonview-node nj-jsonview-hot nj-jsonview-c-num" x="212" y="34" width="84" height="50" rx="5"/>
|
||||
<text class="nj-jsonview-tiny" x="218" y="47" xml:space="preserve">2</text>
|
||||
<text class="nj-jsonview-kind nj-jsonview-xs nj-jsonview-c-num" x="254" y="61" text-anchor="middle" xml:space="preserve">uint</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-sm" x="254" y="77" text-anchor="middle" xml:space="preserve">6380</text>
|
||||
<rect class="nj-jsonview-node nj-jsonview-c-key" x="300" y="34" width="84" height="50" rx="5"/>
|
||||
<text class="nj-jsonview-tiny" x="306" y="47" xml:space="preserve">3</text>
|
||||
<text class="nj-jsonview-kind nj-jsonview-xs nj-jsonview-c-key" x="342" y="61" text-anchor="middle" xml:space="preserve">key</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-sm" x="342" y="77" text-anchor="middle" xml:space="preserve">price</text>
|
||||
<rect class="nj-jsonview-node nj-jsonview-c-num" x="388" y="34" width="84" height="50" rx="5"/>
|
||||
<text class="nj-jsonview-tiny" x="394" y="47" xml:space="preserve">4</text>
|
||||
<text class="nj-jsonview-kind nj-jsonview-xs nj-jsonview-c-num" x="430" y="61" text-anchor="middle" xml:space="preserve">float</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-sm" x="430" y="77" text-anchor="middle" xml:space="preserve">19.90</text>
|
||||
<rect class="nj-jsonview-node nj-jsonview-c-key" x="476" y="34" width="84" height="50" rx="5"/>
|
||||
<text class="nj-jsonview-tiny" x="482" y="47" xml:space="preserve">5</text>
|
||||
<text class="nj-jsonview-kind nj-jsonview-xs nj-jsonview-c-key" x="518" y="61" text-anchor="middle" xml:space="preserve">key</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-sm" x="518" y="77" text-anchor="middle" xml:space="preserve">tags</text>
|
||||
<rect class="nj-jsonview-node nj-jsonview-c-struct" x="564" y="34" width="84" height="50" rx="5"/>
|
||||
<text class="nj-jsonview-tiny" x="570" y="47" xml:space="preserve">6</text>
|
||||
<text class="nj-jsonview-kind nj-jsonview-xs nj-jsonview-c-struct" x="606" y="61" text-anchor="middle" xml:space="preserve">array</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-sm" x="606" y="77" text-anchor="middle" xml:space="preserve">[ ] 2</text>
|
||||
<rect class="nj-jsonview-node nj-jsonview-c-str" x="652" y="34" width="84" height="50" rx="5"/>
|
||||
<text class="nj-jsonview-tiny" x="658" y="47" xml:space="preserve">7</text>
|
||||
<text class="nj-jsonview-kind nj-jsonview-xs nj-jsonview-c-str" x="694" y="61" text-anchor="middle" xml:space="preserve">string</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-sm" x="694" y="77" text-anchor="middle" xml:space="preserve">"eu"</text>
|
||||
<rect class="nj-jsonview-node nj-jsonview-c-dec" x="740" y="34" width="84" height="50" rx="5"/>
|
||||
<text class="nj-jsonview-tiny" x="746" y="47" xml:space="preserve">8</text>
|
||||
<text class="nj-jsonview-kind nj-jsonview-xs nj-jsonview-c-dec" x="782" y="61" text-anchor="middle" xml:space="preserve">string</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-sm" x="782" y="77" text-anchor="middle" xml:space="preserve">a"b</text>
|
||||
<text class="nj-jsonview-note nj-jsonview-strong nj-jsonview-c-own" x="16" y="288" xml:space="preserve">owned storage: allocated on demand, never moves</text>
|
||||
<rect class="nj-jsonview-store" x="10" y="192" width="820" height="54" rx="8"/>
|
||||
<text class="nj-jsonview-note" x="26" y="224" xml:space="preserve">empty: a read-only json_document has none of this</text>
|
||||
<path class="nj-jsonview-sep" d="M16 300 H824"/>
|
||||
<text class="nj-jsonview-mono nj-jsonview-xs" x="16" y="318" xml:space="preserve">view v = root()["tags"] (taken after parse):</text>
|
||||
<text class="nj-jsonview-note nj-jsonview-c-struct nj-jsonview-strong" x="320" y="318" xml:space="preserve">valid, sees every edit</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-xs" x="16" y="338" xml:space="preserve">iterator over tags (taken after parse):</text>
|
||||
<text class="nj-jsonview-note nj-jsonview-c-struct nj-jsonview-strong" x="320" y="338" xml:space="preserve">valid</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-xs nj-jsonview-muted" x="16" y="360" xml:space="preserve">dump(): {"port":6380,"price":19.90,"tags":["eu","a\"b"]}</text>
|
||||
</svg>
|
||||
|
||||
</div>
|
||||
<figcaption>Step 2 of 5. <code>doc.set(doc.root(), "port", 6380)</code> overwrites the node of the integer in place; no member is added or removed, so the object keeps its parsed layout.</figcaption>
|
||||
</figure>
|
||||
<figure class="nj-jsonview-step" data-label="erase an array element">
|
||||
<div class="nj-jsonview-scroll">
|
||||
<svg class="nj-fig nj-jsonview nj-jsonview-editfig" viewBox="0 0 840 372" role="img" aria-labelledby="fig-jve2-title fig-jve2-title-d" xmlns="http://www.w3.org/2000/svg">
|
||||
<title id="fig-jve2-title">Step 3 of 5: doc.erase(doc.root()["tags"], 0)</title>
|
||||
<desc id="fig-jve2-title-d">erase: the array moves to a block of links in owned storage. The dump with number_format::source is {"port":6380,"price":19.90,"tags":["a\"b"]}.</desc>
|
||||
<defs>
|
||||
<marker id="jve2-line" viewBox="0 0 10 10" refX="9" refY="5" markerWidth="6.5" markerHeight="6.5" orient="auto-start-reverse"><path class="nj-jsonview-c-line nj-jsonview-head" d="M0 1 L10 5 L0 9 z"/></marker>
|
||||
<marker id="jve2-own" viewBox="0 0 10 10" refX="9" refY="5" markerWidth="6.5" markerHeight="6.5" orient="auto-start-reverse"><path class="nj-jsonview-c-own nj-jsonview-head" d="M0 1 L10 5 L0 9 z"/></marker>
|
||||
<marker id="jve2-struct" viewBox="0 0 10 10" refX="9" refY="5" markerWidth="6.5" markerHeight="6.5" orient="auto-start-reverse"><path class="nj-jsonview-c-struct nj-jsonview-head" d="M0 1 L10 5 L0 9 z"/></marker>
|
||||
<marker id="jve2-bad" viewBox="0 0 10 10" refX="9" refY="5" markerWidth="6.5" markerHeight="6.5" orient="auto-start-reverse"><path class="nj-jsonview-c-bad nj-jsonview-head" d="M0 1 L10 5 L0 9 z"/></marker>
|
||||
</defs>
|
||||
<text class="nj-jsonview-note nj-jsonview-strong" x="16" y="22" xml:space="preserve">parsed index: fixed, never moves</text>
|
||||
<text class="nj-jsonview-note" x="824" y="22" text-anchor="end" xml:space="preserve">the source text is never written</text>
|
||||
<rect class="nj-jsonview-node nj-jsonview-c-struct" x="36" y="34" width="84" height="50" rx="5"/>
|
||||
<text class="nj-jsonview-tiny" x="42" y="47" xml:space="preserve">0</text>
|
||||
<text class="nj-jsonview-kind nj-jsonview-xs nj-jsonview-c-struct" x="78" y="61" text-anchor="middle" xml:space="preserve">object</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-sm" x="78" y="77" text-anchor="middle" xml:space="preserve">{ } 3</text>
|
||||
<rect class="nj-jsonview-node nj-jsonview-c-key" x="124" y="34" width="84" height="50" rx="5"/>
|
||||
<text class="nj-jsonview-tiny" x="130" y="47" xml:space="preserve">1</text>
|
||||
<text class="nj-jsonview-kind nj-jsonview-xs nj-jsonview-c-key" x="166" y="61" text-anchor="middle" xml:space="preserve">key</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-sm" x="166" y="77" text-anchor="middle" xml:space="preserve">port</text>
|
||||
<rect class="nj-jsonview-node nj-jsonview-c-num" x="212" y="34" width="84" height="50" rx="5"/>
|
||||
<text class="nj-jsonview-tiny" x="218" y="47" xml:space="preserve">2</text>
|
||||
<text class="nj-jsonview-kind nj-jsonview-xs nj-jsonview-c-num" x="254" y="61" text-anchor="middle" xml:space="preserve">uint</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-sm" x="254" y="77" text-anchor="middle" xml:space="preserve">6380</text>
|
||||
<rect class="nj-jsonview-node nj-jsonview-c-key" x="300" y="34" width="84" height="50" rx="5"/>
|
||||
<text class="nj-jsonview-tiny" x="306" y="47" xml:space="preserve">3</text>
|
||||
<text class="nj-jsonview-kind nj-jsonview-xs nj-jsonview-c-key" x="342" y="61" text-anchor="middle" xml:space="preserve">key</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-sm" x="342" y="77" text-anchor="middle" xml:space="preserve">price</text>
|
||||
<rect class="nj-jsonview-node nj-jsonview-c-num" x="388" y="34" width="84" height="50" rx="5"/>
|
||||
<text class="nj-jsonview-tiny" x="394" y="47" xml:space="preserve">4</text>
|
||||
<text class="nj-jsonview-kind nj-jsonview-xs nj-jsonview-c-num" x="430" y="61" text-anchor="middle" xml:space="preserve">float</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-sm" x="430" y="77" text-anchor="middle" xml:space="preserve">19.90</text>
|
||||
<rect class="nj-jsonview-node nj-jsonview-c-key" x="476" y="34" width="84" height="50" rx="5"/>
|
||||
<text class="nj-jsonview-tiny" x="482" y="47" xml:space="preserve">5</text>
|
||||
<text class="nj-jsonview-kind nj-jsonview-xs nj-jsonview-c-key" x="518" y="61" text-anchor="middle" xml:space="preserve">key</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-sm" x="518" y="77" text-anchor="middle" xml:space="preserve">tags</text>
|
||||
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|
||||
<text class="nj-jsonview-tiny" x="570" y="47" xml:space="preserve">6</text>
|
||||
<text class="nj-jsonview-kind nj-jsonview-xs nj-jsonview-c-struct" x="606" y="61" text-anchor="middle" xml:space="preserve">array</text>
|
||||
<text class="nj-jsonview-tiny nj-jsonview-c-struct nj-jsonview-strong" x="643" y="47" text-anchor="end" xml:space="preserve">moved</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-sm" x="606" y="77" text-anchor="middle" xml:space="preserve">[ ] 2</text>
|
||||
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|
||||
<text class="nj-jsonview-tiny" x="658" y="47" xml:space="preserve">7</text>
|
||||
<text class="nj-jsonview-kind nj-jsonview-xs nj-jsonview-c-str" x="694" y="61" text-anchor="middle" xml:space="preserve">string</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-sm" x="694" y="77" text-anchor="middle" xml:space="preserve">"eu"</text>
|
||||
<text class="nj-jsonview-tiny nj-jsonview-c-bad" x="694" y="98" text-anchor="middle" xml:space="preserve">unreferenced</text>
|
||||
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|
||||
<text class="nj-jsonview-tiny" x="746" y="47" xml:space="preserve">8</text>
|
||||
<text class="nj-jsonview-kind nj-jsonview-xs nj-jsonview-c-dec" x="782" y="61" text-anchor="middle" xml:space="preserve">string</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-sm" x="782" y="77" text-anchor="middle" xml:space="preserve">a"b</text>
|
||||
<text class="nj-jsonview-note nj-jsonview-strong nj-jsonview-c-own" x="16" y="288" xml:space="preserve">owned storage: allocated on demand, never moves</text>
|
||||
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|
||||
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|
||||
<text class="nj-jsonview-tiny" x="66" y="207" text-anchor="end" xml:space="preserve">0'</text>
|
||||
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|
||||
<text class="nj-jsonview-mono nj-jsonview-xs" x="43" y="235" text-anchor="middle" xml:space="preserve">[ ] 1</text>
|
||||
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|
||||
<text class="nj-jsonview-tiny" x="123" y="207" text-anchor="end" xml:space="preserve">1'</text>
|
||||
<text class="nj-jsonview-kind nj-jsonview-xs nj-jsonview-c-own" x="100" y="220" text-anchor="middle" xml:space="preserve">link</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-xs" x="100" y="235" text-anchor="middle" xml:space="preserve">→ 8</text>
|
||||
<path class="nj-jsonview-arr nj-jsonview-c-own" d="M100 196 C100 140 782 140 782 84" marker-end="url(#jve2-own)"/>
|
||||
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|
||||
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|
||||
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|
||||
<path class="nj-jsonview-sep" d="M16 300 H824"/>
|
||||
<text class="nj-jsonview-mono nj-jsonview-xs" x="16" y="318" xml:space="preserve">view v = root()["tags"] (taken after parse):</text>
|
||||
<text class="nj-jsonview-note nj-jsonview-c-struct nj-jsonview-strong" x="320" y="318" xml:space="preserve">valid, sees every edit</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-xs" x="16" y="338" xml:space="preserve">iterator over tags (taken after parse):</text>
|
||||
<text class="nj-jsonview-note nj-jsonview-c-bad nj-jsonview-strong" x="320" y="338" xml:space="preserve">invalid: tags no longer uses the parsed layout</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-xs nj-jsonview-muted" x="16" y="360" xml:space="preserve">dump(): {"port":6380,"price":19.90,"tags":["a\"b"]}</text>
|
||||
</svg>
|
||||
|
||||
</div>
|
||||
<figcaption>Step 3 of 5. <code>doc.erase(doc.root()["tags"], 0)</code>: the array is edited for the first time, so its elements move to a block of links in owned storage. The node of <code>"eu"</code> stays in the index, but nothing refers to it any more; iterators over the array taken before are invalid.</figcaption>
|
||||
</figure>
|
||||
<figure class="nj-jsonview-step" data-label="push_back onto the array">
|
||||
<div class="nj-jsonview-scroll">
|
||||
<svg class="nj-fig nj-jsonview nj-jsonview-editfig" viewBox="0 0 840 372" role="img" aria-labelledby="fig-jve3-title fig-jve3-title-d" xmlns="http://www.w3.org/2000/svg">
|
||||
<title id="fig-jve3-title">Step 4 of 5: doc.push_back(doc.root()["tags"], "new")</title>
|
||||
<desc id="fig-jve3-title-d">push_back: a new node, and a link appended to the block. The dump with number_format::source is {"port":6380,"price":19.90,"tags":["a\"b","new"]}.</desc>
|
||||
<defs>
|
||||
<marker id="jve3-line" viewBox="0 0 10 10" refX="9" refY="5" markerWidth="6.5" markerHeight="6.5" orient="auto-start-reverse"><path class="nj-jsonview-c-line nj-jsonview-head" d="M0 1 L10 5 L0 9 z"/></marker>
|
||||
<marker id="jve3-own" viewBox="0 0 10 10" refX="9" refY="5" markerWidth="6.5" markerHeight="6.5" orient="auto-start-reverse"><path class="nj-jsonview-c-own nj-jsonview-head" d="M0 1 L10 5 L0 9 z"/></marker>
|
||||
<marker id="jve3-struct" viewBox="0 0 10 10" refX="9" refY="5" markerWidth="6.5" markerHeight="6.5" orient="auto-start-reverse"><path class="nj-jsonview-c-struct nj-jsonview-head" d="M0 1 L10 5 L0 9 z"/></marker>
|
||||
<marker id="jve3-bad" viewBox="0 0 10 10" refX="9" refY="5" markerWidth="6.5" markerHeight="6.5" orient="auto-start-reverse"><path class="nj-jsonview-c-bad nj-jsonview-head" d="M0 1 L10 5 L0 9 z"/></marker>
|
||||
</defs>
|
||||
<text class="nj-jsonview-note nj-jsonview-strong" x="16" y="22" xml:space="preserve">parsed index: fixed, never moves</text>
|
||||
<text class="nj-jsonview-note" x="824" y="22" text-anchor="end" xml:space="preserve">the source text is never written</text>
|
||||
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|
||||
<text class="nj-jsonview-tiny" x="42" y="47" xml:space="preserve">0</text>
|
||||
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|
||||
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|
||||
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|
||||
<text class="nj-jsonview-tiny" x="130" y="47" xml:space="preserve">1</text>
|
||||
<text class="nj-jsonview-kind nj-jsonview-xs nj-jsonview-c-key" x="166" y="61" text-anchor="middle" xml:space="preserve">key</text>
|
||||
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|
||||
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|
||||
<text class="nj-jsonview-tiny" x="218" y="47" xml:space="preserve">2</text>
|
||||
<text class="nj-jsonview-kind nj-jsonview-xs nj-jsonview-c-num" x="254" y="61" text-anchor="middle" xml:space="preserve">uint</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-sm" x="254" y="77" text-anchor="middle" xml:space="preserve">6380</text>
|
||||
<rect class="nj-jsonview-node nj-jsonview-c-key" x="300" y="34" width="84" height="50" rx="5"/>
|
||||
<text class="nj-jsonview-tiny" x="306" y="47" xml:space="preserve">3</text>
|
||||
<text class="nj-jsonview-kind nj-jsonview-xs nj-jsonview-c-key" x="342" y="61" text-anchor="middle" xml:space="preserve">key</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-sm" x="342" y="77" text-anchor="middle" xml:space="preserve">price</text>
|
||||
<rect class="nj-jsonview-node nj-jsonview-c-num" x="388" y="34" width="84" height="50" rx="5"/>
|
||||
<text class="nj-jsonview-tiny" x="394" y="47" xml:space="preserve">4</text>
|
||||
<text class="nj-jsonview-kind nj-jsonview-xs nj-jsonview-c-num" x="430" y="61" text-anchor="middle" xml:space="preserve">float</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-sm" x="430" y="77" text-anchor="middle" xml:space="preserve">19.90</text>
|
||||
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|
||||
<text class="nj-jsonview-tiny" x="482" y="47" xml:space="preserve">5</text>
|
||||
<text class="nj-jsonview-kind nj-jsonview-xs nj-jsonview-c-key" x="518" y="61" text-anchor="middle" xml:space="preserve">key</text>
|
||||
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|
||||
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|
||||
<text class="nj-jsonview-tiny" x="570" y="47" xml:space="preserve">6</text>
|
||||
<text class="nj-jsonview-kind nj-jsonview-xs nj-jsonview-c-struct" x="606" y="61" text-anchor="middle" xml:space="preserve">array</text>
|
||||
<text class="nj-jsonview-tiny nj-jsonview-c-struct nj-jsonview-strong" x="643" y="47" text-anchor="end" xml:space="preserve">moved</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-sm" x="606" y="77" text-anchor="middle" xml:space="preserve">[ ] 2</text>
|
||||
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|
||||
<text class="nj-jsonview-tiny" x="658" y="47" xml:space="preserve">7</text>
|
||||
<text class="nj-jsonview-kind nj-jsonview-xs nj-jsonview-c-str" x="694" y="61" text-anchor="middle" xml:space="preserve">string</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-sm" x="694" y="77" text-anchor="middle" xml:space="preserve">"eu"</text>
|
||||
<text class="nj-jsonview-tiny nj-jsonview-c-bad" x="694" y="98" text-anchor="middle" xml:space="preserve">unreferenced</text>
|
||||
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|
||||
<text class="nj-jsonview-tiny" x="746" y="47" xml:space="preserve">8</text>
|
||||
<text class="nj-jsonview-kind nj-jsonview-xs nj-jsonview-c-dec" x="782" y="61" text-anchor="middle" xml:space="preserve">string</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-sm" x="782" y="77" text-anchor="middle" xml:space="preserve">a"b</text>
|
||||
<text class="nj-jsonview-note nj-jsonview-strong nj-jsonview-c-own" x="16" y="288" xml:space="preserve">owned storage: allocated on demand, never moves</text>
|
||||
<rect class="nj-jsonview-store" x="10" y="192" width="820" height="54" rx="8"/>
|
||||
<rect class="nj-jsonview-node nj-jsonview-c-struct" x="16" y="196" width="54" height="46" rx="5"/>
|
||||
<text class="nj-jsonview-tiny" x="66" y="207" text-anchor="end" xml:space="preserve">0'</text>
|
||||
<text class="nj-jsonview-kind nj-jsonview-xs nj-jsonview-c-struct" x="43" y="220" text-anchor="middle" xml:space="preserve">header</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-xs" x="43" y="235" text-anchor="middle" xml:space="preserve">[ ] 2</text>
|
||||
<rect class="nj-jsonview-node nj-jsonview-c-own" x="73" y="196" width="54" height="46" rx="5"/>
|
||||
<text class="nj-jsonview-tiny" x="123" y="207" text-anchor="end" xml:space="preserve">1'</text>
|
||||
<text class="nj-jsonview-kind nj-jsonview-xs nj-jsonview-c-own" x="100" y="220" text-anchor="middle" xml:space="preserve">link</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-xs" x="100" y="235" text-anchor="middle" xml:space="preserve">→ 8</text>
|
||||
<path class="nj-jsonview-arr nj-jsonview-c-own" d="M100 196 C100 140 782 140 782 84" marker-end="url(#jve3-own)"/>
|
||||
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|
||||
<text class="nj-jsonview-tiny" x="180" y="207" text-anchor="end" xml:space="preserve">2'</text>
|
||||
<text class="nj-jsonview-kind nj-jsonview-xs nj-jsonview-c-own" x="157" y="220" text-anchor="middle" xml:space="preserve">link</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-xs" x="157" y="235" text-anchor="middle" xml:space="preserve">→ 3'</text>
|
||||
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|
||||
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|
||||
<text class="nj-jsonview-tiny" x="237" y="207" text-anchor="end" xml:space="preserve">3'</text>
|
||||
<text class="nj-jsonview-kind nj-jsonview-xs nj-jsonview-c-str" x="214" y="220" text-anchor="middle" xml:space="preserve">string</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-xs" x="214" y="235" text-anchor="middle" xml:space="preserve">"new"</text>
|
||||
<path class="nj-jsonview-arr nj-jsonview-c-struct nj-jsonview-dash" d="M598 84 C598 140 43 140 43 196" marker-end="url(#jve3-struct)"/>
|
||||
<text class="nj-jsonview-note" x="588" y="282" text-anchor="end" xml:space="preserve">edited text</text>
|
||||
<rect class="nj-jsonview-node nj-jsonview-c-str" x="596" y="264" width="42" height="26" rx="4"/>
|
||||
<text class="nj-jsonview-mono nj-jsonview-sm" x="602" y="282" xml:space="preserve">new</text>
|
||||
<path class="nj-jsonview-sep" d="M16 300 H824"/>
|
||||
<text class="nj-jsonview-mono nj-jsonview-xs" x="16" y="318" xml:space="preserve">view v = root()["tags"] (taken after parse):</text>
|
||||
<text class="nj-jsonview-note nj-jsonview-c-struct nj-jsonview-strong" x="320" y="318" xml:space="preserve">valid, sees every edit</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-xs" x="16" y="338" xml:space="preserve">iterator over tags (taken after parse):</text>
|
||||
<text class="nj-jsonview-note nj-jsonview-c-bad nj-jsonview-strong" x="320" y="338" xml:space="preserve">invalid: tags no longer uses the parsed layout</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-xs nj-jsonview-muted" x="16" y="360" xml:space="preserve">dump(): {"port":6380,"price":19.90,"tags":["a\"b","new"]}</text>
|
||||
</svg>
|
||||
|
||||
</div>
|
||||
<figcaption>Step 4 of 5. <code>doc.push_back(doc.root()["tags"], "new")</code>: the new string gets its own node, its text goes to the buffer of edited text, and a link to it is appended to the block of the array.</figcaption>
|
||||
</figure>
|
||||
<figure class="nj-jsonview-step" data-label="set a new member">
|
||||
<div class="nj-jsonview-scroll">
|
||||
<svg class="nj-fig nj-jsonview nj-jsonview-editfig" viewBox="0 0 840 372" role="img" aria-labelledby="fig-jve4-title fig-jve4-title-d" xmlns="http://www.w3.org/2000/svg">
|
||||
<title id="fig-jve4-title">Step 5 of 5: doc.set(doc.root(), "retries", 3)</title>
|
||||
<desc id="fig-jve4-title-d">set of a new member: the object moves to a block as well. The dump with number_format::source is {"port":6380,"price":19.90,"tags":["a\"b","new"],"retries":3}.</desc>
|
||||
<defs>
|
||||
<marker id="jve4-line" viewBox="0 0 10 10" refX="9" refY="5" markerWidth="6.5" markerHeight="6.5" orient="auto-start-reverse"><path class="nj-jsonview-c-line nj-jsonview-head" d="M0 1 L10 5 L0 9 z"/></marker>
|
||||
<marker id="jve4-own" viewBox="0 0 10 10" refX="9" refY="5" markerWidth="6.5" markerHeight="6.5" orient="auto-start-reverse"><path class="nj-jsonview-c-own nj-jsonview-head" d="M0 1 L10 5 L0 9 z"/></marker>
|
||||
<marker id="jve4-struct" viewBox="0 0 10 10" refX="9" refY="5" markerWidth="6.5" markerHeight="6.5" orient="auto-start-reverse"><path class="nj-jsonview-c-struct nj-jsonview-head" d="M0 1 L10 5 L0 9 z"/></marker>
|
||||
<marker id="jve4-bad" viewBox="0 0 10 10" refX="9" refY="5" markerWidth="6.5" markerHeight="6.5" orient="auto-start-reverse"><path class="nj-jsonview-c-bad nj-jsonview-head" d="M0 1 L10 5 L0 9 z"/></marker>
|
||||
</defs>
|
||||
<text class="nj-jsonview-note nj-jsonview-strong" x="16" y="22" xml:space="preserve">parsed index: fixed, never moves</text>
|
||||
<text class="nj-jsonview-note" x="824" y="22" text-anchor="end" xml:space="preserve">the source text is never written</text>
|
||||
<rect class="nj-jsonview-node nj-jsonview-c-struct" x="36" y="34" width="84" height="50" rx="5"/>
|
||||
<text class="nj-jsonview-tiny" x="42" y="47" xml:space="preserve">0</text>
|
||||
<text class="nj-jsonview-kind nj-jsonview-xs nj-jsonview-c-struct" x="78" y="61" text-anchor="middle" xml:space="preserve">object</text>
|
||||
<text class="nj-jsonview-tiny nj-jsonview-c-struct nj-jsonview-strong" x="115" y="47" text-anchor="end" xml:space="preserve">moved</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-sm" x="78" y="77" text-anchor="middle" xml:space="preserve">{ } 3</text>
|
||||
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|
||||
<text class="nj-jsonview-tiny" x="130" y="47" xml:space="preserve">1</text>
|
||||
<text class="nj-jsonview-kind nj-jsonview-xs nj-jsonview-c-key" x="166" y="61" text-anchor="middle" xml:space="preserve">key</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-sm" x="166" y="77" text-anchor="middle" xml:space="preserve">port</text>
|
||||
<rect class="nj-jsonview-node nj-jsonview-c-num" x="212" y="34" width="84" height="50" rx="5"/>
|
||||
<text class="nj-jsonview-tiny" x="218" y="47" xml:space="preserve">2</text>
|
||||
<text class="nj-jsonview-kind nj-jsonview-xs nj-jsonview-c-num" x="254" y="61" text-anchor="middle" xml:space="preserve">uint</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-sm" x="254" y="77" text-anchor="middle" xml:space="preserve">6380</text>
|
||||
<rect class="nj-jsonview-node nj-jsonview-c-key" x="300" y="34" width="84" height="50" rx="5"/>
|
||||
<text class="nj-jsonview-tiny" x="306" y="47" xml:space="preserve">3</text>
|
||||
<text class="nj-jsonview-kind nj-jsonview-xs nj-jsonview-c-key" x="342" y="61" text-anchor="middle" xml:space="preserve">key</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-sm" x="342" y="77" text-anchor="middle" xml:space="preserve">price</text>
|
||||
<rect class="nj-jsonview-node nj-jsonview-c-num" x="388" y="34" width="84" height="50" rx="5"/>
|
||||
<text class="nj-jsonview-tiny" x="394" y="47" xml:space="preserve">4</text>
|
||||
<text class="nj-jsonview-kind nj-jsonview-xs nj-jsonview-c-num" x="430" y="61" text-anchor="middle" xml:space="preserve">float</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-sm" x="430" y="77" text-anchor="middle" xml:space="preserve">19.90</text>
|
||||
<rect class="nj-jsonview-node nj-jsonview-c-key" x="476" y="34" width="84" height="50" rx="5"/>
|
||||
<text class="nj-jsonview-tiny" x="482" y="47" xml:space="preserve">5</text>
|
||||
<text class="nj-jsonview-kind nj-jsonview-xs nj-jsonview-c-key" x="518" y="61" text-anchor="middle" xml:space="preserve">key</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-sm" x="518" y="77" text-anchor="middle" xml:space="preserve">tags</text>
|
||||
<rect class="nj-jsonview-node nj-jsonview-c-struct" x="564" y="34" width="84" height="50" rx="5"/>
|
||||
<text class="nj-jsonview-tiny" x="570" y="47" xml:space="preserve">6</text>
|
||||
<text class="nj-jsonview-kind nj-jsonview-xs nj-jsonview-c-struct" x="606" y="61" text-anchor="middle" xml:space="preserve">array</text>
|
||||
<text class="nj-jsonview-tiny nj-jsonview-c-struct nj-jsonview-strong" x="643" y="47" text-anchor="end" xml:space="preserve">moved</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-sm" x="606" y="77" text-anchor="middle" xml:space="preserve">[ ] 2</text>
|
||||
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|
||||
<text class="nj-jsonview-tiny" x="658" y="47" xml:space="preserve">7</text>
|
||||
<text class="nj-jsonview-kind nj-jsonview-xs nj-jsonview-c-str" x="694" y="61" text-anchor="middle" xml:space="preserve">string</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-sm" x="694" y="77" text-anchor="middle" xml:space="preserve">"eu"</text>
|
||||
<text class="nj-jsonview-tiny nj-jsonview-c-bad" x="694" y="98" text-anchor="middle" xml:space="preserve">unreferenced</text>
|
||||
<rect class="nj-jsonview-node nj-jsonview-c-dec" x="740" y="34" width="84" height="50" rx="5"/>
|
||||
<text class="nj-jsonview-tiny" x="746" y="47" xml:space="preserve">8</text>
|
||||
<text class="nj-jsonview-kind nj-jsonview-xs nj-jsonview-c-dec" x="782" y="61" text-anchor="middle" xml:space="preserve">string</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-sm" x="782" y="77" text-anchor="middle" xml:space="preserve">a"b</text>
|
||||
<text class="nj-jsonview-note nj-jsonview-strong nj-jsonview-c-own" x="16" y="288" xml:space="preserve">owned storage: allocated on demand, never moves</text>
|
||||
<rect class="nj-jsonview-store" x="10" y="192" width="820" height="54" rx="8"/>
|
||||
<rect class="nj-jsonview-node nj-jsonview-c-struct" x="16" y="196" width="54" height="46" rx="5"/>
|
||||
<text class="nj-jsonview-tiny" x="66" y="207" text-anchor="end" xml:space="preserve">0'</text>
|
||||
<text class="nj-jsonview-kind nj-jsonview-xs nj-jsonview-c-struct" x="43" y="220" text-anchor="middle" xml:space="preserve">header</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-xs" x="43" y="235" text-anchor="middle" xml:space="preserve">[ ] 2</text>
|
||||
<rect class="nj-jsonview-node nj-jsonview-c-own" x="73" y="196" width="54" height="46" rx="5"/>
|
||||
<text class="nj-jsonview-tiny" x="123" y="207" text-anchor="end" xml:space="preserve">1'</text>
|
||||
<text class="nj-jsonview-kind nj-jsonview-xs nj-jsonview-c-own" x="100" y="220" text-anchor="middle" xml:space="preserve">link</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-xs" x="100" y="235" text-anchor="middle" xml:space="preserve">→ 8</text>
|
||||
<path class="nj-jsonview-arr nj-jsonview-c-own" d="M100 196 C100 140 782 140 782 84" marker-end="url(#jve4-own)"/>
|
||||
<rect class="nj-jsonview-node nj-jsonview-c-own" x="130" y="196" width="54" height="46" rx="5"/>
|
||||
<text class="nj-jsonview-tiny" x="180" y="207" text-anchor="end" xml:space="preserve">2'</text>
|
||||
<text class="nj-jsonview-kind nj-jsonview-xs nj-jsonview-c-own" x="157" y="220" text-anchor="middle" xml:space="preserve">link</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-xs" x="157" y="235" text-anchor="middle" xml:space="preserve">→ 3'</text>
|
||||
<path class="nj-jsonview-arr nj-jsonview-c-own" d="M157 242 C157 268 214 268 214 244" marker-end="url(#jve4-own)"/>
|
||||
<rect class="nj-jsonview-node nj-jsonview-c-str" x="187" y="196" width="54" height="46" rx="5"/>
|
||||
<text class="nj-jsonview-tiny" x="237" y="207" text-anchor="end" xml:space="preserve">3'</text>
|
||||
<text class="nj-jsonview-kind nj-jsonview-xs nj-jsonview-c-str" x="214" y="220" text-anchor="middle" xml:space="preserve">string</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-xs" x="214" y="235" text-anchor="middle" xml:space="preserve">"new"</text>
|
||||
<rect class="nj-jsonview-node nj-jsonview-hot nj-jsonview-c-num" x="244" y="196" width="54" height="46" rx="5"/>
|
||||
<text class="nj-jsonview-tiny" x="294" y="207" text-anchor="end" xml:space="preserve">4'</text>
|
||||
<text class="nj-jsonview-kind nj-jsonview-xs nj-jsonview-c-num" x="271" y="220" text-anchor="middle" xml:space="preserve">uint</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-xs" x="271" y="235" text-anchor="middle" xml:space="preserve">3</text>
|
||||
<rect class="nj-jsonview-node nj-jsonview-hot nj-jsonview-c-struct" x="301" y="196" width="54" height="46" rx="5"/>
|
||||
<text class="nj-jsonview-tiny" x="351" y="207" text-anchor="end" xml:space="preserve">5'</text>
|
||||
<text class="nj-jsonview-kind nj-jsonview-xs nj-jsonview-c-struct" x="328" y="220" text-anchor="middle" xml:space="preserve">header</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-xs" x="328" y="235" text-anchor="middle" xml:space="preserve">{ } 4</text>
|
||||
<rect class="nj-jsonview-node nj-jsonview-hot nj-jsonview-c-key" x="358" y="196" width="54" height="46" rx="5"/>
|
||||
<text class="nj-jsonview-tiny" x="408" y="207" text-anchor="end" xml:space="preserve">6'</text>
|
||||
<text class="nj-jsonview-kind nj-jsonview-xs nj-jsonview-c-key" x="385" y="220" text-anchor="middle" xml:space="preserve">key</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-xs" x="385" y="235" text-anchor="middle" xml:space="preserve">port</text>
|
||||
<rect class="nj-jsonview-node nj-jsonview-hot nj-jsonview-c-own" x="415" y="196" width="54" height="46" rx="5"/>
|
||||
<text class="nj-jsonview-tiny" x="465" y="207" text-anchor="end" xml:space="preserve">7'</text>
|
||||
<text class="nj-jsonview-kind nj-jsonview-xs nj-jsonview-c-own" x="442" y="220" text-anchor="middle" xml:space="preserve">link</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-xs" x="442" y="235" text-anchor="middle" xml:space="preserve">→ 2</text>
|
||||
<path class="nj-jsonview-arr nj-jsonview-c-own" d="M442 196 C442 140 254 140 254 84" marker-end="url(#jve4-own)"/>
|
||||
<rect class="nj-jsonview-node nj-jsonview-hot nj-jsonview-c-key" x="472" y="196" width="54" height="46" rx="5"/>
|
||||
<text class="nj-jsonview-tiny" x="522" y="207" text-anchor="end" xml:space="preserve">8'</text>
|
||||
<text class="nj-jsonview-kind nj-jsonview-xs nj-jsonview-c-key" x="499" y="220" text-anchor="middle" xml:space="preserve">key</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-xs" x="499" y="235" text-anchor="middle" xml:space="preserve">price</text>
|
||||
<rect class="nj-jsonview-node nj-jsonview-hot nj-jsonview-c-own" x="529" y="196" width="54" height="46" rx="5"/>
|
||||
<text class="nj-jsonview-tiny" x="579" y="207" text-anchor="end" xml:space="preserve">9'</text>
|
||||
<text class="nj-jsonview-kind nj-jsonview-xs nj-jsonview-c-own" x="556" y="220" text-anchor="middle" xml:space="preserve">link</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-xs" x="556" y="235" text-anchor="middle" xml:space="preserve">→ 4</text>
|
||||
<path class="nj-jsonview-arr nj-jsonview-c-own" d="M556 196 C556 140 430 140 430 84" marker-end="url(#jve4-own)"/>
|
||||
<rect class="nj-jsonview-node nj-jsonview-hot nj-jsonview-c-key" x="586" y="196" width="54" height="46" rx="5"/>
|
||||
<text class="nj-jsonview-tiny" x="636" y="207" text-anchor="end" xml:space="preserve">10'</text>
|
||||
<text class="nj-jsonview-kind nj-jsonview-xs nj-jsonview-c-key" x="613" y="220" text-anchor="middle" xml:space="preserve">key</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-xs" x="613" y="235" text-anchor="middle" xml:space="preserve">tags</text>
|
||||
<rect class="nj-jsonview-node nj-jsonview-hot nj-jsonview-c-own" x="643" y="196" width="54" height="46" rx="5"/>
|
||||
<text class="nj-jsonview-tiny" x="693" y="207" text-anchor="end" xml:space="preserve">11'</text>
|
||||
<text class="nj-jsonview-kind nj-jsonview-xs nj-jsonview-c-own" x="670" y="220" text-anchor="middle" xml:space="preserve">link</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-xs" x="670" y="235" text-anchor="middle" xml:space="preserve">→ 6</text>
|
||||
<path class="nj-jsonview-arr nj-jsonview-c-own" d="M670 196 C670 140 614 140 614 84" marker-end="url(#jve4-own)"/>
|
||||
<rect class="nj-jsonview-node nj-jsonview-hot nj-jsonview-c-key" x="700" y="196" width="54" height="46" rx="5"/>
|
||||
<text class="nj-jsonview-tiny" x="750" y="207" text-anchor="end" xml:space="preserve">12'</text>
|
||||
<text class="nj-jsonview-kind nj-jsonview-xs nj-jsonview-c-key" x="727" y="220" text-anchor="middle" xml:space="preserve">key</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-xs" x="727" y="235" text-anchor="middle" xml:space="preserve">retries</text>
|
||||
<rect class="nj-jsonview-node nj-jsonview-hot nj-jsonview-c-own" x="757" y="196" width="54" height="46" rx="5"/>
|
||||
<text class="nj-jsonview-tiny" x="807" y="207" text-anchor="end" xml:space="preserve">13'</text>
|
||||
<text class="nj-jsonview-kind nj-jsonview-xs nj-jsonview-c-own" x="784" y="220" text-anchor="middle" xml:space="preserve">link</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-xs" x="784" y="235" text-anchor="middle" xml:space="preserve">→ 4'</text>
|
||||
<path class="nj-jsonview-arr nj-jsonview-c-own" d="M784 242 C784 268 271 268 271 244" marker-end="url(#jve4-own)"/>
|
||||
<path class="nj-jsonview-arr nj-jsonview-c-struct nj-jsonview-dash" d="M598 84 C598 140 43 140 43 196" marker-end="url(#jve4-struct)"/>
|
||||
<path class="nj-jsonview-arr nj-jsonview-c-struct nj-jsonview-dash" d="M70 84 C70 140 328 140 328 196" marker-end="url(#jve4-struct)"/>
|
||||
<text class="nj-jsonview-note" x="588" y="282" text-anchor="end" xml:space="preserve">edited text</text>
|
||||
<rect class="nj-jsonview-node nj-jsonview-c-str" x="596" y="264" width="112" height="26" rx="4"/>
|
||||
<text class="nj-jsonview-mono nj-jsonview-sm" x="602" y="282" xml:space="preserve">newretries</text>
|
||||
<path class="nj-jsonview-sep" d="M16 300 H824"/>
|
||||
<text class="nj-jsonview-mono nj-jsonview-xs" x="16" y="318" xml:space="preserve">view v = root()["tags"] (taken after parse):</text>
|
||||
<text class="nj-jsonview-note nj-jsonview-c-struct nj-jsonview-strong" x="320" y="318" xml:space="preserve">valid, sees every edit</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-xs" x="16" y="338" xml:space="preserve">iterator over tags (taken after parse):</text>
|
||||
<text class="nj-jsonview-note nj-jsonview-c-bad nj-jsonview-strong" x="320" y="338" xml:space="preserve">invalid: tags no longer uses the parsed layout</text>
|
||||
<text class="nj-jsonview-mono nj-jsonview-xs nj-jsonview-muted" x="16" y="360" xml:space="preserve">dump(): {"port":6380,"price":19.90,"tags":["a\"b","new"],"retries":3}</text>
|
||||
</svg>
|
||||
|
||||
</div>
|
||||
<figcaption>Step 5 of 5. <code>doc.set(doc.root(), "retries", 3)</code>: a new member, so the object moves to a block too: copies of its keys, links to the values it already had, then the new key and a link to the new integer.</figcaption>
|
||||
</figure>
|
||||
</div>
|
||||
|
||||
<p class="nj-caption" markdown>The node index and the owned storage of the document after each of five edits. Use *Back*, *Next*, or the numbered buttons to switch between the steps; without JavaScript all steps are shown one below the other.</p>
|
||||
|
||||
## Images
|
||||
|
||||
[`save()`](../api/basic_json_document/save.md) writes a document as an *image*: a byte buffer that
|
||||
[`load()`](../api/basic_json_document/load.md) reads back into a document without parsing -- no lexing, no building
|
||||
[`load()`](../api/basic_json_document/load.md) reads back into a document without parsing — no lexing, no building
|
||||
the node index, nothing but copying the nodes and pointing the text and the decoded strings at the image. Where
|
||||
[`parse_copy()`](../api/basic_json_document/parse_copy.md) still has to scan the whole input,
|
||||
[`load()`](../api/basic_json_document/load.md) turns that scan into a copy of the node index alone.
|
||||
|
||||
**Why.** A document that is parsed once and then read many times -- a configuration loaded at startup, a template
|
||||
rendered on every request, a large reference dataset a worker process needs in memory -- pays for parsing once but
|
||||
**Why.** A document that is parsed once and then read many times — a configuration loaded at startup, a template
|
||||
rendered on every request, a large reference dataset a worker process needs in memory — pays for parsing once but
|
||||
can amortize [`save()`](../api/basic_json_document/save.md)'s cost across every later load. That makes images useful
|
||||
for a cache: save a document the first time it is parsed (to a file, a shared-memory segment, an in-process cache),
|
||||
and [`load()`](../api/basic_json_document/load.md) it on every later use instead of parsing the source text again.
