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# Arbitrary Type Conversions
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Every type can be serialized in JSON, not just STL containers and scalar types. Usually, you would do something along those lines:
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```cpp
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namespace ns {
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// a simple struct to model a person
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struct person {
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std::string name;
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std::string address;
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int age;
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};
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} // namespace ns
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ns::person p = {"Ned Flanders", "744 Evergreen Terrace", 60};
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// convert to JSON: copy each value into the JSON object
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json j;
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j["name"] = p.name;
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j["address"] = p.address;
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j["age"] = p.age;
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// ...
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// convert from JSON: copy each value from the JSON object
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ns::person p {
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j["name"].get<std::string>(),
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j["address"].get<std::string>(),
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j["age"].get<int>()
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};
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```
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It works, but that's quite a lot of boilerplate... Fortunately, there's a better way:
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```cpp
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// create a person
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ns::person p {"Ned Flanders", "744 Evergreen Terrace", 60};
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// conversion: person -> json
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json j = p;
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std::cout << j << std::endl;
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// {"address":"744 Evergreen Terrace","age":60,"name":"Ned Flanders"}
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// conversion: json -> person
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auto p2 = j.get<ns::person>();
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// that's it
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assert(p == p2);
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```
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## Basic usage
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To make this work with one of your types, you only need to provide two functions:
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```cpp
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using json = nlohmann::json;
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namespace ns {
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void to_json(json& j, const person& p) {
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j = json{ {"name", p.name}, {"address", p.address}, {"age", p.age} };
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}
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void from_json(const json& j, person& p) {
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j.at("name").get_to(p.name);
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j.at("address").get_to(p.address);
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j.at("age").get_to(p.age);
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}
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} // namespace ns
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```
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That's all! When calling the `json` constructor with your type, your custom `to_json` method will be automatically called.
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Likewise, when calling `get<your_type>()` or `get_to(your_type&)`, the `from_json` method will be called.
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Some important things:
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* Those methods **MUST** be in your type's namespace (which can be the global namespace), or the library will not be able to locate them (in this example, they are in namespace `ns`, where `person` is defined).
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* Those methods **MUST** be available (e.g., proper headers must be included) everywhere you use these conversions. Look at [#1108](https://github.com/nlohmann/json/issues/1108) for errors that may occur otherwise.
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* When using `get<your_type>()`, `your_type` **MUST** be [DefaultConstructible](https://en.cppreference.com/w/cpp/named_req/DefaultConstructible). (There is a way to bypass this requirement described later.)
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* In function `from_json`, use function [`at()`](../api/basic_json/at.md) to access the object values rather than `operator[]`. In case a key does not exist, `at` throws an exception that you can handle, whereas `operator[]` exhibits undefined behavior.
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* You do not need to add serializers or deserializers for STL types like `std::vector`: the library already implements these.
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## Simplify your life with macros
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If you just want to serialize/deserialize some structs, the `to_json`/`from_json` functions can be a lot of boilerplate.
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There are several macros to make your life easier as long as you want to use a JSON object as serialization. The macros are following the naming pattern, and you can choose the macro based on the needed features:
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- All the macros start with `NLOHMANN_DEFINE`.
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- If you want a macro for the derived object, use the [`DERIVED_TYPE`](../api/macros/nlohmann_define_derived_type.md) variant, otherwise use `TYPE`.
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- The `DERIVED_TYPE` variant requires an additional parameter of a base type, which should have the `to_json`/`from_json` functions defined. For instance, with a macro of its own.
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- If you need access to the private fields use [`INTRUSIVE`](../api/macros/nlohmann_define_type_intrusive.md) variant, otherwise use [`NON_INTRUSIVE`](../api/macros/nlohmann_define_type_non_intrusive.md).
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- The `INTRUSIVE` macro should be defined **inside** the target class/struct, `NON_INTRUSIVE` should be defined within the same namespace.
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- If you want to deserialize the incomplete JSONs, use the `WITH_DEFAULTS` variant, which will use the default values for the member variables absent in JSON, the variant without `WITH_DEFAULTS` will raise an exception.
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- If you do not need deserialization at all and only interested in `to_json` function, you can use the `ONLY_SERIALIZE` variant.
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- If you want to use the custom JSON names for member variables, use [`WITH_NAMES`](../api/macros/nlohmann_define_type_with_names.md) variant, otherwise the JSON name of the variable will be the same as its regular name.
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For all the macros, the first parameter is the name of the class/struct. The `DERIVED_TYPE` macros require a second parameter of a base class. All the remaining parameters name the member variables. The `WITH_NAMES` macros require a JSON name before each of the variables.
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| Need access to private members | Need only de-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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| <div style="color: green;">:octicons-check-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_INTRUSIVE_WITH_DEFAULT**](../api/macros/nlohmann_define_type_intrusive.md) |
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| <div style="color: green;">:octicons-check-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_INTRUSIVE_ONLY_SERIALIZE**](../api/macros/nlohmann_define_type_intrusive.md) |
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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: red;">:octicons-x-circle-fill-24:</div> | [**NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE**](../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: 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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For _derived_ classes and structs, use the following macros
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| Need access to private members | Need only de-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_DERIVED_TYPE_INTRUSIVE**](../api/macros/nlohmann_define_derived_type.md) |
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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: green;">:octicons-check-circle-fill-24:</div> | [**NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_WITH_DEFAULT**](../api/macros/nlohmann_define_derived_type.md) |
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| <div style="color: green;">:octicons-check-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_INTRUSIVE_ONLY_SERIALIZE**](../api/macros/nlohmann_define_derived_type.md) |
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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: red;">:octicons-x-circle-fill-24:</div> | [**NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE**](../api/macros/nlohmann_define_derived_type.md) |
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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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!!! 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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??? example
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The `to_json`/`from_json` functions for the `person` struct above can be created with:
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```cpp
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namespace ns {
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NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE(person, name, address, age)
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}
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```
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If you want to inherit the `person` struct and add a field to it, it can be done with:
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```cpp
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namespace ns {
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struct person_derived : person {
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std::string email;
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};
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NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE(person_derived, person, email)
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}
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```
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Here is another example with private members, where `NLOHMANN_DEFINE_TYPE_INTRUSIVE` is needed:
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```cpp
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namespace ns {
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class address {
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private:
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std::string street;
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int housenumber;
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int postcode;
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public:
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NLOHMANN_DEFINE_TYPE_INTRUSIVE(address, street, housenumber, postcode)
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};
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}
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```
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Or in case if you use some naming convention that you do not want to expose to JSON:
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```cpp
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namespace ns {
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class address {
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private:
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std::string m_street;
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int m_housenumber;
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int m_postcode;
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public:
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NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_NAMES(address, "street", m_street,
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"housenumber", m_housenumber,
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"postcode", m_postcode)
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};
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}
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```
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## How do I convert third-party types?
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This requires a bit more advanced technique. But first, let us see how this conversion mechanism works:
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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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It is implemented like this (simplified):
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```cpp
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template <typename T>
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struct adl_serializer {
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static void to_json(json& j, const T& value) {
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// calls the "to_json" method in T's namespace
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}
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static void from_json(const json& j, T& value) {
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// same thing, but with the "from_json" method
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}
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};
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```
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This serializer works fine when you have control over the type's namespace. However, what about `boost::optional` or `std::filesystem::path` (C++17)? Hijacking the `boost` namespace is pretty bad, and it's illegal to add something other than template specializations to `std`...
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To solve this, you need to add a specialization of `adl_serializer` to the `nlohmann` namespace, here's an example:
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```cpp
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// partial specialization (full specialization works too)
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NLOHMANN_JSON_NAMESPACE_BEGIN
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template <typename T>
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struct adl_serializer<boost::optional<T>> {
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static void to_json(json& j, const boost::optional<T>& opt) {
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if (opt == boost::none) {
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j = nullptr;
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} else {
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j = *opt; // this will call adl_serializer<T>::to_json which will
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// find the free function to_json in T's namespace!
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}
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}
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static void from_json(const json& j, boost::optional<T>& opt) {
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if (j.is_null()) {
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opt = boost::none;
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} else {
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opt = j.get<T>(); // same as above, but with
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// adl_serializer<T>::from_json
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}
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}
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};
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NLOHMANN_JSON_NAMESPACE_END
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```
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!!! note "ABI compatibility"
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Use [`NLOHMANN_JSON_NAMESPACE_BEGIN`](../api/macros/nlohmann_json_namespace_begin.md) and `NLOHMANN_JSON_NAMESPACE_END`
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instead of `#!cpp namespace nlohmann { }` in code which may be linked with different versions of this library.