|
||||
@@ -513,10 +1217,10 @@ plus pointers into the received bytes, not a re-parse.
|
||||
**Choosing a check.** [`load()`](../api/basic_json_document/load.md) takes an
|
||||
[`image_check`](../api/basic_json_document/load.md#image_check) that trades validation against speed:
|
||||
`image_check::full` (the default) checks everything the parser itself guarantees, so a checked image is exactly as
|
||||
safe to read and serialize as a freshly parsed document -- the right choice whenever the image did not come straight
|
||||
safe to read and serialize as a freshly parsed document — the right choice whenever the image did not come straight
|
||||
from this process's own [`save()`](../api/basic_json_document/save.md), such as a file or a network peer.
|
||||
`image_check::bounds` only checks structure and bounds -- cheaper, since it skips scanning the text and the decoded
|
||||
strings -- and fits a cache the process trusts, one it wrote and reads back itself. `image_check::none` skips
|
||||
`image_check::bounds` only checks structure and bounds — cheaper, since it skips scanning the text and the decoded
|
||||
strings — and fits a cache the process trusts, one it wrote and reads back itself. `image_check::none` skips
|
||||
validation entirely, for an image trusted as much as the process's own memory. See
|
||||
[`load()`'s Notes](../api/basic_json_document/load.md#notes) for exactly what each level does and does not guarantee.
|
||||
|
||||
@@ -569,7 +1273,7 @@ validation entirely, for an image trusted as much as the process's own memory. S
|
||||
|
||||
The image format is versioned but not yet stable, and may change in an incompatible way before it is declared
|
||||
stable. It is little-endian only (on big-endian targets, `save()` and `load()` do not exist), and tied to the
|
||||
library build that wrote it -- use it to cache a document or to hand one to another process running the *same*
|
||||
library build that wrote it — use it to cache a document or to hand one to another process running the *same*
|
||||
build, not as a long-term storage format; keep the original JSON text if a saved document needs to be readable by
|
||||
a future library version.
|
||||
|
||||
@@ -585,8 +1289,21 @@ either.
|
||||
| **Ownership** | owns every value | owns nothing; you decide what to keep, in your handler | borrows or owns the *text*; the index is always owned by the document | same as `json_document`; edits go to storage the document owns |
|
||||
| **Mutability** | freely mutable | not applicable (a one-shot event stream) | read-only | [`set`](../api/basic_json_document/set.md)/[`push_back`](../api/basic_json_document/push_back.md)/[`insert`](../api/basic_json_document/insert.md)/[`erase`](../api/basic_json_document/erase.md) edit in place; the source text is never rewritten |
|
||||
| **What you get** | a full tree you can read, write, and keep as long as you like | a sequence of callbacks; whatever your handler builds from them | a flat index plus, on demand, [`materialize()`](../api/basic_json_view/materialize.md)d `json`/`ordered_json` values for the parts you actually use | the same, plus [`dump()`](../api/basic_json_view/dump.md) of an edited document that keeps the member order and, with [`number_format::source`](../api/basic_json_view/number_format.md), the spelling of every untouched number |
|
||||
| **Typical use** | general-purpose JSON handling: config, request/response bodies you build or modify, anything you hold onto | validating or projecting a text into your own data structure without ever holding the whole thing as JSON | large or high-volume input where you only need part of it, or need it repeatedly, and can keep the source text (or a copy) alive for as long as the document lives | a document you read, patch a few fields of, and write back -- a configuration file, for instance -- where the rest of it should come back exactly as it was |
|
||||
| **Caching/reload** | not applicable -- re-parse, or roll your own serialization | not applicable | [`save()`](../api/basic_json_document/save.md)/[`load()`](../api/basic_json_document/load.md): cache the parsed index as an image and reload it without parsing | same, saving the document's current -- possibly edited -- state |
|
||||
| **Typical use** | general-purpose JSON handling: config, request/response bodies you build or modify, anything you hold onto | validating or projecting a text into your own data structure without ever holding the whole thing as JSON | large or high-volume input where you only need part of it, or need it repeatedly, and can keep the source text (or a copy) alive for as long as the document lives | a document you read, patch a few fields of, and write back — a configuration file, for instance — where the rest of it should come back exactly as it was |
|
||||
| **Caching/reload** | not applicable — re-parse, or roll your own serialization | not applicable | [`save()`](../api/basic_json_document/save.md)/[`load()`](../api/basic_json_document/load.md): cache the parsed index as an image and reload it without parsing | same, saving the document's current — possibly edited — state |
|
||||
|
||||
<p class="nj-table-caption" markdown>Each column is one way to work with JSON in this library, and each row compares one property of them.</p>
|
||||
|
||||
## See also
|
||||
|
||||
- [`basic_json_document`](../api/basic_json_document/index.md) - the document that owns the parsed index (parse, access, images, edits)
|
||||
- [`basic_json_view`](../api/basic_json_view/index.md) - the non-owning handle to a value in a document
|
||||
- [`json_document`](../api/json_document.md) - and [`ordered_json_document`](../api/ordered_json_document.md) - the read-only documents
|
||||
- [`json_editable_document`](../api/json_editable_document.md) - and [`ordered_json_editable_document`](../api/ordered_json_editable_document.md) - the documents that can be edited in place
|
||||
- [Architecture](../home/architecture.md#node-index-of-json-views) - the layout of the node index
|
||||
- [SAX interface](parsing/sax_interface.md) - event-based parsing without building a tree
|
||||
- [Performance](performance.md) - benchmarks and tuning of parsing and serialization
|
||||
- [Large, Streamed, and Partial Input](parsing/large_and_partial_input.md) - reading only the part of a text you need
|
||||
|
||||
## Version history
|
||||
|
||||
|
||||
@@ -1,4 +1,4 @@
|
||||
<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/json_view/" rel="canonical"><link href=../types/template_parameters/ rel=prev><link href=../../integration/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>Zero-copy JSON views - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.ab4e12ef.min.css><link rel="stylesheet" href="../../assets/external/fonts.googleapis.com/css.61a430c9.css"><style>:root{--md-text-font:"Roboto";--md-code-font:"JetBrains Mono"}</style><link rel=stylesheet href=../../css/custom.css><script>__md_scope=new 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<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=description content="JSON for Modern C++ is a header-only C++11 library that makes JSON a first-class data type."><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/json_view/" rel="canonical"><link href=../types/template_parameters/ rel=prev><link href=../../integration/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>Zero-copy JSON views - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.ab4e12ef.min.css><link rel=stylesheet href=../../css/custom.css><link rel=stylesheet href=../../css/brand.css><link rel=stylesheet href=../../css/fig-sax.css><link rel=stylesheet href=../../css/fig-jsonview.css><link 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<div data-md-component=announce> </div> <header class=md-header data-md-component=header> <nav class="md-header__inner md-grid" aria-label=Header> <a href=../.. title="JSON for Modern C++" class="md-header__button md-logo" aria-label="JSON for Modern C++" data-md-component=logo> <img src=../../images/wordmark-header.svg alt=logo> </a> <label class="md-header__button md-icon" for=__drawer> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M3 6h18v2H3zm0 5h18v2H3zm0 5h18v2H3z"/></svg> </label> <div class=md-header__title data-md-component=header-title> <div class=md-header__ellipsis> <div class=md-header__topic> <span class=md-ellipsis> JSON for Modern C++ </span> </div> <div class=md-header__topic data-md-component=header-topic> <span class=md-ellipsis> Zero-copy JSON views </span> </div> </div> </div> <form class=md-header__option data-md-component=palette> <input class=md-option data-md-color-media="(prefers-color-scheme: light)" data-md-color-scheme=default 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12s-1.44-4.5-3.5-5.45C10.26 6.2 11.11 6 12 6a6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> </form> <script>var palette=__md_get("__palette");if(palette&&palette.color){if("(prefers-color-scheme)"===palette.color.media){var media=matchMedia("(prefers-color-scheme: light)"),input=document.querySelector(media.matches?"[data-md-color-media='(prefers-color-scheme: light)']":"[data-md-color-media='(prefers-color-scheme: dark)']");palette.color.media=input.getAttribute("data-md-color-media"),palette.color.scheme=input.getAttribute("data-md-color-scheme"),palette.color.primary=input.getAttribute("data-md-color-primary"),palette.color.accent=input.getAttribute("data-md-color-accent")}for(var[key,value]of Object.entries(palette.color))document.body.setAttribute("data-md-color-"+key,value)}</script> <label class="md-header__button md-icon" for=__search> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M9.5 3A6.5 6.5 0 0 1 16 9Line truncated
|
||||
<span class=cp>#include</span><span class=w> </span><span class=cpf><nlohmann/json_view.hpp></span>
|
||||
|
||||
<span class=k>using</span><span class=w> </span><span class=n>nlohmann</span><span class=o>::</span><span class=n>json_document</span><span class=p>;</span>
|
||||
@@ -16,7 +16,7 @@
|
||||
<span class=p>}</span>
|
||||
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code><span class=kc>true</span>
|
||||
<span class=p>{</span><span class=nt>"numbers"</span><span class=p>:[</span><span class=mi>1</span><span class=p>,</span><span class=mi>2</span><span class=p>,</span><span class=mi>3</span><span class=p>],</span><span class=nt>"pi"</span><span class=p>:</span><span class=mf>3.14</span><span class=p>}</span>
|
||||
</code></pre></div> </details> <p><a href=../../api/ordered_json_document/ ><code>ordered_json_document</code></a> is the equivalent for <a href=../../api/ordered_json/ ><code>ordered_json</code></a>, just as <a href=../../api/ordered_json/ ><code>ordered_json</code></a> is to <a href=../../api/json/ ><code>json</code></a>.</p> <h2 id=ownership-and-lifetime>Ownership and lifetime<a class=headerlink href=#ownership-and-lifetime title="Permanent link">¶</a></h2> <p>A document either <strong>borrows</strong> the text it was parsed from, or <strong>owns</strong> its own copy of it; call <a href=../../api/basic_json_document/owns_source/ ><code>owns_source()</code></a> to find out which happened. <a href=../../api/basic_json_document/parse/ ><code>parse()</code></a> decides this from the value category and type of its argument (an lvalue <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>string</span></code> is borrowed; an rvalue <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>string</span></code> is moved in, owned without a copy; a stream is read into an owned buffer; and so on -- see <a href=../../api/basic_json_document/parse/#notes><code>parse</code>'s Notes</a> for the full table). <a href=../../api/basic_json_document/parse_copy/ ><code>parse_copy()</code></a> always owns a copy, regardless of the input.</p> <div class="admonition warning"> <p class=admonition-title>A borrowed document depends on your buffer</p> <p>If a document borrows its text, that text <strong>must outlive the document</strong> (and every view taken from it) and must stay <strong>unmodified</strong> for as long as the document exists, since floating-point values and unescaped strings are read from it lazily. Reading or writing through a view after the underlying buffer is gone is undefined behavior, exactly as it would be for a dangling <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>string_view</span></code>.</p> </div> <p>A view is valid only while all of the following hold:</p> <ul> <li>the document is alive,</li> <li>the document has not been re-parsed since the view was taken (with <a href=../../api/basic_json_document/read/ ><code>read()</code></a> or <a href=../../api/basic_json_document/parse/ ><code>parse()</code></a> into it),</li> <li>the document has not had <a href=../../api/basic_json_document/shrink_to_fit/ ><code>shrink_to_fit()</code></a> called on it since, and</li> <li>if the document borrows its source text, that text is still alive and unmodified.</li> </ul> <p>Moving the <code>basic_json_document<></code> itself is fine and does <strong>not</strong> invalidate its views: the index is a separate heap allocation that keeps its address across the move. Take a fresh view from <a href=../../api/basic_json_document/root/ ><code>root()</code></a> whenever any of the other conditions above was not met.</p> <p>A view can dangle, exactly like a <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>string_view</span></code>: nothing stops you from using it after its document is gone. This is deliberate. A view is two pointers and trivially copyable; making it safe (a <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>shared_ptr</span></code> in the document and a <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>weak_ptr</span></code> in each view) would make it larger, add reference counting to every copy, and a <code>lock()</code> to every access -- exactly the costs the view exists to avoid. What is caught is the obvious case: because a view dies with its document, <a href=../../api/basic_json_document/root/ ><code>root()</code></a> is not callable on a temporary document: <code class=highlight><span class=k>auto</span><span class=w> </span><span class=n>v</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>json_document</span><span class=o>::</span><span class=n>parse</span><span class=p>(</span><span class=n>text</span><span class=p>).</span><span class=n>root</span><span class=p>();</span></code> does not compile. Give the document a name first.</p> <details class=example> <summary>Example: borrowed and owned documents, and when views become invalid</summary> <div class=highlight><pre><span></span><code><span class=cp>#include</span><span class=w> </span><span class=cpf><iostream></span>
|
||||
</code></pre></div> </details> <p><a href=../../api/ordered_json_document/ ><code>ordered_json_document</code></a> is the equivalent for <a href=../../api/ordered_json/ ><code>ordered_json</code></a>, just as <a href=../../api/ordered_json/ ><code>ordered_json</code></a> is to <a href=../../api/json/ ><code>json</code></a>.</p> <h2 id=ownership-and-lifetime>Ownership and lifetime<a class=headerlink href=#ownership-and-lifetime title="Permanent link">¶</a></h2> <p>A document either <strong>borrows</strong> the text it was parsed from, or <strong>owns</strong> its own copy of it; call <a href=../../api/basic_json_document/owns_source/ ><code>owns_source()</code></a> to find out which happened. <a href=../../api/basic_json_document/parse/ ><code>parse()</code></a> decides this from the value category and type of its argument (an lvalue <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>string</span></code> is borrowed; an rvalue <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>string</span></code> is moved in, owned without a copy; a stream is read into an owned buffer; and so on — see <a href=../../api/basic_json_document/parse/#notes><code>parse</code>'s Notes</a> for the full table). <a href=../../api/basic_json_document/parse_copy/ ><code>parse_copy()</code></a> always owns a copy, regardless of the input.</p> <div class="admonition warning"> <p class=admonition-title>A borrowed document depends on your buffer</p> <p>If a document borrows its text, that text <strong>must outlive the document</strong> (and every view taken from it) and must stay <strong>unmodified</strong> for as long as the document exists, since floating-point values and unescaped strings are read from it lazily. Reading or writing through a view after the underlying buffer is gone is undefined behavior, exactly as it would be for a dangling <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>string_view</span></code>.</p> </div> <p>A view is valid only while all of the following hold:</p> <ul> <li>the document is alive,</li> <li>the document has not been re-parsed since the view was taken (with <a href=../../api/basic_json_document/read/ ><code>read()</code></a> or <a href=../../api/basic_json_document/parse/ ><code>parse()</code></a> into it),</li> <li>the document has not had <a href=../../api/basic_json_document/shrink_to_fit/ ><code>shrink_to_fit()</code></a> called on it since, and</li> <li>if the document borrows its source text, that text is still alive and unmodified.</li> </ul> <p>Moving the <code>basic_json_document<></code> itself is fine and does <strong>not</strong> invalidate its views: the index is a separate heap allocation that keeps its address across the move. Take a fresh view from <a href=../../api/basic_json_document/root/ ><code>root()</code></a> whenever any of the other conditions above was not met.</p> <div class=nj-fig-wrap> <p><svg class="nj-fig nj-jsonview " viewbox="0 0 840 572" role=img aria-labelledby="fig-jvt-title fig-jvt-title-d" xmlns=http://www.w3.org/2000/svg> <title id=fig-jvt-title>When a json_view becomes invalid</title> <desc id=fig-jvt-title-d>Four timelines. First: a document borrows a std::string; when the string is destroyed, the document and every view taken from it dangle. Second: doc.read() re-parses the document; views taken before are invalid, a fresh view from root() is valid. Third: doc.shrink_to_fit() has the same effect. Fourth: moving the document does not invalidate its views; destroying the moved-to document does.</desc> <defs> <marker id=jvt-bad viewbox="0 0 10 10" refx=9 refy=5 markerwidth=6.5 markerheight=6.5 orient=auto-start-reverse><path class="nj-jsonview-c-bad nj-jsonview-head" d="M0 1 L10 5 L0 9 z"/></marker> </defs> <rect x=210 y=12 width=34 height=12 rx=3 class="nj-jsonview-bar nj-jsonview-c-struct"/> <text class=nj-jsonview-note x=252 y=23 xml:space=preserve>alive / valid</text> <rect class=nj-jsonview-bad x=370 y=12 width=34 height=12 rx=3 /> <text class=nj-jsonview-note x=412 y=23 xml:space=preserve>dangling: using it is undefined behavior</text> <path class="nj-jsonview-arr nj-jsonview-c-line" d="M210 44 L820 44"/> <text class=nj-jsonview-tiny x=820 y=38 text-anchor=end xml:space=preserve>time →</text> <text class="nj-jsonview-h nj-jsonview-sm2" x=16 y=74 xml:space=preserve>The borrowed text is destroyed</text> <text class="nj-jsonview-mono nj-jsonview-sm" x=198 y=99 text-anchor=end xml:space=preserve>std::string text</text> <rect x=210 y=87 width=260 height=16 rx=3 class="nj-jsonview-bar nj-jsonview-c-struct"/> <text class="nj-jsonview-mono nj-jsonview-sm" x=198 y=127 text-anchor=end xml:space=preserve>json_document doc</text> <rect x=270 y=115 width=200 height=16 rx=3 class="nj-jsonview-bar nj-jsonview-c-struct"/> <rect class=nj-jsonview-bad x=470 y=115 width=350 height=16 rx=3 /> <text class="nj-jsonview-mono nj-jsonview-sm" x=198 y=155 text-anchor=endLine truncated
|
||||
<span class=cp>#include</span><span class=w> </span><span class=cpf><string></span>
|
||||
<span class=cp>#include</span><span class=w> </span><span class=cpf><utility></span>
|
||||
<span class=cp>#include</span><span class=w> </span><span class=cpf><nlohmann/json_view.hpp></span>
|
||||
@@ -53,7 +53,7 @@
|
||||
<span class=kc>true</span>
|
||||
<span class=kc>true</span>
|
||||
<span class=kc>true</span>
|
||||
</code></pre></div> </details> <h2 id=what-is-the-same-as-parse>What is the same as <code>parse()</code><a class=headerlink href=#what-is-the-same-as-parse title="Permanent link">¶</a></h2> <ul> <li><strong>Accept/reject.</strong> <a href=../../api/basic_json_document/accept/ ><code>accept()</code></a> and <a href=../../api/basic_json_document/parse/ ><code>parse()</code></a> accept and reject exactly the same inputs as <a href=../../api/basic_json/accept/ ><code>basic_json::accept()</code></a>/<a href=../../api/basic_json/parse/ ><code>basic_json::parse()</code></a>, with the same <code>ignore_comments</code> and <code>ignore_trailing_commas</code> options.</li> <li><strong>Errors.</strong> A failing parse throws the same exception -- the same id, message, and position -- because on failure the library's own parser is run on the same bytes to produce the diagnostic. Valid input never pays for this, and the replay builds no values: it costs about a lexing pass up to the error. <code class=highlight><span class=n>allow_exceptions</span><span class=w> </span><span class=o>==</span><span class=w> </span><span class=nb>false</span></code> skips the replay and gives a <a href=../../api/basic_json_document/is_discarded/ >discarded</a> document instead of throwing, just as it gives a discarded value for <code class=highlight><span class=n>basic_json</span><span class=o>::</span><span class=n>parse</span><span class=p>()</span></code>.</li> <li><strong>Number classification.</strong> An integer literal that does not fit into the 64-bit integer type becomes a floating-point value, exactly as it does for <code class=highlight><span class=n>basic_json</span><span class=o>::</span><span class=n>parse</span><span class=p>()</span></code>.</li> <li><strong>Macros.</strong> <a href=../../api/macros/json_strict_nul_handling/ ><code>JSON_STRICT_NUL_HANDLING</code></a> and <a href=../../api/macros/json_noexception/ ><code>JSON_NOEXCEPTION</code></a>/<a href=../../api/macros/json_throw_user/ ><code>JSON_THROW_USER</code></a> behave the same way they do for <code><nlohmann/json.hpp></code>. These are the macros that change what the parser accepts or how it reports errors. The others either do not apply (<a href=../../api/macros/json_diagnostics/ ><code>JSON_DIAGNOSTICS</code></a> needs parent pointers in <code>basic_json</code> values) or only affect the values <code>materialize()</code> produces, through <code>BasicJsonType</code>.</li> </ul> <h2 id=what-is-different>What is different<a class=headerlink href=#what-is-different title="Permanent link">¶</a></h2> <ul> <li><strong>Only 64-bit integers.</strong> <code>basic_json_document<BasicJsonType></code> requires <code>BasicJsonType::number_integer_t</code> and <code>number_unsigned_t</code> to both be 64 bits wide; this is a compile-time <code class=highlight><span class=k>static_assert</span></code>.</li> <li><strong>A 4 <abbr title="Gibibyte (2^30 bytes)">GiB</abbr> input limit.</strong> An input of 4294967280 bytes (4 <abbr title="Gibibyte (2^30 bytes)">GiB</abbr> minus 16 bytes) or more throws <a href=../../home/exceptions/#jsonexceptionout_of_range416><code>out_of_range.416</code></a>, a limit <code class=highlight><span class=n>basic_json</span><span class=o>::</span><span class=n>parse</span><span class=p>()</span></code> does not have. It keeps an index entry at 16 bytes: offsets and counts are 32 bits wide. The limit is 16 bytes below 2^32, because the parser reads in 16-byte steps, and a position one step past the end must still fit.</li> <li><strong>A stream is always read to its end.</strong> There is no partial/streaming read of an <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>istream</span></code>: the stream is read to its end into a buffer the document owns, and parsing starts after that.</li> <li><strong>No source positions on <code>materialize()</code>.</strong> Even with <a href=../../api/macros/json_diagnostic_positions/ ><code>JSON_DIAGNOSTIC_POSITIONS</code></a> enabled, <a href=../../api/basic_json_view/materialize/ ><code>materialize()</code></a> does not set them. The index has the start offset of every value, but not the end of arrays, objects, and escaped strings, which the positions also need.</li> <li><strong>Objects iterate in document order.</strong> <a href=../../api/basic_json_view/begin/ ><code>begin()</code></a>/ <a href=../../api/basic_json_view/end/ ><code>end()</code></a> and <a href=../../api/basic_json_view/items/ ><code>items()</code></a> visit an object's members in the order they appear in the source text. <code>basic_json</code>'s default <code>object_t</code> is a <code>std::map</code>, which sorts by key, so iterating a <a href=../../api/basic_json_view/materialize/ ><code>materialize()</code></a>d value can print members in a different order than iterating the view they came from.</li> <li><strong>Chained access is safe.</strong> <a href=../../api/basic_json_view/operatLine truncated
|
||||
</code></pre></div> </details> <h2 id=what-is-the-same-as-parse>What is the same as <code>parse()</code><a class=headerlink href=#what-is-the-same-as-parse title="Permanent link">¶</a></h2> <ul> <li><strong>Accept/reject.</strong> <a href=../../api/basic_json_document/accept/ ><code>accept()</code></a> and <a href=../../api/basic_json_document/parse/ ><code>parse()</code></a> accept and reject exactly the same inputs as <a href=../../api/basic_json/accept/ ><code>basic_json::accept()</code></a>/<a href=../../api/basic_json/parse/ ><code>basic_json::parse()</code></a>, with the same <code>ignore_comments</code> and <code>ignore_trailing_commas</code> options.</li> <li><strong>Errors.</strong> A failing parse throws the same exception — the same id, message, and position — because on failure the library's own parser is run on the same bytes to produce the diagnostic. Valid input never pays for this, and the replay builds no values: it costs about a lexing pass up to the error. <code class=highlight><span class=n>allow_exceptions</span><span class=w> </span><span class=o>==</span><span class=w> </span><span class=nb>false</span></code> skips the replay and gives a <a href=../../api/basic_json_document/is_discarded/ >discarded</a> document instead of throwing, just as it gives a discarded value for <code class=highlight><span class=n>basic_json</span><span class=o>::</span><span class=n>parse</span><span class=p>()</span></code>.</li> <li><strong>Number classification.</strong> An integer literal that does not fit into the 64-bit integer type becomes a floating-point value, exactly as it does for <code class=highlight><span class=n>basic_json</span><span class=o>::</span><span class=n>parse</span><span class=p>()</span></code>.</li> <li><strong>Macros.</strong> <a href=../../api/macros/json_strict_nul_handling/ ><code>JSON_STRICT_NUL_HANDLING</code></a> and <a href=../../api/macros/json_noexception/ ><code>JSON_NOEXCEPTION</code></a>/<a href=../../api/macros/json_throw_user/ ><code>JSON_THROW_USER</code></a> behave the same way they do for <code><nlohmann/json.hpp></code>. These are the macros that change what the parser accepts or how it reports errors. The others either do not apply (<a href=../../api/macros/json_diagnostics/ ><code>JSON_DIAGNOSTICS</code></a> needs parent pointers in <code>basic_json</code> values) or only affect the values <code>materialize()</code> produces, through <code>BasicJsonType</code>.</li> </ul> <h2 id=what-is-different>What is different<a class=headerlink href=#what-is-different title="Permanent link">¶</a></h2> <ul> <li><strong>Only 64-bit integers.</strong> <code>basic_json_document<BasicJsonType></code> requires <code>BasicJsonType::number_integer_t</code> and <code>number_unsigned_t</code> to both be 64 bits wide; this is a compile-time <code class=highlight><span class=k>static_assert</span></code>.</li> <li><strong>A 4 <abbr title="Gibibyte (2^30 bytes)">GiB</abbr> input limit.</strong> An input of 4294967280 bytes (4 <abbr title="Gibibyte (2^30 bytes)">GiB</abbr> minus 16 bytes) or more throws <a href=../../home/exceptions/#jsonexceptionout_of_range416><code>out_of_range.416</code></a>, a limit <code class=highlight><span class=n>basic_json</span><span class=o>::</span><span class=n>parse</span><span class=p>()</span></code> does not have. It keeps an index entry at 16 bytes: offsets and counts are 32 bits wide. The limit is 16 bytes below 2^32, because the parser reads in 16-byte steps, and a position one step past the end must still fit.</li> <li><strong>A stream is always read to its end.</strong> There is no partial/streaming read of an <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>istream</span></code>: the stream is read to its end into a buffer the document owns, and parsing starts after that.</li> <li><strong>No source positions on <code>materialize()</code>.</strong> Even with <a href=../../api/macros/json_diagnostic_positions/ ><code>JSON_DIAGNOSTIC_POSITIONS</code></a> enabled, <a href=../../api/basic_json_view/materialize/ ><code>materialize()</code></a> does not set them. The index has the start offset of every value, but not the end of arrays, objects, and escaped strings, which the positions also need.</li> <li><strong>Objects iterate in document order.</strong> <a href=../../api/basic_json_view/begin/ ><code>begin()</code></a>/ <a href=../../api/basic_json_view/end/ ><code>end()</code></a> and <a href=../../api/basic_json_view/items/ ><code>items()</code></a> visit an object's members in the order they appear in the source text. <code>basic_json</code>'s default <code>object_t</code> is a <code>std::map</code>, which sorts by key, so iterating a <a href=../../api/basic_json_view/materialize/ ><code>materialize()</code></a>d value can print members in a different order than iterating the view they came from.</li> <li><strong>Chained access is safe.</strong> <a href=../../api/basic_json_view/operLine truncated
|
||||
<span class=n>json_view</span><span class=w> </span><span class=n>last</span><span class=p>;</span>
|
||||
<span class=k>for</span><span class=w> </span><span class=p>(</span><span class=k>const</span><span class=w> </span><span class=k>auto</span><span class=w> </span><span class=n>item</span><span class=w> </span><span class=o>:</span><span class=w> </span><span class=n>view</span><span class=p>.</span><span class=n>items</span><span class=p>())</span>
|
||||
<span class=p>{</span>
|
||||
@@ -62,7 +62,7 @@
|
||||
<span class=w> </span><span class=n>last</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>item</span><span class=p>.</span><span class=n>value</span><span class=p>();</span>
|
||||
<span class=w> </span><span class=p>}</span>
|
||||
<span class=p>}</span>
|
||||
</code></pre></div> <p>If you cannot trust the source text to have unique keys, check <a href=../../api/basic_json_view/size/ ><code>size()</code></a> against the number of distinct keys, or reject duplicates when iterating.</p> <h2 id=getting-values-out-without-copying>Getting values out without copying<a class=headerlink href=#getting-values-out-without-copying title="Permanent link">¶</a></h2> <p><a href=../../api/basic_json_view/get/ ><code>get<T>()</code></a> converts most <code>T</code> directly from the flat index, without ever building a <code>basic_json</code> value for the conversion: <code class=highlight><span class=kt>bool</span></code>, arithmetic types, <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>nullptr_t</span></code>, <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>string</span></code>/other <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>basic_string</span></code>s (copied once), <code>basic_json_view</code> itself, the standard containers and other types the library converts (<code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>vector</span></code>, <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>map</span></code>, <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>list</span></code>, <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>array</span></code>, <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>pair</span></code>, <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>tuple</span></code>, <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>optional</span></code>, ...), and the types of the <code>NLOHMANN_DEFINE_TYPE_*</code> and <code>NLOHMANN_JSON_SERIALIZE_ENUM</code> macros. Types whose <code>from_json()</code> is written for <code>basic_json</code> only go through <a href=../../api/basic_json_view/materialize/ ><code>materialize()</code></a><code>.get<T>()</code> instead: the subtree is built into a real <code>basic_json</code> value first, exactly as <a href=../../api/basic_json/parse/ ><code>parse()</code></a> would, and converted from there. See <a href=#writing-generic-code>Writing generic code</a> for how to write a <code>from_json()</code> that reads views.</p> <p>Two conversions never copy at all:</p> <ul> <li><a href=../../api/basic_json_view/get_string/ ><code>get_string()</code></a> (equivalently, <code class=highlight><span class=n>get</span><span class=o><</span><span class=n>string_view_t</span><span class=o>></span><span class=p>()</span></code>) returns a string as a <code>string_view_t</code> pointing into the document's <a href=../../api/basic_json_document/source/ ><code>source()</code></a> text -- or, for a string that contains escape sequences, into the document's own buffer of decoded strings -- instead of allocating a new <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>string</span></code>.</li> <li><a href=../../api/basic_json_view/number_token/ ><code>number_token()</code></a> returns a number exactly as it was written in the source, e.g. <code class=highlight><span class=s>"1.50"</span></code>, <code class=highlight><span class=s>"1E2"</span></code>, or an integer with more digits than any number type holds, instead of rounding it into a <code class=highlight><span class=kt>double</span></code>/<code class=highlight><span class=kt>int64_t</span></code> the way <code class=highlight><span class=n>get</span><span class=o><</span><span class=n>T</span><span class=o>></span><span class=p>()</span></code> (and <a href=../../api/basic_json/parse/ ><code>basic_json::parse()</code></a>) would.</li> </ul> <p>Both results are only valid as long as the view -- and, for a string with no escapes, the borrowed source text -- is.</p> <h2 id=writing-generic-code>Writing generic code<a class=headerlink href=#writing-generic-code title="Permanent link">¶</a></h2> <p>A view's type names its <code>BasicJsonType</code> and its <a href=../../api/view_mode/ ><code>view_mode</code></a>: <a href=../../api/json_view/ ><code>json_view</code></a> is <code class=highlight><span class=n>basic_json_view</span><span class=o><</span><span class=n>json</span><span class=p>,</span><span class=w> </span><span class=n>view_mode</span><span class=o>::</span><span class=n>read_only</span><span class=o>></span></code>, and <a href=../../api/json_editable_view/ ><code>json_editable_view</code></a> is <code class=highlight><span class=n>basic_json_view</span><span class=o><</span><span class=n>json</span><span class=p>,</span><span class=w> </span><span class=n>view_mode</span><span class=o>::</span><span class=n>editable</span><span class=o>></span></code>. These are different types. TLine truncated
|
||||
</code></pre></div> <p>If you cannot trust the source text to have unique keys, check <a href=../../api/basic_json_view/size/ ><code>size()</code></a> against the number of distinct keys, or reject duplicates when iterating.</p> <h2 id=getting-values-out-without-copying>Getting values out without copying<a class=headerlink href=#getting-values-out-without-copying title="Permanent link">¶</a></h2> <p><a href=../../api/basic_json_view/get/ ><code>get<T>()</code></a> converts most <code>T</code> directly from the flat index, without ever building a <code>basic_json</code> value for the conversion: <code class=highlight><span class=kt>bool</span></code>, arithmetic types, <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>nullptr_t</span></code>, <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>string</span></code>/other <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>basic_string</span></code>s (copied once), <code>basic_json_view</code> itself, the standard containers and other types the library converts (<code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>vector</span></code>, <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>map</span></code>, <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>list</span></code>, <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>array</span></code>, <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>pair</span></code>, <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>tuple</span></code>, <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>optional</span></code>, ...), and the types of the <code>NLOHMANN_DEFINE_TYPE_*</code> and <code>NLOHMANN_JSON_SERIALIZE_ENUM</code> macros. Types whose <code>from_json()</code> is written for <code>basic_json</code> only go through <a href=../../api/basic_json_view/materialize/ ><code>materialize()</code></a><code>.get<T>()</code> instead: the subtree is built into a real <code>basic_json</code> value first, exactly as <a href=../../api/basic_json/parse/ ><code>parse()</code></a> would, and converted from there. See <a href=#writing-generic-code>Writing generic code</a> for how to write a <code>from_json()</code> that reads views.</p> <p>Two conversions never copy at all:</p> <ul> <li><a href=../../api/basic_json_view/get_string/ ><code>get_string()</code></a> (equivalently, <code class=highlight><span class=n>get</span><span class=o><</span><span class=n>string_view_t</span><span class=o>></span><span class=p>()</span></code>) returns a string as a <code>string_view_t</code> pointing into the document's <a href=../../api/basic_json_document/source/ ><code>source()</code></a> text — or, for a string that contains escape sequences, into the document's own buffer of decoded strings — instead of allocating a new <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>string</span></code>.</li> <li><a href=../../api/basic_json_view/number_token/ ><code>number_token()</code></a> returns a number exactly as it was written in the source, e.g. <code class=highlight><span class=s>"1.50"</span></code>, <code class=highlight><span class=s>"1E2"</span></code>, or an integer with more digits than any number type holds, instead of rounding it into a <code class=highlight><span class=kt>double</span></code>/<code class=highlight><span class=kt>int64_t</span></code> the way <code class=highlight><span class=n>get</span><span class=o><</span><span class=n>T</span><span class=o>></span><span class=p>()</span></code> (and <a href=../../api/basic_json/parse/ ><code>basic_json::parse()</code></a>) would.</li> </ul> <p>Both results are only valid as long as the view — and, for a string with no escapes, the borrowed source text — is.</p> <h2 id=writing-generic-code>Writing generic code<a class=headerlink href=#writing-generic-code title="Permanent link">¶</a></h2> <p>A view's type names its <code>BasicJsonType</code> and its <a href=../../api/view_mode/ ><code>view_mode</code></a>: <a href=../../api/json_view/ ><code>json_view</code></a> is <code class=highlight><span class=n>basic_json_view</span><span class=o><</span><span class=n>json</span><span class=p>,</span><span class=w> </span><span class=n>view_mode</span><span class=o>::</span><span class=n>read_only</span><span class=o>></span></code>, and <a href=../../api/json_editable_view/ ><code>json_editable_view</code></a> is <code class=highlight><span class=n>basic_json_view</span><span class=o><</span><span class=n>json</span><span class=p>,</span><span class=w> </span><span class=n>view_mode</span><span class=o>::</span><span class=n>editable</span><span class=o>></span></code>. These are different typeLine truncated
|
||||
<span class=kt>void</span><span class=w> </span><span class=nf>print_names</span><span class=p>(</span><span class=n>json_editable_view</span><span class=w> </span><span class=n>v</span><span class=p>);</span><span class=w> </span><span class=c1>// also accepts a json_view, but no ordered_json_view</span>
|
||||
</code></pre></div> <p>This is the same pitfall as a <code>to_json</code>/<code>from_json</code> written for <code>json</code> instead of any <code>BasicJsonType</code>, which then does not work for <code>ordered_json</code>. The shortcuts are for declaring documents and views; a function that takes a view names the views it accepts with a template:</p> <div class=highlight><pre><span></span><code><span class=c1>// views of editable documents only</span>
|
||||
<span class=k>template</span><span class=o><</span><span class=k>typename</span><span class=w> </span><span class=nc>BasicJsonType</span><span class=o>></span>
|
||||
@@ -82,7 +82,19 @@
|
||||
<span class=w> </span><span class=n>p</span><span class=p>.</span><span class=n>name</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>j</span><span class=p>.</span><span class=n>at</span><span class=p>(</span><span class=s>"name"</span><span class=p>).</span><span class=k>template</span><span class=w> </span><span class=n>get</span><span class=o><</span><span class=n>std</span><span class=o>::</span><span class=n>string</span><span class=o>></span><span class=p>();</span>
|
||||
<span class=w> </span><span class=n>p</span><span class=p>.</span><span class=n>age</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>j</span><span class=p>.</span><span class=n>at</span><span class=p>(</span><span class=s>"age"</span><span class=p>).</span><span class=k>template</span><span class=w> </span><span class=n>get</span><span class=o><</span><span class=kt>int</span><span class=o>></span><span class=p>();</span>
|
||||
<span class=p>}</span>
|
||||
</code></pre></div> <p>Its body may only use what views and <code>basic_json</code> have in common: the type checks, <code>at()</code>, <code>operator[]</code>, <code>contains()</code>, <code>value()</code>, <code>size()</code>, iteration and <code>items()</code>, and <code>get<T>()</code>/<code>get_to()</code>. An unconstrained <code class=highlight><span class=k>template</span><span class=o><</span><span class=k>typename</span><span class=w> </span><span class=nc>BasicJsonType</span><span class=o>></span><span class=w> </span><span class=kt>void</span><span class=w> </span><span class=n>from_json</span><span class=p>(</span><span class=k>const</span><span class=w> </span><span class=n>BasicJsonType</span><span class=o>&</span><span class=w> </span><span class=n>j</span><span class=p>,</span><span class=w> </span><span class=n>person</span><span class=o>&</span><span class=w> </span><span class=n>p</span><span class=p>)</span></code>, as in <a href=../arbitrary_types/ >arbitrary type conversions</a>, is never called with a view, as it might use <code>basic_json</code> functions a view does not have; such a type is converted through <code>materialize()</code>. A <code>from_json()</code> for <code>json</code> only is converted through <code>materialize()</code> as well.</p> <p>Two things differ when a <code>from_json()</code> reads the view:</p> <ul> <li><strong>Duplicate keys.</strong> Members looked up by name (<code>at()</code>, <code>value()</code>, ... and thus the macros) find the <strong>first</strong> member with a repeated key, see <a href=#duplicate-keys>Duplicate keys</a>; <code class=highlight><span class=n>materialize</span><span class=p>().</span><span class=n>get</span><span class=o><</span><span class=n>T</span><span class=o>></span><span class=p>()</span></code> would see the last one. Containers built from an object, such as <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>map</span></code>, keep the <strong>last</strong> value either way, as <a href=../../api/basic_json/parse/ ><code>parse()</code></a> does.</li> <li><strong><a href=../../api/macros/json_diagnostics/ ><code>JSON_DIAGNOSTICS</code></a>.</strong> Exceptions thrown by the view carry no path to the value.</li> </ul> <p><code>to_json()</code> is unaffected: it writes <code>basic_json</code> values, never views.</p> <h2 id=writing-a-view-back>Writing a view back<a class=headerlink href=#writing-a-view-back title="Permanent link">¶</a></h2> <p><a href=../../api/basic_json_view/dump/ ><code>dump()</code></a> serializes a view directly from the flat index, without ever building a <code>basic_json</code> value. An object's members are written in document order, not sorted by key, and <em>every</em> occurrence of a repeated key is written, not only the last one -- the same two ways <a href=#what-is-different>iteration</a> already differs from a <a href=../../api/basic_json_view/materialize/ ><code>materialize()</code></a>d value, see <a href=#duplicate-keys>Duplicate keys</a>. <code class=highlight><span class=n>materialize</span><span class=p>().</span><span class=n>dump</span><span class=p>()</span></code> gives a different result in both respects for a <code>json_view</code>.</p> <p>By default, numbers are written the way <a href=../../api/basic_json/dump/ ><code>basic_json::dump()</code></a> would. <a href=../../api/basic_json_view/number_format/ ><code>number_format::source</code></a> instead copies every number exactly as it was written in the source text -- a price like <code class=highlight><span class=mf>19.90</span></code>, a long order or account ID with more digits than any number type holds, or a high-precision coordinate -- something <code>basic_json</code> cannot do at all, since parsing already reduces a number to its parsed <code class=highlight><span class=kt>double</span></code>/<code class=highlight><span class=kt>int64_t</span></code> value.</p> <p><a href=../../api/basic_json_view/operator_ltlt/ ><code>operator<<</code></a> writes a view to a stream the way <code>basic_json</code>'s does, using the stream's <code>width</code>/<code>fill</code> for indentation.</p> <h2 id=editing-a-document>Editing a document<a class=headerlink href=#editing-a-document title="Permanent link">¶</a></h2> <p>Everything above is read-only: a <code>json_document</code>/<code>json_view</code> lets you look at a parsed text without copying it, but not change it. <a href=../../api/basic_json_document/ ><code>basic_json_document<BasicJsonType, view_mode::editable></code></a> -- more conveniently spelled <a href=../../api/json_editable_document/ ><code>json_editable_document</code></a> or <a href=../../api/ordered_json_editable_document/ ><code>ordered_json_editable_document</code></a> -- also lets you <a href=../../api/basic_json_document/set/ ><code>set</code></a> a value, <a href=../../api/basic_json_document/push_back/ ><code>push_back</code></a> onto orLine truncated
|
||||
</code></pre></div> <p>Its body may only use what views and <code>basic_json</code> have in common: the type checks, <code>at()</code>, <code>operator[]</code>, <code>contains()</code>, <code>value()</code>, <code>size()</code>, iteration and <code>items()</code>, and <code>get<T>()</code>/<code>get_to()</code>. An unconstrained <code class=highlight><span class=k>template</span><span class=o><</span><span class=k>typename</span><span class=w> </span><span class=nc>BasicJsonType</span><span class=o>></span><span class=w> </span><span class=kt>void</span><span class=w> </span><span class=n>from_json</span><span class=p>(</span><span class=k>const</span><span class=w> </span><span class=n>BasicJsonType</span><span class=o>&</span><span class=w> </span><span class=n>j</span><span class=p>,</span><span class=w> </span><span class=n>person</span><span class=o>&</span><span class=w> </span><span class=n>p</span><span class=p>)</span></code>, as in <a href=../arbitrary_types/ >arbitrary type conversions</a>, is never called with a view, as it might use <code>basic_json</code> functions a view does not have; such a type is converted through <code>materialize()</code>. A <code>from_json()</code> for <code>json</code> only is converted through <code>materialize()</code> as well.</p> <p>Two things differ when a <code>from_json()</code> reads the view:</p> <ul> <li><strong>Duplicate keys.</strong> Members looked up by name (<code>at()</code>, <code>value()</code>, ... and thus the macros) find the <strong>first</strong> member with a repeated key, see <a href=#duplicate-keys>Duplicate keys</a>; <code class=highlight><span class=n>materialize</span><span class=p>().</span><span class=n>get</span><span class=o><</span><span class=n>T</span><span class=o>></span><span class=p>()</span></code> would see the last one. Containers built from an object, such as <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>map</span></code>, keep the <strong>last</strong> value either way, as <a href=../../api/basic_json/parse/ ><code>parse()</code></a> does.</li> <li><strong><a href=../../api/macros/json_diagnostics/ ><code>JSON_DIAGNOSTICS</code></a>.</strong> Exceptions thrown by the view carry no path to the value.</li> </ul> <p><code>to_json()</code> is unaffected: it writes <code>basic_json</code> values, never views.</p> <h2 id=writing-a-view-back>Writing a view back<a class=headerlink href=#writing-a-view-back title="Permanent link">¶</a></h2> <p><a href=../../api/basic_json_view/dump/ ><code>dump()</code></a> serializes a view directly from the flat index, without ever building a <code>basic_json</code> value. An object's members are written in document order, not sorted by key, and <em>every</em> occurrence of a repeated key is written, not only the last one — the same two ways <a href=#what-is-different>iteration</a> already differs from a <a href=../../api/basic_json_view/materialize/ ><code>materialize()</code></a>d value, see <a href=#duplicate-keys>Duplicate keys</a>. <code class=highlight><span class=n>materialize</span><span class=p>().</span><span class=n>dump</span><span class=p>()</span></code> gives a different result in both respects for a <code>json_view</code>.</p> <p>By default, numbers are written the way <a href=../../api/basic_json/dump/ ><code>basic_json::dump()</code></a> would. <a href=../../api/basic_json_view/number_format/ ><code>number_format::source</code></a> instead copies every number exactly as it was written in the source text — a price like <code class=highlight><span class=mf>19.90</span></code>, a long order or account ID with more digits than any number type holds, or a high-precision coordinate — something <code>basic_json</code> cannot do at all, since parsing already reduces a number to its parsed <code class=highlight><span class=kt>double</span></code>/<code class=highlight><span class=kt>int64_t</span></code> value.</p> <p><a href=../../api/basic_json_view/operator_ltlt/ ><code>operator<<</code></a> writes a view to a stream the way <code>basic_json</code>'s does, using the stream's <code>width</code>/<code>fill</code> for indentation.</p> <h2 id=editing-a-document>Editing a document<a class=headerlink href=#editing-a-document title="Permanent link">¶</a></h2> <p>Everything above is read-only: a <code>json_document</code>/<code>json_view</code> lets you look at a parsed text without copying it, but not change it. <a href=../../api/basic_json_document/ ><code>basic_json_document<BasicJsonType, view_mode::editable></code></a> — more conveniently spelled <a href=../../api/json_editable_document/ ><code>json_editable_document</code></a> or <a href=../../api/ordered_json_editable_document/ ><code>ordered_json_editable_document</code></a> — also lets you <a href=../../api/basic_json_document/set/ ><code>set</code></a> a value, <a href=../../api/basic_json_document/push_back/ ><code>push_back</code></a> onLine truncated
|
||||
"name":"cache",
|
||||
"port":6379,
|
||||
"price":<span class=nj-jsonview-bad>19.9</span>,
|
||||
"retries":3}</pre></div> <div><p class=nj-jsonview-cmp-h><code>ordered_json</code></p><p class=nj-jsonview-cmp-v>key order kept, numbers rewritten</p><pre>{"name":"cache",
|
||||
"host":"db2",
|
||||
"port":6379,
|
||||
"price":<span class=nj-jsonview-bad>19.9</span>,
|
||||
"retries":3}</pre></div> <div class=nj-jsonview-cmp--best><p class=nj-jsonview-cmp-h><code>json_editable_document</code></p><p class=nj-jsonview-cmp-v>order and spelling kept</p><pre>{"name":"cache",
|
||||
"host":"db2",
|
||||
"port":6379,
|
||||
"price":<span class=nj-jsonview-ok>19.90</span>,
|
||||
"retries":3}</pre></div> </div> <p>An editable document keeps both. <a href=../../api/basic_json_view/dump/ ><code>dump()</code></a> of an edited document writes members in document order — a member <a href=../../api/basic_json_document/set/ ><code>set</code></a> added goes at the end, exactly where it was inserted, and an <a href=../../api/basic_json_document/erase/ ><code>erase</code></a>d member simply leaves a gap: everything around it keeps its place — and <a href=../../api/basic_json_view/number_format/ ><code>number_format::source</code></a> keeps the exact spelling of every number an edit did not itself touch; a number an edit <em>did</em> touch is written the way <a href=../../api/basic_json/dump/ ><code>BasicJsonType::dump()</code></a> would write it, since there is no source spelling for a brand new value.</p> <details class=example> <summary>Example: patch a configuration, keeping member order and number spellings</summary> <div class=highlight><pre><span></span><code><span class=cp>#include</span><span class=w> </span><span class=cpf><iostream></span>
|
||||
<span class=cp>#include</span><span class=w> </span><span class=cpf><nlohmann/json_view.hpp></span>
|
||||
|
||||
<span class=k>using</span><span class=w> </span><span class=n>nlohmann</span><span class=o>::</span><span class=n>json</span><span class=p>;</span>
|
||||
@@ -116,7 +128,7 @@
|
||||
|
||||
<span class=p>{</span><span class=nt>"name"</span><span class=p>:</span><span class=s2>"cache"</span><span class=p>,</span><span class=nt>"host"</span><span class=p>:</span><span class=s2>"db2"</span><span class=p>,</span><span class=nt>"port"</span><span class=p>:</span><span class=mi>6379</span><span class=p>,</span><span class=nt>"price"</span><span class=p>:</span><span class=mf>19.90</span><span class=p>,</span><span class=nt>"retries"</span><span class=p>:</span><span class=mi>3</span><span class=p>}</span>
|
||||
<span class=p>{</span><span class=nt>"host"</span><span class=p>:</span><span class=s2>"db2"</span><span class=p>,</span><span class=nt>"name"</span><span class=p>:</span><span class=s2>"cache"</span><span class=p>,</span><span class=nt>"port"</span><span class=p>:</span><span class=mi>6379</span><span class=p>,</span><span class=nt>"price"</span><span class=p>:</span><span class=mf>19.9</span><span class=p>,</span><span class=nt>"retries"</span><span class=p>:</span><span class=mi>3</span><span class=p>}</span>
|
||||
</code></pre></div> </details> <h3 id=what-stays-valid-and-what-an-edit-costs>What stays valid, and what an edit costs<a class=headerlink href=#what-stays-valid-and-what-an-edit-costs title="Permanent link">¶</a></h3> <p>The source text itself is <strong>never written</strong>, and the parsed index never moves -- a value keeps the node it was parsed into for as long as it is not itself replaced. So every <a href=../../api/basic_json_view/ >view</a> taken before an edit, including a previously obtained <a href=../../api/basic_json_document/root/ ><code>root()</code></a>, stays valid and, if it still refers to the edited value, sees the edit; a view of a value a later edit drops or replaces just keeps showing what it last held. New values go to storage the document allocates and owns on demand. The one thing an edit does invalidate is the <strong>iterators</strong> taken over an edited array or object: the first time one of its elements is set, appended to, inserted into, or erased, its elements move from the parsed, fixed layout to a growable block of links so that <a href=../../api/basic_json_document/push_back/ ><code>push_back</code></a> can later grow it in amortized constant time -- existing elements are not touched, but an iterator that was walking the old layout no longer matches. A string obtained with <a href=../../api/basic_json_view/get_string/ ><code>get_string()</code></a> is unaffected either way and stays valid across further edits. See <a href=../../api/basic_json_document/#edits><code>basic_json_document</code>'s Edits</a> for the details, and <a href=../../api/basic_json_document/set/#exception-safety><code>set</code>'s Exception safety</a> for what an edit guarantees if it throws (the <em>basic</em> guarantee, not the strong one <code>dump()</code> and the read-only functions provide). How edits are kept in the index is described in the <a href=../../home/architecture/#node-index-of-json-views>architecture overview</a>.</p> <h2 id=images>Images<a class=headerlink href=#images title="Permanent link">¶</a></h2> <p><a href=../../api/basic_json_document/save/ ><code>save()</code></a> writes a document as an <em>image</em>: a byte buffer that <a href=../../api/basic_json_document/load/ ><code>load()</code></a> reads back into a document without parsing -- no lexing, no building the node index, nothing but copying the nodes and pointing the text and the decoded strings at the image. Where <a href=../../api/basic_json_document/parse_copy/ ><code>parse_copy()</code></a> still has to scan the whole input, <a href=../../api/basic_json_document/load/ ><code>load()</code></a> turns that scan into a copy of the node index alone.</p> <p><strong>Why.</strong> A document that is parsed once and then read many times -- a configuration loaded at startup, a template rendered on every request, a large reference dataset a worker process needs in memory -- pays for parsing once but can amortize <a href=../../api/basic_json_document/save/ ><code>save()</code></a>'s cost across every later load. That makes images useful for a cache: save a document the first time it is parsed (to a file, a shared-memory segment, an in-process cache), and <a href=../../api/basic_json_document/load/ ><code>load()</code></a> it on every later use instead of parsing the source text again. They are just as useful for handing a parsed document to another process (or a forked worker) running the same build of the library, since <a href=../../api/basic_json_document/load/ ><code>load()</code></a> turns the transfer into a copy of the node index plus pointers into the received bytes, not a re-parse.</p> <p><strong>Choosing a check.</strong> <a href=../../api/basic_json_document/load/ ><code>load()</code></a> takes an <a href=../../api/basic_json_document/load/#image_check><code>image_check</code></a> that trades validation against speed: <code>image_check::full</code> (the default) checks everything the parser itself guarantees, so a checked image is exactly as safe to read and serialize as a freshly parsed document -- the right choice whenever the image did not come straight from this process's own <a href=../../api/basic_json_document/save/ ><code>save()</code></a>, such as a file or a network peer. <code>image_check::bounds</code> only checks structure and bounds -- cheaper, since it skips scanning the text and the decoded strings -- and fits a cache the process trusts, one it wrote and reads back itself. <code>image_check::none</code> skips validation entirely, for an image trusted as much as the process's own memory. See <a href=../../api/basic_json_document/load/#notes><code>load()</code>'s Notes</a> for exactly what each level does and does not guarantee.</p> <details class=example> <summary>Example: cache a parsed configuration as an image</summary> <div class=highlight><pre><span></span><code><span class=cp>#include</span><span class=w> </span><span class=cpf><cstdint></span>
|
||||
</code></pre></div> </details> <h3 id=what-stays-valid-and-what-an-edit-costs>What stays valid, and what an edit costs<a class=headerlink href=#what-stays-valid-and-what-an-edit-costs title="Permanent link">¶</a></h3> <p>The source text itself is <strong>never written</strong>, and the parsed index never moves — a value keeps the node it was parsed into for as long as it is not itself replaced. So every <a href=../../api/basic_json_view/ >view</a> taken before an edit, including a previously obtained <a href=../../api/basic_json_document/root/ ><code>root()</code></a>, stays valid and, if it still refers to the edited value, sees the edit; a view of a value a later edit drops or replaces just keeps showing what it last held. New values go to storage the document allocates and owns on demand. The one thing an edit does invalidate is the <strong>iterators</strong> taken over an edited array or object: the first time an element is added to it, inserted into it, erased from it, or replaced by a value that spans a different number of nodes, its elements move from the parsed, fixed layout to a growable block of links so that <a href=../../api/basic_json_document/push_back/ ><code>push_back</code></a> can later grow it in amortized constant time — existing elements are not touched, but an iterator that was walking the old layout no longer matches. A string obtained with <a href=../../api/basic_json_view/get_string/ ><code>get_string()</code></a> is unaffected either way and stays valid across further edits. See <a href=../../api/basic_json_document/#edits><code>basic_json_document</code>'s Edits</a> for the details, and <a href=../../api/basic_json_document/set/#exception-safety><code>set</code>'s Exception safety</a> for what an edit guarantees if it throws (the <em>basic</em> guarantee, not the strong one <code>dump()</code> and the read-only functions provide). How edits are kept in the index is described in the <a href=../../home/architecture/#node-index-of-json-views>architecture overview</a>.</p> <p>Step through the edits of a small document (the node fields follow the <a href=../../home/architecture/#node-index-of-json-views>architecture overview</a>):</p> <div class=nj-jsonview-steps data-nj-jsonview-steps> <figure class=nj-jsonview-step data-label="After parsing"> <div class=nj-jsonview-scroll> <svg class="nj-fig nj-jsonview nj-jsonview-editfig" viewbox="0 0 840 372" role=img aria-labelledby="fig-jve0-title fig-jve0-title-d" xmlns=http://www.w3.org/2000/svg> <title id=fig-jve0-title>Step 1 of 5: parsed</title> <desc id=fig-jve0-title-d>After parse: nine nodes, nothing owned yet. The dump with number_format::source is {"port":6379,"price":19.90,"tags":["eu","a\"b"]}.</desc> <defs> <marker id=jve0-line viewbox="0 0 10 10" refx=9 refy=5 markerwidth=6.5 markerheight=6.5 orient=auto-start-reverse><path class="nj-jsonview-c-line nj-jsonview-head" d="M0 1 L10 5 L0 9 z"/></marker> <marker id=jve0-own viewbox="0 0 10 10" refx=9 refy=5 markerwidth=6.5 markerheight=6.5 orient=auto-start-reverse><path class="nj-jsonview-c-own nj-jsonview-head" d="M0 1 L10 5 L0 9 z"/></marker> <marker id=jve0-struct viewbox="0 0 10 10" refx=9 refy=5 markerwidth=6.5 markerheight=6.5 orient=auto-start-reverse><path class="nj-jsonview-c-struct nj-jsonview-head" d="M0 1 L10 5 L0 9 z"/></marker> <marker id=jve0-bad viewbox="0 0 10 10" refx=9 refy=5 markerwidth=6.5 markerheight=6.5 orient=auto-start-reverse><path class="nj-jsonview-c-bad nj-jsonview-head" d="M0 1 L10 5 L0 9 z"/></marker> </defs> <text class="nj-jsonview-note nj-jsonview-strong" x=16 y=22 xml:space=preserve>parsed index: fixed, never moves</text> <text class=nj-jsonview-note x=824 y=22 text-anchor=end xml:space=preserve>the source text is never written</text> <rect x=36 y=34 width=84 height=50 rx=5 class="nj-jsonview-node nj-jsonview-c-struct"/> <text class=nj-jsonview-tiny x=42 y=47 xml:space=preserve>0</text> <text class="nj-jsonview-kind nj-jsonview-xs nj-jsonview-c-struct" x=78 y=61 text-anchor=middle xml:space=preserve>object</text> <text class="nj-jsonview-mono nj-jsonview-sm" x=78 y=77 text-anchor=middle xml:space=preserve>{ } 3</text> <rect x=124 y=34 width=84 height=50 rx=5 class="nj-jsonview-node nj-jsonview-c-key"/> <text class=nj-jsonview-tiny x=130 y=47 xml:space=preserve>1</text> <text class="nj-jsonview-kind nj-jsonview-xs nj-jsonview-c-key" x=166 y=61 text-anchor=middle xml:space=preserve>key</text> <text class="nj-jsonview-mono nj-jsonview-sm" x=166 y=77 text-anchor=middle xml:space=preserve>port</text> <rect x=212 y=34 width=84 height=50 rx=5 class="nj-jsonview-node nj-jsonview-c-num"/> <text class=nj-jsonview-tiny x=218 y=47 xml:space=preserve>2</text> <text class="nj-jsonview-kind nj-jsonview-xs nj-jsonview-c-num" x=254 y=61 text-anchor=middle xml:space=preserve>uint</text> <text class="nj-jsonview-mono nj-jsonview-sm" x=254 y=77 text-anchor=middle xml:space=preserve>6379</text> <rect x=300 Line truncated
|
||||
<span class=cp>#include</span><span class=w> </span><span class=cpf><iostream></span>
|
||||
<span class=cp>#include</span><span class=w> </span><span class=cpf><string></span>
|
||||
<span class=cp>#include</span><span class=w> </span><span class=cpf><vector></span>
|
||||
@@ -149,4 +161,4 @@
|
||||
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code><span class=mi>316</span>
|
||||
<span class=kc>true</span>
|
||||
<span class=kc>true</span>
|
||||
</code></pre></div> </details> <div class="admonition warning"> <p class=admonition-title>Experimental</p> <p>The image format is versioned but not yet stable, and may change in an incompatible way before it is declared stable. It is little-endian only (on big-endian targets, <code>save()</code> and <code>load()</code> do not exist), and tied to the library build that wrote it -- use it to cache a document or to hand one to another process running the <em>same</em> build, not as a long-term storage format; keep the original <abbr title="JavaScript Object Notation">JSON</abbr> text if a saved document needs to be readable by a future library version.</p> </div> <p>The idea of a document you can read without parsing comes from zero-copy formats such as <a href="https://github.com/google/flatbuffers">FlatBuffers</a> and <a href="https://github.com/yandex/yaff">YaFF</a>; the check <a href=../../api/basic_json_document/load/ ><code>load()</code></a> runs follows the idea of FlatBuffers' Verifier. No code is taken from either.</p> <h2 id=choosing-between-json-ordered_json-the-sax-interface-and-json_view>Choosing between <code>json</code>, <code>ordered_json</code>, the <abbr title="Simple API for XML">SAX</abbr> interface, and <code>json_view</code><a class=headerlink href=#choosing-between-json-ordered_json-the-sax-interface-and-json_view title="Permanent link">¶</a></h2> <table> <thead> <tr> <th></th> <th><a href=../../api/json/ ><code>json</code></a> / <a href=../../api/ordered_json/ ><code>ordered_json</code></a></th> <th><a href=../parsing/sax_interface/ ><abbr title="Simple API for XML">SAX</abbr> interface</a></th> <th><a href=../../api/json_document/ ><code>json_document</code></a> / <a href=../../api/json_view/ ><code>json_view</code></a></th> <th><a href=../../api/json_editable_document/ ><code>json_editable_document</code></a> / <a href=../../api/json_editable_view/ ><code>json_editable_view</code></a></th> </tr> </thead> <tbody> <tr> <td><strong>Ownership</strong></td> <td>owns every value</td> <td>owns nothing; you decide what to keep, in your handler</td> <td>borrows or owns the <em>text</em>; the index is always owned by the document</td> <td>same as <code>json_document</code>; edits go to storage the document owns</td> </tr> <tr> <td><strong>Mutability</strong></td> <td>freely mutable</td> <td>not applicable (a one-shot event stream)</td> <td>read-only</td> <td><a href=../../api/basic_json_document/set/ ><code>set</code></a>/<a href=../../api/basic_json_document/push_back/ ><code>push_back</code></a>/<a href=../../api/basic_json_document/insert/ ><code>insert</code></a>/<a href=../../api/basic_json_document/erase/ ><code>erase</code></a> edit in place; the source text is never rewritten</td> </tr> <tr> <td><strong>What you get</strong></td> <td>a full tree you can read, write, and keep as long as you like</td> <td>a sequence of callbacks; whatever your handler builds from them</td> <td>a flat index plus, on demand, <a href=../../api/basic_json_view/materialize/ ><code>materialize()</code></a>d <code>json</code>/<code>ordered_json</code> values for the parts you actually use</td> <td>the same, plus <a href=../../api/basic_json_view/dump/ ><code>dump()</code></a> of an edited document that keeps the member order and, with <a href=../../api/basic_json_view/number_format/ ><code>number_format::source</code></a>, the spelling of every untouched number</td> </tr> <tr> <td><strong>Typical use</strong></td> <td>general-purpose <abbr title="JavaScript Object Notation">JSON</abbr> handling: config, request/response bodies you build or modify, anything you hold onto</td> <td>validating or projecting a text into your own data structure without ever holding the whole thing as <abbr title="JavaScript Object Notation">JSON</abbr></td> <td>large or high-volume input where you only need part of it, or need it repeatedly, and can keep the source text (or a copy) alive for as long as the document lives</td> <td>a document you read, patch a few fields of, and write back -- a configuration file, for instance -- where the rest of it should come back exactly as it was</td> </tr> <tr> <td><strong>Caching/reload</strong></td> <td>not applicable -- re-parse, or roll your own serialization</td> <td>not applicable</td> <td><a href=../../api/basic_json_document/save/ ><code>save()</code></a>/<a href=../../api/basic_json_document/load/ ><code>load()</code></a>: cache the parsed index as an image and reload it without parsing</td> <td>same, saving the document's current -- possibly edited -- state</td> </tr> </tbody> </table> <h2 id=version-history>Version history<a class=headerlink href=#version-history title="Permanent link">¶</a></h2> <ul> <li>Added in version 3.13.0 <span class=unreleased-version title="Not part of a release yet; the latest release is 3.12.0.">unreleased</span>.</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <spanLine truncated
|
||||
</code></pre></div> </details> <div class="admonition warning"> <p class=admonition-title>Experimental</p> <p>The image format is versioned but not yet stable, and may change in an incompatible way before it is declared stable. It is little-endian only (on big-endian targets, <code>save()</code> and <code>load()</code> do not exist), and tied to the library build that wrote it — use it to cache a document or to hand one to another process running the <em>same</em> build, not as a long-term storage format; keep the original <abbr title="JavaScript Object Notation">JSON</abbr> text if a saved document needs to be readable by a future library version.</p> </div> <p>The idea of a document you can read without parsing comes from zero-copy formats such as <a href="https://github.com/google/flatbuffers">FlatBuffers</a> and <a href="https://github.com/yandex/yaff">YaFF</a>; the check <a href=../../api/basic_json_document/load/ ><code>load()</code></a> runs follows the idea of FlatBuffers' Verifier. No code is taken from either.</p> <h2 id=choosing-between-json-ordered_json-the-sax-interface-and-json_view>Choosing between <code>json</code>, <code>ordered_json</code>, the <abbr title="Simple API for XML">SAX</abbr> interface, and <code>json_view</code><a class=headerlink href=#choosing-between-json-ordered_json-the-sax-interface-and-json_view title="Permanent link">¶</a></h2> <table> <thead> <tr> <th></th> <th><a href=../../api/json/ ><code>json</code></a> / <a href=../../api/ordered_json/ ><code>ordered_json</code></a></th> <th><a href=../parsing/sax_interface/ ><abbr title="Simple API for XML">SAX</abbr> interface</a></th> <th><a href=../../api/json_document/ ><code>json_document</code></a> / <a href=../../api/json_view/ ><code>json_view</code></a></th> <th><a href=../../api/json_editable_document/ ><code>json_editable_document</code></a> / <a href=../../api/json_editable_view/ ><code>json_editable_view</code></a></th> </tr> </thead> <tbody> <tr> <td><strong>Ownership</strong></td> <td>owns every value</td> <td>owns nothing; you decide what to keep, in your handler</td> <td>borrows or owns the <em>text</em>; the index is always owned by the document</td> <td>same as <code>json_document</code>; edits go to storage the document owns</td> </tr> <tr> <td><strong>Mutability</strong></td> <td>freely mutable</td> <td>not applicable (a one-shot event stream)</td> <td>read-only</td> <td><a href=../../api/basic_json_document/set/ ><code>set</code></a>/<a href=../../api/basic_json_document/push_back/ ><code>push_back</code></a>/<a href=../../api/basic_json_document/insert/ ><code>insert</code></a>/<a href=../../api/basic_json_document/erase/ ><code>erase</code></a> edit in place; the source text is never rewritten</td> </tr> <tr> <td><strong>What you get</strong></td> <td>a full tree you can read, write, and keep as long as you like</td> <td>a sequence of callbacks; whatever your handler builds from them</td> <td>a flat index plus, on demand, <a href=../../api/basic_json_view/materialize/ ><code>materialize()</code></a>d <code>json</code>/<code>ordered_json</code> values for the parts you actually use</td> <td>the same, plus <a href=../../api/basic_json_view/dump/ ><code>dump()</code></a> of an edited document that keeps the member order and, with <a href=../../api/basic_json_view/number_format/ ><code>number_format::source</code></a>, the spelling of every untouched number</td> </tr> <tr> <td><strong>Typical use</strong></td> <td>general-purpose <abbr title="JavaScript Object Notation">JSON</abbr> handling: config, request/response bodies you build or modify, anything you hold onto</td> <td>validating or projecting a text into your own data structure without ever holding the whole thing as <abbr title="JavaScript Object Notation">JSON</abbr></td> <td>large or high-volume input where you only need part of it, or need it repeatedly, and can keep the source text (or a copy) alive for as long as the document lives</td> <td>a document you read, patch a few fields of, and write back — a configuration file, for instance — where the rest of it should come back exactly as it was</td> </tr> <tr> <td><strong>Caching/reload</strong></td> <td>not applicable — re-parse, or roll your own serialization</td> <td>not applicable</td> <td><a href=../../api/basic_json_document/save/ ><code>save()</code></a>/<a href=../../api/basic_json_document/load/ ><code>load()</code></a>: cache the parsed index as an image and reload it without parsing</td> <td>same, saving the document's current — possibly edited — state</td> </tr> </tbody> </table> <p class=nj-table-caption>Each column is one way to work with <abbr title="JavaScript Object Notation">JSON</abbr> in this library, and each row compares one property of them.</p> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">¶</a></h2> <ul> <li><a href=../../api/basic_json_document/ ><code>basic_json_document</code></a> - the document that owns the parsLine truncated
|
||||
+91
-20
@@ -10,7 +10,11 @@
|
||||
|
||||
[`basic_json_document::parse()`](https://json.nlohmann.me/api/basic_json_document/parse/index.md) parses the same JSON grammar, with the same options, but instead of a tree it builds a flat index of the values it found: one [16-byte entry](https://json.nlohmann.me/home/architecture/#node-index-of-json-views) per value (and one per object key), in document order. Integers are converted while parsing and stored in their entry. Floating-point numbers and strings are referenced in place in the source text: a float is converted when it is read, and only strings that contain escape sequences are decoded, into one shared buffer owned by the document.
|
||||
|
||||
[`basic_json_view`](https://json.nlohmann.me/api/basic_json_view/index.md) is a small, trivially copyable handle (two pointers) into that index. It gives you the read-only, type-inspection part of the `basic_json` interface -- [`type()`](https://json.nlohmann.me/api/basic_json_view/type/index.md) and the `is_*()` predicates, [`size()`](https://json.nlohmann.me/api/basic_json_view/size/index.md)/[`empty()`](https://json.nlohmann.me/api/basic_json_view/empty/index.md) -- as well as element access ([`operator[]`](https://json.nlohmann.me/api/basic_json_view/operator%5B%5D/index.md), [`at`](https://json.nlohmann.me/api/basic_json_view/at/index.md), [`front`](https://json.nlohmann.me/api/basic_json_view/front/index.md)/[`back`](https://json.nlohmann.me/api/basic_json_view/back/index.md)), lookup ([`find`](https://json.nlohmann.me/api/basic_json_view/find/index.md), [`contains`](https://json.nlohmann.me/api/basic_json_view/contains/index.md), [`count`](https://json.nlohmann.me/api/basic_json_view/count/index.md)), and iteration ([`begin`](https://json.nlohmann.me/api/basic_json_view/begin/index.md)/[`end`](https://json.nlohmann.me/api/basic_json_view/end/index.md), [`items`](https://json.nlohmann.me/api/basic_json_view/items/index.md)) -- without ever allocating a `basic_json` value. When you do need an actual `basic_json` value for a subtree, [`materialize()`](https://json.nlohmann.me/api/basic_json_view/materialize/index.md) builds exactly the one [`parse()`](https://json.nlohmann.me/api/basic_json/parse/index.md) would have produced for it.
|
||||
[`basic_json_view`](https://json.nlohmann.me/api/basic_json_view/index.md) is a small, trivially copyable handle (two pointers) into that index. It gives you the read-only, type-inspection part of the `basic_json` interface — [`type()`](https://json.nlohmann.me/api/basic_json_view/type/index.md) and the `is_*()` predicates, [`size()`](https://json.nlohmann.me/api/basic_json_view/size/index.md)/[`empty()`](https://json.nlohmann.me/api/basic_json_view/empty/index.md) — as well as element access ([`operator[]`](https://json.nlohmann.me/api/basic_json_view/operator%5B%5D/index.md), [`at`](https://json.nlohmann.me/api/basic_json_view/at/index.md), [`front`](https://json.nlohmann.me/api/basic_json_view/front/index.md)/[`back`](https://json.nlohmann.me/api/basic_json_view/back/index.md)), lookup ([`find`](https://json.nlohmann.me/api/basic_json_view/find/index.md), [`contains`](https://json.nlohmann.me/api/basic_json_view/contains/index.md), [`count`](https://json.nlohmann.me/api/basic_json_view/count/index.md)), and iteration ([`begin`](https://json.nlohmann.me/api/basic_json_view/begin/index.md)/[`end`](https://json.nlohmann.me/api/basic_json_view/end/index.md), [`items`](https://json.nlohmann.me/api/basic_json_view/items/index.md)) — without ever allocating a `basic_json` value. When you do need an actual `basic_json` value for a subtree, [`materialize()`](https://json.nlohmann.me/api/basic_json_view/materialize/index.md) builds exactly the one [`parse()`](https://json.nlohmann.me/api/basic_json/parse/index.md) would have produced for it.
|
||||
|
||||
The same small text, parsed both ways:
|
||||
|
||||
Top: the tree of separate heap allocations that `basic_json::parse()` builds. Bottom: `json_document::parse()` keeps the source text and builds one flat index of 16-byte nodes that point back into it, plus a buffer of decoded strings.
|
||||
|
||||
## How to use it
|
||||
|
||||
@@ -48,7 +52,7 @@ true
|
||||
|
||||
## Ownership and lifetime
|
||||
|
||||
A document either **borrows** the text it was parsed from, or **owns** its own copy of it; call [`owns_source()`](https://json.nlohmann.me/api/basic_json_document/owns_source/index.md) to find out which happened. [`parse()`](https://json.nlohmann.me/api/basic_json_document/parse/index.md) decides this from the value category and type of its argument (an lvalue `std::string` is borrowed; an rvalue `std::string` is moved in, owned without a copy; a stream is read into an owned buffer; and so on -- see [`parse`'s Notes](https://json.nlohmann.me/api/basic_json_document/parse/#notes) for the full table). [`parse_copy()`](https://json.nlohmann.me/api/basic_json_document/parse_copy/index.md) always owns a copy, regardless of the input.
|
||||
A document either **borrows** the text it was parsed from, or **owns** its own copy of it; call [`owns_source()`](https://json.nlohmann.me/api/basic_json_document/owns_source/index.md) to find out which happened. [`parse()`](https://json.nlohmann.me/api/basic_json_document/parse/index.md) decides this from the value category and type of its argument (an lvalue `std::string` is borrowed; an rvalue `std::string` is moved in, owned without a copy; a stream is read into an owned buffer; and so on — see [`parse`'s Notes](https://json.nlohmann.me/api/basic_json_document/parse/#notes) for the full table). [`parse_copy()`](https://json.nlohmann.me/api/basic_json_document/parse_copy/index.md) always owns a copy, regardless of the input.
|
||||
|
||||
A borrowed document depends on your buffer
|
||||
|
||||
@@ -63,7 +67,9 @@ A view is valid only while all of the following hold:
|
||||
|
||||
Moving the `basic_json_document<>` itself is fine and does **not** invalidate its views: the index is a separate heap allocation that keeps its address across the move. Take a fresh view from [`root()`](https://json.nlohmann.me/api/basic_json_document/root/index.md) whenever any of the other conditions above was not met.
|
||||
|
||||
A view can dangle, exactly like a `std::string_view`: nothing stops you from using it after its document is gone. This is deliberate. A view is two pointers and trivially copyable; making it safe (a `std::shared_ptr` in the document and a `std::weak_ptr` in each view) would make it larger, add reference counting to every copy, and a `lock()` to every access -- exactly the costs the view exists to avoid. What is caught is the obvious case: because a view dies with its document, [`root()`](https://json.nlohmann.me/api/basic_json_document/root/index.md) is not callable on a temporary document: `auto v = json_document::parse(text).root();` does not compile. Give the document a name first.
|
||||
Four timelines of a document and its views, each bar showing when an object is alive or valid and when it dangles. Destroying the borrowed text, re-parsing, or shrinking the document invalidates the views taken before; moving the document does not.
|
||||
|
||||
A view can dangle, exactly like a `std::string_view`: nothing stops you from using it after its document is gone. This is deliberate. A view is two pointers and trivially copyable; making it safe (a `std::shared_ptr` in the document and a `std::weak_ptr` in each view) would make it larger, add reference counting to every copy, and a `lock()` to every access — exactly the costs the view exists to avoid. What is caught is the obvious case: because a view dies with its document, [`root()`](https://json.nlohmann.me/api/basic_json_document/root/index.md) is not callable on a temporary document: `auto v = json_document::parse(text).root();` does not compile. Give the document a name first.
|
||||
|
||||
Example: borrowed and owned documents, and when views become invalid
|
||||
|
||||
@@ -115,7 +121,7 @@ true
|
||||
## What is the same as `parse()`
|
||||
|
||||
- **Accept/reject.** [`accept()`](https://json.nlohmann.me/api/basic_json_document/accept/index.md) and [`parse()`](https://json.nlohmann.me/api/basic_json_document/parse/index.md) accept and reject exactly the same inputs as [`basic_json::accept()`](https://json.nlohmann.me/api/basic_json/accept/index.md)/[`basic_json::parse()`](https://json.nlohmann.me/api/basic_json/parse/index.md), with the same `ignore_comments` and `ignore_trailing_commas` options.
|
||||
- **Errors.** A failing parse throws the same exception -- the same id, message, and position -- because on failure the library's own parser is run on the same bytes to produce the diagnostic. Valid input never pays for this, and the replay builds no values: it costs about a lexing pass up to the error. `allow_exceptions == false` skips the replay and gives a [discarded](https://json.nlohmann.me/api/basic_json_document/is_discarded/index.md) document instead of throwing, just as it gives a discarded value for `basic_json::parse()`.
|
||||
- **Errors.** A failing parse throws the same exception — the same id, message, and position — because on failure the library's own parser is run on the same bytes to produce the diagnostic. Valid input never pays for this, and the replay builds no values: it costs about a lexing pass up to the error. `allow_exceptions == false` skips the replay and gives a [discarded](https://json.nlohmann.me/api/basic_json_document/is_discarded/index.md) document instead of throwing, just as it gives a discarded value for `basic_json::parse()`.
|
||||
- **Number classification.** An integer literal that does not fit into the 64-bit integer type becomes a floating-point value, exactly as it does for `basic_json::parse()`.
|
||||
- **Macros.** [`JSON_STRICT_NUL_HANDLING`](https://json.nlohmann.me/api/macros/json_strict_nul_handling/index.md) and [`JSON_NOEXCEPTION`](https://json.nlohmann.me/api/macros/json_noexception/index.md)/[`JSON_THROW_USER`](https://json.nlohmann.me/api/macros/json_throw_user/index.md) behave the same way they do for `<nlohmann/json.hpp>`. These are the macros that change what the parser accepts or how it reports errors. The others either do not apply ([`JSON_DIAGNOSTICS`](https://json.nlohmann.me/api/macros/json_diagnostics/index.md) needs parent pointers in `basic_json` values) or only affect the values `materialize()` produces, through `BasicJsonType`.
|
||||
|
||||
@@ -129,7 +135,7 @@ true
|
||||
- **Chained access is safe.** [`operator[]`](https://json.nlohmann.me/api/basic_json_view/operator%5B%5D/index.md) with a missing key, an index out of range, or an unresolvable JSON pointer returns a [discarded](https://json.nlohmann.me/api/basic_json_view/is_discarded/index.md) view, and `operator[]` on a discarded view returns a discarded view without throwing: `v["a"]["b"][0]` can be tested once at the end. Type errors on values that exist (a key on an array, an index on an object) still throw, and [`at`](https://json.nlohmann.me/api/basic_json_view/at/index.md) throws for every missing value.
|
||||
- **Duplicate keys: lookups find the first member, `parse()` keeps the last.** See [Duplicate keys](#duplicate-keys) below.
|
||||
- **No [`JSON_DIAGNOSTICS`](https://json.nlohmann.me/api/macros/json_diagnostics/index.md) path.** Exceptions thrown by `basic_json_view`'s own element access and lookup functions never carry the JSON Pointer path `JSON_DIAGNOSTICS` would otherwise add: the view has no `basic_json` value to point at, so the exception is created without one, regardless of how `BasicJsonType` was built.
|
||||
- **Ordering comparisons are not provided** by `basic_json_view` -- there is no `operator<`. [`operator==`](https://json.nlohmann.me/api/basic_json_view/operator_eq/index.md) and [`operator!=`](https://json.nlohmann.me/api/basic_json_view/operator_ne/index.md) are provided, though: two views, or a view and a `BasicJsonType` value, compare equal exactly when [`materialize()`](https://json.nlohmann.me/api/basic_json_view/materialize/index.md) or [`parse()`](https://json.nlohmann.me/api/basic_json/parse/index.md) would produce equal values for them, without ever building a tree to do it. For ordering, too, [`materialize()`](https://json.nlohmann.me/api/basic_json_view/materialize/index.md) is the way to get a value you can compare.
|
||||
- **Ordering comparisons are not provided** by `basic_json_view` — there is no `operator<`. [`operator==`](https://json.nlohmann.me/api/basic_json_view/operator_eq/index.md) and [`operator!=`](https://json.nlohmann.me/api/basic_json_view/operator_ne/index.md) are provided, though: two views, or a view and a `BasicJsonType` value, compare equal exactly when [`materialize()`](https://json.nlohmann.me/api/basic_json_view/materialize/index.md) or [`parse()`](https://json.nlohmann.me/api/basic_json/parse/index.md) would produce equal values for them, without ever building a tree to do it. For ordering, too, [`materialize()`](https://json.nlohmann.me/api/basic_json_view/materialize/index.md) is the way to get a value you can compare.
|
||||
|
||||
## Duplicate keys
|
||||
|
||||
@@ -167,10 +173,10 @@ If you cannot trust the source text to have unique keys, check [`size()`](https:
|
||||
|
||||
Two conversions never copy at all:
|
||||
|
||||
- [`get_string()`](https://json.nlohmann.me/api/basic_json_view/get_string/index.md) (equivalently, `get<string_view_t>()`) returns a string as a `string_view_t` pointing into the document's [`source()`](https://json.nlohmann.me/api/basic_json_document/source/index.md) text -- or, for a string that contains escape sequences, into the document's own buffer of decoded strings -- instead of allocating a new `std::string`.
|
||||
- [`get_string()`](https://json.nlohmann.me/api/basic_json_view/get_string/index.md) (equivalently, `get<string_view_t>()`) returns a string as a `string_view_t` pointing into the document's [`source()`](https://json.nlohmann.me/api/basic_json_document/source/index.md) text — or, for a string that contains escape sequences, into the document's own buffer of decoded strings — instead of allocating a new `std::string`.
|
||||
- [`number_token()`](https://json.nlohmann.me/api/basic_json_view/number_token/index.md) returns a number exactly as it was written in the source, e.g. `"1.50"`, `"1E2"`, or an integer with more digits than any number type holds, instead of rounding it into a `double`/`int64_t` the way `get<T>()` (and [`basic_json::parse()`](https://json.nlohmann.me/api/basic_json/parse/index.md)) would.
|
||||
|
||||
Both results are only valid as long as the view -- and, for a string with no escapes, the borrowed source text -- is.
|
||||
Both results are only valid as long as the view — and, for a string with no escapes, the borrowed source text — is.
|
||||
|
||||
## Writing generic code
|
||||
|
||||
@@ -230,26 +236,64 @@ Two things differ when a `from_json()` reads the view:
|
||||
|
||||
## Writing a view back
|
||||
|
||||
[`dump()`](https://json.nlohmann.me/api/basic_json_view/dump/index.md) serializes a view directly from the flat index, without ever building a `basic_json` value. An object's members are written in document order, not sorted by key, and *every* occurrence of a repeated key is written, not only the last one -- the same two ways [iteration](#what-is-different) already differs from a [`materialize()`](https://json.nlohmann.me/api/basic_json_view/materialize/index.md)d value, see [Duplicate keys](#duplicate-keys). `materialize().dump()` gives a different result in both respects for a `json_view`.
|
||||
[`dump()`](https://json.nlohmann.me/api/basic_json_view/dump/index.md) serializes a view directly from the flat index, without ever building a `basic_json` value. An object's members are written in document order, not sorted by key, and *every* occurrence of a repeated key is written, not only the last one — the same two ways [iteration](#what-is-different) already differs from a [`materialize()`](https://json.nlohmann.me/api/basic_json_view/materialize/index.md)d value, see [Duplicate keys](#duplicate-keys). `materialize().dump()` gives a different result in both respects for a `json_view`.
|
||||
|
||||
By default, numbers are written the way [`basic_json::dump()`](https://json.nlohmann.me/api/basic_json/dump/index.md) would. [`number_format::source`](https://json.nlohmann.me/api/basic_json_view/number_format/index.md) instead copies every number exactly as it was written in the source text -- a price like `19.90`, a long order or account ID with more digits than any number type holds, or a high-precision coordinate -- something `basic_json` cannot do at all, since parsing already reduces a number to its parsed `double`/`int64_t` value.
|
||||
By default, numbers are written the way [`basic_json::dump()`](https://json.nlohmann.me/api/basic_json/dump/index.md) would. [`number_format::source`](https://json.nlohmann.me/api/basic_json_view/number_format/index.md) instead copies every number exactly as it was written in the source text — a price like `19.90`, a long order or account ID with more digits than any number type holds, or a high-precision coordinate — something `basic_json` cannot do at all, since parsing already reduces a number to its parsed `double`/`int64_t` value.
|
||||
|
||||
[`operator<<`](https://json.nlohmann.me/api/basic_json_view/operator_ltlt/index.md) writes a view to a stream the way `basic_json`'s does, using the stream's `width`/`fill` for indentation.
|
||||
|
||||
## Editing a document
|
||||
|
||||
Everything above is read-only: a `json_document`/`json_view` lets you look at a parsed text without copying it, but not change it. [`basic_json_document<BasicJsonType, view_mode::editable>`](https://json.nlohmann.me/api/basic_json_document/index.md) -- more conveniently spelled [`json_editable_document`](https://json.nlohmann.me/api/json_editable_document/index.md) or [`ordered_json_editable_document`](https://json.nlohmann.me/api/ordered_json_editable_document/index.md) -- also lets you [`set`](https://json.nlohmann.me/api/basic_json_document/set/index.md) a value, [`push_back`](https://json.nlohmann.me/api/basic_json_document/push_back/index.md) onto or [`insert`](https://json.nlohmann.me/api/basic_json_document/insert/index.md) into an array, and [`erase`](https://json.nlohmann.me/api/basic_json_document/erase/index.md) an object member or an array element, still without ever building a `basic_json` tree for parts you do not touch.