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## How can I use `get()` for non-default constructible/non-copyable types?
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There is a way if your type is [MoveConstructible](https://en.cppreference.com/w/cpp/named_req/MoveConstructible). You will need to specialize the `adl_serializer` as well, but with a special `from_json` overload:
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```cpp
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struct move_only_type {
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move_only_type() = delete;
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move_only_type(int ii): i(ii) {}
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move_only_type(const move_only_type&) = delete;
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move_only_type(move_only_type&&) = default;
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int i;
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};
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namespace nlohmann {
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template <>
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struct adl_serializer<move_only_type> {
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// note: the return type is no longer 'void', and the method only takes
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// one argument
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static move_only_type from_json(const json& j) {
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return {j.get<int>()};
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}
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// Here's the catch! You must provide a to_json method! Otherwise, you
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// will not be able to convert move_only_type to json, since you fully
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// specialized adl_serializer on that type
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static void to_json(json& j, move_only_type t) {
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j = t.i;
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}
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};
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}
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```
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## Can I write my own serializer? (Advanced use)
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Yes. You might want to take a look at [`unit-udt.cpp`](https://github.com/nlohmann/json/blob/develop/tests/src/unit-udt.cpp) in the test suite, to see a few examples.
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If you write your own serializer, you will need to do a few things:
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- use a different `basic_json` alias than `nlohmann::json` (the last template parameter of `basic_json` is the `JSONSerializer`)
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- use your `basic_json` alias (or a template parameter) in all your `to_json`/`from_json` methods
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- use `nlohmann::to_json` and `nlohmann::from_json` when you need ADL
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Here is an example, without simplifications, that only accepts types with a size <= 32, and uses ADL.
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```cpp
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// You should use void as a second template argument
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// if you don't need compile-time checks on T
|
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template<typename T, typename SFINAE = typename std::enable_if<sizeof(T) <= 32>::type>
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struct less_than_32_serializer {
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template <typename BasicJsonType>
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static void to_json(BasicJsonType& j, T value) {
|
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// we want to use ADL, and call the correct to_json overload
|
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using nlohmann::to_json; // this method is called by adl_serializer,
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// this is where the magic happens
|
||||
to_json(j, value);
|
||||
}
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||||
|
||||
template <typename BasicJsonType>
|
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static void from_json(const BasicJsonType& j, T& value) {
|
||||
// same thing here
|
||||
using nlohmann::from_json;
|
||||
from_json(j, value);
|
||||
}
|
||||
};
|
||||
```
|
||||
|
||||
Be **very** careful when reimplementing your serializer, you can stack overflow if you don't pay attention:
|
||||
|
||||
```cpp
|
||||
template <typename T, void>
|
||||
struct bad_serializer
|
||||
{
|
||||
template <typename BasicJsonType>
|
||||
static void to_json(BasicJsonType& j, const T& value) {
|
||||
// this calls BasicJsonType::json_serializer<T>::to_json(j, value);
|
||||
// if BasicJsonType::json_serializer == bad_serializer ... oops!
|
||||
j = value;
|
||||
}
|
||||
|
||||
template <typename BasicJsonType>
|
||||
static void from_json(const BasicJsonType& j, T& value) {
|
||||
// this calls BasicJsonType::json_serializer<T>::from_json(j, value);
|
||||
// if BasicJsonType::json_serializer == bad_serializer ... oops!
|
||||
value = j.template get<T>(); // oops!
|
||||
}
|
||||
};
|
||||
```
|
||||
@@ -179,4 +179,4 @@
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<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> <!-- 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="July 4, 2026 09:13:25 UTC">July 4, 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> <!-- 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="July 8, 2026 18:18:24 UTC">July 8, 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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|
||||
@@ -0,0 +1,322 @@
|
||||
# Arbitrary Type Conversions
|
||||
|
||||
Every type can be serialized in JSON, not just STL containers and scalar types. Usually, you would do something along those lines:
|
||||
|
||||
```
|
||||
namespace ns {
|
||||
// a simple struct to model a person
|
||||
struct person {
|
||||
std::string name;
|
||||
std::string address;
|
||||
int age;
|
||||
};
|
||||
} // namespace ns
|
||||
|
||||
ns::person p = {"Ned Flanders", "744 Evergreen Terrace", 60};
|
||||
|
||||
// convert to JSON: copy each value into the JSON object
|
||||
json j;
|
||||
j["name"] = p.name;
|
||||
j["address"] = p.address;
|
||||
j["age"] = p.age;
|
||||
|
||||
// ...
|
||||
|
||||
// convert from JSON: copy each value from the JSON object
|
||||
ns::person p {
|
||||
j["name"].get<std::string>(),
|
||||
j["address"].get<std::string>(),
|
||||
j["age"].get<int>()
|
||||
};
|
||||
```
|
||||
|
||||
It works, but that's quite a lot of boilerplate... Fortunately, there's a better way:
|
||||
|
||||
```
|
||||
// create a person
|
||||
ns::person p {"Ned Flanders", "744 Evergreen Terrace", 60};
|
||||
|
||||
// conversion: person -> json
|
||||
json j = p;
|
||||
|
||||
std::cout << j << std::endl;
|
||||
// {"address":"744 Evergreen Terrace","age":60,"name":"Ned Flanders"}
|
||||
|
||||
// conversion: json -> person
|
||||
auto p2 = j.get<ns::person>();
|
||||
|
||||
// that's it
|
||||
assert(p == p2);
|
||||
```
|
||||
|
||||
## Basic usage
|
||||
|
||||
To make this work with one of your types, you only need to provide two functions:
|
||||
|
||||
```
|
||||
using json = nlohmann::json;
|
||||
|
||||
namespace ns {
|
||||
void to_json(json& j, const person& p) {
|
||||
j = json{ {"name", p.name}, {"address", p.address}, {"age", p.age} };
|
||||
}
|
||||
|
||||
void from_json(const json& j, person& p) {
|
||||
j.at("name").get_to(p.name);
|
||||
j.at("address").get_to(p.address);
|
||||
j.at("age").get_to(p.age);
|
||||
}
|
||||
} // namespace ns
|
||||
```
|
||||
|
||||
That's all! When calling the `json` constructor with your type, your custom `to_json` method will be automatically called. Likewise, when calling `get<your_type>()` or `get_to(your_type&)`, the `from_json` method will be called.
|
||||
|
||||
Some important things:
|
||||
|
||||
- Those methods **MUST** be in your type's namespace (which can be the global namespace), or the library will not be able to locate them (in this example, they are in namespace `ns`, where `person` is defined).
|
||||
- Those methods **MUST** be available (e.g., proper headers must be included) everywhere you use these conversions. Look at [#1108](https://github.com/nlohmann/json/issues/1108) for errors that may occur otherwise.
|
||||
- When using `get<your_type>()`, `your_type` **MUST** be [DefaultConstructible](https://en.cppreference.com/w/cpp/named_req/DefaultConstructible). (There is a way to bypass this requirement described later.)
|
||||
- In function `from_json`, use function [`at()`](https://json.nlohmann.me/api/basic_json/at/index.md) to access the object values rather than `operator[]`. In case a key does not exist, `at` throws an exception that you can handle, whereas `operator[]` exhibits undefined behavior.
|
||||
- You do not need to add serializers or deserializers for STL types like `std::vector`: the library already implements these.
|
||||
|
||||
## Simplify your life with macros
|
||||
|
||||
If you just want to serialize/deserialize some structs, the `to_json`/`from_json` functions can be a lot of boilerplate.
|
||||
|
||||
There are several macros to make your life easier as long as you want to use a JSON object as serialization. The macros are following the naming pattern, and you can choose the macro based on the needed features:
|
||||
|
||||
- All the macros start with `NLOHMANN_DEFINE`.
|
||||
- If you want a macro for the derived object, use the [`DERIVED_TYPE`](https://json.nlohmann.me/api/macros/nlohmann_define_derived_type/index.md) variant, otherwise use `TYPE`.
|
||||
- The `DERIVED_TYPE` variant requires an additional parameter of a base type, which should have the `to_json`/`from_json` functions defined. For instance, with a macro of its own.
|
||||
- If you need access to the private fields use [`INTRUSIVE`](https://json.nlohmann.me/api/macros/nlohmann_define_type_intrusive/index.md) variant, otherwise use [`NON_INTRUSIVE`](https://json.nlohmann.me/api/macros/nlohmann_define_type_non_intrusive/index.md).
|
||||
- The `INTRUSIVE` macro should be defined **inside** the target class/struct, `NON_INTRUSIVE` should be defined within the same namespace.