|
||||
Everything above is read-only: a `json_document`/`json_view` lets you look at a parsed text without copying it, but not change it. [`basic_json_document<BasicJsonType, view_mode::editable>`](https://json.nlohmann.me/api/basic_json_document/index.md) — more conveniently spelled [`json_editable_document`](https://json.nlohmann.me/api/json_editable_document/index.md) or [`ordered_json_editable_document`](https://json.nlohmann.me/api/ordered_json_editable_document/index.md) — also lets you [`set`](https://json.nlohmann.me/api/basic_json_document/set/index.md) a value, [`push_back`](https://json.nlohmann.me/api/basic_json_document/push_back/index.md) onto or [`insert`](https://json.nlohmann.me/api/basic_json_document/insert/index.md) into an array, and [`erase`](https://json.nlohmann.me/api/basic_json_document/erase/index.md) an object member or an array element, still without ever building a `basic_json` tree for parts you do not touch.
|
||||
|
||||
The [mode](https://json.nlohmann.me/api/view_mode/index.md) defaults to `view_mode::read_only`, so `json_document`/`ordered_json_document` are unaffected -- they carry none of the bookkeeping edits need, and calling `set`/`push_back`/`insert`/`erase` on one is a compile error, not a runtime one.
|
||||
The [mode](https://json.nlohmann.me/api/view_mode/index.md) defaults to `view_mode::read_only`, so `json_document`/`ordered_json_document` are unaffected — they carry none of the bookkeeping edits need, and calling `set`/`push_back`/`insert`/`erase` on one is a compile error, not a runtime one.
|
||||
|
||||
### Why: editing without reformatting
|
||||
|
||||
The `19.90` price from [above](#writing-a-view-back) is exactly the kind of value that makes editing a `json` or `ordered_json` value in place lossy. Say you parse a configuration file, patch one field, and write it back:
|
||||
|
||||
- **`json`** re-sorts every key on the way in (`object_t` is a `std::map`) and rewrites every number to its shortest round-trip form on the way out -- a one-field patch turns into a diff that reorders the whole file and rewrites `19.90` to `19.9`.
|
||||
- **`json`** re-sorts every key on the way in (`object_t` is a `std::map`) and rewrites every number to its shortest round-trip form on the way out — a one-field patch turns into a diff that reorders the whole file and rewrites `19.90` to `19.9`.
|
||||
- **`ordered_json`** keeps the key order, but still rewrites every number the same way: parsing has already reduced it to a `double`/`int64_t`, and there is no way back to how it was spelled in the source text.
|
||||
|
||||
An editable document keeps both. [`dump()`](https://json.nlohmann.me/api/basic_json_view/dump/index.md) of an edited document writes members in document order -- a member [`set`](https://json.nlohmann.me/api/basic_json_document/set/index.md) added goes at the end, exactly where it was inserted, and an [`erase`](https://json.nlohmann.me/api/basic_json_document/erase/index.md)d member simply leaves a gap: everything around it keeps its place -- and [`number_format::source`](https://json.nlohmann.me/api/basic_json_view/number_format/index.md) keeps the exact spelling of every number an edit did not itself touch; a number an edit *did* touch is written the way [`BasicJsonType::dump()`](https://json.nlohmann.me/api/basic_json/dump/index.md) would write it, since there is no source spelling for a brand new value.
|
||||
The patch of the example below (set `"host"`, add `"retries"`) written back by each of them:
|
||||
|
||||
`json`
|
||||
|
||||
keys sorted, numbers rewritten
|
||||
|
||||
```
|
||||
{"host":"db2",
|
||||
"name":"cache",
|
||||
"port":6379,
|
||||
"price":19.9,
|
||||
"retries":3}
|
||||
```
|
||||
|
||||
`ordered_json`
|
||||
|
||||
key order kept, numbers rewritten
|
||||
|
||||
```
|
||||
{"name":"cache",
|
||||
"host":"db2",
|
||||
"port":6379,
|
||||
"price":19.9,
|
||||
"retries":3}
|
||||
```
|
||||
|
||||
`json_editable_document`
|
||||
|
||||
order and spelling kept
|
||||
|
||||
```
|
||||
{"name":"cache",
|
||||
"host":"db2",
|
||||
"port":6379,
|
||||
"price":19.90,
|
||||
"retries":3}
|
||||
```
|
||||
|
||||
An editable document keeps both. [`dump()`](https://json.nlohmann.me/api/basic_json_view/dump/index.md) of an edited document writes members in document order — a member [`set`](https://json.nlohmann.me/api/basic_json_document/set/index.md) added goes at the end, exactly where it was inserted, and an [`erase`](https://json.nlohmann.me/api/basic_json_document/erase/index.md)d member simply leaves a gap: everything around it keeps its place — and [`number_format::source`](https://json.nlohmann.me/api/basic_json_view/number_format/index.md) keeps the exact spelling of every number an edit did not itself touch; a number an edit *did* touch is written the way [`BasicJsonType::dump()`](https://json.nlohmann.me/api/basic_json/dump/index.md) would write it, since there is no source spelling for a brand new value.
|
||||
|
||||
Example: patch a configuration, keeping member order and number spellings
|
||||
|
||||
@@ -297,15 +341,29 @@ Output:
|
||||
|
||||
### What stays valid, and what an edit costs
|
||||
|
||||
The source text itself is **never written**, and the parsed index never moves -- a value keeps the node it was parsed into for as long as it is not itself replaced. So every [view](https://json.nlohmann.me/api/basic_json_view/index.md) taken before an edit, including a previously obtained [`root()`](https://json.nlohmann.me/api/basic_json_document/root/index.md), stays valid and, if it still refers to the edited value, sees the edit; a view of a value a later edit drops or replaces just keeps showing what it last held. New values go to storage the document allocates and owns on demand. The one thing an edit does invalidate is the **iterators** taken over an edited array or object: the first time one of its elements is set, appended to, inserted into, or erased, its elements move from the parsed, fixed layout to a growable block of links so that [`push_back`](https://json.nlohmann.me/api/basic_json_document/push_back/index.md) can later grow it in amortized constant time -- existing elements are not touched, but an iterator that was walking the old layout no longer matches. A string obtained with [`get_string()`](https://json.nlohmann.me/api/basic_json_view/get_string/index.md) is unaffected either way and stays valid across further edits. See [`basic_json_document`'s Edits](https://json.nlohmann.me/api/basic_json_document/#edits) for the details, and [`set`'s Exception safety](https://json.nlohmann.me/api/basic_json_document/set/#exception-safety) for what an edit guarantees if it throws (the *basic* guarantee, not the strong one `dump()` and the read-only functions provide). How edits are kept in the index is described in the [architecture overview](https://json.nlohmann.me/home/architecture/#node-index-of-json-views).
|
||||
The source text itself is **never written**, and the parsed index never moves — a value keeps the node it was parsed into for as long as it is not itself replaced. So every [view](https://json.nlohmann.me/api/basic_json_view/index.md) taken before an edit, including a previously obtained [`root()`](https://json.nlohmann.me/api/basic_json_document/root/index.md), stays valid and, if it still refers to the edited value, sees the edit; a view of a value a later edit drops or replaces just keeps showing what it last held. New values go to storage the document allocates and owns on demand. The one thing an edit does invalidate is the **iterators** taken over an edited array or object: the first time an element is added to it, inserted into it, erased from it, or replaced by a value that spans a different number of nodes, its elements move from the parsed, fixed layout to a growable block of links so that [`push_back`](https://json.nlohmann.me/api/basic_json_document/push_back/index.md) can later grow it in amortized constant time — existing elements are not touched, but an iterator that was walking the old layout no longer matches. A string obtained with [`get_string()`](https://json.nlohmann.me/api/basic_json_view/get_string/index.md) is unaffected either way and stays valid across further edits. See [`basic_json_document`'s Edits](https://json.nlohmann.me/api/basic_json_document/#edits) for the details, and [`set`'s Exception safety](https://json.nlohmann.me/api/basic_json_document/set/#exception-safety) for what an edit guarantees if it throws (the *basic* guarantee, not the strong one `dump()` and the read-only functions provide). How edits are kept in the index is described in the [architecture overview](https://json.nlohmann.me/home/architecture/#node-index-of-json-views).
|
||||
|
||||
Step through the edits of a small document (the node fields follow the [architecture overview](https://json.nlohmann.me/home/architecture/#node-index-of-json-views)):
|
||||
|
||||
Step 1 of 5. After parsing: nine nodes in the parsed index, no owned storage yet.
|
||||
|
||||
Step 2 of 5. `doc.set(doc.root(), "port", 6380)` overwrites the node of the integer in place; no member is added or removed, so the object keeps its parsed layout.
|
||||
|
||||
Step 3 of 5. `doc.erase(doc.root()["tags"], 0)`: the array is edited for the first time, so its elements move to a block of links in owned storage. The node of `"eu"` stays in the index, but nothing refers to it any more; iterators over the array taken before are invalid.
|
||||
|
||||
Step 4 of 5. `doc.push_back(doc.root()["tags"], "new")`: the new string gets its own node, its text goes to the buffer of edited text, and a link to it is appended to the block of the array.
|
||||
|
||||
Step 5 of 5. `doc.set(doc.root(), "retries", 3)`: a new member, so the object moves to a block too: copies of its keys, links to the values it already had, then the new key and a link to the new integer.
|
||||
|
||||
The node index and the owned storage of the document after each of five edits. Use *Back*, *Next*, or the numbered buttons to switch between the steps; without JavaScript all steps are shown one below the other.
|
||||
|
||||
## Images
|
||||
|
||||
[`save()`](https://json.nlohmann.me/api/basic_json_document/save/index.md) writes a document as an *image*: a byte buffer that [`load()`](https://json.nlohmann.me/api/basic_json_document/load/index.md) reads back into a document without parsing -- no lexing, no building the node index, nothing but copying the nodes and pointing the text and the decoded strings at the image. Where [`parse_copy()`](https://json.nlohmann.me/api/basic_json_document/parse_copy/index.md) still has to scan the whole input, [`load()`](https://json.nlohmann.me/api/basic_json_document/load/index.md) turns that scan into a copy of the node index alone.
|
||||
[`save()`](https://json.nlohmann.me/api/basic_json_document/save/index.md) writes a document as an *image*: a byte buffer that [`load()`](https://json.nlohmann.me/api/basic_json_document/load/index.md) reads back into a document without parsing — no lexing, no building the node index, nothing but copying the nodes and pointing the text and the decoded strings at the image. Where [`parse_copy()`](https://json.nlohmann.me/api/basic_json_document/parse_copy/index.md) still has to scan the whole input, [`load()`](https://json.nlohmann.me/api/basic_json_document/load/index.md) turns that scan into a copy of the node index alone.
|
||||
|
||||
**Why.** A document that is parsed once and then read many times -- a configuration loaded at startup, a template rendered on every request, a large reference dataset a worker process needs in memory -- pays for parsing once but can amortize [`save()`](https://json.nlohmann.me/api/basic_json_document/save/index.md)'s cost across every later load. That makes images useful for a cache: save a document the first time it is parsed (to a file, a shared-memory segment, an in-process cache), and [`load()`](https://json.nlohmann.me/api/basic_json_document/load/index.md) it on every later use instead of parsing the source text again. They are just as useful for handing a parsed document to another process (or a forked worker) running the same build of the library, since [`load()`](https://json.nlohmann.me/api/basic_json_document/load/index.md) turns the transfer into a copy of the node index plus pointers into the received bytes, not a re-parse.
|
||||
**Why.** A document that is parsed once and then read many times — a configuration loaded at startup, a template rendered on every request, a large reference dataset a worker process needs in memory — pays for parsing once but can amortize [`save()`](https://json.nlohmann.me/api/basic_json_document/save/index.md)'s cost across every later load. That makes images useful for a cache: save a document the first time it is parsed (to a file, a shared-memory segment, an in-process cache), and [`load()`](https://json.nlohmann.me/api/basic_json_document/load/index.md) it on every later use instead of parsing the source text again. They are just as useful for handing a parsed document to another process (or a forked worker) running the same build of the library, since [`load()`](https://json.nlohmann.me/api/basic_json_document/load/index.md) turns the transfer into a copy of the node index plus pointers into the received bytes, not a re-parse.
|
||||
|
||||
**Choosing a check.** [`load()`](https://json.nlohmann.me/api/basic_json_document/load/index.md) takes an [`image_check`](https://json.nlohmann.me/api/basic_json_document/load/#image_check) that trades validation against speed: `image_check::full` (the default) checks everything the parser itself guarantees, so a checked image is exactly as safe to read and serialize as a freshly parsed document -- the right choice whenever the image did not come straight from this process's own [`save()`](https://json.nlohmann.me/api/basic_json_document/save/index.md), such as a file or a network peer. `image_check::bounds` only checks structure and bounds -- cheaper, since it skips scanning the text and the decoded strings -- and fits a cache the process trusts, one it wrote and reads back itself. `image_check::none` skips validation entirely, for an image trusted as much as the process's own memory. See [`load()`'s Notes](https://json.nlohmann.me/api/basic_json_document/load/#notes) for exactly what each level does and does not guarantee.
|
||||
**Choosing a check.** [`load()`](https://json.nlohmann.me/api/basic_json_document/load/index.md) takes an [`image_check`](https://json.nlohmann.me/api/basic_json_document/load/#image_check) that trades validation against speed: `image_check::full` (the default) checks everything the parser itself guarantees, so a checked image is exactly as safe to read and serialize as a freshly parsed document — the right choice whenever the image did not come straight from this process's own [`save()`](https://json.nlohmann.me/api/basic_json_document/save/index.md), such as a file or a network peer. `image_check::bounds` only checks structure and bounds — cheaper, since it skips scanning the text and the decoded strings — and fits a cache the process trusts, one it wrote and reads back itself. `image_check::none` skips validation entirely, for an image trusted as much as the process's own memory. See [`load()`'s Notes](https://json.nlohmann.me/api/basic_json_document/load/#notes) for exactly what each level does and does not guarantee.
|
||||
|
||||
Example: cache a parsed configuration as an image
|
||||
|
||||
@@ -352,7 +410,7 @@ true
|
||||
|
||||
Experimental
|
||||
|
||||
The image format is versioned but not yet stable, and may change in an incompatible way before it is declared stable. It is little-endian only (on big-endian targets, `save()` and `load()` do not exist), and tied to the library build that wrote it -- use it to cache a document or to hand one to another process running the *same* build, not as a long-term storage format; keep the original JSON text if a saved document needs to be readable by a future library version.
|
||||
The image format is versioned but not yet stable, and may change in an incompatible way before it is declared stable. It is little-endian only (on big-endian targets, `save()` and `load()` do not exist), and tied to the library build that wrote it — use it to cache a document or to hand one to another process running the *same* build, not as a long-term storage format; keep the original JSON text if a saved document needs to be readable by a future library version.
|
||||
|
||||
The idea of a document you can read without parsing comes from zero-copy formats such as [FlatBuffers](https://github.com/google/flatbuffers) and [YaFF](https://github.com/yandex/yaff); the check [`load()`](https://json.nlohmann.me/api/basic_json_document/load/index.md) runs follows the idea of FlatBuffers' Verifier. No code is taken from either.
|
||||
|
||||
@@ -363,8 +421,21 @@ The idea of a document you can read without parsing comes from zero-copy formats
|
||||
| **Ownership** | owns every value | owns nothing; you decide what to keep, in your handler | borrows or owns the *text*; the index is always owned by the document | same as `json_document`; edits go to storage the document owns |
|
||||
| **Mutability** | freely mutable | not applicable (a one-shot event stream) | read-only | [`set`](https://json.nlohmann.me/api/basic_json_document/set/index.md)/[`push_back`](https://json.nlohmann.me/api/basic_json_document/push_back/index.md)/[`insert`](https://json.nlohmann.me/api/basic_json_document/insert/index.md)/[`erase`](https://json.nlohmann.me/api/basic_json_document/erase/index.md) edit in place; the source text is never rewritten |
|
||||
| **What you get** | a full tree you can read, write, and keep as long as you like | a sequence of callbacks; whatever your handler builds from them | a flat index plus, on demand, [`materialize()`](https://json.nlohmann.me/api/basic_json_view/materialize/index.md)d `json`/`ordered_json` values for the parts you actually use | the same, plus [`dump()`](https://json.nlohmann.me/api/basic_json_view/dump/index.md) of an edited document that keeps the member order and, with [`number_format::source`](https://json.nlohmann.me/api/basic_json_view/number_format/index.md), the spelling of every untouched number |
|
||||
| **Typical use** | general-purpose JSON handling: config, request/response bodies you build or modify, anything you hold onto | validating or projecting a text into your own data structure without ever holding the whole thing as JSON | large or high-volume input where you only need part of it, or need it repeatedly, and can keep the source text (or a copy) alive for as long as the document lives | a document you read, patch a few fields of, and write back -- a configuration file, for instance -- where the rest of it should come back exactly as it was |
|
||||
| **Caching/reload** | not applicable -- re-parse, or roll your own serialization | not applicable | [`save()`](https://json.nlohmann.me/api/basic_json_document/save/index.md)/[`load()`](https://json.nlohmann.me/api/basic_json_document/load/index.md): cache the parsed index as an image and reload it without parsing | same, saving the document's current -- possibly edited -- state |
|
||||
| **Typical use** | general-purpose JSON handling: config, request/response bodies you build or modify, anything you hold onto | validating or projecting a text into your own data structure without ever holding the whole thing as JSON | large or high-volume input where you only need part of it, or need it repeatedly, and can keep the source text (or a copy) alive for as long as the document lives | a document you read, patch a few fields of, and write back — a configuration file, for instance — where the rest of it should come back exactly as it was |
|
||||
| **Caching/reload** | not applicable — re-parse, or roll your own serialization | not applicable | [`save()`](https://json.nlohmann.me/api/basic_json_document/save/index.md)/[`load()`](https://json.nlohmann.me/api/basic_json_document/load/index.md): cache the parsed index as an image and reload it without parsing | same, saving the document's current — possibly edited — state |
|
||||
|
||||
Each column is one way to work with JSON in this library, and each row compares one property of them.
|
||||
|
||||
## See also
|
||||
|
||||
- [`basic_json_document`](https://json.nlohmann.me/api/basic_json_document/index.md) - the document that owns the parsed index (parse, access, images, edits)
|
||||
- [`basic_json_view`](https://json.nlohmann.me/api/basic_json_view/index.md) - the non-owning handle to a value in a document
|
||||
- [`json_document`](https://json.nlohmann.me/api/json_document/index.md) - and [`ordered_json_document`](https://json.nlohmann.me/api/ordered_json_document/index.md) - the read-only documents
|
||||
- [`json_editable_document`](https://json.nlohmann.me/api/json_editable_document/index.md) - and [`ordered_json_editable_document`](https://json.nlohmann.me/api/ordered_json_editable_document/index.md) - the documents that can be edited in place
|
||||
- [Architecture](https://json.nlohmann.me/home/architecture/#node-index-of-json-views) - the layout of the node index
|
||||
- [SAX interface](https://json.nlohmann.me/features/parsing/sax_interface/index.md) - event-based parsing without building a tree
|
||||
- [Performance](https://json.nlohmann.me/features/performance/index.md) - benchmarks and tuning of parsing and serialization
|
||||
- [Large, Streamed, and Partial Input](https://json.nlohmann.me/features/parsing/large_and_partial_input/index.md) - reading only the part of a text you need
|
||||
|
||||
## Version history
|
||||
|
||||
|
||||
@@ -322,3 +322,17 @@ These macros are defined by the library and contain the version numbers accordin
|
||||
[Semantic Versioning 2.0.0](https://semver.org/spec/v2.0.0.html).
|
||||
|
||||
See [full documentation of `NLOHMANN_JSON_VERSION_MAJOR`, `NLOHMANN_JSON_VERSION_MINOR`, and `NLOHMANN_JSON_VERSION_PATCH`](../api/macros/nlohmann_json_version_major.md).
|
||||
|
||||
## See also
|
||||
|
||||
- [Macros](../api/macros/index.md) - reference of all macros
|
||||
- [Runtime Assertions](assertions.md) - runtime assertions in detail
|
||||
- [`nlohmann` Namespace](namespace.md) - the library namespace and its macros
|
||||
- [Working Without Exceptions](no_exceptions.md) - using the library without exceptions
|
||||
- [Modules](modules.md) - macros and C++20 modules
|
||||
- [Exceptions](../home/exceptions.md) - all exceptions and their error codes
|
||||
- [CMake](../integration/cmake.md) - set macros via CMake options
|
||||
- [Compile times](../integration/compile_times.md) - reduce compile times with macros
|
||||
- [Architecture](../home/architecture.md) - how the library is put together
|
||||
- [Specializing enum conversion](enum_conversion.md) - enum macros
|
||||
- [Arbitrary Type Conversions](arbitrary_types.md) - type conversion macros
|
||||
@@ -1 +1 @@
|
||||
<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/macros/" rel="canonical"><link href=../enum_conversion/ rel=prev><link href=../trailing_commas/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>Supported Macros - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.ab4e12ef.min.css><link rel="stylesheet" href="../../assets/external/fonts.googleapis.com/css.61a430c9.css"><style>:root{--md-text-font:"Roboto";--md-code-font:"JetBrains Mono"}</style><link rel=stylesheet href=../../css/custom.css><script>__md_scope=new 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xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 8a3 3 0 0 0 3-3 3 3 0 0 0-3-3 3 3 0 0 0-3 3 3 3 0 0 0 3 3m0 3.54C9.64 9.35 6.5 8 3 8v11c3.5 0 6.64 1.35 9 3.54 2.36-2.19 5.5-3.54 9-3.54V8c-3.5 0-6.64 1.35-9 3.54"/></svg> </a> <label class="md-header__button md-icon" for=__drawer> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M3 6h18v2H3zm0 5h18v2H3zm0 5h18v2H3z"/></svg> </label> <div class=md-header__title data-md-component=header-title> <div class=md-header__ellipsis> <div class=md-header__topic> <span class=md-ellipsis> JSON for Modern C++ </span> </div> <div class=md-header__topic data-md-component=header-topic> <span class=md-ellipsis> Supported Macros </span> </div> </div> </div> <form class=md-header__option data-md-component=palette> <input class=md-option data-md-color-media="(prefers-color-scheme: light)" data-md-color-scheme=default data-md-color-primary=indigo data-md-color-accent=indigo aria-label="Switch to dark mode" type=radio 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15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> </form> <script>var palette=__md_get("__palette");if(palette&&palette.color){if("(prefers-color-scheme)"===palette.color.media){var media=matchMedia("(prefers-color-scheme: light)"),input=document.querySelector(media.matches?"[data-md-color-media='(prefers-color-scheme: light)']":"[data-md-color-media='(prefers-color-scheme: dark)']");palette.color.media=input.getAttribute("data-md-color-media"),palette.color.scheme=input.getAttribute("data-md-color-scheme"),palette.color.primary=input.getAttribute("data-md-color-primary"),palette.color.accent=input.getAttribute("data-md-color-accent")}for(var[key,value]of Object.entries(palette.color))document.body.setAttribute("data-md-color-"+key,value)}</script> <label class="md-header__button md-icon" for=__search> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M9.5 3A6.5 6.5 0 0 1 16 9.5c0 1.61-.59 3.09-1.56 4.23l.27.27h.79l5 5-1.5 1.5-5-5v-.79l-.27-.27A6.52 6.52 0 0 1 9.5 16 6.5 6.5 0 0 1 3 9.5 6.5 6.5 0 0 1 9.5 3m0 2C7 5 5 7 5 9.5S7 14 9.5 14 14 12 14 9.5 12 5 9.5 5"/></svg> </labelLine truncated
|
||||
<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=description content="JSON for Modern C++ is a header-only C++11 library that makes JSON a first-class data type."><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/macros/" rel="canonical"><link href=../enum_conversion/ rel=prev><link href=../trailing_commas/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>Supported Macros - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.ab4e12ef.min.css><link rel=stylesheet href=../../css/custom.css><link rel=stylesheet href=../../css/brand.css><link rel=stylesheet href=../../css/fig-sax.css><link rel=stylesheet href=../../css/fig-jsonview.css><link rel=stylesheet href=../../css/fig-exceptions.css><link rel=stylesheet href=../../css/fig-pointer.css><link rel=stylesheet href=../../css/fig-charts.css><link rel=stylesheet href=../../css/fig-integration.css><link rel=stylesheet href=../../css/fig-macros.css><script>__md_scope=new URL("../..",location),__md_hash=e=>[...e].reduce(((e,_)=>(e<<5)-e+_.charCodeAt(0)),0),__md_get=(e,_=localStorage,t=__md_scope)=>JSON.parse(_.getItem(t.pathname+"."+e)),__md_set=(e,_,t=localStorage,a=__md_scope)=>{try{t.setItem(a.pathname+"."+e,JSON.stringify(_))}catch(e){}}</script></head> <body dir=ltr data-md-color-scheme=default data-md-color-primary=custom data-md-color-accent=custom> <input class=md-toggle data-md-toggle=drawer type=checkbox id=__drawer autocomplete=off> <input class=md-toggle data-md-toggle=search type=checkbox id=__search autocomplete=off> <label class=md-overlay for=__drawer></label> <div data-md-component=skip> <a href=#supported-macros class=md-skip> Skip to content </a> </div> <div data-md-component=announce> </div> <header class=md-header data-md-component=header> <nav class="md-header__inner md-grid" aria-label=Header> <a href=../.. title="JSON for Modern C++" class="md-header__button md-logo" aria-label="JSON for Modern C++" data-md-component=logo> <img src=../../images/wordmark-header.svg alt=logo> </a> <label class="md-header__button md-icon" for=__drawer> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M3 6h18v2H3zm0 5h18v2H3zm0 5h18v2H3z"/></svg> </label> <div class=md-header__title data-md-component=header-title> <div class=md-header__ellipsis> <div class=md-header__topic> <span class=md-ellipsis> JSON for Modern C++ </span> </div> <div class=md-header__topic data-md-component=header-topic> <span class=md-ellipsis> Supported Macros </span> </div> </div> </div> <form class=md-header__option data-md-component=palette> <input class=md-option data-md-color-media="(prefers-color-scheme: light)" data-md-color-scheme=default data-md-color-primary=custom data-md-color-accent=custom aria-label="Switch to dark mode" type=radio name=__palette id=__palette_0> <label class="md-header__button md-icon" title="Switch to dark mode" for=__palette_1 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 8a4 4 0 0 0-4 4 4 4 0 0 0 4 4 4 4 0 0 0 4-4 4 4 0 0 0-4-4m0 10a6 6 0 0 1-6-6 6 6 0 0 1 6-6 6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> <input class=md-option data-md-color-media="(prefers-color-scheme: dark)" data-md-color-scheme=slate data-md-color-primary=custom data-md-color-accent=custom aria-label="Switch to light mode" type=radio name=__palette id=__palette_1> <label class="md-header__button md-icon" title="Switch to light mode" for=__palette_0 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 18c-.89 0-1.74-.2-2.5-.55C11.56 16.5 13 14.42 13 12s-1.44-4.5-3.5-5.45C10.26 6.2 11.11 6 12 6a6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> </form> <script>var palette=__md_get("__palette");if(palette&&palette.color){if("(prefers-color-scheme)"===palette.color.media){var media=matchMedia("(prefers-color-scheme: light)"),input=document.querySelector(media.matches?"[data-md-color-media='(prefers-color-scheme: light)']":"[data-md-color-media='(prefers-color-scheme: dark)']");palette.color.media=input.getAttribute("data-md-color-media"),palette.color.scheme=input.getAttribute("data-md-color-scheme"),palette.color.primary=input.getAttribute("data-md-color-primary"),palette.color.accent=input.getAttribute("data-md-color-accent")}for(var[key,value]of Object.entries(palette.color))document.body.setAttribute("data-md-color-"+key,value)}</script> <label class="md-header__button md-icon" for=__search> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M9.5 3A6.5 6.5 0 0 1 16 9.5c0 1.61-.59 3.09-1.56 Line truncated
|
||||
@@ -245,3 +245,17 @@ A strict variant [`NLOHMANN_JSON_SERIALIZE_ENUM_STRICT`](https://json.nlohmann.m
|
||||
These macros are defined by the library and contain the version numbers according to [Semantic Versioning 2.0.0](https://semver.org/spec/v2.0.0.html).
|
||||
|
||||
See [full documentation of `NLOHMANN_JSON_VERSION_MAJOR`, `NLOHMANN_JSON_VERSION_MINOR`, and `NLOHMANN_JSON_VERSION_PATCH`](https://json.nlohmann.me/api/macros/nlohmann_json_version_major/index.md).
|
||||
|
||||
## See also
|
||||
|
||||
- [Macros](https://json.nlohmann.me/api/macros/index.md) - reference of all macros
|
||||
- [Runtime Assertions](https://json.nlohmann.me/features/assertions/index.md) - runtime assertions in detail
|
||||
- [`nlohmann` Namespace](https://json.nlohmann.me/features/namespace/index.md) - the library namespace and its macros
|
||||
- [Working Without Exceptions](https://json.nlohmann.me/features/no_exceptions/index.md) - using the library without exceptions
|
||||
- [Modules](https://json.nlohmann.me/features/modules/index.md) - macros and C++20 modules
|
||||
- [Exceptions](https://json.nlohmann.me/home/exceptions/index.md) - all exceptions and their error codes
|
||||
- [CMake](https://json.nlohmann.me/integration/cmake/index.md) - set macros via CMake options
|
||||
- [Compile times](https://json.nlohmann.me/integration/compile_times/index.md) - reduce compile times with macros
|
||||
- [Architecture](https://json.nlohmann.me/home/architecture/index.md) - how the library is put together
|
||||
- [Specializing enum conversion](https://json.nlohmann.me/features/enum_conversion/index.md) - enum macros
|
||||
- [Arbitrary Type Conversions](https://json.nlohmann.me/features/arbitrary_types/index.md) - type conversion macros
|
||||
+75
-2
@@ -12,6 +12,78 @@ easier to read and write for object-shaped documents.
|
||||
The function [`merge_diff`](../api/basic_json/merge_diff.md) creates a merge patch that changes one JSON value into
|
||||
another.
|
||||
|
||||
The example below applies one patch to one document. Here is the document before:
|
||||
|
||||
```json
|
||||
{
|
||||
"title": "Goodbye!",
|
||||
"author": {
|
||||
"givenName": "John",
|
||||
"familyName": "Doe"
|
||||
},
|
||||
"tags": ["example", "sample"],
|
||||
"content": "This will be unchanged"
|
||||
}
|
||||
```
|
||||
|
||||
This is the patch:
|
||||
|
||||
```json
|
||||
{
|
||||
"title": "Hello!",
|
||||
"phoneNumber": "+01-123-456-7890",
|
||||
"author": {
|
||||
"familyName": null
|
||||
},
|
||||
"tags": ["example"]
|
||||
}
|
||||
```
|
||||
|
||||
And this is the document after applying it:
|
||||
|
||||
```json
|
||||
{
|
||||
"title": "Hello!",
|
||||
"author": {
|
||||
"givenName": "John"
|
||||
},
|
||||
"tags": ["example"],
|
||||
"content": "This will be unchanged",
|
||||
"phoneNumber": "+01-123-456-7890"
|
||||
}
|
||||
```
|
||||
|
||||
The table goes through the members one by one. "Member" is the name of an object member in the patch (and in the
|
||||
document); it is not a [JSON Pointer](json_pointer.md) and nothing of it is passed to the function: the whole patch is
|
||||
passed to [`merge_patch`](../api/basic_json/merge_patch.md).
|
||||
|
||||
<div class="nj-pointer-mpt" markdown>
|
||||
|
||||
| Rule | Member | Before | Patch | After |
|
||||
|------|--------|--------|-------|-------|
|
||||
| <span class="nj-pointer-mp nj-pointer-mp--set">change value</span> | `title` | `#!json "Goodbye!"` | `#!json "Hello!"` | `#!json "Hello!"` |
|
||||
| <span class="nj-pointer-mp nj-pointer-mp--add">add key</span> | `phoneNumber` | <span class="nj-pointer-mp-none">(no member)</span> | `#!json "+01-123-456-7890"` | `#!json "+01-123-456-7890"` |
|
||||
| <span class="nj-pointer-mp nj-pointer-mp--rec">object merges recursively</span> | `author` | `#!json {"givenName": "John", "familyName": "Doe"}` | `#!json {"familyName": null}` | `#!json {"givenName": "John"}` |
|
||||
| <span class="nj-pointer-mp nj-pointer-mp--del">null removes key</span> | `familyName` inside `author` | `#!json "Doe"` | `#!json null` | <span class="nj-pointer-mp-none">(removed)</span> |
|
||||
| <span class="nj-pointer-mp nj-pointer-mp--arr">array replaced whole</span> | `tags` | `#!json ["example", "sample"]` | `#!json ["example"]` | `#!json ["example"]` |
|
||||
| <span class="nj-pointer-mp nj-pointer-mp--keep">unchanged key</span> | `content` | `#!json "This will be unchanged"` | <span class="nj-pointer-mp-none">(no member)</span> | `#!json "This will be unchanged"` |
|
||||
|
||||
</div>
|
||||
|
||||
An array in the patch replaces the array in the target, so a merge patch cannot change a single element. Each member of
|
||||
the patch is applied like this; a patch that is not an object replaces the whole target.
|
||||
|
||||
```mermaid
|
||||
flowchart LR
|
||||
M["member of the patch"] --> N{{"value null?"}}
|
||||
N -->|yes| D["delete the key"]
|
||||
N -->|no| O{{"value an object?"}}
|
||||
O -->|yes| R["merge recursively into the target's value"]
|
||||
O -->|no| S["set the key to the value"]
|
||||
```
|
||||
|
||||
<p class="nj-caption" markdown>How one member of a merge patch is applied: a `null` value deletes the key, an object is merged recursively, and any other value replaces the value of the key.</p>
|
||||
|
||||
!!! tip "Not a general deep merge"
|
||||
|
||||
A merge patch is not a general deep merge: a `#!json null` value in the patch deletes the key from the target.
|
||||
@@ -101,6 +173,7 @@ another.
|
||||
|
||||
- [JSON Patch and Diff](json_patch.md) - a more expressive alternative that describes a sequence of operations
|
||||
- [JSON Pointer](json_pointer.md) - the addressing scheme used by JSON Patch
|
||||
- Function [`merge_patch`](../api/basic_json/merge_patch.md)
|
||||
- Function [`merge_diff`](../api/basic_json/merge_diff.md)
|
||||
- [`merge_patch`](../api/basic_json/merge_patch.md) - apply a JSON Merge Patch to a value
|
||||
- [`merge_diff`](../api/basic_json/merge_diff.md) - create a JSON Merge Patch from two values
|
||||
- Function [`update`](../api/basic_json/update.md) - merge objects, optionally recursively
|
||||
- [Modifying values](modifying_values.md) - simple in-place modifications
|
||||
@@ -1,4 +1,35 @@
|
||||
<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/merge_patch/" rel="canonical"><link href=../iterators/ rel=prev><link href=../json_patch/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>JSON Merge Patch - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.ab4e12ef.min.css><link rel="stylesheet" href="../../assets/external/fonts.googleapis.com/css.61a430c9.css"><style>:root{--md-text-font:"Roboto";--md-code-font:"JetBrains Mono"}</style><link rel=stylesheet href=../../css/custom.css><script>__md_scope=new URL("../..",location),__md_hash=e=>[...e].reduce(((e,_)=>(e<<5)-e+_.charCodeAt(0)),0),__md_get=(e,_=localStorage,t=__md_scope)=>JSON.parse(_.getItem(t.pathname+"."+e)),__md_set=(e,_,t=localStorage,a=__md_scope)=>{try{t.setItem(a.pathname+"."+e,JSON.stringify(_))}catch(e){}}</script></head> <body dir=ltr data-md-color-scheme=default data-md-color-primary=indigo data-md-color-accent=indigo> <input class=md-toggle data-md-toggle=drawer type=checkbox id=__drawer autocomplete=off> <input class=md-toggle data-md-toggle=search type=checkbox id=__search autocomplete=off> <label class=md-overlay for=__drawer></label> <div data-md-component=skip> <a href=#json-merge-patch class=md-skip> Skip to content </a> </div> <div data-md-component=announce> </div> <header class=md-header data-md-component=header> <nav class="md-header__inner md-grid" aria-label=Header> <a href=../.. title="JSON for Modern C++" class="md-header__button md-logo" aria-label="JSON for Modern C++" data-md-component=logo> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 8a3 3 0 0 0 3-3 3 3 0 0 0-3-3 3 3 0 0 0-3 3 3 3 0 0 0 3 3m0 3.54C9.64 9.35 6.5 8 3 8v11c3.5 0 6.64 1.35 9 3.54 2.36-2.19 5.5-3.54 9-3.54V8c-3.5 0-6.64 1.35-9 3.54"/></svg> </a> <label class="md-header__button md-icon" for=__drawer> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M3 6h18v2H3zm0 5h18v2H3zm0 5h18v2H3z"/></svg> </label> <div class=md-header__title data-md-component=header-title> <div class=md-header__ellipsis> <div class=md-header__topic> <span class=md-ellipsis> JSON for Modern C++ </span> </div> <div class=md-header__topic data-md-component=header-topic> <span class=md-ellipsis> JSON Merge Patch </span> </div> </div> </div> <form class=md-header__option data-md-component=palette> <input class=md-option data-md-color-media="(prefers-color-scheme: light)" data-md-color-scheme=default data-md-color-primary=indigo data-md-color-accent=indigo aria-label="Switch to dark mode" type=radio 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<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=description content="JSON for Modern C++ is a header-only C++11 library that makes JSON a first-class data type."><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/merge_patch/" rel="canonical"><link href=../iterators/ rel=prev><link href=../json_patch/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>JSON Merge Patch - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.ab4e12ef.min.css><link rel=stylesheet href=../../css/custom.css><link rel=stylesheet href=../../css/brand.css><link rel=stylesheet href=../../css/fig-sax.css><link rel=stylesheet href=../../css/fig-jsonview.css><link rel=stylesheet 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data-md-component=announce> </div> <header class=md-header data-md-component=header> <nav class="md-header__inner md-grid" aria-label=Header> <a href=../.. title="JSON for Modern C++" class="md-header__button md-logo" aria-label="JSON for Modern C++" data-md-component=logo> <img src=../../images/wordmark-header.svg alt=logo> </a> <label class="md-header__button md-icon" for=__drawer> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M3 6h18v2H3zm0 5h18v2H3zm0 5h18v2H3z"/></svg> </label> <div class=md-header__title data-md-component=header-title> <div class=md-header__ellipsis> <div class=md-header__topic> <span class=md-ellipsis> JSON for Modern C++ </span> </div> <div class=md-header__topic data-md-component=header-topic> <span class=md-ellipsis> JSON Merge Patch </span> </div> </div> </div> <form class=md-header__option data-md-component=palette> <input class=md-option data-md-color-media="(prefers-color-scheme: light)" data-md-color-scheme=default 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0 24 24"><path d="M9.5 3A6.5 6.5 0 0 1 16 9.5c0 1.61-.59 3.09-1.56 4.23l.Line truncated
|
||||
<span class=w> </span><span class=nt>"title"</span><span class=p>:</span><span class=w> </span><span class=s2>"Goodbye!"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"author"</span><span class=p>:</span><span class=w> </span><span class=p>{</span>
|
||||
<span class=w> </span><span class=nt>"givenName"</span><span class=p>:</span><span class=w> </span><span class=s2>"John"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"familyName"</span><span class=p>:</span><span class=w> </span><span class=s2>"Doe"</span>
|
||||
<span class=w> </span><span class=p>},</span>
|
||||
<span class=w> </span><span class=nt>"tags"</span><span class=p>:</span><span class=w> </span><span class=p>[</span><span class=s2>"example"</span><span class=p>,</span><span class=w> </span><span class=s2>"sample"</span><span class=p>],</span>
|
||||
<span class=w> </span><span class=nt>"content"</span><span class=p>:</span><span class=w> </span><span class=s2>"This will be unchanged"</span>
|
||||
<span class=p>}</span>
|
||||
</code></pre></div> <p>This is the patch:</p> <div class=highlight><pre><span></span><code><span class=p>{</span>
|
||||
<span class=w> </span><span class=nt>"title"</span><span class=p>:</span><span class=w> </span><span class=s2>"Hello!"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"phoneNumber"</span><span class=p>:</span><span class=w> </span><span class=s2>"+01-123-456-7890"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"author"</span><span class=p>:</span><span class=w> </span><span class=p>{</span>
|
||||
<span class=w> </span><span class=nt>"familyName"</span><span class=p>:</span><span class=w> </span><span class=kc>null</span>
|
||||
<span class=w> </span><span class=p>},</span>
|
||||
<span class=w> </span><span class=nt>"tags"</span><span class=p>:</span><span class=w> </span><span class=p>[</span><span class=s2>"example"</span><span class=p>]</span>
|
||||
<span class=p>}</span>
|
||||
</code></pre></div> <p>And this is the document after applying it:</p> <div class=highlight><pre><span></span><code><span class=p>{</span>
|
||||
<span class=w> </span><span class=nt>"title"</span><span class=p>:</span><span class=w> </span><span class=s2>"Hello!"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"author"</span><span class=p>:</span><span class=w> </span><span class=p>{</span>
|
||||
<span class=w> </span><span class=nt>"givenName"</span><span class=p>:</span><span class=w> </span><span class=s2>"John"</span>
|
||||
<span class=w> </span><span class=p>},</span>
|
||||
<span class=w> </span><span class=nt>"tags"</span><span class=p>:</span><span class=w> </span><span class=p>[</span><span class=s2>"example"</span><span class=p>],</span>
|
||||
<span class=w> </span><span class=nt>"content"</span><span class=p>:</span><span class=w> </span><span class=s2>"This will be unchanged"</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"phoneNumber"</span><span class=p>:</span><span class=w> </span><span class=s2>"+01-123-456-7890"</span>
|
||||
<span class=p>}</span>
|
||||
</code></pre></div> <p>The table goes through the members one by one. "Member" is the name of an object member in the patch (and in the document); it is not a <a href=../json_pointer/ ><abbr title="JavaScript Object Notation">JSON</abbr> Pointer</a> and nothing of it is passed to the function: the whole patch is passed to <a href=../../api/basic_json/merge_patch/ ><code>merge_patch</code></a>.</p> <div class=nj-pointer-mpt> <table> <thead> <tr> <th>Rule</th> <th>Member</th> <th>Before</th> <th>Patch</th> <th>After</th> </tr> </thead> <tbody> <tr> <td><span class="nj-pointer-mp nj-pointer-mp--set">change value</span></td> <td><code>title</code></td> <td><code class=highlight><span class=s2>"Goodbye!"</span></code></td> <td><code class=highlight><span class=s2>"Hello!"</span></code></td> <td><code class=highlight><span class=s2>"Hello!"</span></code></td> </tr> <tr> <td><span class="nj-pointer-mp nj-pointer-mp--add">add key</span></td> <td><code>phoneNumber</code></td> <td><span class=nj-pointer-mp-none>(no member)</span></td> <td><code class=highlight><span class=s2>"+01-123-456-7890"</span></code></td> <td><code class=highlight><span class=s2>"+01-123-456-7890"</span></code></td> </tr> <tr> <td><span class="nj-pointer-mp nj-pointer-mp--rec">object merges recursively</span></td> <td><code>author</code></td> <td><code class=highlight><span class=p>{</span><span class=nt>"givenName"</span><span class=p>:</span><span class=w> </span><span class=s2>"John"</span><span class=p>,</span><span class=w> </span><span class=nt>"familyName"</span><span class=p>:</span><span class=w> </span><span class=s2>"Doe"</span><span class=p>}</span></code></td> <td><code class=highlight><span class=p>{</span><span class=nt>"familyName"</span><span class=p>:</span><span class=w> </span><span class=kc>null</span><span class=p>}</span></code></td> <td><code class=highlight><span class=p>{</span><span class=nt>"givenName"</span><span class=p>:</span><span class=w> </span><span class=s2>"John"</span><span class=p>}</span></code></td> </tr> <tr> <td><span class="nj-pointer-mp nj-pointer-mp--del">null removes key</span></td> <td><code>familyName</code> inside <code>author</code></td> <td><code class=highlight><span class=s2>"Doe"</span></code></td> <td><code class=highlight><span class=kc>null</span></code></td> <td><span class=nj-pointer-mp-none>(removed)</span></td> </tr> <tr> <td><span class="nj-pointer-mp nj-pointer-mp--arr">array replaced whole</span></td> <td><code>tags</code></td> <td><code class=highlight><span class=p>[</span><span class=s2>"example"</span><span class=p>,</span><span class=w> </span><span class=s2>"sample"</span><span class=p>]</span></code></td> <td><code class=highlight><span class=p>[</span><span class=s2>"example"</span><span class=p>]</span></code></td> <td><code class=highlight><span class=p>[</span><span class=s2>"example"</span><span class=p>]</span></code></td> </tr> <tr> <td><span class="nj-pointer-mp nj-pointer-mp--keep">unchanged key</span></td> <td><code>content</code></td> <td><code class=highlight><span class=s2>"This will be unchanged"</span></code></td> <td><span class=nj-pointer-mp-none>(no member)</span></td> <td><code class=highlight><span class=s2>"This will be unchanged"</span></code></td> </tr> </tbody> </table> </div> <p>An array in the patch replaces the array in the target, so a merge patch cannot change a single element. Each member of the patch is applied like this; a patch that is not an object replaces the whole target.</p> <pre class=mermaid><code>flowchart LR
|
||||
M["member of the patch"] --> N{{"value null?"}}
|
||||
N -->|yes| D["delete the key"]
|
||||
N -->|no| O{{"value an object?"}}
|
||||
O -->|yes| R["merge recursively into the target's value"]
|
||||
O -->|no| S["set the key to the value"]</code></pre> <p class=nj-caption>How one member of a merge patch is applied: a <code>null</code> value deletes the key, an object is merged recursively, and any other value replaces the value of the key.</p> <div class="admonition tip"> <p class=admonition-title>Not a general deep merge</p> <p>A merge patch is not a general deep merge: a <code class=highlight><span class=kc>null</span></code> value in the patch deletes the key from the target. To merge two objects recursively (e.g., defaults and user settings), use <a href=../../api/basic_json/update/ ><code>update</code></a> with <code>merge_objects</code> set to <code class=highlight><span class=nb>true</span></code>; see <a href=../modifying_values/#merging-objects>Merging objects</a>.</p> </div> <div class="admonition note"> <p class=admonition-title>Limitations of <abbr title="Request for Comments">RFC</abbr> 7396</p> <ul> <li>A patch cannot set a member to <code class=highlight><span class=kc>null</span></code>, because <code class=highlight><span class=kc>null</span></code> removes the member: <code class=highlight><span class=p>{</span><span class=nt>"a"</span><span class=p>:</span><span class=mi>1</span><span class=p>}</span></code> patched with <code class=highlight><span class=p>{</span><span class=nt>"a"</span><span class=p>:</span><span class=kc>null</span><span class=p>}</span></code> gives <code class=highlight><span class=p>{}</span></code>.</li> <li>Arrays are replaced as a whole, never merged, and a <code class=highlight><span class=kc>null</span></code> inside an array is kept.</li> <li><code class=highlight><span class=kc>null</span></code> members of the target that the patch does not mention are preserved.</li> <li>A <code class=highlight><span class=kc>null</span></code> that would remove a member of a not yet existing object still creates that object: <code class=highlight><span class=p>{}</span></code> patched with <code class=highlight><span class=p>{</span><span class=nt>"a"</span><span class=p>:{</span><span class=nt>"b"</span><span class=p>:</span><span class=kc>null</span><span class=p>}}</span></code> gives <code class=highlight><span class=p>{</span><span class=nt>"a"</span><span class=p>:{}}</span></code>.</li> <li>A patch that is not an object replaces the whole target (a <code class=highlight><span class=kc>null</span></code> patch makes the target <code class=highlight><span class=kc>null</span></code>); an object patch applied to a non-object target starts from an empty object.</li> <li><a href=../../api/basic_json/merge_diff/#notes><code>merge_diff</code></a> has a matching limitation for <code class=highlight><span class=kc>null</span></code> members.</li> </ul> </div> <details class=example> <summary>Example</summary> <p>The following code shows how a <abbr title="JavaScript Object Notation">JSON</abbr> Merge Patch is applied to a <abbr title="JavaScript Object Notation">JSON</abbr> document.</p> <div class=highlight><pre><span></span><code><span class=cp>#include</span><span class=w> </span><span class=cpf><iostream></span>
|
||||
<span class=cp>#include</span><span class=w> </span><span class=cpf><nlohmann/json.hpp></span>
|
||||
<span class=cp>#include</span><span class=w> </span><span class=cpf><iomanip></span><span class=w> </span><span class=c1>// for std::setw</span>
|
||||
|
||||
@@ -50,4 +81,4 @@
|
||||
<span class=w> </span><span class=p>],</span>
|
||||
<span class=w> </span><span class=nt>"title"</span><span class=p>:</span><span class=w> </span><span class=s2>"Hello!"</span>
|
||||
<span class=p>}</span>
|
||||
</code></pre></div> </details> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">¶</a></h2> <ul> <li><a href=../json_patch/ ><abbr title="JavaScript Object Notation">JSON</abbr> Patch and Diff</a> - a more expressive alternative that describes a sequence of operations</li> <li><a href=../json_pointer/ ><abbr title="JavaScript Object Notation">JSON</abbr> Pointer</a> - the addressing scheme used by <abbr title="JavaScript Object Notation">JSON</abbr> Patch</li> <li>Function <a href=../../api/basic_json/merge_patch/ ><code>merge_patch</code></a></li> <li>Function <a href=../../api/basic_json/merge_diff/ ><code>merge_diff</code></a></li> <li>Function <a href=../../api/basic_json/update/ ><code>update</code></a> - merge objects, optionally recursively</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 26, 2026 16:11:55 UTC">October 26, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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||||
</code></pre></div> </details> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">¶</a></h2> <ul> <li><a href=../json_patch/ ><abbr title="JavaScript Object Notation">JSON</abbr> Patch and Diff</a> - a more expressive alternative that describes a sequence of operations</li> <li><a href=../json_pointer/ ><abbr title="JavaScript Object Notation">JSON</abbr> Pointer</a> - the addressing scheme used by <abbr title="JavaScript Object Notation">JSON</abbr> Patch</li> <li><a href=../../api/basic_json/merge_patch/ ><code>merge_patch</code></a> - apply a <abbr title="JavaScript Object Notation">JSON</abbr> Merge Patch to a value</li> <li><a href=../../api/basic_json/merge_diff/ ><code>merge_diff</code></a> - create a <abbr title="JavaScript Object Notation">JSON</abbr> Merge Patch from two values</li> <li>Function <a href=../../api/basic_json/update/ ><code>update</code></a> - merge objects, optionally recursively</li> <li><a href=../modifying_values/ >Modifying values</a> - simple in-place modifications</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../iterators/ class="md-footer__link md-footer__link--prev" aria-label="Previous: Iterators"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> Iterators </div> </div> </a> <a href=../json_patch/ class="md-footer__link md-footer__link--next" aria-label="Next: JSON Patch and Diff"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> JSON Patch and Diff </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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|
||||
@@ -6,6 +6,71 @@ Instead of using [JSON Pointer](https://json.nlohmann.me/features/json_pointer/i
|
||||
|
||||
The function [`merge_diff`](https://json.nlohmann.me/api/basic_json/merge_diff/index.md) creates a merge patch that changes one JSON value into another.