|
||||
- If you want to deserialize the incomplete JSONs, use the `WITH_DEFAULTS` variant, which will use the default values for the member variables absent in JSON, the variant without `WITH_DEFAULTS` will raise an exception.
|
||||
- If you do not need deserialization at all and only interested in `to_json` function, you can use the `ONLY_SERIALIZE` variant.
|
||||
- If you want to use the custom JSON names for member variables, use [`WITH_NAMES`](https://json.nlohmann.me/api/macros/nlohmann_define_type_with_names/index.md) variant, otherwise the JSON name of the variable will be the same as its regular name.
|
||||
|
||||
For all the macros, the first parameter is the name of the class/struct. The `DERIVED_TYPE` macros require a second parameter of a base class. All the remaining parameters name the member variables. The `WITH_NAMES` macros require a JSON name before each of the variables.
|
||||
|
||||
| Need access to private members | Need only de-serialization | Allow missing values when de-serializing | macro |
|
||||
| ------------------------------ | -------------------------- | ---------------------------------------- | ---------------------------------------------------------------------------------------------------------------------------------------- |
|
||||
| | | | [**NLOHMANN_DEFINE_TYPE_INTRUSIVE**](https://json.nlohmann.me/api/macros/nlohmann_define_type_intrusive/index.md) |
|
||||
| | | | [**NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_DEFAULT**](https://json.nlohmann.me/api/macros/nlohmann_define_type_intrusive/index.md) |
|
||||
| | | | [**NLOHMANN_DEFINE_TYPE_INTRUSIVE_ONLY_SERIALIZE**](https://json.nlohmann.me/api/macros/nlohmann_define_type_intrusive/index.md) |
|
||||
| | | | [**NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE**](https://json.nlohmann.me/api/macros/nlohmann_define_type_non_intrusive/index.md) |
|
||||
| | | | [**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) |
|
||||
|
||||
For *derived* classes and structs, use the following macros
|
||||
|
||||
| Need access to private members | Need only de-serialization | Allow missing values when de-serializing | macro |
|
||||
| ------------------------------ | -------------------------- | ---------------------------------------- | ------------------------------------------------------------------------------------------------------------------------------------------ |
|
||||
| | | | [**NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE**](https://json.nlohmann.me/api/macros/nlohmann_define_derived_type/index.md) |
|
||||
| | | | [**NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_WITH_DEFAULT**](https://json.nlohmann.me/api/macros/nlohmann_define_derived_type/index.md) |
|
||||
| | | | [**NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_ONLY_SERIALIZE**](https://json.nlohmann.me/api/macros/nlohmann_define_derived_type/index.md) |
|
||||
| | | | [**NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE**](https://json.nlohmann.me/api/macros/nlohmann_define_derived_type/index.md) |
|
||||
| | | | [**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) |
|
||||
|
||||
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.
|
||||
|
||||
Example
|
||||
|
||||
The `to_json`/`from_json` functions for the `person` struct above can be created with:
|
||||
|
||||
```
|
||||
namespace ns {
|
||||
NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE(person, name, address, age)
|
||||
}
|
||||
```
|
||||
|
||||
If you want to inherit the `person` struct and add a field to it, it can be done with:
|
||||
|
||||
```
|
||||
namespace ns {
|
||||
struct person_derived : person {
|
||||
std::string email;
|
||||
};
|
||||
|
||||
NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE(person_derived, person, email)
|
||||
}
|
||||
```
|
||||
|
||||
Here is another example with private members, where `NLOHMANN_DEFINE_TYPE_INTRUSIVE` is needed:
|
||||
|
||||
```
|
||||
namespace ns {
|
||||
class address {
|
||||
private:
|
||||
std::string street;
|
||||
int housenumber;
|
||||
int postcode;
|
||||
|
||||
public:
|
||||
NLOHMANN_DEFINE_TYPE_INTRUSIVE(address, street, housenumber, postcode)
|
||||
};
|
||||
}
|
||||
```
|
||||
|
||||
Or in case if you use some naming convention that you do not want to expose to JSON:
|
||||
|
||||
```
|
||||
namespace ns {
|
||||
class address {
|
||||
private:
|
||||
std::string m_street;
|
||||
int m_housenumber;
|
||||
int m_postcode;
|
||||
|
||||
public:
|
||||
NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_NAMES(address, "street", m_street,
|
||||
"housenumber", m_housenumber,
|
||||
"postcode", m_postcode)
|
||||
};
|
||||
}
|
||||
```
|
||||
|
||||
## How do I convert third-party types?
|
||||
|
||||
This requires a bit more advanced technique. But first, let us see how this conversion mechanism works:
|
||||
|
||||
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):
|
||||
|
||||
```
|
||||
template <typename T>
|
||||
struct adl_serializer {
|
||||
static void to_json(json& j, const T& value) {
|
||||
// calls the "to_json" method in T's namespace
|
||||
}
|
||||
|
||||
static void from_json(const json& j, T& value) {
|
||||
// same thing, but with the "from_json" method
|
||||
}
|
||||
};
|
||||
```
|
||||
|
||||
This serializer works fine when you have control over the type's namespace. However, what about `boost::optional` or `std::filesystem::path` (C++17)? Hijacking the `boost` namespace is pretty bad, and it's illegal to add something other than template specializations to `std`...
|
||||
|
||||
To solve this, you need to add a specialization of `adl_serializer` to the `nlohmann` namespace, here's an example:
|
||||
|
||||
```
|
||||
// partial specialization (full specialization works too)
|
||||
NLOHMANN_JSON_NAMESPACE_BEGIN
|
||||
template <typename T>
|
||||
struct adl_serializer<boost::optional<T>> {
|
||||
static void to_json(json& j, const boost::optional<T>& opt) {
|
||||
if (opt == boost::none) {
|
||||
j = nullptr;
|
||||
} else {
|
||||
j = *opt; // this will call adl_serializer<T>::to_json which will
|
||||
// find the free function to_json in T's namespace!
|
||||
}
|
||||
}
|
||||
|
||||
static void from_json(const json& j, boost::optional<T>& opt) {
|
||||
if (j.is_null()) {
|
||||
opt = boost::none;
|
||||
} else {
|
||||
opt = j.get<T>(); // same as above, but with
|
||||
// adl_serializer<T>::from_json
|
||||
}
|
||||
}
|
||||
};
|
||||
NLOHMANN_JSON_NAMESPACE_END
|
||||
```
|
||||
|
||||
ABI compatibility
|
||||
|
||||
Use [`NLOHMANN_JSON_NAMESPACE_BEGIN`](https://json.nlohmann.me/api/macros/nlohmann_json_namespace_begin/index.md) and `NLOHMANN_JSON_NAMESPACE_END` instead of `namespace nlohmann { }` in code which may be linked with different versions of this library.
|
||||
|
||||
## How can I use `get()` for non-default constructible/non-copyable types?
|
||||
|
||||
There is a way if your type is [MoveConstructible](https://en.cppreference.com/w/cpp/named_req/MoveConstructible). You will need to specialize the `adl_serializer` as well, but with a special `from_json` overload:
|
||||
|
||||
```
|
||||
struct move_only_type {
|
||||
move_only_type() = delete;
|
||||
move_only_type(int ii): i(ii) {}
|
||||
move_only_type(const move_only_type&) = delete;
|
||||
move_only_type(move_only_type&&) = default;
|
||||
|
||||
int i;
|
||||
};
|
||||
|
||||
namespace nlohmann {
|
||||
template <>
|
||||
struct adl_serializer<move_only_type> {
|
||||
// note: the return type is no longer 'void', and the method only takes
|
||||
// one argument
|
||||
static move_only_type from_json(const json& j) {
|
||||
return {j.get<int>()};
|
||||
}
|
||||
|
||||
// Here's the catch! You must provide a to_json method! Otherwise, you
|
||||
// will not be able to convert move_only_type to json, since you fully
|
||||
// specialized adl_serializer on that type
|
||||
static void to_json(json& j, move_only_type t) {
|
||||
j = t.i;
|
||||
}
|
||||
};
|
||||
}
|
||||
```
|
||||
|
||||
## Can I write my own serializer? (Advanced use)
|
||||
|
||||
Yes. You might want to take a look at [`unit-udt.cpp`](https://github.com/nlohmann/json/blob/develop/tests/src/unit-udt.cpp) in the test suite, to see a few examples.
|
||||
|
||||
If you write your own serializer, you will need to do a few things:
|
||||
|
||||
- use a different `basic_json` alias than `nlohmann::json` (the last template parameter of `basic_json` is the `JSONSerializer`)
|
||||
- use your `basic_json` alias (or a template parameter) in all your `to_json`/`from_json` methods
|
||||
- use `nlohmann::to_json` and `nlohmann::from_json` when you need ADL
|
||||
|
||||
Here is an example, without simplifications, that only accepts types with a size \<= 32, and uses ADL.
|
||||
|
||||
```
|
||||
// You should use void as a second template argument
|
||||
// if you don't need compile-time checks on T
|
||||
template<typename T, typename SFINAE = typename std::enable_if<sizeof(T) <= 32>::type>
|
||||
struct less_than_32_serializer {
|
||||
template <typename BasicJsonType>
|
||||
static void to_json(BasicJsonType& j, T value) {
|
||||
// we want to use ADL, and call the correct to_json overload
|
||||
using nlohmann::to_json; // this method is called by adl_serializer,
|
||||
// this is where the magic happens
|
||||
to_json(j, value);
|
||||
}
|
||||
|
||||
template <typename BasicJsonType>
|
||||
static void from_json(const BasicJsonType& j, T& value) {
|
||||
// same thing here
|
||||
using nlohmann::from_json;
|
||||
from_json(j, value);
|
||||
}
|
||||
};
|
||||
```
|
||||
|
||||
Be **very** careful when reimplementing your serializer, you can stack overflow if you don't pay attention:
|
||||
|
||||
```
|
||||
template <typename T, void>
|
||||
struct bad_serializer
|
||||
{
|
||||
template <typename BasicJsonType>
|
||||
static void to_json(BasicJsonType& j, const T& value) {
|
||||
// this calls BasicJsonType::json_serializer<T>::to_json(j, value);
|
||||
// if BasicJsonType::json_serializer == bad_serializer ... oops!
|
||||
j = value;
|
||||
}
|
||||
|
||||
template <typename BasicJsonType>
|
||||
static void from_json(const BasicJsonType& j, T& value) {
|
||||
// this calls BasicJsonType::json_serializer<T>::from_json(j, value);
|
||||
// if BasicJsonType::json_serializer == bad_serializer ... oops!
|
||||
value = j.template get<T>(); // oops!
|
||||
}
|
||||
};
|
||||
```
|
||||
@@ -0,0 +1,144 @@
|
||||
# Runtime Assertions
|
||||
|
||||
The code contains numerous debug assertions to ensure class invariants are valid or to detect undefined behavior.
|
||||
Whereas the former class invariants are nothing to be concerned with, the latter checks for undefined behavior are to
|
||||
detect bugs in client code.
|
||||
|
||||
## Switch off runtime assertions
|
||||
|
||||
Runtime assertions can be switched off by defining the preprocessor macro `NDEBUG` (see the
|
||||
[documentation of assert](https://en.cppreference.com/w/cpp/error/assert)) which is the default for release builds.
|
||||
|
||||
## Change assertion behavior
|
||||
|
||||
The behavior of runtime assertions can be changed by defining macro [`JSON_ASSERT(x)`](../api/macros/json_assert.md)
|
||||
before including the `json.hpp` header.
|
||||
|
||||
## Function with runtime assertions
|
||||
|
||||
### Unchecked object access to a const value
|
||||
|
||||
Function [`operator[]`](../api/basic_json/operator%5B%5D.md) implements unchecked access for objects. Whereas a missing
|
||||
key is added in the case of non-const objects, accessing a const object with a missing key is undefined behavior (think
|
||||
of a dereferenced null pointer) and yields a runtime assertion.
|
||||
|
||||
If you are not sure whether an element in an object exists, use checked access with the
|
||||
[`at` function](../api/basic_json/at.md) or call the [`contains` function](../api/basic_json/contains.md) before.
|
||||
|
||||
See also the documentation on [element access](element_access/index.md).
|
||||
|
||||
??? example "Example 1: Missing object key"
|
||||
|
||||
The following code will trigger an assertion at runtime:
|
||||
|
||||
```cpp
|
||||
#include <nlohmann/json.hpp>
|
||||
|
||||
using json = nlohmann::json;
|
||||
|
||||
int main()
|
||||
{
|
||||
const json j = {{"key", "value"}};
|
||||
auto v = j["missing"];
|
||||
}
|
||||
```
|
||||
|
||||
Output:
|
||||
|
||||
```
|
||||
Assertion failed: (m_value.object->find(key) != m_value.object->end()), function operator[], file json.hpp, line 2144.
|
||||
```
|
||||
|
||||
### Constructing from an uninitialized iterator range
|
||||
|
||||
Constructing a JSON value from an iterator range (see [constructor](../api/basic_json/basic_json.md)) with an
|
||||
uninitialized iterator is undefined behavior and yields a runtime assertion.
|
||||
|
||||
??? example "Example 2: Uninitialized iterator range"
|
||||
|
||||
The following code will trigger an assertion at runtime:
|
||||
|
||||
```cpp
|
||||
#include <nlohmann/json.hpp>
|
||||
|
||||
using json = nlohmann::json;
|
||||
|
||||
int main()
|
||||
{
|
||||
json::iterator it1, it2;
|
||||
json j(it1, it2);
|
||||
}
|
||||
```
|
||||
|
||||
Output:
|
||||
|
||||
```
|
||||
Assertion failed: (m_object != nullptr), function operator++, file iter_impl.hpp, line 368.
|
||||
```
|
||||
|
||||
### Operations on uninitialized iterators
|
||||
|
||||
Any operation on uninitialized iterators (i.e., iterators that are not associated with any JSON value) is undefined
|
||||
behavior and yields a runtime assertion.
|
||||
|
||||
??? example "Example 3: Uninitialized iterator"
|
||||
|
||||
The following code will trigger an assertion at runtime:
|
||||
|
||||
```cpp
|
||||
#include <nlohmann/json.hpp>
|
||||
|
||||
using json = nlohmann::json;
|
||||
|
||||
int main()
|
||||
{
|
||||
json::iterator it;
|
||||
++it;
|
||||
}
|
||||
```
|
||||
|
||||
Output:
|
||||
|
||||
```
|
||||
Assertion failed: (m_object != nullptr), function operator++, file iter_impl.hpp, line 368.
|
||||
```
|
||||
|
||||
## Changes
|
||||
|
||||
### Reading from a null `FILE` or `char` pointer
|
||||
|
||||
Reading from a null `#!cpp FILE` or `#!cpp char` pointer in C++ is undefined behavior. Until version 3.12.0, this
|
||||
library asserted that the pointer was not `nullptr` using a runtime assertion. If assertions were disabled, this would
|
||||
result in undefined behavior. Since version 3.12.0, this library checks for `nullptr` and throws a
|
||||
[`parse_error.101`](../home/exceptions.md#jsonexceptionparse_error101) to prevent the undefined behavior.
|
||||
|
||||
??? example "Example 4: Reading from null pointer"
|
||||
|
||||
The following code will trigger an assertion at runtime:
|
||||
|
||||
```cpp
|
||||
#include <iostream>
|
||||
#include <nlohmann/json.hpp>
|
||||
|
||||
using json = nlohmann::json;
|
||||
|
||||
int main()
|
||||
{
|
||||
std::FILE* f = std::fopen("nonexistent_file.json", "r");
|
||||
try {
|
||||
json j = json::parse(f);
|
||||
} catch (std::exception& e) {
|
||||
std::cerr << e.what() << std::endl;
|
||||
}
|
||||
}
|
||||
```
|
||||
|
||||
Output:
|
||||
|
||||
```
|
||||
[json.exception.parse_error.101] parse error: attempting to parse an empty input; check that your input string or stream contains the expected JSON
|
||||
```
|
||||
|
||||
## See also
|
||||
|
||||
- [JSON_ASSERT](../api/macros/json_assert.md) - control behavior of runtime assertions
|
||||
@@ -43,4 +43,4 @@
|
||||
<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="July 4, 2026 09:13:25 UTC">July 4, 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/ >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="July 8, 2026 18:18:24 UTC">July 8, 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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|
||||
@@ -0,0 +1,132 @@
|
||||
# Runtime Assertions
|
||||
|
||||
The code contains numerous debug assertions to ensure class invariants are valid or to detect undefined behavior. Whereas the former class invariants are nothing to be concerned with, the latter checks for undefined behavior are to detect bugs in client code.
|
||||
|
||||
## Switch off runtime assertions
|
||||
|
||||
Runtime assertions can be switched off by defining the preprocessor macro `NDEBUG` (see the [documentation of assert](https://en.cppreference.com/w/cpp/error/assert)) which is the default for release builds.