|
||||
|
||||
The example below applies one patch to one document. Here is the document before:
|
||||
|
||||
```
|
||||
{
|
||||
"title": "Goodbye!",
|
||||
"author": {
|
||||
"givenName": "John",
|
||||
"familyName": "Doe"
|
||||
},
|
||||
"tags": ["example", "sample"],
|
||||
"content": "This will be unchanged"
|
||||
}
|
||||
```
|
||||
|
||||
This is the patch:
|
||||
|
||||
```
|
||||
{
|
||||
"title": "Hello!",
|
||||
"phoneNumber": "+01-123-456-7890",
|
||||
"author": {
|
||||
"familyName": null
|
||||
},
|
||||
"tags": ["example"]
|
||||
}
|
||||
```
|
||||
|
||||
And this is the document after applying it:
|
||||
|
||||
```
|
||||
{
|
||||
"title": "Hello!",
|
||||
"author": {
|
||||
"givenName": "John"
|
||||
},
|
||||
"tags": ["example"],
|
||||
"content": "This will be unchanged",
|
||||
"phoneNumber": "+01-123-456-7890"
|
||||
}
|
||||
```
|
||||
|
||||
The table goes through the members one by one. "Member" is the name of an object member in the patch (and in the document); it is not a [JSON Pointer](https://json.nlohmann.me/features/json_pointer/index.md) and nothing of it is passed to the function: the whole patch is passed to [`merge_patch`](https://json.nlohmann.me/api/basic_json/merge_patch/index.md).
|
||||
|
||||
| Rule | Member | Before | Patch | After |
|
||||
| ------------------------- | ---------------------------- | -------------------------------------------- | ---------------------- | -------------------------- |
|
||||
| change value | `title` | `"Goodbye!"` | `"Hello!"` | `"Hello!"` |
|
||||
| add key | `phoneNumber` | (no member) | `"+01-123-456-7890"` | `"+01-123-456-7890"` |
|
||||
| object merges recursively | `author` | `{"givenName": "John", "familyName": "Doe"}` | `{"familyName": null}` | `{"givenName": "John"}` |
|
||||
| null removes key | `familyName` inside `author` | `"Doe"` | `null` | (removed) |
|
||||
| array replaced whole | `tags` | `["example", "sample"]` | `["example"]` | `["example"]` |
|
||||
| unchanged key | `content` | `"This will be unchanged"` | (no member) | `"This will be unchanged"` |
|
||||
|
||||
An array in the patch replaces the array in the target, so a merge patch cannot change a single element. Each member of the patch is applied like this; a patch that is not an object replaces the whole target.
|
||||
|
||||
```
|
||||
flowchart LR
|
||||
M["member of the patch"] --> N{{"value null?"}}
|
||||
N -->|yes| D["delete the key"]
|
||||
N -->|no| O{{"value an object?"}}
|
||||
O -->|yes| R["merge recursively into the target's value"]
|
||||
O -->|no| S["set the key to the value"]
|
||||
```
|
||||
|
||||
How one member of a merge patch is applied: a `null` value deletes the key, an object is merged recursively, and any other value replaces the value of the key.
|
||||
|
||||
Not a general deep merge
|
||||
|
||||
A merge patch is not a general deep merge: a `null` value in the patch deletes the key from the target. To merge two objects recursively (e.g., defaults and user settings), use [`update`](https://json.nlohmann.me/api/basic_json/update/index.md) with `merge_objects` set to `true`; see [Merging objects](https://json.nlohmann.me/features/modifying_values/#merging-objects).
|
||||
@@ -87,6 +152,7 @@ Output:
|
||||
|
||||
- [JSON Patch and Diff](https://json.nlohmann.me/features/json_patch/index.md) - a more expressive alternative that describes a sequence of operations
|
||||
- [JSON Pointer](https://json.nlohmann.me/features/json_pointer/index.md) - the addressing scheme used by JSON Patch
|
||||
- Function [`merge_patch`](https://json.nlohmann.me/api/basic_json/merge_patch/index.md)
|
||||
- Function [`merge_diff`](https://json.nlohmann.me/api/basic_json/merge_diff/index.md)
|
||||
- [`merge_patch`](https://json.nlohmann.me/api/basic_json/merge_patch/index.md) - apply a JSON Merge Patch to a value
|
||||
- [`merge_diff`](https://json.nlohmann.me/api/basic_json/merge_diff/index.md) - create a JSON Merge Patch from two values
|
||||
- Function [`update`](https://json.nlohmann.me/api/basic_json/update/index.md) - merge objects, optionally recursively
|
||||
- [Modifying values](https://json.nlohmann.me/features/modifying_values/index.md) - simple in-place modifications
|
||||
@@ -140,10 +140,14 @@ editable document types, such as [`json_editable_document`](../api/json_editable
|
||||
|
||||
## See also
|
||||
|
||||
- [`push_back`](../api/basic_json/push_back.md) / [`emplace_back`](../api/basic_json/emplace_back.md) - append to an array
|
||||
- [`push_back`](../api/basic_json/push_back.md) - / [`emplace_back`](../api/basic_json/emplace_back.md) - append to an array
|
||||
- [`emplace`](../api/basic_json/emplace.md) - insert into an object if the key is absent
|
||||
- [`update`](../api/basic_json/update.md) - merge objects (shallow, or recursive with `merge_objects`)
|
||||
- [`merge_patch`](../api/basic_json/merge_patch.md) - apply an RFC 7396 merge patch
|
||||
- [`erase`](../api/basic_json/erase.md) / [`clear`](../api/basic_json/clear.md) - remove elements
|
||||
- [JSON Patch and Diff](json_patch.md) and [JSON Merge Patch](merge_patch.md) - structured modifications
|
||||
- [`erase`](../api/basic_json/erase.md) - / [`clear`](../api/basic_json/clear.md) - remove elements
|
||||
- [JSON Patch and Diff](json_patch.md) - and [JSON Merge Patch](merge_patch.md) - structured modifications
|
||||
- [Editing a document](json_view.md#editing-a-document) - change a parsed document in place
|
||||
- [`insert`](../api/basic_json/insert.md) - insert elements at an iterator position or merge a range
|
||||
- [`operator+=`](../api/basic_json/operator+=.md) - shorthand for `push_back`
|
||||
- [Element Access](element_access/index.md) - read elements
|
||||
- [Iterators](iterators.md) - iterate over elements
|
||||
@@ -1,4 +1,4 @@
|
||||
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<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=description content="JSON for Modern C++ is a header-only C++11 library that makes JSON a first-class data type."><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/modifying_values/" rel="canonical"><link href=../json_pointer/ rel=prev><link href=../modules/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>Modifying values - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.ab4e12ef.min.css><link rel=stylesheet href=../../css/custom.css><link rel=stylesheet href=../../css/brand.css><link rel=stylesheet href=../../css/fig-sax.css><link rel=stylesheet href=../../css/fig-jsonview.css><link rel=stylesheet 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0 24 24"><path d="M9.5 3A6.5 6.5 0 0 1 16 9.5c0 1.61-.59 3.09-1.56 4Line truncated
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<span class=n>j</span><span class=p>.</span><span class=n>push_back</span><span class=p>(</span><span class=mi>1</span><span class=p>);</span><span class=w> </span><span class=c1>// [1]</span>
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<span class=n>j</span><span class=p>.</span><span class=n>push_back</span><span class=p>(</span><span class=mi>2</span><span class=p>);</span><span class=w> </span><span class=c1>// [1,2]</span>
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<span class=n>j</span><span class=p>.</span><span class=n>emplace_back</span><span class=p>(</span><span class=mi>3</span><span class=p>);</span><span class=w> </span><span class=c1>// [1,2,3]</span>
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@@ -60,4 +60,4 @@
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<span class=n>json</span><span class=w> </span><span class=n>a</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=p>{</span><span class=mi>1</span><span class=p>,</span><span class=w> </span><span class=mi>2</span><span class=p>,</span><span class=w> </span><span class=mi>3</span><span class=p>,</span><span class=w> </span><span class=mi>4</span><span class=p>};</span>
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<span class=n>a</span><span class=p>.</span><span class=n>erase</span><span class=p>(</span><span class=mi>1</span><span class=p>);</span><span class=w> </span><span class=c1>// [1,3,4] (erase by index)</span>
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</code></pre></div> <h2 id=editing-a-document>Editing a document<a class=headerlink href=#editing-a-document title="Permanent link">¶</a></h2> <p>The operations above work on a <code>basic_json</code> value. A <abbr title="JavaScript Object Notation">JSON</abbr> text that was parsed into a <a href=../json_view/#editing-a-document>document</a> can also be changed in place, without building a <code>basic_json</code> tree for the parts that are not touched: <a href=../../api/basic_json_document/set/ ><code>set</code></a> replaces a value, adds or replaces an object member, or replaces an array element (also through a <abbr title="JavaScript Object Notation">JSON</abbr> pointer), <a href=../../api/basic_json_document/push_back/ ><code>push_back</code></a> appends to an array, <a href=../../api/basic_json_document/insert/ ><code>insert</code></a> inserts into an array (unlike <code>basic_json::insert</code>, it has no object overload; members are added with <code>set</code>), and <a href=../../api/basic_json_document/erase/ ><code>erase</code></a> removes a member or an element. These functions are available on the editable document types, such as <a href=../../api/json_editable_document/ ><code>json_editable_document</code></a>.</p> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">¶</a></h2> <ul> <li><a href=../../api/basic_json/push_back/ ><code>push_back</code></a> / <a href=../../api/basic_json/emplace_back/ ><code>emplace_back</code></a> - append to an array</li> <li><a href=../../api/basic_json/emplace/ ><code>emplace</code></a> - insert into an object if the key is absent</li> <li><a href=../../api/basic_json/update/ ><code>update</code></a> - merge objects (shallow, or recursive with <code>merge_objects</code>)</li> <li><a href=../../api/basic_json/merge_patch/ ><code>merge_patch</code></a> - apply an <abbr title="Request for Comments">RFC</abbr> 7396 merge patch</li> <li><a href=../../api/basic_json/erase/ ><code>erase</code></a> / <a href=../../api/basic_json/clear/ ><code>clear</code></a> - remove elements</li> <li><a href=../json_patch/ ><abbr title="JavaScript Object Notation">JSON</abbr> Patch and Diff</a> and <a href=../merge_patch/ ><abbr title="JavaScript Object Notation">JSON</abbr> Merge Patch</a> - structured modifications</li> <li><a href=../json_view/#editing-a-document>Editing a document</a> - change a parsed document in place</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 25, 2026 05:18:07 UTC">October 25, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2025 Fonticons, Inc.--><path d="M173.9 397.4c0 2-2.3 3.6-5.2 3.6-3.3.3-5.6-1.3-5.6-3.6 0-2 2.3-3.6 5.2-3.6 3-.3 5.6 1.3 5.6 3.6m-31.1-4.5c-.7 2 1.3 4.3 4.3 4.9 2.6 1 5.6 0 6.2-2s-1.3-4.3-4.3-5.2c-2.6-.7-5.5.3-6.2 2.3m44.2-1.7c-2.9.7-4.9 2.6-4.6 4.9.3 2 2.9 3.3 5.9 2.6 2.9-.7 4.9-2.6 4.6-4.6-.3-1.9-3-3.2-5.9-2.9M252.8 8C114.1 8 8 113.3 8 252c0 110.9 69.8 205.8 169.5 239.2 12.8 2.3 17.3-5.6 17.3-12.1 0-6.2-.3-40.4-.3Line truncated
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</code></pre></div> <h2 id=editing-a-document>Editing a document<a class=headerlink href=#editing-a-document title="Permanent link">¶</a></h2> <p>The operations above work on a <code>basic_json</code> value. A <abbr title="JavaScript Object Notation">JSON</abbr> text that was parsed into a <a href=../json_view/#editing-a-document>document</a> can also be changed in place, without building a <code>basic_json</code> tree for the parts that are not touched: <a href=../../api/basic_json_document/set/ ><code>set</code></a> replaces a value, adds or replaces an object member, or replaces an array element (also through a <abbr title="JavaScript Object Notation">JSON</abbr> pointer), <a href=../../api/basic_json_document/push_back/ ><code>push_back</code></a> appends to an array, <a href=../../api/basic_json_document/insert/ ><code>insert</code></a> inserts into an array (unlike <code>basic_json::insert</code>, it has no object overload; members are added with <code>set</code>), and <a href=../../api/basic_json_document/erase/ ><code>erase</code></a> removes a member or an element. These functions are available on the editable document types, such as <a href=../../api/json_editable_document/ ><code>json_editable_document</code></a>.</p> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">¶</a></h2> <ul> <li><a href=../../api/basic_json/push_back/ ><code>push_back</code></a> - / <a href=../../api/basic_json/emplace_back/ ><code>emplace_back</code></a> - append to an array</li> <li><a href=../../api/basic_json/emplace/ ><code>emplace</code></a> - insert into an object if the key is absent</li> <li><a href=../../api/basic_json/update/ ><code>update</code></a> - merge objects (shallow, or recursive with <code>merge_objects</code>)</li> <li><a href=../../api/basic_json/merge_patch/ ><code>merge_patch</code></a> - apply an <abbr title="Request for Comments">RFC</abbr> 7396 merge patch</li> <li><a href=../../api/basic_json/erase/ ><code>erase</code></a> - / <a href=../../api/basic_json/clear/ ><code>clear</code></a> - remove elements</li> <li><a href=../json_patch/ ><abbr title="JavaScript Object Notation">JSON</abbr> Patch and Diff</a> - and <a href=../merge_patch/ ><abbr title="JavaScript Object Notation">JSON</abbr> Merge Patch</a> - structured modifications</li> <li><a href=../json_view/#editing-a-document>Editing a document</a> - change a parsed document in place</li> <li><a href=../../api/basic_json/insert/ ><code>insert</code></a> - insert elements at an iterator position or merge a range</li> <li><a href=../../api/basic_json/operator%2B%3D/ ><code>operator+=</code></a> - shorthand for <code>push_back</code></li> <li><a href=../element_access/ >Element Access</a> - read elements</li> <li><a href=../iterators/ >Iterators</a> - iterate over elements</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../json_pointer/ class="md-footer__link md-footer__link--prev" aria-label="Previous: JSON Pointer"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> JSON Pointer </div> </div> </a> <a href=../modules/ class="md-footer__link md-footer__link--next" aria-label="Next: Modules"> <div class=md-footeLine truncated
|
||||
@@ -117,10 +117,14 @@ The operations above work on a `basic_json` value. A JSON text that was parsed i
|
||||
|
||||
## See also
|
||||
|
||||
- [`push_back`](https://json.nlohmann.me/api/basic_json/push_back/index.md) / [`emplace_back`](https://json.nlohmann.me/api/basic_json/emplace_back/index.md) - append to an array
|
||||
- [`push_back`](https://json.nlohmann.me/api/basic_json/push_back/index.md) - / [`emplace_back`](https://json.nlohmann.me/api/basic_json/emplace_back/index.md) - append to an array
|
||||
- [`emplace`](https://json.nlohmann.me/api/basic_json/emplace/index.md) - insert into an object if the key is absent
|
||||
- [`update`](https://json.nlohmann.me/api/basic_json/update/index.md) - merge objects (shallow, or recursive with `merge_objects`)
|
||||
- [`merge_patch`](https://json.nlohmann.me/api/basic_json/merge_patch/index.md) - apply an RFC 7396 merge patch
|
||||
- [`erase`](https://json.nlohmann.me/api/basic_json/erase/index.md) / [`clear`](https://json.nlohmann.me/api/basic_json/clear/index.md) - remove elements
|
||||
- [JSON Patch and Diff](https://json.nlohmann.me/features/json_patch/index.md) and [JSON Merge Patch](https://json.nlohmann.me/features/merge_patch/index.md) - structured modifications
|
||||
- [`erase`](https://json.nlohmann.me/api/basic_json/erase/index.md) - / [`clear`](https://json.nlohmann.me/api/basic_json/clear/index.md) - remove elements
|
||||
- [JSON Patch and Diff](https://json.nlohmann.me/features/json_patch/index.md) - and [JSON Merge Patch](https://json.nlohmann.me/features/merge_patch/index.md) - structured modifications
|
||||
- [Editing a document](https://json.nlohmann.me/features/json_view/#editing-a-document) - change a parsed document in place
|
||||
- [`insert`](https://json.nlohmann.me/api/basic_json/insert/index.md) - insert elements at an iterator position or merge a range
|
||||
- [`operator+=`](https://json.nlohmann.me/api/basic_json/operator%2B%3D/index.md) - shorthand for `push_back`
|
||||
- [Element Access](https://json.nlohmann.me/features/element_access/index.md) - read elements
|
||||
- [Iterators](https://json.nlohmann.me/features/iterators/index.md) - iterate over elements
|
||||
@@ -4,6 +4,8 @@ This library has experimental support for C++ modules, introduced in C++20. The
|
||||
|
||||
Please be aware that the module is experimental and a full test is outstanding, and the exported symbols are subject to change.
|
||||
|
||||
Importing the module instead of including the header reduces the compile time of every translation unit that uses the library, at the price of building the module once. The [Compile times](../integration/compile_times.md#use-c20-modules) page has measurements and the one-off cost.
|
||||
|
||||
## Requirements
|
||||
The `nlohmann.json` module requires that the build system is configured to build and resolve modules when imported. Obviously, as modules were introduced in C++20, this feature can only be used in C++20 and subsequent versions.
|
||||
|
||||
@@ -66,3 +68,12 @@ C++20 modules support is exercised in CI against current GCC and Clang on Ubuntu
|
||||
- **MSVC** could fail with `C2039: 'json_sax_dom_callback_parser' is not a member of ... detail`; fixed by exporting the required internal symbols from `json.cppm` (see [Exported symbols](#exported-symbols) above). ([issue #3970](https://github.com/nlohmann/json/issues/3970))
|
||||
|
||||
If you hit a different module-related build failure, search [existing issues](https://github.com/nlohmann/json/issues?q=is%3Aissue+modules) before filing a new one.
|
||||
|
||||
## See also
|
||||
|
||||
- [Supported Macros](macros.md) - macros are not exported by modules
|
||||
- [Macros](../api/macros/index.md) - reference of all macros
|
||||
- [`nlohmann` Namespace](namespace.md) - the library namespace
|
||||
- [CMake](../integration/cmake.md) - enable the module build in CMake
|
||||
- [Compile times](../integration/compile_times.md) - reduce compile times
|
||||
- [`basic_json`](../api/basic_json/index.md) - the main class and all its member functions
|
||||
@@ -1,4 +1,4 @@
|
||||
<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/modules/" rel="canonical"><link href=../modifying_values/ rel=prev><link href=../namespace/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>Modules - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.ab4e12ef.min.css><link rel="stylesheet" href="../../assets/external/fonts.googleapis.com/css.61a430c9.css"><style>:root{--md-text-font:"Roboto";--md-code-font:"JetBrains Mono"}</style><link rel=stylesheet href=../../css/custom.css><script>__md_scope=new 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xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 8a3 3 0 0 0 3-3 3 3 0 0 0-3-3 3 3 0 0 0-3 3 3 3 0 0 0 3 3m0 3.54C9.64 9.35 6.5 8 3 8v11c3.5 0 6.64 1.35 9 3.54 2.36-2.19 5.5-3.54 9-3.54V8c-3.5 0-6.64 1.35-9 3.54"/></svg> </a> <label class="md-header__button md-icon" for=__drawer> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M3 6h18v2H3zm0 5h18v2H3zm0 5h18v2H3z"/></svg> </label> <div class=md-header__title data-md-component=header-title> <div class=md-header__ellipsis> <div class=md-header__topic> <span class=md-ellipsis> JSON for Modern C++ </span> </div> <div class=md-header__topic data-md-component=header-topic> <span class=md-ellipsis> Modules </span> </div> </div> </div> <form class=md-header__option data-md-component=palette> <input class=md-option data-md-color-media="(prefers-color-scheme: light)" data-md-color-scheme=default data-md-color-primary=indigo data-md-color-accent=indigo aria-label="Switch to dark mode" type=radio name=__palette id=__palette_0> <label class="md-header__button md-icon" title="Switch to dark mode" for=__palette_1 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 8a4 4 0 0 0-4 4 4 4 0 0 0 4 4 4 4 0 0 0 4-4 4 4 0 0 0-4-4m0 10a6 6 0 0 1-6-6 6 6 0 0 1 6-6 6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> <input class=md-option data-md-color-media="(prefers-color-scheme: dark)" data-md-color-scheme=slate data-md-color-primary=indigo data-md-color-accent=indigo aria-label="Switch to light mode" type=radio name=__palette id=__palette_1> <label class="md-header__button md-icon" title="Switch to light mode" for=__palette_0 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 18c-.89 0-1.74-.2-2.5-.55C11.56 16.5 13 14.42 13 12s-1.44-4.5-3.5-5.45C10.26 6.2 11.11 6 12 6a6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> </form> <script>var palette=__md_get("__palette");if(palette&&palette.color){if("(prefers-color-scheme)"===palette.color.media){var media=matchMedia("(prefers-color-scheme: light)"),input=document.querySelector(media.matches?"[data-md-color-media='(prefers-color-scheme: light)']":"[data-md-color-media='(prefers-color-scheme: dark)']");palette.color.media=input.getAttribute("data-md-color-media"),palette.color.scheme=input.getAttribute("data-md-color-scheme"),palette.color.primary=input.getAttribute("data-md-color-primary"),palette.color.accent=input.getAttribute("data-md-color-accent")}for(var[key,value]of Object.entries(palette.color))document.body.setAttribute("data-md-color-"+key,value)}</script> <label class="md-header__button md-icon" for=__search> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M9.5 3A6.5 6.5 0 0 1 16 9.5c0 1.61-.59 3.09-1.56 4.23l.27.27h.79l5 5-1.5 1.5-5-5v-.79l-.27-.27A6.52 6.52 0 0 1 9.5 16 6.5 6.5 0 0 1 3 9.5 6.5 6.5 0 0 1 9.5 3m0 2C7 5 5 7 5 9.5S7 14 9.5 14 14 12 14 9.5 12 5 9.5 5"/></svg> </label> <div class=md-search data-md-Line truncated
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||||
<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=description content="JSON for Modern C++ is a header-only C++11 library that makes JSON a first-class data type."><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/modules/" rel="canonical"><link href=../modifying_values/ rel=prev><link href=../namespace/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>Modules - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.ab4e12ef.min.css><link rel=stylesheet href=../../css/custom.css><link rel=stylesheet href=../../css/brand.css><link rel=stylesheet href=../../css/fig-sax.css><link rel=stylesheet href=../../css/fig-jsonview.css><link rel=stylesheet href=../../css/fig-exceptions.css><link rel=stylesheet href=../../css/fig-pointer.css><link rel=stylesheet href=../../css/fig-charts.css><link rel=stylesheet href=../../css/fig-integration.css><link rel=stylesheet href=../../css/fig-macros.css><script>__md_scope=new URL("../..",location),__md_hash=e=>[...e].reduce(((e,_)=>(e<<5)-e+_.charCodeAt(0)),0),__md_get=(e,_=localStorage,t=__md_scope)=>JSON.parse(_.getItem(t.pathname+"."+e)),__md_set=(e,_,t=localStorage,a=__md_scope)=>{try{t.setItem(a.pathname+"."+e,JSON.stringify(_))}catch(e){}}</script></head> <body dir=ltr data-md-color-scheme=default data-md-color-primary=custom data-md-color-accent=custom> <input class=md-toggle data-md-toggle=drawer type=checkbox id=__drawer autocomplete=off> <input class=md-toggle data-md-toggle=search type=checkbox id=__search autocomplete=off> <label class=md-overlay for=__drawer></label> <div data-md-component=skip> <a href=#modules class=md-skip> Skip to content </a> </div> <div data-md-component=announce> </div> <header class=md-header data-md-component=header> <nav class="md-header__inner md-grid" aria-label=Header> <a href=../.. title="JSON for Modern C++" class="md-header__button md-logo" aria-label="JSON for Modern C++" data-md-component=logo> <img src=../../images/wordmark-header.svg alt=logo> </a> <label class="md-header__button md-icon" for=__drawer> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M3 6h18v2H3zm0 5h18v2H3zm0 5h18v2H3z"/></svg> </label> <div class=md-header__title data-md-component=header-title> <div class=md-header__ellipsis> <div class=md-header__topic> <span class=md-ellipsis> JSON for Modern C++ </span> </div> <div class=md-header__topic data-md-component=header-topic> <span class=md-ellipsis> Modules </span> </div> </div> </div> <form class=md-header__option data-md-component=palette> <input class=md-option data-md-color-media="(prefers-color-scheme: light)" data-md-color-scheme=default data-md-color-primary=custom data-md-color-accent=custom aria-label="Switch to dark mode" type=radio name=__palette id=__palette_0> <label class="md-header__button md-icon" title="Switch to dark mode" for=__palette_1 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 8a4 4 0 0 0-4 4 4 4 0 0 0 4 4 4 4 0 0 0 4-4 4 4 0 0 0-4-4m0 10a6 6 0 0 1-6-6 6 6 0 0 1 6-6 6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> <input class=md-option data-md-color-media="(prefers-color-scheme: dark)" data-md-color-scheme=slate data-md-color-primary=custom data-md-color-accent=custom aria-label="Switch to light mode" type=radio name=__palette id=__palette_1> <label class="md-header__button md-icon" title="Switch to light mode" for=__palette_0 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 18c-.89 0-1.74-.2-2.5-.55C11.56 16.5 13 14.42 13 12s-1.44-4.5-3.5-5.45C10.26 6.2 11.11 6 12 6a6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> </form> <script>var palette=__md_get("__palette");if(palette&&palette.color){if("(prefers-color-scheme)"===palette.color.media){var media=matchMedia("(prefers-color-scheme: light)"),input=document.querySelector(media.matches?"[data-md-color-media='(prefers-color-scheme: light)']":"[data-md-color-media='(prefers-color-scheme: dark)']");palette.color.media=input.getAttribute("data-md-color-media"),palette.color.scheme=input.getAttribute("data-md-color-scheme"),palette.color.primary=input.getAttribute("data-md-color-primary"),palette.color.accent=input.getAttribute("data-md-color-accent")}for(var[key,value]of Object.entries(palette.color))document.body.setAttribute("data-md-color-"+key,value)}</script> <label class="md-header__button md-icon" for=__search> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M9.5 3A6.5 6.5 0 0 1 16 9.5c0 1.61-.59 3.09-1.56 4.23l.27.27h.79l5 5-1.5 1.5-5-5Line truncated
|
||||
<span class=k>import</span><span class=w> </span><span class=n>nlohmann</span><span class=p>.</span><span class=n>json</span><span class=p>;</span>
|
||||
|
||||
<span class=k>using</span><span class=w> </span><span class=n>nlohmann</span><span class=o>::</span><span class=n>json</span><span class=p>;</span>
|
||||
@@ -7,4 +7,4 @@
|
||||
|
||||
<span class=n>std</span><span class=o>::</span><span class=n>ifstream</span><span class=w> </span><span class=n>f</span><span class=p>(</span><span class=s>"example.json"</span><span class=p>);</span>
|
||||
<span class=n>json</span><span class=w> </span><span class=n>data</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>parse</span><span class=p>(</span><span class=n>f</span><span class=p>);</span>
|
||||
</code></pre></div></p> <h2 id=modules-do-not-export-macros>Modules do not export macros<a class=headerlink href=#modules-do-not-export-macros title="Permanent link">¶</a></h2> <p>It should be noted that as modules do not export macros, the <code>nlohmann.json</code> module will not export any macros.</p> <h2 id=exported-symbols>Exported symbols<a class=headerlink href=#exported-symbols title="Permanent link">¶</a></h2> <p>Only the following symbols are exported from <code>nlohmann.json</code>:</p> <ul> <li><code>nlohmann::adl_serializer</code></li> <li><code>nlohmann::basic_json</code></li> <li><code>nlohmann::basic_json_document</code></li> <li><code>nlohmann::basic_json_view</code></li> <li><code>nlohmann::json</code></li> <li><code>nlohmann::json_document</code></li> <li><code>nlohmann::json_pointer</code></li> <li><code>nlohmann::json_view</code></li> <li><code>nlohmann::ordered_json</code></li> <li><code>nlohmann::ordered_json_document</code></li> <li><code>nlohmann::ordered_json_view</code></li> <li><code>nlohmann::ordered_map</code></li> <li><code>nlohmann::to_string</code></li> <li><code>nlohmann::literals::json_literals::operator""_json</code></li> <li><code>nlohmann::literals::json_literals::operator""_json_pointer</code></li> </ul> <p>The module always exports the two user-defined string literals, even if <a href=../../api/macros/json_no_automatic_udls/ ><code>JSON_NO_AUTOMATIC_UDLS</code></a> is defined when building it.</p> <p>Additionally, the following <code>nlohmann::detail</code> symbols are exported, solely to work around an <abbr title="Microsoft Visual C++">MSVC</abbr> compilation issue (<a href="https://github.com/nlohmann/json/issues/3970">#3970</a>). They are implementation details, not part of the public <abbr title="Application Programming Interface">API</abbr>, and should not be used directly:</p> <ul> <li><code>nlohmann::detail::json_sax_dom_callback_parser</code></li> <li><code>nlohmann::detail::unknown_size</code></li> </ul> <h2 id=known-issues>Known issues<a class=headerlink href=#known-issues title="Permanent link">¶</a></h2> <p>C++20 modules support is exercised in <abbr title="Continuous Integration">CI</abbr> against current <abbr title="GNU Compiler Collection">GCC</abbr> and Clang on Ubuntu, and the default <abbr title="Microsoft Visual C++">MSVC</abbr> toolset on Windows Server 2022 — there is no documented minimum compiler version, unlike feature-test-macro-gated features such as <a href=../../api/macros/json_has_ranges/ ><code>JSON_HAS_RANGES</code></a>.</p> <div class="admonition info"> <p class=admonition-title>Known compiler issues</p> <ul> <li><strong><abbr title="GNU Compiler Collection">GCC</abbr></strong> may emit "redefinition" errors when <code>#include <nlohmann/json.hpp></code> appears in a module preamble together with other imports. This is an upstream <abbr title="GNU Compiler Collection">GCC</abbr> bug, not yet resolved as of <abbr title="GNU Compiler Collection">GCC</abbr> 16. Workarounds: include <code>nlohmann/json.hpp</code> before other <code>#include</code>s, use <code>import nlohmann.json;</code> instead, or upgrade <abbr title="GNU Compiler Collection">GCC</abbr>. (<a href="https://github.com/nlohmann/json/issues/5103">issue #5103</a>)</li> <li><strong><abbr title="Microsoft Visual C++">MSVC</abbr></strong> could fail with <code>C2039: 'json_sax_dom_callback_parser' is not a member of ... detail</code>; fixed by exporting the required internal symbols from <code>json.cppm</code> (see <a href=#exported-symbols>Exported symbols</a> above). (<a href="https://github.com/nlohmann/json/issues/3970">issue #3970</a>)</li> </ul> <p>If you hit a different module-related build failure, search <a href="https://github.com/nlohmann/json/issues?q=is%3Aissue+modules">existing issues</a> before filing a new one.</p> </div> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 25, 2026 11:07:51 UTC">October 25, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <scrLine truncated
|
||||
</code></pre></div></p> <h2 id=modules-do-not-export-macros>Modules do not export macros<a class=headerlink href=#modules-do-not-export-macros title="Permanent link">¶</a></h2> <p>It should be noted that as modules do not export macros, the <code>nlohmann.json</code> module will not export any macros.</p> <h2 id=exported-symbols>Exported symbols<a class=headerlink href=#exported-symbols title="Permanent link">¶</a></h2> <p>Only the following symbols are exported from <code>nlohmann.json</code>:</p> <ul> <li><code>nlohmann::adl_serializer</code></li> <li><code>nlohmann::basic_json</code></li> <li><code>nlohmann::basic_json_document</code></li> <li><code>nlohmann::basic_json_view</code></li> <li><code>nlohmann::json</code></li> <li><code>nlohmann::json_document</code></li> <li><code>nlohmann::json_pointer</code></li> <li><code>nlohmann::json_view</code></li> <li><code>nlohmann::ordered_json</code></li> <li><code>nlohmann::ordered_json_document</code></li> <li><code>nlohmann::ordered_json_view</code></li> <li><code>nlohmann::ordered_map</code></li> <li><code>nlohmann::to_string</code></li> <li><code>nlohmann::literals::json_literals::operator""_json</code></li> <li><code>nlohmann::literals::json_literals::operator""_json_pointer</code></li> </ul> <p>The module always exports the two user-defined string literals, even if <a href=../../api/macros/json_no_automatic_udls/ ><code>JSON_NO_AUTOMATIC_UDLS</code></a> is defined when building it.</p> <p>Additionally, the following <code>nlohmann::detail</code> symbols are exported, solely to work around an <abbr title="Microsoft Visual C++">MSVC</abbr> compilation issue (<a href="https://github.com/nlohmann/json/issues/3970">#3970</a>). They are implementation details, not part of the public <abbr title="Application Programming Interface">API</abbr>, and should not be used directly:</p> <ul> <li><code>nlohmann::detail::json_sax_dom_callback_parser</code></li> <li><code>nlohmann::detail::unknown_size</code></li> </ul> <h2 id=known-issues>Known issues<a class=headerlink href=#known-issues title="Permanent link">¶</a></h2> <p>C++20 modules support is exercised in <abbr title="Continuous Integration">CI</abbr> against current <abbr title="GNU Compiler Collection">GCC</abbr> and Clang on Ubuntu, and the default <abbr title="Microsoft Visual C++">MSVC</abbr> toolset on Windows Server 2022 — there is no documented minimum compiler version, unlike feature-test-macro-gated features such as <a href=../../api/macros/json_has_ranges/ ><code>JSON_HAS_RANGES</code></a>.</p> <div class="admonition info"> <p class=admonition-title>Known compiler issues</p> <ul> <li><strong><abbr title="GNU Compiler Collection">GCC</abbr></strong> may emit "redefinition" errors when <code>#include <nlohmann/json.hpp></code> appears in a module preamble together with other imports. This is an upstream <abbr title="GNU Compiler Collection">GCC</abbr> bug, not yet resolved as of <abbr title="GNU Compiler Collection">GCC</abbr> 16. Workarounds: include <code>nlohmann/json.hpp</code> before other <code>#include</code>s, use <code>import nlohmann.json;</code> instead, or upgrade <abbr title="GNU Compiler Collection">GCC</abbr>. (<a href="https://github.com/nlohmann/json/issues/5103">issue #5103</a>)</li> <li><strong><abbr title="Microsoft Visual C++">MSVC</abbr></strong> could fail with <code>C2039: 'json_sax_dom_callback_parser' is not a member of ... detail</code>; fixed by exporting the required internal symbols from <code>json.cppm</code> (see <a href=#exported-symbols>Exported symbols</a> above). (<a href="https://github.com/nlohmann/json/issues/3970">issue #3970</a>)</li> </ul> <p>If you hit a different module-related build failure, search <a href="https://github.com/nlohmann/json/issues?q=is%3Aissue+modules">existing issues</a> before filing a new one.</p> </div> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">¶</a></h2> <ul> <li><a href=../macros/ >Supported Macros</a> - macros are not exported by modules</li> <li><a href=../../api/macros/ >Macros</a> - reference of all macros</li> <li><a href=../namespace/ ><code>nlohmann</code> Namespace</a> - the library namespace</li> <li><a href=../../integration/cmake/ >CMake</a> - enable the module build in CMake</li> <li><a href=../../integration/compile_times/ >Compile times</a> - reduce compile times</li> <li><a href=../../api/basic_json/ ><code>basic_json</code></a> - the main class and all its member functions</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.Line truncated
|
||||
@@ -4,6 +4,8 @@ This library has experimental support for C++ modules, introduced in C++20. The
|
||||
|
||||
Please be aware that the module is experimental and a full test is outstanding, and the exported symbols are subject to change.