|
||||
|
||||
## Change assertion behavior
|
||||
|
||||
The behavior of runtime assertions can be changed by defining macro [`JSON_ASSERT(x)`](https://json.nlohmann.me/api/macros/json_assert/index.md) before including the `json.hpp` header.
|
||||
|
||||
## Function with runtime assertions
|
||||
|
||||
### Unchecked object access to a const value
|
||||
|
||||
Function [`operator[]`](https://json.nlohmann.me/api/basic_json/operator%5B%5D/index.md) implements unchecked access for objects. Whereas a missing key is added in the case of non-const objects, accessing a const object with a missing key is undefined behavior (think of a dereferenced null pointer) and yields a runtime assertion.
|
||||
|
||||
If you are not sure whether an element in an object exists, use checked access with the [`at` function](https://json.nlohmann.me/api/basic_json/at/index.md) or call the [`contains` function](https://json.nlohmann.me/api/basic_json/contains/index.md) before.
|
||||
|
||||
See also the documentation on [element access](https://json.nlohmann.me/features/element_access/index.md).
|
||||
|
||||
Example 1: Missing object key
|
||||
|
||||
The following code will trigger an assertion at runtime:
|
||||
|
||||
```
|
||||
#include <nlohmann/json.hpp>
|
||||
|
||||
using json = nlohmann::json;
|
||||
|
||||
int main()
|
||||
{
|
||||
const json j = {{"key", "value"}};
|
||||
auto v = j["missing"];
|
||||
}
|
||||
```
|
||||
|
||||
Output:
|
||||
|
||||
```
|
||||
Assertion failed: (m_value.object->find(key) != m_value.object->end()), function operator[], file json.hpp, line 2144.
|
||||
```
|
||||
|
||||
### Constructing from an uninitialized iterator range
|
||||
|
||||
Constructing a JSON value from an iterator range (see [constructor](https://json.nlohmann.me/api/basic_json/basic_json/index.md)) with an uninitialized iterator is undefined behavior and yields a runtime assertion.
|
||||
|
||||
Example 2: Uninitialized iterator range
|
||||
|
||||
The following code will trigger an assertion at runtime:
|
||||
|
||||
```
|
||||
#include <nlohmann/json.hpp>
|
||||
|
||||
using json = nlohmann::json;
|
||||
|
||||
int main()
|
||||
{
|
||||
json::iterator it1, it2;
|
||||
json j(it1, it2);
|
||||
}
|
||||
```
|
||||
|
||||
Output:
|
||||
|
||||
```
|
||||
Assertion failed: (m_object != nullptr), function operator++, file iter_impl.hpp, line 368.
|
||||
```
|
||||
|
||||
### Operations on uninitialized iterators
|
||||
|
||||
Any operation on uninitialized iterators (i.e., iterators that are not associated with any JSON value) is undefined behavior and yields a runtime assertion.
|
||||
|
||||
Example 3: Uninitialized iterator
|
||||
|
||||
The following code will trigger an assertion at runtime:
|
||||
|
||||
```
|
||||
#include <nlohmann/json.hpp>
|
||||
|
||||
using json = nlohmann::json;
|
||||
|
||||
int main()
|
||||
{
|
||||
json::iterator it;
|
||||
++it;
|
||||
}
|
||||
```
|
||||
|
||||
Output:
|
||||
|
||||
```
|
||||
Assertion failed: (m_object != nullptr), function operator++, file iter_impl.hpp, line 368.
|
||||
```
|
||||
|
||||
## Changes
|
||||
|
||||
### Reading from a null `FILE` or `char` pointer
|
||||
|
||||
Reading from a null `FILE` or `char` pointer in C++ is undefined behavior. Until version 3.12.0, this library asserted that the pointer was not `nullptr` using a runtime assertion. If assertions were disabled, this would result in undefined behavior. Since version 3.12.0, this library checks for `nullptr` and throws a [`parse_error.101`](https://json.nlohmann.me/home/exceptions/#jsonexceptionparse_error101) to prevent the undefined behavior.
|
||||
|
||||
Example 4: Reading from null pointer
|
||||
|
||||
The following code will trigger an assertion at runtime:
|
||||
|
||||
```
|
||||
#include <iostream>
|
||||
#include <nlohmann/json.hpp>
|
||||
|
||||
using json = nlohmann::json;
|
||||
|
||||
int main()
|
||||
{
|
||||
std::FILE* f = std::fopen("nonexistent_file.json", "r");
|
||||
try {
|
||||
json j = json::parse(f);
|
||||
} catch (std::exception& e) {
|
||||
std::cerr << e.what() << std::endl;
|
||||
}
|
||||
}
|
||||
```
|
||||
|
||||
Output:
|
||||
|
||||
```
|
||||
[json.exception.parse_error.101] parse error: attempting to parse an empty input; check that your input string or stream contains the expected JSON
|
||||
```
|
||||
|
||||
## See also
|
||||
|
||||
- [JSON_ASSERT](https://json.nlohmann.me/api/macros/json_assert/index.md) - control behavior of runtime assertions
|
||||
@@ -0,0 +1,52 @@
|
||||
# Binary Formats
|
||||
|
||||
Though JSON is a ubiquitous data format, it is not a very compact format suitable for data exchange, for instance, over
|
||||
a network. Hence, the library supports
|
||||
|
||||
- [BJData](bjdata.md) (Binary JData),
|
||||
- [BSON](bson.md) (Binary JSON),
|
||||
- [CBOR](cbor.md) (Concise Binary Object Representation),
|
||||
- [MessagePack](messagepack.md), and
|
||||
- [UBJSON](ubjson.md) (Universal Binary JSON)
|
||||
|
||||
to efficiently encode JSON values to byte vectors and to decode such vectors.
|
||||
|
||||
## Comparison
|
||||
|
||||
### Completeness
|
||||
|
||||
| Format | Serialization | Deserialization |
|
||||
|-------------|-----------------------------------------------|----------------------------------------------|
|
||||
| BJData | complete | complete |
|
||||
| BSON | incomplete: top-level value must be an object | incomplete, but all JSON types are supported |
|
||||
| CBOR | complete | incomplete, but all JSON types are supported |
|
||||
| MessagePack | complete | complete |
|
||||
| UBJSON | complete | complete |
|
||||
|
||||
### Binary values
|
||||
|
||||
| Format | Binary values | Binary subtypes |
|
||||
|-------------|---------------|-----------------|
|
||||
| BJData | not supported | not supported |
|
||||
| BSON | supported | supported |
|
||||
| CBOR | supported | supported |
|
||||
| MessagePack | supported | supported |
|
||||
| UBJSON | not supported | not supported |
|
||||
|
||||
See [binary values](../binary_values.md) for more information.
|
||||
|
||||
### Sizes
|
||||
|
||||
| Format | canada.json | twitter.json | citm_catalog.json | jeopardy.json |
|
||||
|--------------------|-------------|--------------|-------------------|---------------|
|
||||
| BJData | 53.2 % | 91.1 % | 78.1 % | 96.6 % |
|
||||
| BJData (size) | 58.6 % | 92.1 % | 86.7 % | 97.4 % |
|
||||
| BJData (size+type) | 58.6 % | 92.1 % | 86.5 % | 97.4 % |
|
||||
| BSON | 85.8 % | 95.2 % | 95.8 % | 106.7 % |
|
||||
| CBOR | 50.5 % | 86.3 % | 68.4 % | 88.0 % |
|
||||
| MessagePack | 50.5 % | 86.0 % | 68.5 % | 87.9 % |
|
||||
| UBJSON | 53.2 % | 91.3 % | 78.2 % | 96.6 % |
|
||||
| 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.
|
||||
@@ -0,0 +1,206 @@
|
||||
# BJData
|
||||
|
||||
The [BJData format](https://neurojson.org) was derived from and improved upon
|
||||
[Universal Binary JSON(UBJSON)](https://ubjson.org) specification (Draft 12). Specifically, it introduces an optimized
|
||||
array container for efficient storage of N-dimensional packed arrays (**ND-arrays**); it also adds 5 new type markers -
|
||||
`[u] - uint16`, `[m] - uint32`, `[M] - uint64`, `[h] - float16` and `[B] - byte` - to unambiguously map common binary
|
||||
numeric types; furthermore, it uses little-endian (LE) to store all numerics instead of big-endian (BE) as in UBJSON to
|
||||
avoid unnecessary conversions on commonly available platforms.
|
||||
|
||||
Compared to other binary JSON-like formats such as MessagePack and CBOR, both BJData and UBJSON demonstrate a rare
|
||||
combination of being both binary and **quasi-human-readable**. This is because all semantic elements in BJData and
|
||||
UBJSON, including the data-type markers and name/string types, are directly human-readable. Data stored in the
|
||||
BJData/UBJSON format is not only compact in size, fast to read/write, but also can be directly searched or read using
|
||||
simple processing.
|
||||
|
||||
!!! abstract "References"
|
||||
|
||||
- [BJData Specification](https://neurojson.org/bjdata/draft2)
|
||||
|
||||
## Serialization
|
||||
|
||||
The library uses the following mapping from JSON values types to BJData types according to the BJData specification:
|
||||
|
||||
| JSON value type | value/range | BJData type | marker |
|
||||
|-----------------|-------------------------------------------|----------------|--------|
|
||||
| null | `null` | null | `Z` |
|
||||
| boolean | `true` | true | `T` |
|
||||
| boolean | `false` | false | `F` |
|
||||
| number_integer | -9223372036854775808..-2147483649 | int64 | `L` |
|
||||
| number_integer | -2147483648..-32769 | int32 | `l` |
|
||||
| number_integer | -32768..-129 | int16 | `I` |
|
||||
| number_integer | -128..127 | int8 | `i` |
|
||||
| number_integer | 128..255 | uint8 | `U` |
|
||||
| number_integer | 256..32767 | int16 | `I` |
|
||||
| number_integer | 32768..65535 | uint16 | `u` |
|
||||
| number_integer | 65536..2147483647 | int32 | `l` |
|
||||
| number_integer | 2147483648..4294967295 | uint32 | `m` |
|
||||
| number_integer | 4294967296..9223372036854775807 | int64 | `L` |
|
||||
| number_integer | 9223372036854775808..18446744073709551615 | uint64 | `M` |
|
||||
| number_unsigned | 0..127 | int8 | `i` |
|
||||
| number_unsigned | 128..255 | uint8 | `U` |
|
||||
| number_unsigned | 256..32767 | int16 | `I` |
|
||||
| number_unsigned | 32768..65535 | uint16 | `u` |
|
||||
| number_unsigned | 65536..2147483647 | int32 | `l` |
|
||||
| number_unsigned | 2147483648..4294967295 | uint32 | `m` |
|
||||
| number_unsigned | 4294967296..9223372036854775807 | int64 | `L` |
|
||||
| number_unsigned | 9223372036854775808..18446744073709551615 | uint64 | `M` |
|
||||
| number_float | *any value* | float64 | `D` |
|
||||
| string | *with shortest length indicator* | string | `S` |
|
||||
| array | *see notes on optimized format/ND-array* | array | `[` |
|
||||
| object | *see notes on optimized format* | map | `{` |
|
||||
| binary | *see notes on binary values* | array | `[$B` |
|
||||
|
||||
!!! success "Complete mapping"
|
||||
|
||||
The mapping is **complete** in the sense that any JSON value type can be converted to a BJData value.
|
||||
|
||||
Any BJData output created by `to_bjdata` can be successfully parsed by `from_bjdata`.
|
||||
|
||||
!!! warning "Size constraints"
|
||||
|
||||
The following values can **not** be converted to a BJData value:
|
||||
|
||||
- strings with more than 18446744073709551615 bytes, i.e., $2^{64}-1$ bytes (theoretical)
|
||||
|
||||
!!! info "Unused BJData markers"
|
||||
|
||||
The following markers are not used in the conversion:
|
||||
|
||||
- `Z`: no-op values are not created.
|
||||
- `C`: single-byte strings are serialized with `S` markers.
|
||||
|
||||
!!! info "NaN/infinity handling"
|
||||
|
||||
If NaN or Infinity are stored inside a JSON number, they are serialized properly. This behavior differs from the
|
||||
`dump()` function which serializes NaN or Infinity to `#!json null`.
|
||||
|
||||
!!! info "Endianness"
|
||||
|
||||
A breaking difference between BJData and UBJSON is the endianness of numerical values. In BJData, all numerical data
|
||||
types (integers `UiuImlML` and floating-point values `hdD`) are stored in the little-endian (LE) byte order as
|
||||
opposed to big-endian as used by UBJSON. Adopting LE to store numeric records avoids unnecessary byte swapping on
|
||||
most modern computers where LE is used as the default byte order.
|
||||
|
||||
!!! info "Optimized formats"
|
||||
|
||||
Optimized formats for containers are supported via two parameters of
|
||||
[`to_bjdata`](../../api/basic_json/to_bjdata.md):
|
||||
|
||||
- Parameter `use_size` adds size information to the beginning of a container and removes the closing marker.
|
||||
- Parameter `use_type` further checks whether all elements of a container have the same type and adds the type
|
||||
marker to the beginning of the container. The `use_type` parameter must only be used together with
|
||||
`use_size = true`.
|
||||
|
||||
Note that `use_size = true` alone may result in larger representations - the benefit of this parameter is that the
|
||||
receiving side is immediately informed of the number of elements in the container.
|
||||
|
||||
!!! info "ND-array optimized format"
|
||||
|
||||
BJData extends UBJSON's optimized array **size** marker to support ND-arrays of uniform numerical data types
|
||||
(referred to as *packed arrays*). For example, the 2-D `uint8` integer array `[[1,2],[3,4],[5,6]]`, stored as nested
|
||||
optimized array in UBJSON `[ [$U#i2 1 2 [$U#i2 3 4 [$U#i2 5 6 ]`, can be further compressed in BJData to
|
||||
`[$U#[$i#i2 2 3 1 2 3 4 5 6` or `[$U#[i2 i3] 1 2 3 4 5 6`.
|
||||
|
||||
To maintain type and size information, ND-arrays are converted to JSON objects following the **annotated array
|
||||
format** (defined in the [JData specification (Draft 3)][JDataAAFmt]), when parsed using
|
||||
[`from_bjdata`](../../api/basic_json/from_bjdata.md). For example, the above 2-D `uint8` array can be parsed and
|
||||
accessed as
|
||||
|
||||
```json
|
||||
{
|
||||
"_ArrayType_": "uint8",
|
||||
"_ArraySize_": [2,3],
|
||||
"_ArrayData_": [1,2,3,4,5,6]
|
||||
}
|
||||
```
|
||||
|
||||
Likewise, when a JSON object in the above form is serialized using
|
||||
[`to_bjdata`](../../api/basic_json/to_bjdata.md), it is automatically converted into a compact BJData ND-array. The
|
||||
only exception is, that when the 1-dimensional vector stored in `"_ArraySize_"` contains a single integer or two
|
||||
integers with one being 1, a regular 1-D optimized array is generated.
|
||||
|
||||
The current version of this library does not yet support automatic detection of and conversion from a nested JSON
|
||||
array input to a BJData ND-array.
|
||||
|
||||
[JDataAAFmt]: https://github.com/NeuroJSON/jdata/blob/master/JData_specification.md#annotated-storage-of-n-d-arrays
|
||||
|
||||
!!! info "Restrictions in optimized data types for arrays and objects"
|
||||
|
||||
Due to diminished space saving, hampered readability, and increased security risks, in BJData, the allowed data
|
||||
types following the `$` marker in an optimized array and object container are restricted to
|
||||
**non-zero-fixed-length** data types. Therefore, the valid optimized type markers can only be one of
|
||||
`UiuImlMLhdDCB`. This also means other variable (`[{SH`) or zero-length types (`TFN`) can not be used in an
|
||||
optimized array or object in BJData.
|
||||
|
||||
!!! info "Binary values"
|
||||
|
||||
BJData provides a dedicated `B` marker (defined in the [BJData specification (Draft 3)][BJDataBinArr]) that is used
|
||||
in optimized arrays to designate binary data. This means that, unlike UBJSON, binary data can be both serialized and
|
||||
deserialized.
|
||||
|
||||
To preserve compatibility with BJData Draft 2, the Draft 3 optimized binary array must be explicitly enabled using
|
||||
the `version` parameter of [`to_bjdata`](../../api/basic_json/to_bjdata.md).
|
||||
|
||||
In Draft2 mode (default), if the JSON data contains the binary type, the value stored as a list of integers, as
|
||||
suggested by the BJData documentation. In particular, this means that the serialization and the deserialization of
|
||||
JSON containing binary values into BJData and back will result in a different JSON object.