|
||||
|
||||
Importing the module instead of including the header reduces the compile time of every translation unit that uses the library, at the price of building the module once. The [Compile times](https://json.nlohmann.me/integration/compile_times/#use-c20-modules) page has measurements and the one-off cost.
|
||||
|
||||
## Requirements
|
||||
|
||||
The `nlohmann.json` module requires that the build system is configured to build and resolve modules when imported. Obviously, as modules were introduced in C++20, this feature can only be used in C++20 and subsequent versions.
|
||||
@@ -67,3 +69,12 @@ Known compiler issues
|
||||
- **MSVC** could fail with `C2039: 'json_sax_dom_callback_parser' is not a member of ... detail`; fixed by exporting the required internal symbols from `json.cppm` (see [Exported symbols](#exported-symbols) above). ([issue #3970](https://github.com/nlohmann/json/issues/3970))
|
||||
|
||||
If you hit a different module-related build failure, search [existing issues](https://github.com/nlohmann/json/issues?q=is%3Aissue+modules) before filing a new one.
|
||||
|
||||
## See also
|
||||
|
||||
- [Supported Macros](https://json.nlohmann.me/features/macros/index.md) - macros are not exported by modules
|
||||
- [Macros](https://json.nlohmann.me/api/macros/index.md) - reference of all macros
|
||||
- [`nlohmann` Namespace](https://json.nlohmann.me/features/namespace/index.md) - the library namespace
|
||||
- [CMake](https://json.nlohmann.me/integration/cmake/index.md) - enable the module build in CMake
|
||||
- [Compile times](https://json.nlohmann.me/integration/compile_times/index.md) - reduce compile times
|
||||
- [`basic_json`](https://json.nlohmann.me/api/basic_json/index.md) - the main class and all its member functions
|
||||
@@ -38,6 +38,22 @@ For example, the namespace name for version 3.11.2 with `JSON_DIAGNOSTICS` defin
|
||||
nlohmann::json_abi_diag_v3_11_2
|
||||
```
|
||||
|
||||
The name is assembled from these segments; pick a version and macros to see the resulting name:
|
||||
|
||||
<figure class="nj-integration-ns" data-nj-integration-ns aria-label="How the inline namespace name is assembled from its segments">
|
||||
<div class="nj-integration-ns__strip" aria-live="polite">
|
||||
<span class="nj-integration-ns__seg nj-integration-ns__seg--root"><code>nlohmann</code><small>root namespace</small></span>
|
||||
<span class="nj-integration-ns__seg nj-integration-ns__seg--sep"><code>::</code><small> </small></span>
|
||||
<span class="nj-integration-ns__seg nj-integration-ns__seg--abi"><code>json_abi</code><small>always</small></span>
|
||||
<span class="nj-integration-ns__seg nj-integration-ns__seg--tag"><code>_diag</code><small>JSON_DIAGNOSTICS=1</small></span>
|
||||
<span class="nj-integration-ns__seg nj-integration-ns__seg--ver"><code>_v3_11_2</code><small>version 3.11.2</small></span>
|
||||
</div>
|
||||
<figcaption>Version 3.11.2 with <code>JSON_DIAGNOSTICS</code> defined to <code>1</code>. ABI tags are appended in the order of the
|
||||
list above, the version suffix comes last.</figcaption>
|
||||
</figure>
|
||||
|
||||
<p class="nj-caption" markdown>The name of the inline namespace, assembled from its segments. Select a version and macros to see which ABI tags appear; a tag the chosen version does not have is disabled.</p>
|
||||
|
||||
## Purpose
|
||||
|
||||
Several incompatibilities have been observed. Amongst the most common ones is linking code compiled with different
|
||||
@@ -55,6 +71,8 @@ graph
|
||||
app --> library
|
||||
```
|
||||
|
||||
<p class="nj-caption" markdown>The application uses the library with `JSON_DIAGNOSTICS=1` and links `some library`, which uses the same library version with `JSON_DIAGNOSTICS=0`. Arrows point from a component to what it uses.</p>
|
||||
|
||||
In releases prior to 3.11.0, mixing any version of the JSON library with different `JSON_DIAGNOSTICS` settings would
|
||||
result in a crashing application. If `some_library` never passes instances of JSON library types to the application,
|
||||
this scenario became safe in version 3.11.0 and above due to the inline namespace yielding distinct symbol names.
|
||||
@@ -102,6 +120,15 @@ follows:
|
||||
Overriding the namespace and mixing ABI-incompatible versions will result in crashes or incorrect behavior. You
|
||||
have been warned!
|
||||
|
||||
## See also
|
||||
|
||||
- [`NLOHMANN_JSON_NAMESPACE`](../api/macros/nlohmann_json_namespace.md) - the library namespace as macro
|
||||
- [`NLOHMANN_JSON_NAMESPACE_BEGIN`, `NLOHMANN_JSON_NAMESPACE_END`](../api/macros/nlohmann_json_namespace_begin.md) - open and close the library namespace
|
||||
- [`NLOHMANN_JSON_NAMESPACE_NO_VERSION`](../api/macros/nlohmann_json_namespace_no_version.md) - disable the version component
|
||||
- [Supported Macros](macros.md) - all supported macros
|
||||
- [Migration Guide](../integration/migration_guide.md) - do not hard-code the library namespace
|
||||
- [`basic_json`](../api/basic_json/index.md) - the main class and all its member functions
|
||||
|
||||
## Version history
|
||||
|
||||
- Introduced inline namespace (`json_v3_11_0[_abi-tag]*`) in version 3.11.0.
|
||||
|
||||
@@ -1,5 +1,5 @@
|
||||
<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/namespace/" rel="canonical"><link href=../modules/ rel=prev><link href=../no_exceptions/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>nlohmann Namespace - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.ab4e12ef.min.css><link rel="stylesheet" href="../../assets/external/fonts.googleapis.com/css.61a430c9.css"><style>:root{--md-text-font:"Roboto";--md-code-font:"JetBrains Mono"}</style><link rel=stylesheet href=../../css/custom.css><script>__md_scope=new 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xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 8a3 3 0 0 0 3-3 3 3 0 0 0-3-3 3 3 0 0 0-3 3 3 3 0 0 0 3 3m0 3.54C9.64 9.35 6.5 8 3 8v11c3.5 0 6.64 1.35 9 3.54 2.36-2.19 5.5-3.54 9-3.54V8c-3.5 0-6.64 1.35-9 3.54"/></svg> </a> <label class="md-header__button md-icon" for=__drawer> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M3 6h18v2H3zm0 5h18v2H3zm0 5h18v2H3z"/></svg> </label> <div class=md-header__title data-md-component=header-title> <div class=md-header__ellipsis> <div class=md-header__topic> <span class=md-ellipsis> JSON for Modern C++ </span> </div> <div class=md-header__topic data-md-component=header-topic> <span class=md-ellipsis> nlohmann Namespace </span> </div> </div> </div> <form class=md-header__option data-md-component=palette> <input class=md-option data-md-color-media="(prefers-color-scheme: light)" data-md-color-scheme=default data-md-color-primary=indigo data-md-color-accent=indigo aria-label="Switch to dark mode" type=radio 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6.52 0 0 1 9.5 16 6.5 6.5 0 0 1 3 9.5 6.5 6.5 0 0 1 9.5 3m0 2C7 5 5 7 5 9.5S7 14 9.5 14 14 12 14 9.5 12 5 9.5 5"/></svg> </label>Line truncated
|
||||
</code></pre></div> <h2 id=purpose>Purpose<a class=headerlink href=#purpose title="Permanent link">¶</a></h2> <p>Several incompatibilities have been observed. Amongst the most common ones is linking code compiled with different definitions of <a href=../../api/macros/json_diagnostics/ ><code>JSON_DIAGNOSTICS</code></a>. This is illustrated in the diagram below.</p> <pre class=mermaid><code>graph
|
||||
<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=description content="JSON for Modern C++ is a header-only C++11 library that makes JSON a first-class data type."><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/namespace/" rel="canonical"><link href=../modules/ rel=prev><link href=../no_exceptions/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>nlohmann Namespace - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.ab4e12ef.min.css><link rel=stylesheet href=../../css/custom.css><link rel=stylesheet href=../../css/brand.css><link rel=stylesheet href=../../css/fig-sax.css><link rel=stylesheet href=../../css/fig-jsonview.css><link rel=stylesheet 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data-md-component=announce> </div> <header class=md-header data-md-component=header> <nav class="md-header__inner md-grid" aria-label=Header> <a href=../.. title="JSON for Modern C++" class="md-header__button md-logo" aria-label="JSON for Modern C++" data-md-component=logo> <img src=../../images/wordmark-header.svg alt=logo> </a> <label class="md-header__button md-icon" for=__drawer> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M3 6h18v2H3zm0 5h18v2H3zm0 5h18v2H3z"/></svg> </label> <div class=md-header__title data-md-component=header-title> <div class=md-header__ellipsis> <div class=md-header__topic> <span class=md-ellipsis> JSON for Modern C++ </span> </div> <div class=md-header__topic data-md-component=header-topic> <span class=md-ellipsis> nlohmann Namespace </span> </div> </div> </div> <form class=md-header__option data-md-component=palette> <input class=md-option data-md-color-media="(prefers-color-scheme: light)" data-md-color-scheme=default 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</code></pre></div> <p>The name is assembled from these segments; pick a version and macros to see the resulting name:</p> <figure class=nj-integration-ns data-nj-integration-ns aria-label="How the inline namespace name is assembled from its segments"> <div class=nj-integration-ns__strip aria-live=polite> <span class="nj-integration-ns__seg nj-integration-ns__seg--root"><code>nlohmann</code><small>root namespace</small></span> <span class="nj-integration-ns__seg nj-integration-ns__seg--sep"><code>::</code><small> </small></span> <span class="nj-integration-ns__seg nj-integration-ns__seg--abi"><code>json_abi</code><small>always</small></span> <span class="nj-integration-ns__seg nj-integration-ns__seg--tag"><code>_diag</code><small>JSON_DIAGNOSTICS=1</small></span> <span class="nj-integration-ns__seg nj-integration-ns__seg--ver"><code>_v3_11_2</code><small>version 3.11.2</small></span> </div> <figcaption>Version 3.11.2 with <code>JSON_DIAGNOSTICS</code> defined to <code>1</code>. ABI tags are appended in the order of the list above, the version suffix comes last.</figcaption> </figure> <p class=nj-caption>The name of the inline namespace, assembled from its segments. Select a version and macros to see which <abbr title="Application Binary Interface">ABI</abbr> tags appear; a tag the chosen version does not have is disabled.</p> <h2 id=purpose>Purpose<a class=headerlink href=#purpose title="Permanent link">¶</a></h2> <p>Several incompatibilities have been observed. Amongst the most common ones is linking code compiled with different definitions of <a href=../../api/macros/json_diagnostics/ ><code>JSON_DIAGNOSTICS</code></a>. This is illustrated in the diagram below.</p> <pre class=mermaid><code>graph
|
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json["<strong>nlohmann_json (v3.10.5)</strong><br>JSON_DIAGNOSTICS=0"]
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json_diag["<strong>nlohmann_json (v3.10.5)</strong><br>JSON_DIAGNOSTICS=1"]
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library["<strong>some library</strong>"]
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@@ -7,6 +7,6 @@
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|
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library --> json
|
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app --> json_diag
|
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app --> library</code></pre> <p>In releases prior to 3.11.0, mixing any version of the <abbr title="JavaScript Object Notation">JSON</abbr> library with different <code>JSON_DIAGNOSTICS</code> settings would result in a crashing application. If <code>some_library</code> never passes instances of <abbr title="JavaScript Object Notation">JSON</abbr> library types to the application, this scenario became safe in version 3.11.0 and above due to the inline namespace yielding distinct symbol names.</p> <h2 id=limitations>Limitations<a class=headerlink href=#limitations title="Permanent link">¶</a></h2> <p>Neither the compiler nor the linker will issue as much as a warning when translation units – intended to be linked together and that include different versions and/or configurations of the <abbr title="JavaScript Object Notation">JSON</abbr> library – exchange and use library types.</p> <p>There is an exception when forward declarations are used (i.e., when including <code>json_fwd.hpp</code>) in which case the linker may complain about undefined references.</p> <h2 id=disabling-the-version-component>Disabling the version component<a class=headerlink href=#disabling-the-version-component title="Permanent link">¶</a></h2> <p>Different versions are not necessarily <abbr title="Application Binary Interface">ABI</abbr>-incompatible, but the project does not actively track changes in the <abbr title="Application Binary Interface">ABI</abbr> and recommends that all parts of a codebase exchanging library types be built with the same version. Users can, <strong>at their own risk</strong>, disable the version component of the inline namespace, allowing different versions – but not configurations – to be used in cases where the linker would otherwise output undefined reference errors.</p> <p>To do so, define <a href=../../api/macros/nlohmann_json_namespace_no_version/ ><code>NLOHMANN_JSON_NAMESPACE_NO_VERSION</code></a> to <code>1</code>.</p> <p>This applies to version 3.11.2 and above only; versions 3.11.0 and 3.11.1 can apply the technique described in the next section to emulate the effect of the <code>NLOHMANN_JSON_NAMESPACE_NO_VERSION</code> macro.</p> <div class="admonition danger"> <p class=admonition-title>Use at your own risk</p> <p>Disabling the namespace version component and mixing <abbr title="Application Binary Interface">ABI</abbr>-incompatible versions will result in crashes or incorrect behavior. You have been warned!</p> </div> <h2 id=disabling-the-inline-namespace-completely>Disabling the inline namespace completely<a class=headerlink href=#disabling-the-inline-namespace-completely title="Permanent link">¶</a></h2> <p>When interoperability with code using a pre-3.11.0 version of the library is required, users can, <strong>at their own risk</strong> restore the old namespace layout by redefining <a href=../../api/macros/nlohmann_json_namespace_begin/ ><code>NLOHMANN_JSON_NAMESPACE_BEGIN, NLOHMANN_JSON_NAMESPACE_END</code></a> as follows:</p> <div class=highlight><pre><span></span><code><span class=cp>#define NLOHMANN_JSON_NAMESPACE_BEGIN namespace nlohmann {</span>
|
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app --> library</code></pre> <p class=nj-caption>The application uses the library with <code>JSON_DIAGNOSTICS=1</code> and links <code>some library</code>, which uses the same library version with <code>JSON_DIAGNOSTICS=0</code>. Arrows point from a component to what it uses.</p> <p>In releases prior to 3.11.0, mixing any version of the <abbr title="JavaScript Object Notation">JSON</abbr> library with different <code>JSON_DIAGNOSTICS</code> settings would result in a crashing application. If <code>some_library</code> never passes instances of <abbr title="JavaScript Object Notation">JSON</abbr> library types to the application, this scenario became safe in version 3.11.0 and above due to the inline namespace yielding distinct symbol names.</p> <h2 id=limitations>Limitations<a class=headerlink href=#limitations title="Permanent link">¶</a></h2> <p>Neither the compiler nor the linker will issue as much as a warning when translation units – intended to be linked together and that include different versions and/or configurations of the <abbr title="JavaScript Object Notation">JSON</abbr> library – exchange and use library types.</p> <p>There is an exception when forward declarations are used (i.e., when including <code>json_fwd.hpp</code>) in which case the linker may complain about undefined references.</p> <h2 id=disabling-the-version-component>Disabling the version component<a class=headerlink href=#disabling-the-version-component title="Permanent link">¶</a></h2> <p>Different versions are not necessarily <abbr title="Application Binary Interface">ABI</abbr>-incompatible, but the project does not actively track changes in the <abbr title="Application Binary Interface">ABI</abbr> and recommends that all parts of a codebase exchanging library types be built with the same version. Users can, <strong>at their own risk</strong>, disable the version component of the inline namespace, allowing different versions – but not configurations – to be used in cases where the linker would otherwise output undefined reference errors.</p> <p>To do so, define <a href=../../api/macros/nlohmann_json_namespace_no_version/ ><code>NLOHMANN_JSON_NAMESPACE_NO_VERSION</code></a> to <code>1</code>.</p> <p>This applies to version 3.11.2 and above only; versions 3.11.0 and 3.11.1 can apply the technique described in the next section to emulate the effect of the <code>NLOHMANN_JSON_NAMESPACE_NO_VERSION</code> macro.</p> <div class="admonition danger"> <p class=admonition-title>Use at your own risk</p> <p>Disabling the namespace version component and mixing <abbr title="Application Binary Interface">ABI</abbr>-incompatible versions will result in crashes or incorrect behavior. You have been warned!</p> </div> <h2 id=disabling-the-inline-namespace-completely>Disabling the inline namespace completely<a class=headerlink href=#disabling-the-inline-namespace-completely title="Permanent link">¶</a></h2> <p>When interoperability with code using a pre-3.11.0 version of the library is required, users can, <strong>at their own risk</strong> restore the old namespace layout by redefining <a href=../../api/macros/nlohmann_json_namespace_begin/ ><code>NLOHMANN_JSON_NAMESPACE_BEGIN, NLOHMANN_JSON_NAMESPACE_END</code></a> as follows:</p> <div class=highlight><pre><span></span><code><span class=cp>#define NLOHMANN_JSON_NAMESPACE_BEGIN namespace nlohmann {</span>
|
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<span class=cp>#define NLOHMANN_JSON_NAMESPACE_END }</span>
|
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</code></pre></div> <div class="admonition danger"> <p class=admonition-title>Use at your own risk</p> <p>Overriding the namespace and mixing <abbr title="Application Binary Interface">ABI</abbr>-incompatible versions will result in crashes or incorrect behavior. You have been warned!</p> </div> <h2 id=version-history>Version history<a class=headerlink href=#version-history title="Permanent link">¶</a></h2> <ul> <li>Introduced inline namespace (<code>json_v3_11_0[_abi-tag]*</code>) in version 3.11.0.</li> <li>Changed structure of inline namespace in version 3.11.2.</li> <li>Added <abbr title="Application Binary Interface">ABI</abbr> tag <code>_dp</code> (<a href=../../api/macros/json_diagnostic_positions/ ><code>JSON_DIAGNOSTIC_POSITIONS</code></a>) in version 3.12.0.</li> <li>Added <abbr title="Application Binary Interface">ABI</abbr> tags <code>_bics</code> (<a href=../../api/macros/json_brace_init_copy_semantics/ ><code>JSON_BRACE_INIT_COPY_SEMANTICS</code></a>), <code>_psp</code> (<a href=../../api/macros/json_precise_stream_position/ ><code>JSON_PRECISE_STREAM_POSITION</code></a>), <code>_snul</code> (<a href=../../api/macros/json_strict_nul_handling/ ><code>JSON_STRICT_NUL_HANDLING</code></a>), <code>_sbu8</code> (<a href=../../api/macros/json_strict_binary_utf8/ ><code>JSON_STRICT_BINARY_UTF8</code></a>), <code>_ekmo</code> (<a href=../../api/macros/json_use_objects_for_enum_keyed_maps/ ><code>JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS</code></a>), <code>_dsbs</code> (<a href=../../api/macros/json_disable_stream_bulk_scan/ ><code>JSON_DISABLE_STREAM_BULK_SCAN</code></a>), <code>_dufio</code> (<a href=../../api/macros/json_disable_unlocked_file_io/ ><code>JSON_DISABLE_UNLOCKED_FILE_IO</code></a>), and <code>_ns</code> (<a href=../../api/macros/json_no_simd/ ><code>JSON_NO_SIMD</code></a>) in version 3.13.0 <span class=unreleased-version title="Not part of a release yet; the latest release is 3.12.0.">unreleased</span>.</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 26, 2026 19:13:31 UTC">October 26, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2025 Fonticons, Inc.--><path d="M173.9 397.4c0 2-2.3 3.6-5.2 3.6-3.3.3-5.6-1.3-5.6-3.6 0-2 2.3-3.6 5.2-3.6 3-.3 5.6 1.3 5.6 3.6m-31.1-4.5c-.7 2 1.3 4.3 4.3 4.9 2.6 1 5.6 0 6.2-2s-1.3-4.3-4.3-5.2c-2.6-.7-5.5.3-6.2 2.3m44.2-1.7c-2.9.7-4.9 2.6-4.6 4.9.3 2 2.9 3.3 5.9 2.6 2.9-.7 4.9-2.6 4.6-4.6-.3-1.9-3-3.2-5.9-2.9M252.8 8C114.1 8 8 113.3 8 252c0 110.9 69.8 205.8 169.5 239.2 12.8 2.3 17.3-5.6 17.3-12.1 0-6.2-.3-40.4-.3-61.4 0 0-70 15-84.7-29.8 0 0-11.4-29.1-27.8-36.6 0 0-22.9-15.7 1.6-15.4 0 0 24.9 2 38.6 25.8 21.9 38.6 58.6 27.5 72.9 20.9 2.3-16 8.8-27.1 16-33.7-55.9-6.2-112.3-14.3-112.3-110.5 0-27.5 7.6-41.3 23.6-58.9-2.6-6.5-11.1-33.3 2.6-67.9 20.9-6.5 69 27 69 27 20-5.6 41.5-8.5 62.8-8.5s42.8 2.9 62.8 8.5c0 0 48.1-33.6 69-27 13.7 34.7 5.2 61.4 2.6 67.9 16 17.7 25.8 31.5 25.8 58.9 0 96.5-58.9 104.2-114.8 110.5 9.2 7.9 17 22.9 17 46.4 0 33.7-.3 75.4-.3 83.6 0 6.5 4.6 14.4 17.3 12Line truncated
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||||
</code></pre></div> <div class="admonition danger"> <p class=admonition-title>Use at your own risk</p> <p>Overriding the namespace and mixing <abbr title="Application Binary Interface">ABI</abbr>-incompatible versions will result in crashes or incorrect behavior. You have been warned!</p> </div> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">¶</a></h2> <ul> <li><a href=../../api/macros/nlohmann_json_namespace/ ><code>NLOHMANN_JSON_NAMESPACE</code></a> - the library namespace as macro</li> <li><a href=../../api/macros/nlohmann_json_namespace_begin/ ><code>NLOHMANN_JSON_NAMESPACE_BEGIN</code>, <code>NLOHMANN_JSON_NAMESPACE_END</code></a> - open and close the library namespace</li> <li><a href=../../api/macros/nlohmann_json_namespace_no_version/ ><code>NLOHMANN_JSON_NAMESPACE_NO_VERSION</code></a> - disable the version component</li> <li><a href=../macros/ >Supported Macros</a> - all supported macros</li> <li><a href=../../integration/migration_guide/ >Migration Guide</a> - do not hard-code the library namespace</li> <li><a href=../../api/basic_json/ ><code>basic_json</code></a> - the main class and all its member functions</li> </ul> <h2 id=version-history>Version history<a class=headerlink href=#version-history title="Permanent link">¶</a></h2> <ul> <li>Introduced inline namespace (<code>json_v3_11_0[_abi-tag]*</code>) in version 3.11.0.</li> <li>Changed structure of inline namespace in version 3.11.2.</li> <li>Added <abbr title="Application Binary Interface">ABI</abbr> tag <code>_dp</code> (<a href=../../api/macros/json_diagnostic_positions/ ><code>JSON_DIAGNOSTIC_POSITIONS</code></a>) in version 3.12.0.</li> <li>Added <abbr title="Application Binary Interface">ABI</abbr> tags <code>_bics</code> (<a href=../../api/macros/json_brace_init_copy_semantics/ ><code>JSON_BRACE_INIT_COPY_SEMANTICS</code></a>), <code>_psp</code> (<a href=../../api/macros/json_precise_stream_position/ ><code>JSON_PRECISE_STREAM_POSITION</code></a>), <code>_snul</code> (<a href=../../api/macros/json_strict_nul_handling/ ><code>JSON_STRICT_NUL_HANDLING</code></a>), <code>_sbu8</code> (<a href=../../api/macros/json_strict_binary_utf8/ ><code>JSON_STRICT_BINARY_UTF8</code></a>), <code>_ekmo</code> (<a href=../../api/macros/json_use_objects_for_enum_keyed_maps/ ><code>JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS</code></a>), <code>_dsbs</code> (<a href=../../api/macros/json_disable_stream_bulk_scan/ ><code>JSON_DISABLE_STREAM_BULK_SCAN</code></a>), <code>_dufio</code> (<a href=../../api/macros/json_disable_unlocked_file_io/ ><code>JSON_DISABLE_UNLOCKED_FILE_IO</code></a>), and <code>_ns</code> (<a href=../../api/macros/json_no_simd/ ><code>JSON_NO_SIMD</code></a>) in version 3.13.0 <span class=unreleased-version title="Not part of a release yet; the latest release is 3.12.0.">unreleased</span>.</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../modules/ class="md-footer__link md-footer__link--prev" aria-label="Previous: Modules"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> Modules </div> </div> </a> <a href=../no_exceptions/ class="md-footer__link md-footer__link--next" aria-label="Next: Working Without ELine truncated
|
||||
@@ -27,6 +27,14 @@ For example, the namespace name for version 3.11.2 with `JSON_DIAGNOSTICS` defin
|
||||
nlohmann::json_abi_diag_v3_11_2
|
||||
```
|
||||
|
||||
The name is assembled from these segments; pick a version and macros to see the resulting name:
|
||||
|
||||
`nlohmann`root namespace `::` `json_abi`always `_diag`JSON_DIAGNOSTICS=1 `_v3_11_2`version 3.11.2
|
||||
|
||||
Version 3.11.2 with `JSON_DIAGNOSTICS` defined to `1`. ABI tags are appended in the order of the list above, the version suffix comes last.
|
||||
|
||||
The name of the inline namespace, assembled from its segments. Select a version and macros to see which ABI tags appear; a tag the chosen version does not have is disabled.
|
||||
|
||||
## Purpose
|
||||
|
||||
Several incompatibilities have been observed. Amongst the most common ones is linking code compiled with different definitions of [`JSON_DIAGNOSTICS`](https://json.nlohmann.me/api/macros/json_diagnostics/index.md). This is illustrated in the diagram below.
|
||||
@@ -43,6 +51,8 @@ graph
|
||||
app --> library
|
||||
```
|
||||
|
||||
The application uses the library with `JSON_DIAGNOSTICS=1` and links `some library`, which uses the same library version with `JSON_DIAGNOSTICS=0`. Arrows point from a component to what it uses.
|
||||
|
||||
In releases prior to 3.11.0, mixing any version of the JSON library with different `JSON_DIAGNOSTICS` settings would result in a crashing application. If `some_library` never passes instances of JSON library types to the application, this scenario became safe in version 3.11.0 and above due to the inline namespace yielding distinct symbol names.
|
||||
|
||||
## Limitations
|
||||
@@ -76,6 +86,15 @@ Use at your own risk
|
||||
|
||||
Overriding the namespace and mixing ABI-incompatible versions will result in crashes or incorrect behavior. You have been warned!
|
||||
|
||||
## See also
|
||||
|
||||
- [`NLOHMANN_JSON_NAMESPACE`](https://json.nlohmann.me/api/macros/nlohmann_json_namespace/index.md) - the library namespace as macro
|
||||
- [`NLOHMANN_JSON_NAMESPACE_BEGIN`, `NLOHMANN_JSON_NAMESPACE_END`](https://json.nlohmann.me/api/macros/nlohmann_json_namespace_begin/index.md) - open and close the library namespace
|
||||
- [`NLOHMANN_JSON_NAMESPACE_NO_VERSION`](https://json.nlohmann.me/api/macros/nlohmann_json_namespace_no_version/index.md) - disable the version component
|
||||
- [Supported Macros](https://json.nlohmann.me/features/macros/index.md) - all supported macros
|
||||
- [Migration Guide](https://json.nlohmann.me/integration/migration_guide/index.md) - do not hard-code the library namespace
|
||||
- [`basic_json`](https://json.nlohmann.me/api/basic_json/index.md) - the main class and all its member functions
|
||||
|
||||
## Version history
|
||||
|
||||
- Introduced inline namespace (`json_v3_11_0[_abi-tag]*`) in version 3.11.0.
|
||||
|
||||
@@ -240,5 +240,10 @@ With exceptions off, you can replace the `abort()` by your own code, for example
|
||||
|
||||
- [Exceptions](../home/exceptions.md) - all exceptions and their ids
|
||||
- [Parsing and exceptions](parsing/parse_exceptions.md) - the parsing alternatives in detail
|
||||
- [Parsing untrusted input](parsing/untrusted_input.md)
|
||||
- [`JSON_NOEXCEPTION`](../api/macros/json_noexception.md)
|
||||
- [Parsing untrusted input](parsing/untrusted_input.md) - harden an application that parses data it does not control
|
||||
- [`JSON_NOEXCEPTION`](../api/macros/json_noexception.md) - disable exceptions in the library
|
||||
- [`exception`](../api/basic_json/exception.md) - base class of all exceptions thrown by the library
|
||||
- [`JSON_THROW_USER`](../api/macros/json_throw_user.md) - replace the throw statements of the library
|
||||
- [`parse`](../api/basic_json/parse.md) - deserialize from a compatible input
|
||||
- [`accept`](../api/basic_json/accept.md) - check whether the input is valid JSON
|
||||
- [Supported Macros](macros.md) - all supported macros
|
||||
@@ -1,4 +1,4 @@
|
||||
<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/no_exceptions/" rel="canonical"><link href=../namespace/ rel=prev><link href=../object_order/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>Working Without Exceptions - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.ab4e12ef.min.css><link rel="stylesheet" href="../../assets/external/fonts.googleapis.com/css.61a430c9.css"><style>:root{--md-text-font:"Roboto";--md-code-font:"JetBrains Mono"}</style><link rel=stylesheet href=../../css/custom.css><script>__md_scope=new 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6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> </form> <script>var palette=__md_get("__palette");if(palette&&palette.color){if("(prefers-color-scheme)"===palette.color.media){var media=matchMedia("(prefers-color-scheme: light)"),input=document.querySelector(media.matches?"[data-md-color-media='(prefers-color-scheme: light)']":"[data-md-color-media='(prefers-color-scheme: dark)']");palette.color.media=input.getAttribute("data-md-color-media"),palette.color.scheme=input.getAttribute("data-md-color-scheme"),palette.color.primary=input.getAttribute("data-md-color-primary"),palette.color.accent=input.getAttribute("data-md-color-accent")}for(var[key,value]of Object.entries(palette.color))document.body.setAttribute("data-md-color-"+key,value)}</script> <label class="md-header__button md-icon" for=__search> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M9.5 3A6.5 6.5 0 0 1 16 9.5c0 1.61-.59 3.09-1.56 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|
||||
<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=description content="JSON for Modern C++ is a header-only C++11 library that makes JSON a first-class data type."><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/no_exceptions/" rel="canonical"><link href=../namespace/ rel=prev><link href=../object_order/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>Working Without Exceptions - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.ab4e12ef.min.css><link rel=stylesheet href=../../css/custom.css><link rel=stylesheet href=../../css/brand.css><link rel=stylesheet href=../../css/fig-sax.css><link rel=stylesheet href=../../css/fig-jsonview.css><link rel=stylesheet 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data-md-component=announce> </div> <header class=md-header data-md-component=header> <nav class="md-header__inner md-grid" aria-label=Header> <a href=../.. title="JSON for Modern C++" class="md-header__button md-logo" aria-label="JSON for Modern C++" data-md-component=logo> <img src=../../images/wordmark-header.svg alt=logo> </a> <label class="md-header__button md-icon" for=__drawer> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M3 6h18v2H3zm0 5h18v2H3zm0 5h18v2H3z"/></svg> </label> <div class=md-header__title data-md-component=header-title> <div class=md-header__ellipsis> <div class=md-header__topic> <span class=md-ellipsis> JSON for Modern C++ </span> </div> <div class=md-header__topic data-md-component=header-topic> <span class=md-ellipsis> Working Without Exceptions </span> </div> </div> </div> <form class=md-header__option data-md-component=palette> <input class=md-option data-md-color-media="(prefers-color-scheme: light)" data-md-color-scheme=default data-md-color-primary=custom data-md-color-accent=custom aria-label="Switch to dark mode" type=radio name=__palette id=__palette_0> <label class="md-header__button md-icon" title="Switch to dark mode" for=__palette_1 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 8a4 4 0 0 0-4 4 4 4 0 0 0 4 4 4 4 0 0 0 4-4 4 4 0 0 0-4-4m0 10a6 6 0 0 1-6-6 6 6 0 0 1 6-6 6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> <input class=md-option data-md-color-media="(prefers-color-scheme: dark)" data-md-color-scheme=slate data-md-color-primary=custom data-md-color-accent=custom aria-label="Switch to light mode" type=radio name=__palette id=__palette_1> <label class="md-header__button md-icon" title="Switch to light mode" for=__palette_0 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 18c-.89 0-1.74-.2-2.5-.55C11.56 16.5 13 14.42 13 12s-1.44-4.5-3.5-5.45C10.26 6.2 11.11 6 12 6a6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> </form> <script>var palette=__md_get("__palette");if(palette&&palette.color){if("(prefers-color-scheme)"===palette.color.media){var media=matchMedia("(prefers-color-scheme: light)"),input=document.querySelector(media.matches?"[data-md-color-media='(prefers-color-scheme: light)']":"[data-md-color-media='(prefers-color-scheme: dark)']");palette.color.media=input.getAttribute("data-md-color-media"),palette.color.scheme=input.getAttribute("data-md-color-scheme"),palette.color.primary=input.getAttribute("data-md-color-primary"),palette.color.accent=input.getAttribute("data-md-color-accent")}for(var[key,value]of Object.entries(palette.color))document.body.setAttribute("data-md-color-"+key,value)}</script> <label class="md-header__button md-icon" for=__search> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M9.5 3A6.5 6.5 0 0 1 Line truncated
|
||||
<span class=cp>#include</span><span class=w> </span><span class=cpf><iostream></span>
|
||||
<span class=cp>#include</span><span class=w> </span><span class=cpf><vector></span>
|
||||
<span class=cp>#include</span><span class=w> </span><span class=cpf><nlohmann/json.hpp></span>
|
||||
@@ -129,4 +129,4 @@ get_ptr<string*>: name = 1, age = 0
|
||||
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code>person: Mary, 42
|
||||
not a person: {"age":"forty-two","name":"Mary"}
|
||||
not a person: {"name":"Mary"}
|
||||
</code></pre></div> </details> <h2 id=overriding-what-happens-instead-of-an-exception>Overriding what happens instead of an exception<a class=headerlink href=#overriding-what-happens-instead-of-an-exception title="Permanent link">¶</a></h2> <p>With exceptions off, you can replace the <code>abort()</code> by your own code, for example to log the message first. See <a href=../../home/exceptions/#switch-off-exceptions>Switch off exceptions</a> and <a href=../../api/macros/json_throw_user/ ><code>JSON_THROW_USER</code></a>. The handler must not return normally.</p> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">¶</a></h2> <ul> <li><a href=../../home/exceptions/ >Exceptions</a> - all exceptions and their ids</li> <li><a href=../parsing/parse_exceptions/ >Parsing and exceptions</a> - the parsing alternatives in detail</li> <li><a href=../parsing/untrusted_input/ >Parsing untrusted input</a></li> <li><a href=../../api/macros/json_noexception/ ><code>JSON_NOEXCEPTION</code></a></li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 25, 2026 11:07:13 UTC">October 25, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2025 Fonticons, Inc.--><path d="M173.9 397.4c0 2-2.3 3.6-5.2 3.6-3.3.3-5.6-1.3-5.6-3.6 0-2 2.3-3.6 5.2-3.6 3-.3 5.6 1.3 5.6 3.6m-31.1-4.5c-.7 2 1.3 4.3 4.3 4.9 2.6 1 5.6 0 6.2-2s-1.3-4.3-4.3-5.2c-2.6-.7-5.5.3-6.2 2.3m44.2-1.7c-2.9.7-4.9 2.6-4.6 4.9.3 2 2.9 3.3 5.9 2.6 2.9-.7 4.9-2.6 4.6-4.6-.3-1.9-3-3.2-5.9-2.9M252.8 8C114.1 8 8 113.3 8 252c0 110.9 69.8 205.8 169.5 239.2 12.8 2.3 17.3-5.6 17.3-12.1 0-6.2-.3-40.4-.3-61.4 0 0-70 15-84.7-29.8 0 0-11.4-29.1-27.8-36.6 0 0-22.9-15.7 1.6-15.4 0 0 24.9 2 38.6 25.8 21.9 38.6 58.6 27.5 72.9 20.9 2.3-16 8.8-27.1 16-33.7-55.9-6.2-112.3-14.3-112.3-110.5 0-27.5 7.6-41.3 23.6-58.9-2.6-6.5-11.1-33.3 2.6-67.9 20.9-6.5 69 27 69 27 20-5.6 41.5-8.5 62.8-8.5s42.8 2.9 62.8 8.5c0 0 48.1-33.6 69-27 13.7 34.7 5.2 61.4 2.6 67.9 16 17.7 25.8 31.5 25.8 58.9 0 96.5-58.9 104.2-114.8 110.5 9.2 7.9 17 22.9 17 46.4 0 33.7-.3 75.4-.3 83.6 0 6.5 4.6 14.4 17.3 12.1C436.2 457.8 504 362.9 504 252 504 113.3 391.5 8 252.8 8M105.2 352.9c-1.3 1-1 3.3.7 5.2 1.6 1.6 3.9 2.3 5.2 1 1.3-1 1-3.3-.7-5.2-1.6-1.6-3.9-2.3-5.2-1m-10.8-8.1c-.7 1.3.3 2.9 2.3 3.9 1.6 1 3.6.7 4.3-.7.7-1.3-.3-2.9-2.3-3.9-2-.6-3.6-.3-4.3.7m32.4 35.6c-1.6 1.3-1 4.3 1.3 6.2 2.3 2.3 5.2 2.6 6.5 1 1.3-1.3.7-4.3-1.3-6.2-2.2-2.3-5.2-2.6-6.5-1m-11.4-14.7c-1.6 1-1.6 3.6 0 5.9s4.3 3.3 5.6 2.3c1.6-1.3 1.6-3.9 0-6.2-1.4-2.3-4-3.3-5.6-2"/></svg> </a> <a href="https://www.linkedin.com/in/nielslohmann/" target="_blank" rel="noopener" title="www.linkedin.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 448 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2025 Fonticons, Inc.--><path d="M416 32H31.9C14.3 32 0 46.5 0 64.3v383.4C0 465.5 14.3 480 31.9 480H416cLine truncated
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||||
</code></pre></div> </details> <h2 id=overriding-what-happens-instead-of-an-exception>Overriding what happens instead of an exception<a class=headerlink href=#overriding-what-happens-instead-of-an-exception title="Permanent link">¶</a></h2> <p>With exceptions off, you can replace the <code>abort()</code> by your own code, for example to log the message first. See <a href=../../home/exceptions/#switch-off-exceptions>Switch off exceptions</a> and <a href=../../api/macros/json_throw_user/ ><code>JSON_THROW_USER</code></a>. The handler must not return normally.</p> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">¶</a></h2> <ul> <li><a href=../../home/exceptions/ >Exceptions</a> - all exceptions and their ids</li> <li><a href=../parsing/parse_exceptions/ >Parsing and exceptions</a> - the parsing alternatives in detail</li> <li><a href=../parsing/untrusted_input/ >Parsing untrusted input</a> - harden an application that parses data it does not control</li> <li><a href=../../api/macros/json_noexception/ ><code>JSON_NOEXCEPTION</code></a> - disable exceptions in the library</li> <li><a href=../../api/basic_json/exception/ ><code>exception</code></a> - base class of all exceptions thrown by the library</li> <li><a href=../../api/macros/json_throw_user/ ><code>JSON_THROW_USER</code></a> - replace the throw statements of the library</li> <li><a href=../../api/basic_json/parse/ ><code>parse</code></a> - deserialize from a compatible input</li> <li><a href=../../api/basic_json/accept/ ><code>accept</code></a> - check whether the input is valid <abbr title="JavaScript Object Notation">JSON</abbr></li> <li><a href=../macros/ >Supported Macros</a> - all supported macros</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../namespace/ class="md-footer__link md-footer__link--prev" aria-label="Previous: nlohmann Namespace"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> nlohmann Namespace </div> </div> </a> <a href=../object_order/ class="md-footer__link md-footer__link--next" aria-label="Next: Object Order"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> Object Order </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2025 Fonticons, Inc.--><path d="M173.9 397.4c0 2-2.3 3.6-5.2 3.6-3.3.3-5.6-1.3-5.6-3.6 0-2 2.3-3.6 5.2-3.6 3-.3 5.6 1.3 5.6 3.6m-31.1-4.5c-.7 2 1.3 4.3 4.3 4.9 2.6 1 5.6 0 6.2-2s-1.3-4.3-4.3-5.2Line truncated
|
||||
@@ -218,5 +218,10 @@ With exceptions off, you can replace the `abort()` by your own code, for example
|
||||
|
||||
- [Exceptions](https://json.nlohmann.me/home/exceptions/index.md) - all exceptions and their ids
|
||||
- [Parsing and exceptions](https://json.nlohmann.me/features/parsing/parse_exceptions/index.md) - the parsing alternatives in detail
|
||||
- [Parsing untrusted input](https://json.nlohmann.me/features/parsing/untrusted_input/index.md)
|
||||
- [`JSON_NOEXCEPTION`](https://json.nlohmann.me/api/macros/json_noexception/index.md)
|
||||
- [Parsing untrusted input](https://json.nlohmann.me/features/parsing/untrusted_input/index.md) - harden an application that parses data it does not control
|
||||
- [`JSON_NOEXCEPTION`](https://json.nlohmann.me/api/macros/json_noexception/index.md) - disable exceptions in the library
|
||||
- [`exception`](https://json.nlohmann.me/api/basic_json/exception/index.md) - base class of all exceptions thrown by the library
|
||||
- [`JSON_THROW_USER`](https://json.nlohmann.me/api/macros/json_throw_user/index.md) - replace the throw statements of the library
|
||||
- [`parse`](https://json.nlohmann.me/api/basic_json/parse/index.md) - deserialize from a compatible input
|
||||
- [`accept`](https://json.nlohmann.me/api/basic_json/accept/index.md) - check whether the input is valid JSON
|
||||
- [Supported Macros](https://json.nlohmann.me/features/macros/index.md) - all supported macros
|
||||
@@ -136,3 +136,11 @@ Assume file `input.json` contains the JSON object above:
|
||||
"two": 2
|
||||
}
|
||||
```
|
||||
|
||||
## See also
|
||||
|
||||
- [`ordered_json`](../api/ordered_json.md) - JSON type that preserves insertion order
|
||||
- [`ordered_map`](../api/ordered_map.md) - the container behind `ordered_json`
|
||||
- [`object_t`](../api/basic_json/object_t.md) - type used to store JSON objects
|
||||
- [Iterators](iterators.md) - iteration order of object keys
|
||||
- [Element Access](element_access/index.md) - access elements by key
|
||||
@@ -1,4 +1,4 @@
|
||||
<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/object_order/" rel="canonical"><link href=../no_exceptions/ rel=prev><link href=../performance/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>Object Order - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.ab4e12ef.min.css><link rel="stylesheet" href="../../assets/external/fonts.googleapis.com/css.61a430c9.css"><style>:root{--md-text-font:"Roboto";--md-code-font:"JetBrains Mono"}</style><link rel=stylesheet href=../../css/custom.css><script>__md_scope=new 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<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=description content="JSON for Modern C++ is a header-only C++11 library that makes JSON a first-class data type."><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/object_order/" rel="canonical"><link href=../no_exceptions/ rel=prev><link href=../performance/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>Object Order - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.ab4e12ef.min.css><link rel=stylesheet href=../../css/custom.css><link rel=stylesheet href=../../css/brand.css><link rel=stylesheet href=../../css/fig-sax.css><link rel=stylesheet href=../../css/fig-jsonview.css><link rel=stylesheet href=../../css/fig-exceptions.css><link rel=stylesheet href=../../css/fig-pointer.css><link rel=stylesheet href=../../css/fig-charts.css><link rel=stylesheet href=../../css/fig-integration.css><link rel=stylesheet href=../../css/fig-macros.css><script>__md_scope=new URL("../..",location),__md_hash=e=>[...e].reduce(((e,_)=>(e<<5)-e+_.charCodeAt(0)),0),__md_get=(e,_=localStorage,t=__md_scope)=>JSON.parse(_.getItem(t.pathname+"."+e)),__md_set=(e,_,t=localStorage,a=__md_scope)=>{try{t.setItem(a.pathname+"."+e,JSON.stringify(_))}catch(e){}}</script></head> <body dir=ltr data-md-color-scheme=default data-md-color-primary=custom data-md-color-accent=custom> <input class=md-toggle data-md-toggle=drawer type=checkbox id=__drawer autocomplete=off> <input class=md-toggle data-md-toggle=search type=checkbox id=__search autocomplete=off> <label class=md-overlay for=__drawer></label> <div data-md-component=skip> <a href=#object-order class=md-skip> Skip to content </a> </div> <div data-md-component=announce> </div> <header class=md-header data-md-component=header> <nav class="md-header__inner md-grid" aria-label=Header> <a href=../.. title="JSON for Modern C++" class="md-header__button md-logo" aria-label="JSON for Modern C++" data-md-component=logo> <img src=../../images/wordmark-header.svg alt=logo> </a> <label class="md-header__button md-icon" for=__drawer> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M3 6h18v2H3zm0 5h18v2H3zm0 5h18v2H3z"/></svg> </label> <div class=md-header__title data-md-component=header-title> <div class=md-header__ellipsis> <div class=md-header__topic> <span class=md-ellipsis> JSON for Modern C++ </span> </div> <div class=md-header__topic data-md-component=header-topic> <span class=md-ellipsis> Object Order </span> </div> </div> </div> <form class=md-header__option data-md-component=palette> <input class=md-option data-md-color-media="(prefers-color-scheme: light)" data-md-color-scheme=default data-md-color-primary=custom data-md-color-accent=custom aria-label="Switch to dark mode" type=radio name=__palette id=__palette_0> <label class="md-header__button md-icon" title="Switch to dark mode" for=__palette_1 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 8a4 4 0 0 0-4 4 4 4 0 0 0 4 4 4 4 0 0 0 4-4 4 4 0 0 0-4-4m0 10a6 6 0 0 1-6-6 6 6 0 0 1 6-6 6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> <input class=md-option data-md-color-media="(prefers-color-scheme: dark)" data-md-color-scheme=slate data-md-color-primary=custom data-md-color-accent=custom aria-label="Switch to light mode" type=radio name=__palette id=__palette_1> <label class="md-header__button md-icon" title="Switch to light mode" for=__palette_0 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 18c-.89 0-1.74-.2-2.5-.55C11.56 16.5 13 14.42 13 12s-1.44-4.5-3.5-5.45C10.26 6.2 11.11 6 12 6a6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> </form> <script>var palette=__md_get("__palette");if(palette&&palette.color){if("(prefers-color-scheme)"===palette.color.media){var media=matchMedia("(prefers-color-scheme: light)"),input=document.querySelector(media.matches?"[data-md-color-media='(prefers-color-scheme: light)']":"[data-md-color-media='(prefers-color-scheme: dark)']");palette.color.media=input.getAttribute("data-md-color-media"),palette.color.scheme=input.getAttribute("data-md-color-scheme"),palette.color.primary=input.getAttribute("data-md-color-primary"),palette.color.accent=input.getAttribute("data-md-color-accent")}for(var[key,value]of Object.entries(palette.color))document.body.setAttribute("data-md-color-"+key,value)}</script> <label class="md-header__button md-icon" for=__search> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M9.5 3A6.5 6.5 0 0 1 16 9.5c0 1.61-.59 3.09-1.56 4.23l.27.27hLine truncated
|
||||
<span class=cp>#include</span><span class=w> </span><span class=cpf><nlohmann/json.hpp></span>
|
||||
|
||||
<span class=k>using</span><span class=w> </span><span class=n>nlohmann</span><span class=o>::</span><span class=n>json</span><span class=p>;</span>
|
||||
@@ -57,4 +57,4 @@
|
||||
<span class=w> </span><span class=nt>"three"</span><span class=p>:</span><span class=w> </span><span class=mi>3</span><span class=p>,</span>
|
||||
<span class=w> </span><span class=nt>"two"</span><span class=p>:</span><span class=w> </span><span class=mi>2</span>
|
||||
<span class=p>}</span>
|
||||
</code></pre></div> </details> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 25, 2026 11:07:51 UTC">October 25, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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</code></pre></div> </details> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">¶</a></h2> <ul> <li><a href=../../api/ordered_json/ ><code>ordered_json</code></a> - <abbr title="JavaScript Object Notation">JSON</abbr> type that preserves insertion order</li> <li><a href=../../api/ordered_map/ ><code>ordered_map</code></a> - the container behind <code>ordered_json</code></li> <li><a href=../../api/basic_json/object_t/ ><code>object_t</code></a> - type used to store <abbr title="JavaScript Object Notation">JSON</abbr> objects</li> <li><a href=../iterators/ >Iterators</a> - iteration order of object keys</li> <li><a href=../element_access/ >Element Access</a> - access elements by key</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../no_exceptions/ class="md-footer__link md-footer__link--prev" aria-label="Previous: Working Without Exceptions"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> Working Without Exceptions </div> </div> </a> <a href=../performance/ class="md-footer__link md-footer__link--next" aria-label="Next: Performance"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> Performance </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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|
||||
@@ -127,3 +127,11 @@ The output will be:
|
||||
"two": 2
|
||||
}
|
||||
```
|
||||
|
||||
## See also
|
||||
|
||||
- [`ordered_json`](https://json.nlohmann.me/api/ordered_json/index.md) - JSON type that preserves insertion order
|
||||
- [`ordered_map`](https://json.nlohmann.me/api/ordered_map/index.md) - the container behind `ordered_json`
|
||||
- [`object_t`](https://json.nlohmann.me/api/basic_json/object_t/index.md) - type used to store JSON objects
|
||||
- [Iterators](https://json.nlohmann.me/features/iterators/index.md) - iteration order of object keys
|
||||
- [Element Access](https://json.nlohmann.me/features/element_access/index.md) - access elements by key
|
||||
+11
-2
@@ -13,6 +13,8 @@ flowchart LR
|
||||
A --> V["bool: is the input valid JSON?"]
|
||||
```
|
||||
|
||||
<p class="nj-caption" markdown>The three entry points for parsing: `parse()` builds a `basic_json` value, optionally filtered by a parser callback, `sax_parse()` delivers events to a SAX handler, and `accept()` only checks whether the input is valid.</p>
|
||||
|
||||
## Input
|
||||
|
||||
The [`parse`](../../api/basic_json/parse.md) function reads a JSON value from an input. The input can be
|
||||
@@ -34,7 +36,7 @@ json data = json::parse(f);
|
||||
```
|
||||
|
||||
The input must be encoded in UTF-8; other encodings are not supported. The exception is wide-character input, which
|
||||
is read as UTF-16 or UTF-32 -- depending on the character type -- and converted to UTF-8; see the
|
||||
is read as UTF-16 or UTF-32 — depending on the character type — and converted to UTF-8; see the
|
||||
[`parse`](../../api/basic_json/parse.md) page for an example. A single input may contain only one JSON value.
|
||||
Inputs consisting of multiple values separated by newlines are handled by the [JSON Lines](json_lines.md) format; input
|
||||
that arrives in chunks or contains several values is covered in
|
||||
@@ -154,4 +156,11 @@ options.
|
||||
- [Zero-copy JSON views](../json_view.md) - parse into a read-only document and read it through views
|
||||
- [parsing and exceptions](parse_exceptions.md) - control error handling
|
||||
- [parsing untrusted input](untrusted_input.md) - what to consider when parsing input from untrusted sources
|
||||
- [Zero-copy JSON views](../json_view.md) - parse into a document and read it through a view
|
||||
- [`operator>>`](../../api/operator_gtgt.md) - deserialize from stream
|
||||
- [`json_sax`](../../api/json_sax/index.md) - documentation of the SAX interface
|
||||
- [`parse_error`](../../api/basic_json/parse_error.md) - exception thrown on a parse error
|
||||
- [Comments](../comments.md) - ignore `//` and `/* */` comments while parsing
|
||||
- [Trailing Commas](../trailing_commas.md) - accept trailing commas in arrays and objects
|
||||
- [Serialization](../serialization.md) - pretty-printing, UTF-8 handling, and number output
|
||||
- [Working Without Exceptions](../no_exceptions.md) - error handling when exceptions are disabled
|
||||
- [Exceptions](../../home/exceptions.md) - list of exceptions and error codes
|
||||
@@ -1,17 +1,17 @@
|
||||
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0 24 24"><path d="M9.5 3A6.5 6.5 0 0 1 16 9.5c0 1.61-.59 3.09-1.56 4.23l.27.27h.79l5 5-1.5 1.5-5-5v-.79l-Line truncated
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I["JSON input"] --> P["parse()"]
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||||
I --> S["sax_parse()"]
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I --> A["accept()"]
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P -->|"optional parser callback filters values"| D["basic_json value (DOM)"]
|
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S --> H["events delivered to a user SAX handler"]
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A --> V["bool: is the input valid JSON?"]</code></pre> <h2 id=input>Input<a class=headerlink href=#input title="Permanent link">¶</a></h2> <p>The <a href=../../api/basic_json/parse/ ><code>parse</code></a> function reads a <abbr title="JavaScript Object Notation">JSON</abbr> value from an input. The input can be</p> <ul> <li>a string (<code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>string</span></code>, C string, or string literal),</li> <li>a wide string (<code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>u16string</span></code>, <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>u32string</span></code>, or <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>wstring</span></code>, or a wide string literal),</li> <li>a <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>istream</span></code> (e.g., an <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>ifstream</span></code> reading from a file),</li> <li>a <code class=highlight><span class=kt>FILE</span><span class=o>*</span></code> pointer,</li> <li>a pair of iterators over a contiguous range (e.g., a <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>vector</span><span class=o><</span><span class=n>std</span><span class=o>::</span><span class=kt>uint8_t</span><span class=o>></span></code>), or</li> <li>a contiguous container.</li> </ul> <div class=highlight><pre><span></span><code><span class=c1>// parse from a string</span>
|
||||
A --> V["bool: is the input valid JSON?"]</code></pre> <p class=nj-caption>The three entry points for parsing: <code>parse()</code> builds a <code>basic_json</code> value, optionally filtered by a parser callback, <code>sax_parse()</code> delivers events to a <abbr title="Simple API for XML">SAX</abbr> handler, and <code>accept()</code> only checks whether the input is valid.</p> <h2 id=input>Input<a class=headerlink href=#input title="Permanent link">¶</a></h2> <p>The <a href=../../api/basic_json/parse/ ><code>parse</code></a> function reads a <abbr title="JavaScript Object Notation">JSON</abbr> value from an input. The input can be</p> <ul> <li>a string (<code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>string</span></code>, C string, or string literal),</li> <li>a wide string (<code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>u16string</span></code>, <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>u32string</span></code>, or <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>wstring</span></code>, or a wide string literal),</li> <li>a <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>istream</span></code> (e.g., an <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>ifstream</span></code> reading from a file),</li> <li>a <code class=highlight><span class=kt>FILE</span><span class=o>*</span></code> pointer,</li> <li>a pair of iterators over a contiguous range (e.g., a <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>vector</span><span class=o><</span><span class=n>std</span><span class=o>::</span><span class=kt>uint8_t</span><span class=o>></span></code>), or</li> <li>a contiguous container.</li> </ul> <div class=highlight><pre><span></span><code><span class=c1>// parse from a string</span>
|
||||
<span class=n>json</span><span class=w> </span><span class=n>j</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>parse</span><span class=p>(</span><span class=sa>R</span><span class=s>"</span><span class=dl>(</span><span class=s>{"happy": true, "pi": 3.141}</span><span class=dl>)</span><span class=s>"</span><span class=p>);</span>
|
||||
|
||||
<span class=c1>// parse from a file</span>
|
||||
<span class=n>std</span><span class=o>::</span><span class=n>ifstream</span><span class=w> </span><span class=n>f</span><span class=p>(</span><span class=s>"example.json"</span><span class=p>);</span>
|
||||
<span class=n>json</span><span class=w> </span><span class=n>data</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>parse</span><span class=p>(</span><span class=n>f</span><span class=p>);</span>
|
||||
</code></pre></div> <p>The input must be encoded in <abbr title="Unicode Transformation Format">UTF</abbr>-8; other encodings are not supported. The exception is wide-character input, which is read as <abbr title="Unicode Transformation Format">UTF</abbr>-16 or <abbr title="Unicode Transformation Format">UTF</abbr>-32 -- depending on the character type -- and converted to <abbr title="Unicode Transformation Format">UTF</abbr>-8; see the <a href=../../api/basic_json/parse/ ><code>parse</code></a> page for an example. A single input may contain only one <abbr title="JavaScript Object Notation">JSON</abbr> value. Inputs consisting of multiple values separated by newlines are handled by the <a href=json_lines/ ><abbr title="JavaScript Object Notation">JSON</abbr> Lines</a> format; input that arrives in chunks or contains several values is covered in <a href=large_and_partial_input/ >Large, streamed, and partial input</a>.</p> <p>By default, the library rejects comments and trailing commas. Both can be enabled with parameters of the <code>parse</code> function — see <a href=../comments/ >comments</a> and <a href=../trailing_commas/ >trailing commas</a>.</p> <p><a href=../../api/basic_json_document/parse/ ><code>basic_json_document::parse</code></a> accepts the same inputs with the same options, but keeps the result in a document rather than in a <code>basic_json</code> value; see <a href=#documents-and-views>Documents and views</a> below.</p> <h2 id=strictness-and-trailing-data>Strictness and trailing data<a class=headerlink href=#strictness-and-trailing-data title="Permanent link">¶</a></h2> <p><a href=../../api/basic_json/parse/ ><code>parse</code></a> reads a single <abbr title="JavaScript Object Notation">JSON</abbr> value and requires the whole input to be consumed: any non-whitespace data after the value is reported as a parse error. Use it when you want to guarantee that an input is exactly one complete <abbr title="JavaScript Object Notation">JSON</abbr> document.</p> <p><a href=../../api/operator_gtgt/ ><code>operator>></code></a> follows relaxed <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>istream</span></code> semantics instead: it parses one <abbr title="JavaScript Object Notation">JSON</abbr> value and leaves the stream positioned right after it, without requiring the rest of the stream to be consumed. This is what makes it possible to read several concatenated values from the same stream, but it also means that "a valid document followed by trailing bytes" is accepted rather than rejected. If you are validating conformance, or need to reject any input that is not exactly one <abbr title="JavaScript Object Notation">JSON</abbr> document, prefer <code>parse</code>.</p> <p>When using <code>operator>></code> to read several concatenated values this way, a value that is a number must be followed by whitespace, because <code>operator>></code> consumes the character that terminates a number, unless <a href=../../api/macros/json_precise_stream_position/ ><code>JSON_PRECISE_STREAM_POSITION</code></a> is defined to <code>1</code> — see the <a href=../../api/operator_gtgt/#notes><code>operator>></code> notes</a> for details and examples.</p> <h2 id=input-sources-and-performance>Input sources and performance<a class=headerlink href=#input-sources-and-performance title="Permanent link">¶</a></h2> <p>All inputs produce the same values and the same exceptions, but they are read in different ways, which makes some of them faster than others:</p> <table> <thead> <tr> <th>input</th> <th>how it is read</th> </tr> </thead> <tbody> <tr> <td>strings, string views, string literals, contiguous containers, pointer or iterator ranges over contiguous memory</td> <td>the whole input is scanned in bulk: runs of string characters, numbers, and indentation are recognized several bytes at a time</td> </tr> <tr> <td><code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>istream</span></code> (<code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>ifstream</span></code>, <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>istringstream</span></code>, ...)</td> <td>the characters the stream buffer holds are scanned in bulk like a string; the stream buffer is asked for more when they are used up</td> </tr> <tr> <td><code class=highlight><span class=kt>FILE</span><span class=o>*</span></code></td> <td>one character at a time, with the file locked once for the whole parse</td> </tr> <tr> <td>other iterator ranges (e.g., <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>list</span><span class=o><</span><span class=kt>char</span><span class=o>></span></code>, <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>istreambuf_iterator</span><span class=o><</span><span class=kt>char</span><span class=o>></spLine truncated
|
||||
</code></pre></div> <p>The input must be encoded in <abbr title="Unicode Transformation Format">UTF</abbr>-8; other encodings are not supported. The exception is wide-character input, which is read as <abbr title="Unicode Transformation Format">UTF</abbr>-16 or <abbr title="Unicode Transformation Format">UTF</abbr>-32 — depending on the character type — and converted to <abbr title="Unicode Transformation Format">UTF</abbr>-8; see the <a href=../../api/basic_json/parse/ ><code>parse</code></a> page for an example. A single input may contain only one <abbr title="JavaScript Object Notation">JSON</abbr> value. Inputs consisting of multiple values separated by newlines are handled by the <a href=json_lines/ ><abbr title="JavaScript Object Notation">JSON</abbr> Lines</a> format; input that arrives in chunks or contains several values is covered in <a href=large_and_partial_input/ >Large, streamed, and partial input</a>.</p> <p>By default, the library rejects comments and trailing commas. Both can be enabled with parameters of the <code>parse</code> function — see <a href=../comments/ >comments</a> and <a href=../trailing_commas/ >trailing commas</a>.</p> <p><a href=../../api/basic_json_document/parse/ ><code>basic_json_document::parse</code></a> accepts the same inputs with the same options, but keeps the result in a document rather than in a <code>basic_json</code> value; see <a href=#documents-and-views>Documents and views</a> below.</p> <h2 id=strictness-and-trailing-data>Strictness and trailing data<a class=headerlink href=#strictness-and-trailing-data title="Permanent link">¶</a></h2> <p><a href=../../api/basic_json/parse/ ><code>parse</code></a> reads a single <abbr title="JavaScript Object Notation">JSON</abbr> value and requires the whole input to be consumed: any non-whitespace data after the value is reported as a parse error. Use it when you want to guarantee that an input is exactly one complete <abbr title="JavaScript Object Notation">JSON</abbr> document.</p> <p><a href=../../api/operator_gtgt/ ><code>operator>></code></a> follows relaxed <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>istream</span></code> semantics instead: it parses one <abbr title="JavaScript Object Notation">JSON</abbr> value and leaves the stream positioned right after it, without requiring the rest of the stream to be consumed. This is what makes it possible to read several concatenated values from the same stream, but it also means that "a valid document followed by trailing bytes" is accepted rather than rejected. If you are validating conformance, or need to reject any input that is not exactly one <abbr title="JavaScript Object Notation">JSON</abbr> document, prefer <code>parse</code>.</p> <p>When using <code>operator>></code> to read several concatenated values this way, a value that is a number must be followed by whitespace, because <code>operator>></code> consumes the character that terminates a number, unless <a href=../../api/macros/json_precise_stream_position/ ><code>JSON_PRECISE_STREAM_POSITION</code></a> is defined to <code>1</code> — see the <a href=../../api/operator_gtgt/#notes><code>operator>></code> notes</a> for details and examples.</p> <h2 id=input-sources-and-performance>Input sources and performance<a class=headerlink href=#input-sources-and-performance title="Permanent link">¶</a></h2> <p>All inputs produce the same values and the same exceptions, but they are read in different ways, which makes some of them faster than others:</p> <table> <thead> <tr> <th>input</th> <th>how it is read</th> </tr> </thead> <tbody> <tr> <td>strings, string views, string literals, contiguous containers, pointer or iterator ranges over contiguous memory</td> <td>the whole input is scanned in bulk: runs of string characters, numbers, and indentation are recognized several bytes at a time</td> </tr> <tr> <td><code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>istream</span></code> (<code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>ifstream</span></code>, <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>istringstream</span></code>, ...)</td> <td>the characters the stream buffer holds are scanned in bulk like a string; the stream buffer is asked for more when they are used up</td> </tr> <tr> <td><code class=highlight><span class=kt>FILE</span><span class=o>*</span></code></td> <td>one character at a time, with the file locked once for the whole parse</td> </tr> <tr> <td>other iterator ranges (e.g., <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>list</span><span class=o><</span><span class=kt>char</span><span class=o>></span></code>, <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>istreambuf_iterator</span><span class=o><</span><span class=kt>char</span><span class=o>></Line truncated
|
||||
|
||||
<span class=n>json_document</span><span class=w> </span><span class=n>doc</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>json_document</span><span class=o>::</span><span class=n>parse</span><span class=p>(</span><span class=sa>R</span><span class=s>"</span><span class=dl>(</span><span class=s>{"happy": true, "pi": 3.141}</span><span class=dl>)</span><span class=s>"</span><span class=p>);</span>
|
||||
<span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=n>doc</span><span class=p>.</span><span class=n>root</span><span class=p>()[</span><span class=s>"happy"</span><span class=p>].</span><span class=n>get</span><span class=o><</span><span class=kt>bool</span><span class=o>></span><span class=p>()</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=sc>'\n'</span><span class=p>;</span>
|
||||
</code></pre></div> <p>See <a href=../json_view/ >Zero-copy <abbr title="JavaScript Object Notation">JSON</abbr> views</a> for the concept and the trade-offs.</p> <h2 id=exceptions>Exceptions<a class=headerlink href=#exceptions title="Permanent link">¶</a></h2> <p>When the input is not valid <abbr title="JavaScript Object Notation">JSON</abbr>, the <code>parse</code> function throws an exception by default. If exceptions are undesired or unavailable, the parser can instead return a discarded value, or <a href=../../api/basic_json/accept/ ><code>accept</code></a> can be used to only check whether an input is valid <abbr title="JavaScript Object Notation">JSON</abbr>. See <a href=parse_exceptions/ >parsing and exceptions</a> for the available options.</p> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">¶</a></h2> <ul> <li><a href=../../api/basic_json/parse/ ><code>parse</code></a> - deserialize from a compatible input</li> <li><a href=../../api/basic_json_document/parse/ ><code>basic_json_document::parse</code></a> - deserialize into a document instead of a value</li> <li><a href=../../api/basic_json/accept/ ><code>accept</code></a> - check if the input is valid <abbr title="JavaScript Object Notation">JSON</abbr></li> <li><a href=../../api/basic_json/sax_parse/ ><code>sax_parse</code></a> - generate <abbr title="Simple API for XML">SAX</abbr> events</li> <li><a href=json_lines/ ><abbr title="JavaScript Object Notation">JSON</abbr> Lines</a> - parse newline-delimited <abbr title="JavaScript Object Notation">JSON</abbr></li> <li><a href=large_and_partial_input/ >Large, streamed, and partial input</a> - chunked, concatenated, and very large input</li> <li><a href=parser_callbacks/ >parser callbacks</a> - influence the parsing by a callback function</li> <li><a href=sax_interface/ ><abbr title="Simple API for XML">SAX</abbr> interface</a> - implement a custom <abbr title="Simple API for XML">SAX</abbr> handler</li> <li><a href=../json_view/ >Zero-copy <abbr title="JavaScript Object Notation">JSON</abbr> views</a> - parse into a read-only document and read it through views</li> <li><a href=parse_exceptions/ >parsing and exceptions</a> - control error handling</li> <li><a href=untrusted_input/ >parsing untrusted input</a> - what to consider when parsing input from untrusted sources</li> <li><a href=../json_view/ >Zero-copy <abbr title="JavaScript Object Notation">JSON</abbr> views</a> - parse into a document and read it through a view</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 25, 2026 11:07:13 UTC">October 25, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2025 Fonticons, Inc.--><path d="M173.9 397.4c0 2-2.3 3.6-5.2 3.6-3.3.3-5.6-1.3-5.6-3.6 0-2 2.3-3.6 5.2-3.6 3-.3 5.6 1.3 5.6 3.6m-31.1-4.5c-.7 2 1.3 4.3 4.3 4.9 2.6 1 5.6 0 6.2-2s-1.3-4.3-4.3-5.2c-2.6-.7-5.5.3-6.2 2.3m44.2-1.7c-2.9.7-4.9 2.6-4.6 4.9.3 2 2.9 3.3 5.9 2.6 2.9-.7 4.9-2.6 4.6-4.6-.3-1.9-3-3.2-5.9-2.9M252.8 8C114.1 8 8 1Line truncated
|
||||
</code></pre></div> <p>See <a href=../json_view/ >Zero-copy <abbr title="JavaScript Object Notation">JSON</abbr> views</a> for the concept and the trade-offs.</p> <h2 id=exceptions>Exceptions<a class=headerlink href=#exceptions title="Permanent link">¶</a></h2> <p>When the input is not valid <abbr title="JavaScript Object Notation">JSON</abbr>, the <code>parse</code> function throws an exception by default. If exceptions are undesired or unavailable, the parser can instead return a discarded value, or <a href=../../api/basic_json/accept/ ><code>accept</code></a> can be used to only check whether an input is valid <abbr title="JavaScript Object Notation">JSON</abbr>. See <a href=parse_exceptions/ >parsing and exceptions</a> for the available options.</p> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">¶</a></h2> <ul> <li><a href=../../api/basic_json/parse/ ><code>parse</code></a> - deserialize from a compatible input</li> <li><a href=../../api/basic_json_document/parse/ ><code>basic_json_document::parse</code></a> - deserialize into a document instead of a value</li> <li><a href=../../api/basic_json/accept/ ><code>accept</code></a> - check if the input is valid <abbr title="JavaScript Object Notation">JSON</abbr></li> <li><a href=../../api/basic_json/sax_parse/ ><code>sax_parse</code></a> - generate <abbr title="Simple API for XML">SAX</abbr> events</li> <li><a href=json_lines/ ><abbr title="JavaScript Object Notation">JSON</abbr> Lines</a> - parse newline-delimited <abbr title="JavaScript Object Notation">JSON</abbr></li> <li><a href=large_and_partial_input/ >Large, streamed, and partial input</a> - chunked, concatenated, and very large input</li> <li><a href=parser_callbacks/ >parser callbacks</a> - influence the parsing by a callback function</li> <li><a href=sax_interface/ ><abbr title="Simple API for XML">SAX</abbr> interface</a> - implement a custom <abbr title="Simple API for XML">SAX</abbr> handler</li> <li><a href=../json_view/ >Zero-copy <abbr title="JavaScript Object Notation">JSON</abbr> views</a> - parse into a read-only document and read it through views</li> <li><a href=parse_exceptions/ >parsing and exceptions</a> - control error handling</li> <li><a href=untrusted_input/ >parsing untrusted input</a> - what to consider when parsing input from untrusted sources</li> <li><a href=../../api/operator_gtgt/ ><code>operator>></code></a> - deserialize from stream</li> <li><a href=../../api/json_sax/ ><code>json_sax</code></a> - documentation of the <abbr title="Simple API for XML">SAX</abbr> interface</li> <li><a href=../../api/basic_json/parse_error/ ><code>parse_error</code></a> - exception thrown on a parse error</li> <li><a href=../comments/ >Comments</a> - ignore <code>//</code> and <code>/* */</code> comments while parsing</li> <li><a href=../trailing_commas/ >Trailing Commas</a> - accept trailing commas in arrays and objects</li> <li><a href=../serialization/ >Serialization</a> - pretty-printing, <abbr title="Unicode Transformation Format">UTF</abbr>-8 handling, and number output</li> <li><a href=../no_exceptions/ >Working Without Exceptions</a> - error handling when exceptions are disabled</li> <li><a href=../../home/exceptions/ >Exceptions</a> - list of exceptions and error codes</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../performance/ class="md-footer__link md-footer__link--preLine truncated
|
||||
@@ -12,6 +12,8 @@ flowchart LR
|
||||
A --> V["bool: is the input valid JSON?"]
|
||||
```
|
||||
|
||||
The three entry points for parsing: `parse()` builds a `basic_json` value, optionally filtered by a parser callback, `sax_parse()` delivers events to a SAX handler, and `accept()` only checks whether the input is valid.
|
||||
|
||||
## Input
|
||||
|
||||
The [`parse`](https://json.nlohmann.me/api/basic_json/parse/index.md) function reads a JSON value from an input. The input can be
|
||||
@@ -32,7 +34,7 @@ std::ifstream f("example.json");
|
||||
json data = json::parse(f);
|
||||
```
|
||||
|
||||
The input must be encoded in UTF-8; other encodings are not supported. The exception is wide-character input, which is read as UTF-16 or UTF-32 -- depending on the character type -- and converted to UTF-8; see the [`parse`](https://json.nlohmann.me/api/basic_json/parse/index.md) page for an example. A single input may contain only one JSON value. Inputs consisting of multiple values separated by newlines are handled by the [JSON Lines](https://json.nlohmann.me/features/parsing/json_lines/index.md) format; input that arrives in chunks or contains several values is covered in [Large, streamed, and partial input](https://json.nlohmann.me/features/parsing/large_and_partial_input/index.md).
|
||||
The input must be encoded in UTF-8; other encodings are not supported. The exception is wide-character input, which is read as UTF-16 or UTF-32 — depending on the character type — and converted to UTF-8; see the [`parse`](https://json.nlohmann.me/api/basic_json/parse/index.md) page for an example. A single input may contain only one JSON value. Inputs consisting of multiple values separated by newlines are handled by the [JSON Lines](https://json.nlohmann.me/features/parsing/json_lines/index.md) format; input that arrives in chunks or contains several values is covered in [Large, streamed, and partial input](https://json.nlohmann.me/features/parsing/large_and_partial_input/index.md).
|
||||
|
||||
By default, the library rejects comments and trailing commas. Both can be enabled with parameters of the `parse` function — see [comments](https://json.nlohmann.me/features/comments/index.md) and [trailing commas](https://json.nlohmann.me/features/trailing_commas/index.md).
|
||||
|
||||
@@ -103,4 +105,11 @@ When the input is not valid JSON, the `parse` function throws an exception by de
|
||||
- [Zero-copy JSON views](https://json.nlohmann.me/features/json_view/index.md) - parse into a read-only document and read it through views
|
||||
- [parsing and exceptions](https://json.nlohmann.me/features/parsing/parse_exceptions/index.md) - control error handling
|
||||
- [parsing untrusted input](https://json.nlohmann.me/features/parsing/untrusted_input/index.md) - what to consider when parsing input from untrusted sources
|
||||
- [Zero-copy JSON views](https://json.nlohmann.me/features/json_view/index.md) - parse into a document and read it through a view
|
||||
- [`operator>>`](https://json.nlohmann.me/api/operator_gtgt/index.md) - deserialize from stream
|
||||
- [`json_sax`](https://json.nlohmann.me/api/json_sax/index.md) - documentation of the SAX interface
|
||||
- [`parse_error`](https://json.nlohmann.me/api/basic_json/parse_error/index.md) - exception thrown on a parse error
|
||||
- [Comments](https://json.nlohmann.me/features/comments/index.md) - ignore `//` and `/* */` comments while parsing
|
||||
- [Trailing Commas](https://json.nlohmann.me/features/trailing_commas/index.md) - accept trailing commas in arrays and objects
|
||||
- [Serialization](https://json.nlohmann.me/features/serialization/index.md) - pretty-printing, UTF-8 handling, and number output
|
||||
- [Working Without Exceptions](https://json.nlohmann.me/features/no_exceptions/index.md) - error handling when exceptions are disabled
|
||||
- [Exceptions](https://json.nlohmann.me/home/exceptions/index.md) - list of exceptions and error codes
|
||||
@@ -87,5 +87,12 @@ JSON Lines input with more than one value is treated as invalid JSON by the [`pa
|
||||
}
|
||||
```
|
||||
|
||||
A value that is a number must be followed by whitespace -- see the [notes](../../api/operator_gtgt.md#notes) of
|
||||
A value that is a number must be followed by whitespace — see the [notes](../../api/operator_gtgt.md#notes) of
|
||||
`operator>>` for details.
|
||||
|
||||
## See also
|
||||
|
||||
- [Parsing](index.md) - overview of the parsing functions
|
||||
- [Large, Streamed, and Partial Input](large_and_partial_input.md) - chunked, concatenated, and very large input
|
||||
- [`parse`](../../api/basic_json/parse.md) - deserialize from a compatible input
|
||||
- [`operator>>`](../../api/operator_gtgt.md) - deserialize from stream
|
||||
@@ -1,4 +1,4 @@
|
||||
<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/parsing/json_lines/" rel="canonical"><link href=../ rel=prev><link href=../large_and_partial_input/ rel=next><link rel=icon href=../../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>JSON Lines - JSON for Modern C++</title><link rel=stylesheet href=../../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../../assets/stylesheets/palette.ab4e12ef.min.css><link rel="stylesheet" href="../../../assets/external/fonts.googleapis.com/css.61a430c9.css"><style>:root{--md-text-font:"Roboto";--md-code-font:"JetBrains Mono"}</style><link rel=stylesheet href=../../../css/custom.css><script>__md_scope=new 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<span class=p>{</span><span class=nt>"name"</span><span class=p>:</span><span class=w> </span><span class=s2>"Alexa"</span><span class=p>,</span><span class=w> </span><span class=nt>"wins"</span><span class=p>:</span><span class=w> </span><span class=p>[[</span><span class=s2>"two pair"</span><span class=p>,</span><span class=w> </span><span class=s2>"4♠"</span><span class=p>],</span><span class=w> </span><span class=p>[</span><span class=s2>"two pair"</span><span class=p>,</span><span class=w> </span><span class=s2>"9♠"</span><span class=p>]]}</span>
|
||||
<span class=p>{</span><span class=nt>"name"</span><span class=p>:</span><span class=w> </span><span class=s2>"May"</span><span class=p>,</span><span class=w> </span><span class=nt>"wins"</span><span class=p>:</span><span class=w> </span><span class=p>[]}</span>
|
||||
<span class=p>{</span><span class=nt>"name"</span><span class=p>:</span><span class=w> </span><span class=s2>"Deloise"</span><span class=p>,</span><span class=w> </span><span class=nt>"wins"</span><span class=p>:</span><span class=w> </span><span class=p>[[</span><span class=s2>"three of a kind"</span><span class=p>,</span><span class=w> </span><span class=s2>"5♣"</span><span class=p>]]}</span>
|
||||
@@ -39,4 +39,4 @@
|
||||
<span class=w> </span><span class=n>input</span><span class=w> </span><span class=o>>></span><span class=w> </span><span class=n>j</span><span class=p>;</span>
|
||||
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=n>j</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>endl</span><span class=p>;</span>
|
||||
<span class=p>}</span>
|
||||
</code></pre></div> <p>A value that is a number must be followed by whitespace -- see the <a href=../../../api/operator_gtgt/#notes>notes</a> of <code>operator>></code> for details.</p> </div> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 2, 2026 09:32:15 UTC">October 2, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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|
||||
</code></pre></div> <p>A value that is a number must be followed by whitespace — see the <a href=../../../api/operator_gtgt/#notes>notes</a> of <code>operator>></code> for details.</p> </div> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">¶</a></h2> <ul> <li><a href=../ >Parsing</a> - overview of the parsing functions</li> <li><a href=../large_and_partial_input/ >Large, Streamed, and Partial Input</a> - chunked, concatenated, and very large input</li> <li><a href=../../../api/basic_json/parse/ ><code>parse</code></a> - deserialize from a compatible input</li> <li><a href=../../../api/operator_gtgt/ ><code>operator>></code></a> - deserialize from stream</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../ class="md-footer__link md-footer__link--prev" aria-label="Previous: Parsing"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> Parsing </div> </div> </a> <a href=../large_and_partial_input/ class="md-footer__link md-footer__link--next" aria-label="Next: Large, Streamed, and Partial Input"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> Large, Streamed, and Partial Input </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2025 Fonticons, Inc.--><path d="M173.9 397.4c0 2-2.3 3.6-5.2 3.6-3.3.3-5.6-1.3-5.6-3.6 0-2 2.3-3.6 5.2-3.6 3-.3 5.6 1.3 5.6 3.6m-31.1-4.5c-.7 2 1.3 4.3 4.3 4.9 2.6 1 5.6 0 6.2-2s-1.3-4.3-4.3-5.2c-2.6-.7-5.5.3-6.2 2.3m44.2-1.7c-2.9.7-4.9 2.6-4.6 4.9.3 2 2.9 3.3 5.9 2.6 2.9-.7 4.9-2.6 4.6-4.6-.3-1.9-3-3.2-5.9-2.9M252.8 8C114.1 8 8 113.3 8 252c0 110.9 69.8 205.8 169.5 239.2 12.8 2.3 17.3-5.6 17.3-12.1 0-6.2-.3-40.4-.3-61.4 0 0-70 15-84.7-29.8 0 0-11.4-29.1-27.8-36.6 0 0-22.9-15.7 1.6-15.4 0 0 24.9 2 38.6 25.8 21.9 38.6 58.6 27.5 72.9 20.9 2.3-16 8.8-27.1 16-33.7-55.9-6.2-112.3-14.3-112.3-110.5 0-27.5 7.6-41.3 23.6-58.9-2.6-6.5-11.1-33.3 2.6-67.9 20.9-6.5 69 27 69 27 20-5.6 41.5-8.5 62.8-8.5s42.8 2.9 62.8 8.5c0 0 48.1-33.6 69-27 13.7 34.7 5.2 61.4 2.6 67.9 16 17.7 25.8 31.5 25.8 58.9 0 96.5-58.9 104.2-114.8 110.5 9.2 7.9 17 22.9 17 46.4 0 33.7-.3 75.4-.3 83.6 0 6.5 4.6 14.4 17.3 12.1C436.2 457.8 504 362.9 504 252 504 113.3 391.5 8 252.8 8M105.2 352.9c-1.3 1-1 3.3.7 5.2 1.6 1.6 3.9 2.3 5.2 1 1.3-1 1-3.3-.7-5.2-1.6-1.6-3.9-2.3-5.2-1m-10.8-8.1c-.7 1.3.3 2.9 2.3 3.9 1.6 1 3.6.7 4.3-.7.7-1.3-.3-2.9-2.3-3.9-2-.6-3.6-.3-4.3.7m32.4 35.6c-1.6 1.3-1 4.3 1.3 6.2 2.3 2.3 5.2 2.6 6Line truncated
|
||||
@@ -79,4 +79,11 @@ while (input >> std::ws && input.peek() != std::char_traits<char>::eof())
|
||||
}
|
||||
```
|
||||
|
||||
A value that is a number must be followed by whitespace -- see the [notes](https://json.nlohmann.me/api/operator_gtgt/#notes) of `operator>>` for details.
|
||||
A value that is a number must be followed by whitespace — see the [notes](https://json.nlohmann.me/api/operator_gtgt/#notes) of `operator>>` for details.
|
||||
|
||||
## See also
|
||||
|
||||
- [Parsing](https://json.nlohmann.me/features/parsing/index.md) - overview of the parsing functions
|
||||
- [Large, Streamed, and Partial Input](https://json.nlohmann.me/features/parsing/large_and_partial_input/index.md) - chunked, concatenated, and very large input
|
||||
- [`parse`](https://json.nlohmann.me/api/basic_json/parse/index.md) - deserialize from a compatible input
|
||||
- [`operator>>`](https://json.nlohmann.me/api/operator_gtgt/index.md) - deserialize from stream
|
||||
@@ -17,6 +17,8 @@ flowchart TD
|
||||
H -->|"yes"| I["stop early with SAX, or read through a view"]
|
||||
```
|
||||
|
||||
<p class="nj-caption" markdown>Questions about the input and the approach this page describes for each; an input can match more than one question.</p>
|
||||
|
||||
## Several values in one stream
|
||||
|
||||
For newline-delimited input, read line by line and parse each line, as described in [JSON Lines](json_lines.md). For
|
||||
@@ -182,6 +184,64 @@ exception is one past the buffer size in the first case:
|
||||
The example feeds a document in five chunks. The first four are reported as incomplete, and a syntax error is reported as
|
||||
such, not as missing data.