|
||||
|
||||
[BJDataBinArr]: https://github.com/NeuroJSON/bjdata/blob/master/Binary_JData_Specification.md#optimized-binary-array
|
||||
|
||||
??? example
|
||||
|
||||
```cpp
|
||||
--8<-- "examples/to_bjdata.cpp"
|
||||
```
|
||||
|
||||
Output:
|
||||
|
||||
```c
|
||||
--8<-- "examples/to_bjdata.output"
|
||||
```
|
||||
|
||||
## Deserialization
|
||||
|
||||
The library maps BJData types to JSON value types as follows:
|
||||
|
||||
| BJData type | JSON value type | marker |
|
||||
|-------------|------------------------------------------|----------|
|
||||
| no-op | *no value, next value is read* | `N` |
|
||||
| null | `null` | `Z` |
|
||||
| false | `false` | `F` |
|
||||
| true | `true` | `T` |
|
||||
| float16 | number_float | `h` |
|
||||
| float32 | number_float | `d` |
|
||||
| float64 | number_float | `D` |
|
||||
| uint8 | number_unsigned | `U` |
|
||||
| int8 | number_integer | `i` |
|
||||
| uint16 | number_unsigned | `u` |
|
||||
| int16 | number_integer | `I` |
|
||||
| uint32 | number_unsigned | `m` |
|
||||
| int32 | number_integer | `l` |
|
||||
| uint64 | number_unsigned | `M` |
|
||||
| int64 | number_integer | `L` |
|
||||
| byte | number_unsigned | `B` |
|
||||
| string | string | `S` |
|
||||
| char | string | `C` |
|
||||
| array | array (optimized values are supported) | `[` |
|
||||
| ND-array | object (in JData annotated array format) | `[$.#[.` |
|
||||
| object | object (optimized values are supported) | `{` |
|
||||
| binary | binary (strongly-typed byte array) | `[$B` |
|
||||
|
||||
!!! success "Complete mapping"
|
||||
|
||||
The mapping is **complete** in the sense that any BJData value can be converted to a JSON value.
|
||||
|
||||
??? example
|
||||
|
||||
```cpp
|
||||
--8<-- "examples/from_bjdata.cpp"
|
||||
```
|
||||
|
||||
Output:
|
||||
|
||||
```json
|
||||
--8<-- "examples/from_bjdata.output"
|
||||
```
|
||||
@@ -95,4 +95,4 @@
|
||||
<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="July 4, 2026 09:13:25 UTC">July 4, 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> <!-- 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="July 8, 2026 18:18:24 UTC">July 8, 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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|
||||
@@ -0,0 +1,254 @@
|
||||
# BJData
|
||||
|
||||
The [BJData format](https://neurojson.org) was derived from and improved upon [Universal Binary JSON(UBJSON)](https://ubjson.org) specification (Draft 12). Specifically, it introduces an optimized array container for efficient storage of N-dimensional packed arrays (**ND-arrays**); it also adds 5 new type markers - `[u] - uint16`, `[m] - uint32`, `[M] - uint64`, `[h] - float16` and `[B] - byte` - to unambiguously map common binary numeric types; furthermore, it uses little-endian (LE) to store all numerics instead of big-endian (BE) as in UBJSON to avoid unnecessary conversions on commonly available platforms.
|
||||
|
||||
Compared to other binary JSON-like formats such as MessagePack and CBOR, both BJData and UBJSON demonstrate a rare combination of being both binary and **quasi-human-readable**. This is because all semantic elements in BJData and UBJSON, including the data-type markers and name/string types, are directly human-readable. Data stored in the BJData/UBJSON format is not only compact in size, fast to read/write, but also can be directly searched or read using simple processing.
|
||||
|
||||
References
|
||||
|
||||
- [BJData Specification](https://neurojson.org/bjdata/draft2)
|
||||
|
||||
## Serialization
|
||||
|
||||
The library uses the following mapping from JSON values types to BJData types according to the BJData specification:
|
||||
|
||||
| JSON value type | value/range | BJData type | marker |
|
||||
| --------------- | ----------------------------------------- | ----------- | ------ |
|
||||
| null | `null` | null | `Z` |
|
||||
| boolean | `true` | true | `T` |
|
||||
| boolean | `false` | false | `F` |
|
||||
| number_integer | -9223372036854775808..-2147483649 | int64 | `L` |
|
||||
| number_integer | -2147483648..-32769 | int32 | `l` |
|
||||
| number_integer | -32768..-129 | int16 | `I` |
|
||||
| number_integer | -128..127 | int8 | `i` |
|
||||
| number_integer | 128..255 | uint8 | `U` |
|
||||
| number_integer | 256..32767 | int16 | `I` |
|
||||
| number_integer | 32768..65535 | uint16 | `u` |
|
||||
| number_integer | 65536..2147483647 | int32 | `l` |
|
||||
| number_integer | 2147483648..4294967295 | uint32 | `m` |
|
||||
| number_integer | 4294967296..9223372036854775807 | int64 | `L` |
|
||||
| number_integer | 9223372036854775808..18446744073709551615 | uint64 | `M` |
|
||||
| number_unsigned | 0..127 | int8 | `i` |
|
||||
| number_unsigned | 128..255 | uint8 | `U` |
|
||||
| number_unsigned | 256..32767 | int16 | `I` |
|
||||
| number_unsigned | 32768..65535 | uint16 | `u` |
|
||||
| number_unsigned | 65536..2147483647 | int32 | `l` |
|
||||
| number_unsigned | 2147483648..4294967295 | uint32 | `m` |
|
||||
| number_unsigned | 4294967296..9223372036854775807 | int64 | `L` |
|
||||
| number_unsigned | 9223372036854775808..18446744073709551615 | uint64 | `M` |
|
||||
| number_float | *any value* | float64 | `D` |
|
||||
| string | *with shortest length indicator* | string | `S` |
|
||||
| array | *see notes on optimized format/ND-array* | array | `[` |
|
||||
| object | *see notes on optimized format* | map | `{` |
|
||||
| binary | *see notes on binary values* | array | `[$B` |
|
||||
|
||||
Complete mapping
|
||||
|
||||
The mapping is **complete** in the sense that any JSON value type can be converted to a BJData value.
|
||||
|
||||
Any BJData output created by `to_bjdata` can be successfully parsed by `from_bjdata`.
|
||||
|
||||
Size constraints
|
||||
|
||||
The following values can **not** be converted to a BJData value:
|
||||
|
||||
- strings with more than 18446744073709551615 bytes, i.e., 2^{64}-1 bytes (theoretical)
|
||||
|
||||
Unused BJData markers
|
||||
|
||||
The following markers are not used in the conversion:
|
||||
|
||||
- `Z`: no-op values are not created.
|
||||
- `C`: single-byte strings are serialized with `S` markers.
|
||||
|
||||
NaN/infinity handling
|
||||
|
||||
If NaN or Infinity are stored inside a JSON number, they are serialized properly. This behavior differs from the `dump()` function which serializes NaN or Infinity to `null`.
|
||||
|
||||
Endianness
|
||||
|
||||
A breaking difference between BJData and UBJSON is the endianness of numerical values. In BJData, all numerical data types (integers `UiuImlML` and floating-point values `hdD`) are stored in the little-endian (LE) byte order as opposed to big-endian as used by UBJSON. Adopting LE to store numeric records avoids unnecessary byte swapping on most modern computers where LE is used as the default byte order.
|
||||
|
||||
Optimized formats
|
||||
|
||||
Optimized formats for containers are supported via two parameters of [`to_bjdata`](https://json.nlohmann.me/api/basic_json/to_bjdata/index.md):
|
||||
|
||||
- Parameter `use_size` adds size information to the beginning of a container and removes the closing marker.
|
||||
- Parameter `use_type` further checks whether all elements of a container have the same type and adds the type marker to the beginning of the container. The `use_type` parameter must only be used together with `use_size = true`.
|
||||
|
||||
Note that `use_size = true` alone may result in larger representations - the benefit of this parameter is that the receiving side is immediately informed of the number of elements in the container.
|
||||
|
||||
ND-array optimized format
|
||||
|
||||
BJData extends UBJSON's optimized array **size** marker to support ND-arrays of uniform numerical data types (referred to as *packed arrays*). For example, the 2-D `uint8` integer array `[[1,2],[3,4],[5,6]]`, stored as nested optimized array in UBJSON `[ [$U#i2 1 2 [$U#i2 3 4 [$U#i2 5 6 ]`, can be further compressed in BJData to `[$U#[$i#i2 2 3 1 2 3 4 5 6` or `[$U#[i2 i3] 1 2 3 4 5 6`.