|
||||
|
||||
The chunks of the example, and the bytes each attempt parses again:
|
||||
|
||||
<div class="nj-fig-wrap" markdown>
|
||||
<svg class="nj-fig nj-fig--sax-buf" viewBox="0 0 700 442" role="img" aria-labelledby="fig-sax-buf-title fig-sax-buf-desc" xmlns="http://www.w3.org/2000/svg">
|
||||
<title id="fig-sax-buf-title">Buffering chunks until the value is complete: five chunks of {"name": "Mary", "list": [1, 2, 3]} appended to a buffer.</title>
|
||||
<desc id="fig-sax-buf-desc">After chunks 1 to 4 the buffer has 4, 12, 19, and 30 bytes, and parsing it fails at byte 5, 13, 20, and 31: one past the end, so the input is incomplete and the program waits for more. After chunk 5 the buffer of 35 bytes parses as one complete value. For the input [1, ] of 5 bytes the error is at byte 5, before the end, so it is a real syntax error. Every attempt parses the buffer from byte 1, so 4 + 12 + 19 + 30 + 35 = 100 bytes are parsed for a 35-byte value, and the work grows quadratically.</desc>
|
||||
<text class="fg-h" x="16" y="24">chunk</text>
|
||||
<text class="fg-h" x="96" y="24">buffer, parsed from byte 1 each time</text>
|
||||
<text class="fg-h" x="420" y="24">result</text>
|
||||
<text class="fg-note fg-note--l" x="16" y="67">chunk 1</text>
|
||||
<rect class="nj-sax-new" x="96" y="50" width="36.6" height="24" rx="4"/>
|
||||
<text class="nj-sax-buf" x="96" y="67" xml:space="preserve">{"na</text>
|
||||
<path class="nj-sax-caret nj-sax-s-wait" d="M133.8 76 l-5 8 h10 z"/>
|
||||
<text class="nj-sax-status nj-sax-s-wait" x="420" y="61">error at byte 5 = size + 1</text>
|
||||
<text class="fg-note fg-note--l" x="420" y="77">incomplete: wait for more</text>
|
||||
<text class="fg-note fg-note--l" x="16" y="117">chunk 2</text>
|
||||
<rect class="nj-sax-old" x="93" y="100" width="36.6" height="24" rx="4"/>
|
||||
<rect class="nj-sax-new" x="129.6" y="100" width="70.2" height="24" rx="4"/>
|
||||
<text class="nj-sax-buf" x="96" y="117" xml:space="preserve">{"name": "Ma</text>
|
||||
<path class="nj-sax-caret nj-sax-s-wait" d="M201 126 l-5 8 h10 z"/>
|
||||
<text class="nj-sax-status nj-sax-s-wait" x="420" y="111">error at byte 13 = size + 1</text>
|
||||
<text class="fg-note fg-note--l" x="420" y="127">incomplete: wait for more</text>
|
||||
<text class="fg-note fg-note--l" x="16" y="167">chunk 3</text>
|
||||
<rect class="nj-sax-old" x="93" y="150" width="103.8" height="24" rx="4"/>
|
||||
<rect class="nj-sax-new" x="196.8" y="150" width="61.8" height="24" rx="4"/>
|
||||
<text class="nj-sax-buf" x="96" y="167" xml:space="preserve">{"name": "Mary", "l</text>
|
||||
<path class="nj-sax-caret nj-sax-s-wait" d="M259.8 176 l-5 8 h10 z"/>
|
||||
<text class="nj-sax-status nj-sax-s-wait" x="420" y="161">error at byte 20 = size + 1</text>
|
||||
<text class="fg-note fg-note--l" x="420" y="177">incomplete: wait for more</text>
|
||||
<text class="fg-note fg-note--l" x="16" y="217">chunk 4</text>
|
||||
<rect class="nj-sax-old" x="93" y="200" width="162.6" height="24" rx="4"/>
|
||||
<rect class="nj-sax-new" x="255.6" y="200" width="95.4" height="24" rx="4"/>
|
||||
<text class="nj-sax-buf" x="96" y="217" xml:space="preserve">{"name": "Mary", "list": [1, 2</text>
|
||||
<path class="nj-sax-caret nj-sax-s-wait" d="M352.2 226 l-5 8 h10 z"/>
|
||||
<text class="nj-sax-status nj-sax-s-wait" x="420" y="211">error at byte 31 = size + 1</text>
|
||||
<text class="fg-note fg-note--l" x="420" y="227">incomplete: wait for more</text>
|
||||
<text class="fg-note fg-note--l" x="16" y="267">chunk 5</text>
|
||||
<rect class="nj-sax-old" x="93" y="250" width="255" height="24" rx="4"/>
|
||||
<rect class="nj-sax-new" x="348" y="250" width="45" height="24" rx="4"/>
|
||||
<text class="nj-sax-buf" x="96" y="267" xml:space="preserve">{"name": "Mary", "list": [1, 2, 3]}</text>
|
||||
<text class="nj-sax-status nj-sax-s-ok" x="420" y="261">parsed, size 35</text>
|
||||
<text class="fg-note fg-note--l" x="420" y="277">complete: one value</text>
|
||||
<path class="nj-sax-sep" d="M16 290 H684"/>
|
||||
<text class="fg-note fg-note--l" x="16" y="325">other input</text>
|
||||
<rect class="nj-sax-new" x="96" y="308" width="45" height="24" rx="4"/>
|
||||
<text class="nj-sax-buf" x="96" y="325" xml:space="preserve">[1, ]</text>
|
||||
<path class="nj-sax-caret nj-sax-s-err" d="M133.8 334 l-5 8 h10 z"/>
|
||||
<text class="nj-sax-status nj-sax-s-err" x="420" y="319">error at byte 5, size 5</text>
|
||||
<text class="fg-note fg-note--l" x="420" y="335">before the end: a real error</text>
|
||||
<text class="fg-note fg-note--l" x="16" y="366">Bytes parsed: 4 + 12 + 19 + 30 + 35 = 100 for a value of 35 bytes.</text>
|
||||
<text class="fg-note fg-note--l" x="16" y="386">Every attempt starts again at byte 1, so the total work grows</text>
|
||||
<text class="fg-note fg-note--l" x="16" y="406">quadratically with the size of the value.</text>
|
||||
</svg>
|
||||
|
||||
</div>
|
||||
|
||||
<p class="nj-caption" markdown>Each row is one parse attempt on the growing buffer: an error at the byte after the end of the buffer means the value is incomplete, an error before the end is a real syntax error. The last line adds up the bytes parsed over all attempts.</p>
|
||||
|
||||
!!! warning "Limits of buffering"
|
||||
|
||||
- The buffer must contain exactly one value. Data that belongs to the next value is a syntax error. Use framing if
|
||||
@@ -195,6 +255,34 @@ such, not as missing data.
|
||||
|
||||
The input does not have to become one huge `json` value. The two tools below process it piece by piece.
|
||||
|
||||
How the memory use develops while a large array of records is parsed (illustrative, not a measurement):
|
||||
|
||||
<div class="nj-fig-wrap" markdown>
|
||||
<svg class="nj-fig nj-fig--sax-mem" viewBox="0 0 660 312" role="img" aria-labelledby="fig-sax-mem-title fig-sax-mem-desc" xmlns="http://www.w3.org/2000/svg">
|
||||
<title id="fig-sax-mem-title">Illustrative memory use over time while a large array of records is parsed, not a measurement.</title>
|
||||
<desc id="fig-sax-mem-desc">Four curves. D​OM, parse(): rises steadily until the whole tree is in memory. json_document: starts at about the size of the text, which it keeps, and grows a little while its index is built. Parser callback that discards every record: stays flat at the size of one record. S​AX: stays flat and lowest, because no value is built and only what the handler keeps is in memory.</desc>
|
||||
<path class="nj-sax-axis" d="M56 36 V272 H448"/>
|
||||
<text class="fg-h" x="16" y="24">memory</text>
|
||||
<text class="fg-note fg-note--l" x="56" y="294">time: records parsed</text>
|
||||
<text class="fg-note fg-note--r nj-sax-illus" x="648" y="24">illustrative, not a measurement</text>
|
||||
<path class="nj-sax-curve nj-sax-c-dom" d="M56 268 L440 52"/>
|
||||
<path class="nj-sax-curve nj-sax-c-doc" d="M56 168 L440 146"/>
|
||||
<path class="nj-sax-curve nj-sax-c-cb" d="M56 250 L88 238 L88 250 L120 238 L120 250 L152 238 L152 250 L184 238 L184 250 L216 238 L216 250 L248 238 L248 250 L280 238 L280 250 L312 238 L312 250 L344 238 L344 250 L376 238 L376 250 L408 238 L408 250 L440 238 L440 250"/>
|
||||
<path class="nj-sax-curve nj-sax-c-sax" d="M56 264 H440"/>
|
||||
<text class="nj-sax-lab nj-sax-c-dom" x="456" y="48">D​OM: parse()</text>
|
||||
<text class="fg-note fg-note--l" x="456" y="65">the whole tree</text>
|
||||
<text class="nj-sax-lab nj-sax-c-doc" x="456" y="142">json_document</text>
|
||||
<text class="fg-note fg-note--l" x="456" y="159">the text plus its index</text>
|
||||
<text class="nj-sax-lab nj-sax-c-cb" x="456" y="227">callback, discard</text>
|
||||
<text class="fg-note fg-note--l" x="456" y="244">one record at a time</text>
|
||||
<text class="nj-sax-lab nj-sax-c-sax" x="456" y="269">S​AX</text>
|
||||
<text class="fg-note fg-note--l" x="456" y="286">only what the handler keeps</text>
|
||||
</svg>
|
||||
|
||||
</div>
|
||||
|
||||
<p class="nj-caption" markdown>Memory use over the number of records parsed: `parse()` keeps the whole tree, `json_document` the text plus its index, a discarding callback one record at a time, and SAX only what the handler keeps.</p>
|
||||
|
||||
### Handle and discard elements with a callback
|
||||
|
||||
A [parser callback](parser_callbacks.md) is called for every finished value. If it returns `#!cpp false`, the value is
|
||||
@@ -368,3 +456,8 @@ and gives read-only [views](../../api/basic_json_view/index.md) of the parts you
|
||||
- [SAX interface](sax_interface.md) - event-based parsing
|
||||
- [Zero-copy JSON views](../json_view.md) - read parts of a large document without building a tree
|
||||
- [Untrusted input](untrusted_input.md) - limits for input you do not control
|
||||
- [`operator>>`](../../api/operator_gtgt.md) - deserialize from stream
|
||||
- [`parse`](../../api/basic_json/parse.md) - deserialize from a compatible input
|
||||
- [`sax_parse`](../../api/basic_json/sax_parse.md) - generate SAX events
|
||||
- [`JSON_PRECISE_STREAM_POSITION`](../../api/macros/json_precise_stream_position.md) - leave the stream positioned right after a parsed number
|
||||
- [`JSON_DISABLE_STREAM_BULK_SCAN`](../../api/macros/json_disable_stream_bulk_scan.md) - read a `std::istream` one character at a time
|
||||
@@ -1,4 +1,4 @@
|
||||
<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/parsing/large_and_partial_input/" rel="canonical"><link href=../json_lines/ rel=prev><link href=../parse_exceptions/ rel=next><link rel=icon href=../../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>Large, Streamed, and Partial Input - JSON for Modern C++</title><link rel=stylesheet href=../../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../../assets/stylesheets/palette.ab4e12ef.min.css><link rel="stylesheet" href="../../../assets/external/fonts.googleapis.com/css.61a430c9.css"><style>:root{--md-text-font:"Roboto";--md-code-font:"JetBrains Mono"}</style><link rel=stylesheet href=../../../css/custom.css><script>__md_scope=new 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href=#large-streamed-and-partial-input class=md-skip> Skip to content </a> </div> <div data-md-component=announce> </div> <header class=md-header data-md-component=header> <nav class="md-header__inner md-grid" aria-label=Header> <a href=../../.. title="JSON for Modern C++" class="md-header__button md-logo" aria-label="JSON for Modern C++" data-md-component=logo> <img src=../../../images/wordmark-header.svg alt=logo> </a> <label class="md-header__button md-icon" for=__drawer> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M3 6h18v2H3zm0 5h18v2H3zm0 5h18v2H3z"/></svg> </label> <div class=md-header__title data-md-component=header-title> <div class=md-header__ellipsis> <div class=md-header__topic> <span class=md-ellipsis> JSON for Modern C++ </span> </div> <div class=md-header__topic data-md-component=header-topic> <span class=md-ellipsis> Large, Streamed, and Partial Input </span> </div> </div> </div> <form class=md-header__option data-md-component=palette> <input 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class="md-header__button md-icon" for=__searchLine truncated
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||||
A["my input is..."] --> B{"several JSON values in one stream?"}
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||||
B -->|"yes"| C["read them one at a time"]
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||||
A --> D{"arriving in chunks (socket, pipe)?"}
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@@ -6,7 +6,7 @@
|
||||
A --> F{"too large to keep in memory?"}
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F -->|"yes"| G["discard parts with a callback, or use SAX"]
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||||
A --> H{"only a small part is needed?"}
|
||||
H -->|"yes"| I["stop early with SAX, or read through a view"]</code></pre> <h2 id=several-values-in-one-stream>Several values in one stream<a class=headerlink href=#several-values-in-one-stream title="Permanent link">¶</a></h2> <p>For newline-delimited input, read line by line and parse each line, as described in <a href=../json_lines/ ><abbr title="JavaScript Object Notation">JSON</abbr> Lines</a>. For values that are simply concatenated, <a href=../../../api/operator_gtgt/ ><code>operator>></code></a> reads one value and leaves the stream positioned after it. Check for the end of the stream before each read, because a read at the end throws:</p> <details class=example> <summary>Example: read concatenated values</summary> <div class=highlight><pre><span></span><code><span class=cp>#include</span><span class=w> </span><span class=cpf><iostream></span>
|
||||
H -->|"yes"| I["stop early with SAX, or read through a view"]</code></pre> <p class=nj-caption>Questions about the input and the approach this page describes for each; an input can match more than one question.</p> <h2 id=several-values-in-one-stream>Several values in one stream<a class=headerlink href=#several-values-in-one-stream title="Permanent link">¶</a></h2> <p>For newline-delimited input, read line by line and parse each line, as described in <a href=../json_lines/ ><abbr title="JavaScript Object Notation">JSON</abbr> Lines</a>. For values that are simply concatenated, <a href=../../../api/operator_gtgt/ ><code>operator>></code></a> reads one value and leaves the stream positioned after it. Check for the end of the stream before each read, because a read at the end throws:</p> <details class=example> <summary>Example: read concatenated values</summary> <div class=highlight><pre><span></span><code><span class=cp>#include</span><span class=w> </span><span class=cpf><iostream></span>
|
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<span class=cp>#include</span><span class=w> </span><span class=cpf><sstream></span>
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<span class=cp>#include</span><span class=w> </span><span class=cpf><nlohmann/json.hpp></span>
|
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@@ -108,7 +108,7 @@ chunk ist": [1, 2 -> incomplete
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chunk , 3]} -> complete
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{"list":[1,2,3],"name":"Mary"}
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[json.exception.parse_error.101] parse error at line 1, column 5: syntax error while parsing value - unexpected ']'; expected '[', '{', or a literal
|
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</code></pre></div> </details> <p>The example feeds a document in five chunks. The first four are reported as incomplete, and a syntax error is reported as such, not as missing data.</p> <div class="admonition warning"> <p class=admonition-title>Limits of buffering</p> <ul> <li>The buffer must contain exactly one value. Data that belongs to the next value is a syntax error. Use framing if values follow each other without a separator.</li> <li>A number at the end of the buffer is a complete value, even if more digits are still to come (<code>12</code> may be the start of <code>123</code>). Buffering cannot detect this; use framing for top-level numbers.</li> <li>Every attempt parses the buffer from the beginning, so the total work grows quadratically with the size of the value. Limit the size of the buffer for input you do not trust (see <a href=../untrusted_input/ >untrusted input</a>).</li> </ul> </div> <h2 id=input-too-large-for-memory>Input too large for memory<a class=headerlink href=#input-too-large-for-memory title="Permanent link">¶</a></h2> <p>The input does not have to become one huge <code>json</code> value. The two tools below process it piece by piece.</p> <h3 id=handle-and-discard-elements-with-a-callback>Handle and discard elements with a callback<a class=headerlink href=#handle-and-discard-elements-with-a-callback title="Permanent link">¶</a></h3> <p>A <a href=../parser_callbacks/ >parser callback</a> is called for every finished value. If it returns <code class=highlight><span class=nb>false</span></code>, the value is discarded and its memory is released at once. For a file that consists of one huge array (or one huge object) of records, handle each record at depth 1 and discard it:</p> <details class=example> <summary>Example: handle and discard each record of a large array</summary> <div class=highlight><pre><span></span><code><span class=cp>#include</span><span class=w> </span><span class=cpf><iostream></span>
|
||||
</code></pre></div> </details> <p>The example feeds a document in five chunks. The first four are reported as incomplete, and a syntax error is reported as such, not as missing data.</p> <p>The chunks of the example, and the bytes each attempt parses again:</p> <div class=nj-fig-wrap> <p><svg class="nj-fig nj-fig--sax-buf" viewbox="0 0 700 442" role=img aria-labelledby="fig-sax-buf-title fig-sax-buf-desc" xmlns=http://www.w3.org/2000/svg> <title id=fig-sax-buf-title>Buffering chunks until the value is complete: five chunks of {"name": "Mary", "list": [1, 2, 3]} appended to a buffer.</title> <desc id=fig-sax-buf-desc>After chunks 1 to 4 the buffer has 4, 12, 19, and 30 bytes, and parsing it fails at byte 5, 13, 20, and 31: one past the end, so the input is incomplete and the program waits for more. After chunk 5 the buffer of 35 bytes parses as one complete value. For the input [1, ] of 5 bytes the error is at byte 5, before the end, so it is a real syntax error. Every attempt parses the buffer from byte 1, so 4 + 12 + 19 + 30 + 35 = 100 bytes are parsed for a 35-byte value, and the work grows quadratically.</desc> <text class=fg-h x=16 y=24>chunk</text> <text class=fg-h x=96 y=24>buffer, parsed from byte 1 each time</text> <text class=fg-h x=420 y=24>result</text> <text class="fg-note fg-note--l" x=16 y=67>chunk 1</text> <rect class=nj-sax-new x=96 y=50 width=36.6 height=24 rx=4 /> <text class=nj-sax-buf x=96 y=67 xml:space=preserve>{"na</text> <path class="nj-sax-caret nj-sax-s-wait" d="M133.8 76 l-5 8 h10 z"/> <text class="nj-sax-status nj-sax-s-wait" x=420 y=61>error at byte 5 = size + 1</text> <text class="fg-note fg-note--l" x=420 y=77>incomplete: wait for more</text> <text class="fg-note fg-note--l" x=16 y=117>chunk 2</text> <rect class=nj-sax-old x=93 y=100 width=36.6 height=24 rx=4 /> <rect class=nj-sax-new x=129.6 y=100 width=70.2 height=24 rx=4 /> <text class=nj-sax-buf x=96 y=117 xml:space=preserve>{"name": "Ma</text> <path class="nj-sax-caret nj-sax-s-wait" d="M201 126 l-5 8 h10 z"/> <text class="nj-sax-status nj-sax-s-wait" x=420 y=111>error at byte 13 = size + 1</text> <text class="fg-note fg-note--l" x=420 y=127>incomplete: wait for more</text> <text class="fg-note fg-note--l" x=16 y=167>chunk 3</text> <rect class=nj-sax-old x=93 y=150 width=103.8 height=24 rx=4 /> <rect class=nj-sax-new x=196.8 y=150 width=61.8 height=24 rx=4 /> <text class=nj-sax-buf x=96 y=167 xml:space=preserve>{"name": "Mary", "l</text> <path class="nj-sax-caret nj-sax-s-wait" d="M259.8 176 l-5 8 h10 z"/> <text class="nj-sax-status nj-sax-s-wait" x=420 y=161>error at byte 20 = size + 1</text> <text class="fg-note fg-note--l" x=420 y=177>incomplete: wait for more</text> <text class="fg-note fg-note--l" x=16 y=217>chunk 4</text> <rect class=nj-sax-old x=93 y=200 width=162.6 height=24 rx=4 /> <rect class=nj-sax-new x=255.6 y=200 width=95.4 height=24 rx=4 /> <text class=nj-sax-buf x=96 y=217 xml:space=preserve>{"name": "Mary", "list": [1, 2</text> <path class="nj-sax-caret nj-sax-s-wait" d="M352.2 226 l-5 8 h10 z"/> <text class="nj-sax-status nj-sax-s-wait" x=420 y=211>error at byte 31 = size + 1</text> <text class="fg-note fg-note--l" x=420 y=227>incomplete: wait for more</text> <text class="fg-note fg-note--l" x=16 y=267>chunk 5</text> <rect class=nj-sax-old x=93 y=250 width=255 height=24 rx=4 /> <rect class=nj-sax-new x=348 y=250 width=45 height=24 rx=4 /> <text class=nj-sax-buf x=96 y=267 xml:space=preserve>{"name": "Mary", "list": [1, 2, 3]}</text> <text class="nj-sax-status nj-sax-s-ok" x=420 y=261>parsed, size 35</text> <text class="fg-note fg-note--l" x=420 y=277>complete: one value</text> <path class=nj-sax-sep d="M16 290 H684"/> <text class="fg-note fg-note--l" x=16 y=325>other input</text> <rect class=nj-sax-new x=96 y=308 width=45 height=24 rx=4 /> <text class=nj-sax-buf x=96 y=325 xml:space=preserve>[1, ]</text> <path class="nj-sax-caret nj-sax-s-err" d="M133.8 334 l-5 8 h10 z"/> <text class="nj-sax-status nj-sax-s-err" x=420 y=319>error at byte 5, size 5</text> <text class="fg-note fg-note--l" x=420 y=335>before the end: a real error</text> <text class="fg-note fg-note--l" x=16 y=366>Bytes parsed: 4 + 12 + 19 + 30 + 35 = 100 for a value of 35 bytes.</text> <text class="fg-note fg-note--l" x=16 y=386>Every attempt starts again at byte 1, so the total work grows</text> <text class="fg-note fg-note--l" x=16 y=406>quadratically with the size of the value.</text> </svg></p> </div> <p class=nj-caption>Each row is one parse attempt on the growing buffer: an error at the byte after the end of the buffer means the value is incomplete, an error before the end is a real syntax error. The last line adds up the bytes parsed over all attempts.</p> <div class="admonition warning"> <p class=admonition-title>Limits of buffering</p> <ul> <li>The buffer must contain exactly one value. Data that bLine truncated
|
||||
<span class=cp>#include</span><span class=w> </span><span class=cpf><sstream></span>
|
||||
<span class=cp>#include</span><span class=w> </span><span class=cpf><nlohmann/json.hpp></span>
|
||||
|
||||
@@ -222,4 +222,4 @@ result: []
|
||||
<span class=p>}</span>
|
||||
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code>parse completed: false
|
||||
id: 42
|
||||
</code></pre></div> </details> <p>Note that <code>sax_parse</code> returns <code class=highlight><span class=nb>false</span></code> both when a handler stopped the parse and when the input is invalid, so record the reason in the handler (here: <code>id</code>) if you need to tell the two apart.</p> <h3 id=read-parts-of-a-large-document-with-a-view>Read parts of a large document with a view<a class=headerlink href=#read-parts-of-a-large-document-with-a-view title="Permanent link">¶</a></h3> <p>If the whole text is available and you only need to read some of it, a <a href=../../json_view/ ><code>json_document</code></a> from <code><nlohmann/json_view.hpp></code> (added in version 3.13.0 <span class=unreleased-version title="Not part of a release yet; the latest release is 3.12.0.">unreleased</span>) avoids the <code>basic_json</code> tree altogether: it keeps the text, builds a flat index of the values, and gives read-only <a href=../../../api/basic_json_view/ >views</a> of the parts you look at. Only what you <a href=../../../api/basic_json_view/materialize/ ><code>materialize()</code></a> becomes a <code>basic_json</code> value. See <a href=../../json_view/ >Zero-copy <abbr title="JavaScript Object Notation">JSON</abbr> views</a> for details and for the lifetime rules.</p> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">¶</a></h2> <ul> <li><a href=../ >Parsing</a> - overview of the parsing functions</li> <li><a href=../json_lines/ ><abbr title="JavaScript Object Notation">JSON</abbr> Lines</a> - newline-delimited <abbr title="JavaScript Object Notation">JSON</abbr></li> <li><a href=../parser_callbacks/ >Parser callbacks</a> - discard values while parsing</li> <li><a href=../sax_interface/ ><abbr title="Simple API for XML">SAX</abbr> interface</a> - event-based parsing</li> <li><a href=../../json_view/ >Zero-copy <abbr title="JavaScript Object Notation">JSON</abbr> views</a> - read parts of a large document without building a tree</li> <li><a href=../untrusted_input/ >Untrusted input</a> - limits for input you do not control</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 25, 2026 11:07:13 UTC">October 25, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2025 Fonticons, Inc.--><path d="M173.9 397.4c0 2-2.3 3.6-5.2 3.6-3.3.3-5.6-1.3-5.6-3.6 0-2 2.3-3.6 5.2-3.6 3-.3 5.6 1.3 5.6 3.6m-31.1-4.5c-.7 2 1.3 4.3 4.3 4.9 2.6 1 5.6 0 6.2-2s-1.3-4.3-4.3-5.2c-2.6-.7-5.5.3-6.2 2.3m44.2-1.7c-2.9.7-4.9 2.6-4.6 4.9.3 2 2.9 3.3 5.9 2.6 2.9-.7 4.9-2.6 4.6-4.6-.3-1.9-3-3.2-5.9-2.9M252.8 8C114.1 8 8 113.3 8 252c0 110.9 69.8 205.8 169.5 239.2 12.8 2.3 17.3-5.6 17.3-12.1 0-6.2-.3-40.4-.3-61.4 0 0-70 15-84.7-29.8 0 0-11.4-29.1-27.8-36.6 0 0-22.9-15.7 1.6-15.4 0 0 24.9 2 38.6 25.8 21.9 38.6 58.6 27.5 72.9 20.9 2.3-16 8.8-27.1 16-33.7-55.9-6.2-112.3-14.3-112.3-110.5 0-27.5 7.6-41.3 23.6-58.9-2.6-6.5-11.1-33.3 2.6-67.9 20.9-6.5 69 27 69 27 20-5.6 41.5-8.5 62.8-8.5s42.8 2.9 62.8 8.5c0 0 48.1-33.6 69-27 13.Line truncated
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||||
</code></pre></div> </details> <p>Note that <code>sax_parse</code> returns <code class=highlight><span class=nb>false</span></code> both when a handler stopped the parse and when the input is invalid, so record the reason in the handler (here: <code>id</code>) if you need to tell the two apart.</p> <h3 id=read-parts-of-a-large-document-with-a-view>Read parts of a large document with a view<a class=headerlink href=#read-parts-of-a-large-document-with-a-view title="Permanent link">¶</a></h3> <p>If the whole text is available and you only need to read some of it, a <a href=../../json_view/ ><code>json_document</code></a> from <code><nlohmann/json_view.hpp></code> (added in version 3.13.0 <span class=unreleased-version title="Not part of a release yet; the latest release is 3.12.0.">unreleased</span>) avoids the <code>basic_json</code> tree altogether: it keeps the text, builds a flat index of the values, and gives read-only <a href=../../../api/basic_json_view/ >views</a> of the parts you look at. Only what you <a href=../../../api/basic_json_view/materialize/ ><code>materialize()</code></a> becomes a <code>basic_json</code> value. See <a href=../../json_view/ >Zero-copy <abbr title="JavaScript Object Notation">JSON</abbr> views</a> for details and for the lifetime rules.</p> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">¶</a></h2> <ul> <li><a href=../ >Parsing</a> - overview of the parsing functions</li> <li><a href=../json_lines/ ><abbr title="JavaScript Object Notation">JSON</abbr> Lines</a> - newline-delimited <abbr title="JavaScript Object Notation">JSON</abbr></li> <li><a href=../parser_callbacks/ >Parser callbacks</a> - discard values while parsing</li> <li><a href=../sax_interface/ ><abbr title="Simple API for XML">SAX</abbr> interface</a> - event-based parsing</li> <li><a href=../../json_view/ >Zero-copy <abbr title="JavaScript Object Notation">JSON</abbr> views</a> - read parts of a large document without building a tree</li> <li><a href=../untrusted_input/ >Untrusted input</a> - limits for input you do not control</li> <li><a href=../../../api/operator_gtgt/ ><code>operator>></code></a> - deserialize from stream</li> <li><a href=../../../api/basic_json/parse/ ><code>parse</code></a> - deserialize from a compatible input</li> <li><a href=../../../api/basic_json/sax_parse/ ><code>sax_parse</code></a> - generate <abbr title="Simple API for XML">SAX</abbr> events</li> <li><a href=../../../api/macros/json_precise_stream_position/ ><code>JSON_PRECISE_STREAM_POSITION</code></a> - leave the stream positioned right after a parsed number</li> <li><a href=../../../api/macros/json_disable_stream_bulk_scan/ ><code>JSON_DISABLE_STREAM_BULK_SCAN</code></a> - read a <code>std::istream</code> one character at a time</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../json_lines/ class="md-footer__link md-footer__link--prev" aria-label="Previous: JSON Lines"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> JSON Lines </div> </div> </a> <a href=../parse_exceptions/ class="md-footer__link md-footer__link--next" aria-label="Next: Parsing and Exceptions"> <div class=md-foLine truncated
|
||||
@@ -14,6 +14,8 @@ flowchart TD
|
||||
H -->|"yes"| I["stop early with SAX, or read through a view"]
|
||||
```
|
||||
|
||||
Questions about the input and the approach this page describes for each; an input can match more than one question.
|
||||
|
||||
## Several values in one stream
|
||||
|
||||
For newline-delimited input, read line by line and parse each line, as described in [JSON Lines](https://json.nlohmann.me/features/parsing/json_lines/index.md). For values that are simply concatenated, [`operator>>`](https://json.nlohmann.me/api/operator_gtgt/index.md) reads one value and leaves the stream positioned after it. Check for the end of the stream before each read, because a read at the end throws:
|
||||
@@ -162,6 +164,10 @@ chunk , 3]} -> complete
|
||||
|
||||
The example feeds a document in five chunks. The first four are reported as incomplete, and a syntax error is reported as such, not as missing data.
|
||||
|
||||
The chunks of the example, and the bytes each attempt parses again:
|
||||
|
||||
Each row is one parse attempt on the growing buffer: an error at the byte after the end of the buffer means the value is incomplete, an error before the end is a real syntax error. The last line adds up the bytes parsed over all attempts.
|
||||
|
||||
Limits of buffering
|
||||
|
||||
- The buffer must contain exactly one value. Data that belongs to the next value is a syntax error. Use framing if values follow each other without a separator.
|
||||
@@ -172,6 +178,10 @@ Limits of buffering
|
||||
|
||||
The input does not have to become one huge `json` value. The two tools below process it piece by piece.
|
||||
|
||||
How the memory use develops while a large array of records is parsed (illustrative, not a measurement):
|
||||
|
||||
Memory use over the number of records parsed: `parse()` keeps the whole tree, `json_document` the text plus its index, a discarding callback one record at a time, and SAX only what the handler keeps.
|
||||
|
||||
### Handle and discard elements with a callback
|
||||
|
||||
A [parser callback](https://json.nlohmann.me/features/parsing/parser_callbacks/index.md) is called for every finished value. If it returns `false`, the value is discarded and its memory is released at once. For a file that consists of one huge array (or one huge object) of records, handle each record at depth 1 and discard it:
|
||||
@@ -330,3 +340,8 @@ If the whole text is available and you only need to read some of it, a [`json_do
|
||||
- [SAX interface](https://json.nlohmann.me/features/parsing/sax_interface/index.md) - event-based parsing
|
||||
- [Zero-copy JSON views](https://json.nlohmann.me/features/json_view/index.md) - read parts of a large document without building a tree
|
||||
- [Untrusted input](https://json.nlohmann.me/features/parsing/untrusted_input/index.md) - limits for input you do not control
|
||||
- [`operator>>`](https://json.nlohmann.me/api/operator_gtgt/index.md) - deserialize from stream
|
||||
- [`parse`](https://json.nlohmann.me/api/basic_json/parse/index.md) - deserialize from a compatible input
|
||||
- [`sax_parse`](https://json.nlohmann.me/api/basic_json/sax_parse/index.md) - generate SAX events
|
||||
- [`JSON_PRECISE_STREAM_POSITION`](https://json.nlohmann.me/api/macros/json_precise_stream_position/index.md) - leave the stream positioned right after a parsed number
|
||||
- [`JSON_DISABLE_STREAM_BULK_SCAN`](https://json.nlohmann.me/api/macros/json_disable_stream_bulk_scan/index.md) - read a `std::istream` one character at a time
|
||||
@@ -280,3 +280,14 @@ acceptable, for example for configuration files. Note that `start_pos()` is a 0-
|
||||
For binary formats such as CBOR, there are no lines; `byte` is the position in the byte sequence. To find out how many
|
||||
bytes a successfully parsed value used, see
|
||||
[Several values in one stream](large_and_partial_input.md#several-values-in-one-stream).
|
||||
|
||||
## See also
|
||||
|
||||
- [`parse`](../../api/basic_json/parse.md) - deserialize from a compatible input
|
||||
- [`accept`](../../api/basic_json/accept.md) - check if the input is valid JSON
|
||||
- [`sax_parse`](../../api/basic_json/sax_parse.md) - generate SAX events
|
||||
- [`parse_error`](../../api/basic_json/parse_error.md) - exception thrown on a parse error
|
||||
- [`JSON_DIAGNOSTIC_POSITIONS`](../../api/macros/json_diagnostic_positions.md) - record start and end positions of parsed values
|
||||
- [Working Without Exceptions](../no_exceptions.md) - error handling when exceptions are disabled
|
||||
- [Exceptions](../../home/exceptions.md) - list of exceptions and error codes
|
||||
- [Parsing Untrusted Input](untrusted_input.md) - limits and checks for input you do not control
|
||||
@@ -1,4 +1,4 @@
|
||||
<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/parsing/parse_exceptions/" rel="canonical"><link href=../large_and_partial_input/ rel=prev><link href=../parser_callbacks/ rel=next><link rel=icon href=../../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>Parsing and Exceptions - JSON for Modern C++</title><link rel=stylesheet href=../../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../../assets/stylesheets/palette.ab4e12ef.min.css><link rel="stylesheet" href="../../../assets/external/fonts.googleapis.com/css.61a430c9.css"><style>:root{--md-text-font:"Roboto";--md-code-font:"JetBrains Mono"}</style><link rel=stylesheet href=../../../css/custom.css><script>__md_scope=new 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data-md-component=logo> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 8a3 3 0 0 0 3-3 3 3 0 0 0-3-3 3 3 0 0 0-3 3 3 3 0 0 0 3 3m0 3.54C9.64 9.35 6.5 8 3 8v11c3.5 0 6.64 1.35 9 3.54 2.36-2.19 5.5-3.54 9-3.54V8c-3.5 0-6.64 1.35-9 3.54"/></svg> </a> <label class="md-header__button md-icon" for=__drawer> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M3 6h18v2H3zm0 5h18v2H3zm0 5h18v2H3z"/></svg> </label> <div class=md-header__title data-md-component=header-title> <div class=md-header__ellipsis> <div class=md-header__topic> <span class=md-ellipsis> JSON for Modern C++ </span> </div> <div class=md-header__topic data-md-component=header-topic> <span class=md-ellipsis> Parsing and Exceptions </span> </div> </div> </div> <form class=md-header__option data-md-component=palette> <input class=md-option data-md-color-media="(prefers-color-scheme: light)" data-md-color-scheme=default data-md-color-primary=indigo data-md-color-accent=indigo 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<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=description content="JSON for Modern C++ is a header-only C++11 library that makes JSON a first-class data type."><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/parsing/parse_exceptions/" rel="canonical"><link href=../large_and_partial_input/ rel=prev><link href=../parser_callbacks/ rel=next><link rel=icon href=../../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>Parsing and Exceptions - JSON for Modern C++</title><link rel=stylesheet href=../../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../../assets/stylesheets/palette.ab4e12ef.min.css><link rel=stylesheet href=../../../css/custom.css><link rel=stylesheet href=../../../css/brand.css><link rel=stylesheet href=../../../css/fig-sax.css><link rel=stylesheet 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data-md-color-media="(prefers-color-scheme: light)" data-md-color-scheme=default data-md-color-primary=custom data-md-color-accent=custom aria-label="Switch to dark mode" type=radio name=__palette id=__palette_0> <label class="md-header__button md-icon" title="Switch to dark mode" for=__palette_1 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 8a4 4 0 0 0-4 4 4 4 0 0 0 4 4 4 4 0 0 0 4-4 4 4 0 0 0-4-4m0 10a6 6 0 0 1-6-6 6 6 0 0 1 6-6 6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> <input class=md-option data-md-color-media="(prefers-color-scheme: dark)" data-md-color-scheme=slate data-md-color-primary=custom data-md-color-accent=custom aria-label="Switch to light mode" type=radio name=__palette id=__palette_1> <label class="md-header__button md-icon" title="Switch to light mode" for=__palette_0 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 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<span class=k>try</span>
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<span class=p>{</span>
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<span class=w> </span><span class=n>j</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>parse</span><span class=p>(</span><span class=n>my_input</span><span class=p>);</span>
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@@ -157,4 +157,4 @@ byte 20 = line 3, column 8
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<span class=p>}</span>
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</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code>[json.exception.type_error.302] (bytes 29-40) type must be number, but is string
|
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the value starts at line 3, column 10
|
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</code></pre></div> </details> <p>Positions are only available for values created by <a href=../../../api/basic_json/parse/ ><code>parse</code></a>, they are not updated when a value is changed, and the macro makes every value 16 bytes larger on 64-bit platforms, so use it where the cost is acceptable, for example for configuration files. Note that <code>start_pos()</code> is a 0-based index into the input, whereas <code>byte</code> of a <code>parse_error</code> starts at 1.</p> <p>For binary formats such as <abbr title="Concise Binary Object Representation">CBOR</abbr>, there are no lines; <code>byte</code> is the position in the byte sequence. To find out how many bytes a successfully parsed value used, see <a href=../large_and_partial_input/#several-values-in-one-stream>Several values in one stream</a>.</p> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 25, 2026 11:07:13 UTC">October 25, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2025 Fonticons, Inc.--><path d="M173.9 397.4c0 2-2.3 3.6-5.2 3.6-3.3.3-5.6-1.3-5.6-3.6 0-2 2.3-3.6 5.2-3.6 3-.3 5.6 1.3 5.6 3.6m-31.1-4.5c-.7 2 1.3 4.3 4.3 4.9 2.6 1 5.6 0 6.2-2s-1.3-4.3-4.3-5.2c-2.6-.7-5.5.3-6.2 2.3m44.2-1.7c-2.9.7-4.9 2.6-4.6 4.9.3 2 2.9 3.3 5.9 2.6 2.9-.7 4.9-2.6 4.6-4.6-.3-1.9-3-3.2-5.9-2.9M252.8 8C114.1 8 8 113.3 8 252c0 110.9 69.8 205.8 169.5 239.2 12.8 2.3 17.3-5.6 17.3-12.1 0-6.2-.3-40.4-.3-61.4 0 0-70 15-84.7-29.8 0 0-11.4-29.1-27.8-36.6 0 0-22.9-15.7 1.6-15.4 0 0 24.9 2 38.6 25.8 21.9 38.6 58.6 27.5 72.9 20.9 2.3-16 8.8-27.1 16-33.7-55.9-6.2-112.3-14.3-112.3-110.5 0-27.5 7.6-41.3 23.6-58.9-2.6-6.5-11.1-33.3 2.6-67.9 20.9-6.5 69 27 69 27 20-5.6 41.5-8.5 62.8-8.5s42.8 2.9 62.8 8.5c0 0 48.1-33.6 69-27 13.7 34.7 5.2 61.4 2.6 67.9 16 17.7 25.8 31.5 25.8 58.9 0 96.5-58.9 104.2-114.8 110.5 9.2 7.9 17 22.9 17 46.4 0 33.7-.3 75.4-.3 83.6 0 6.5 4.6 14.4 17.3 12.1C436.2 457.8 504 362.9 504 252 504 113.3 391.5 8 252.8 8M105.2 352.9c-1.3 1-1 3.3.7 5.2 1.6 1.6 3.9 2.3 5.2 1 1.3-1 1-3.3-.7-5.2-1.6-1.6-3.9-2.3-5.2-1m-10.8-8.1c-.7 1.3.3 2.9 2.3 3.9 1.6 1 3.6.7 4.3-.7.7-1.3-.3-2.9-2.3-3.9-2-.6-3.6-.3-4.3.7m32.4 35.6c-1.6 1.3-1 4.3 1.3 6.2 2.3 2.3 5.2 2.6 6.5 1 1.3-1.3.7-4.3-1.3-6.2-2.2-2.3-5.2-2.6-6.5-1m-11.4-14.7c-1.6 1-1.6 3.6 0 5.9s4.3 3.3 5.6 2.3c1.6-1.3 1.6-3.9 0-6.2-1.4-2.3-4-3.3-5.6-2"/></svg> </a> <a href="https://www.linkedin.com/in/nielslohmann/" target="_blank" rel="noopener" title="www.linkedin.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 448 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2025 Fonticons, Inc.--><path d="M416 32H31.9C14.3 32 0 46.5 0 64.3v383.4C0 465.5 14.3 480 31.9 480H416c17.6 0 32-14.5 32-32.3V64.3c0-17.8-14.4-32.3-32-32.3M135.4 416H69V202.2h66.5V416zM102.2 96a38.5 38.5 0 1 1 0 77 38.5 38.5 0 1 1 0-77m282.1 320h-66.4V312c0-24.8-.5-56.7-34.5-56.7-34.6 0-39.9 27-39.9 54.9V416h-66.4V202.2hLine truncated
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</code></pre></div> </details> <p>Positions are only available for values created by <a href=../../../api/basic_json/parse/ ><code>parse</code></a>, they are not updated when a value is changed, and the macro makes every value 16 bytes larger on 64-bit platforms, so use it where the cost is acceptable, for example for configuration files. Note that <code>start_pos()</code> is a 0-based index into the input, whereas <code>byte</code> of a <code>parse_error</code> starts at 1.</p> <p>For binary formats such as <abbr title="Concise Binary Object Representation">CBOR</abbr>, there are no lines; <code>byte</code> is the position in the byte sequence. To find out how many bytes a successfully parsed value used, see <a href=../large_and_partial_input/#several-values-in-one-stream>Several values in one stream</a>.</p> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">¶</a></h2> <ul> <li><a href=../../../api/basic_json/parse/ ><code>parse</code></a> - deserialize from a compatible input</li> <li><a href=../../../api/basic_json/accept/ ><code>accept</code></a> - check if the input is valid <abbr title="JavaScript Object Notation">JSON</abbr></li> <li><a href=../../../api/basic_json/sax_parse/ ><code>sax_parse</code></a> - generate <abbr title="Simple API for XML">SAX</abbr> events</li> <li><a href=../../../api/basic_json/parse_error/ ><code>parse_error</code></a> - exception thrown on a parse error</li> <li><a href=../../../api/macros/json_diagnostic_positions/ ><code>JSON_DIAGNOSTIC_POSITIONS</code></a> - record start and end positions of parsed values</li> <li><a href=../../no_exceptions/ >Working Without Exceptions</a> - error handling when exceptions are disabled</li> <li><a href=../../../home/exceptions/ >Exceptions</a> - list of exceptions and error codes</li> <li><a href=../untrusted_input/ >Parsing Untrusted Input</a> - limits and checks for input you do not control</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../large_and_partial_input/ class="md-footer__link md-footer__link--prev" aria-label="Previous: Large, Streamed, and Partial Input"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> Large, Streamed, and Partial Input </div> </div> </a> <a href=../parser_callbacks/ class="md-footer__link md-footer__link--next" aria-label="Next: Parser Callbacks"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> Parser Callbacks </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/frLine truncated
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