|
||||
|
||||
To maintain type and size information, ND-arrays are converted to JSON objects following the **annotated array format** (defined in the [JData specification (Draft 3)](https://github.com/NeuroJSON/jdata/blob/master/JData_specification.md#annotated-storage-of-n-d-arrays)), when parsed using [`from_bjdata`](https://json.nlohmann.me/api/basic_json/from_bjdata/index.md). For example, the above 2-D `uint8` array can be parsed and accessed as
|
||||
|
||||
```
|
||||
{
|
||||
"_ArrayType_": "uint8",
|
||||
"_ArraySize_": [2,3],
|
||||
"_ArrayData_": [1,2,3,4,5,6]
|
||||
}
|
||||
```
|
||||
|
||||
Likewise, when a JSON object in the above form is serialized using [`to_bjdata`](https://json.nlohmann.me/api/basic_json/to_bjdata/index.md), it is automatically converted into a compact BJData ND-array. The only exception is, that when the 1-dimensional vector stored in `"_ArraySize_"` contains a single integer or two integers with one being 1, a regular 1-D optimized array is generated.
|
||||
|
||||
The current version of this library does not yet support automatic detection of and conversion from a nested JSON array input to a BJData ND-array.
|
||||
|
||||
Restrictions in optimized data types for arrays and objects
|
||||
|
||||
Due to diminished space saving, hampered readability, and increased security risks, in BJData, the allowed data types following the `$` marker in an optimized array and object container are restricted to **non-zero-fixed-length** data types. Therefore, the valid optimized type markers can only be one of `UiuImlMLhdDCB`. This also means other variable (`[{SH`) or zero-length types (`TFN`) can not be used in an optimized array or object in BJData.
|
||||
|
||||
Binary values
|
||||
|
||||
BJData provides a dedicated `B` marker (defined in the [BJData specification (Draft 3)](https://github.com/NeuroJSON/bjdata/blob/master/Binary_JData_Specification.md#optimized-binary-array)) that is used in optimized arrays to designate binary data. This means that, unlike UBJSON, binary data can be both serialized and deserialized.
|
||||
|
||||
To preserve compatibility with BJData Draft 2, the Draft 3 optimized binary array must be explicitly enabled using the `version` parameter of [`to_bjdata`](https://json.nlohmann.me/api/basic_json/to_bjdata/index.md).
|
||||
|
||||
In Draft2 mode (default), if the JSON data contains the binary type, the value stored as a list of integers, as suggested by the BJData documentation. In particular, this means that the serialization and the deserialization of JSON containing binary values into BJData and back will result in a different JSON object.
|
||||
|
||||
Example
|
||||
|
||||
```
|
||||
#include <iostream>
|
||||
#include <iomanip>
|
||||
#include <nlohmann/json.hpp>
|
||||
|
||||
using json = nlohmann::json;
|
||||
using namespace nlohmann::literals;
|
||||
|
||||
// function to print BJData's diagnostic format
|
||||
void print_byte(uint8_t byte)
|
||||
{
|
||||
if (32 < byte and byte < 128)
|
||||
{
|
||||
std::cout << (char)byte;
|
||||
}
|
||||
else
|
||||
{
|
||||
std::cout << (int)byte;
|
||||
}
|
||||
}
|
||||
|
||||
int main()
|
||||
{
|
||||
// create a JSON value
|
||||
json j = R"({"compact": true, "schema": false})"_json;
|
||||
|
||||
// serialize it to BJData
|
||||
std::vector<std::uint8_t> v = json::to_bjdata(j);
|
||||
|
||||
// print the vector content
|
||||
for (auto& byte : v)
|
||||
{
|
||||
print_byte(byte);
|
||||
}
|
||||
std::cout << std::endl;
|
||||
|
||||
// create an array of numbers
|
||||
json array = {1, 2, 3, 4, 5, 6, 7, 8};
|
||||
|
||||
// serialize it to BJData using default representation
|
||||
std::vector<std::uint8_t> v_array = json::to_bjdata(array);
|
||||
// serialize it to BJData using size optimization
|
||||
std::vector<std::uint8_t> v_array_size = json::to_bjdata(array, true);
|
||||
// serialize it to BJData using type optimization
|
||||
std::vector<std::uint8_t> v_array_size_and_type = json::to_bjdata(array, true, true);
|
||||
|
||||
// print the vector contents
|
||||
for (auto& byte : v_array)
|
||||
{
|
||||
print_byte(byte);
|
||||
}
|
||||
std::cout << std::endl;
|
||||
|
||||
for (auto& byte : v_array_size)
|
||||
{
|
||||
print_byte(byte);
|
||||
}
|
||||
std::cout << std::endl;
|
||||
|
||||
for (auto& byte : v_array_size_and_type)
|
||||
{
|
||||
print_byte(byte);
|
||||
}
|
||||
std::cout << std::endl;
|
||||
}
|
||||
```
|
||||
|
||||
Output:
|
||||
|
||||
```
|
||||
{i7compactTi6schemaF}
|
||||
[i1i2i3i4i5i6i7i8]
|
||||
[#i8i1i2i3i4i5i6i7i8
|
||||
[$i#i812345678
|
||||
```
|
||||
|
||||
## Deserialization
|
||||
|
||||
The library maps BJData types to JSON value types as follows:
|
||||
|
||||
| BJData type | JSON value type | marker |
|
||||
| ----------- | ---------------------------------------- | -------- |
|
||||
| no-op | *no value, next value is read* | `N` |
|
||||
| null | `null` | `Z` |
|
||||
| false | `false` | `F` |
|
||||
| true | `true` | `T` |
|
||||
| float16 | number_float | `h` |
|
||||
| float32 | number_float | `d` |
|
||||
| float64 | number_float | `D` |
|
||||
| uint8 | number_unsigned | `U` |
|
||||
| int8 | number_integer | `i` |
|
||||
| uint16 | number_unsigned | `u` |
|
||||
| int16 | number_integer | `I` |
|
||||
| uint32 | number_unsigned | `m` |
|
||||
| int32 | number_integer | `l` |
|
||||
| uint64 | number_unsigned | `M` |
|
||||
| int64 | number_integer | `L` |
|
||||
| byte | number_unsigned | `B` |
|
||||
| string | string | `S` |
|
||||
| char | string | `C` |
|
||||
| array | array (optimized values are supported) | `[` |
|
||||
| ND-array | object (in JData annotated array format) | `[$.#[.` |
|
||||
| object | object (optimized values are supported) | `{` |
|
||||
| binary | binary (strongly-typed byte array) | `[$B` |
|
||||
|
||||
Complete mapping
|
||||
|
||||
The mapping is **complete** in the sense that any BJData value can be converted to a JSON value.
|
||||
|
||||
Example
|
||||
|
||||
```
|
||||
#include <iostream>
|
||||
#include <iomanip>
|
||||
#include <nlohmann/json.hpp>
|
||||
|
||||
using json = nlohmann::json;
|
||||
|
||||
int main()
|
||||
{
|
||||
// create byte vector
|
||||
std::vector<std::uint8_t> v = {0x7B, 0x69, 0x07, 0x63, 0x6F, 0x6D, 0x70, 0x61,
|
||||
0x63, 0x74, 0x54, 0x69, 0x06, 0x73, 0x63, 0x68,
|
||||
0x65, 0x6D, 0x61, 0x69, 0x00, 0x7D
|
||||
};
|
||||
|
||||
// deserialize it with BJData
|
||||
json j = json::from_bjdata(v);
|
||||
|
||||
// print the deserialized JSON value
|
||||
std::cout << std::setw(2) << j << std::endl;
|
||||
}
|
||||
```
|
||||
|
||||
Output:
|
||||
|
||||
```
|
||||
{
|
||||
"compact": true,
|
||||
"schema": 0
|
||||
}
|
||||
```
|
||||
@@ -0,0 +1,98 @@
|
||||
# BSON
|
||||
|
||||
BSON, short for Binary JSON, is a binary-encoded serialization of JSON-like documents. Like JSON, BSON supports the
|
||||
embedding of documents and arrays within other documents and arrays. BSON also contains extensions that allow
|
||||
representation of data types that are not part of the JSON spec. For example, BSON has a Date type and a BinData type.
|
||||
|
||||
!!! abstract "References"
|
||||
|
||||
- [BSON Website](http://bsonspec.org) - the main source on BSON
|
||||
- [BSON Specification](http://bsonspec.org/spec.html) - the specification
|
||||
|
||||
|
||||
## Serialization
|
||||
|
||||
The library uses the following mapping from JSON values types to BSON types:
|
||||
|
||||
| JSON value type | value/range | BSON type | marker |
|
||||
|-----------------|-------------------------------------------|-----------|--------|
|
||||
| null | `null` | null | 0x0A |
|
||||
| boolean | `true`, `false` | boolean | 0x08 |
|
||||
| number_integer | -9223372036854775808..-2147483649 | int64 | 0x12 |
|
||||
| number_integer | -2147483648..2147483647 | int32 | 0x10 |
|
||||
| number_integer | 2147483648..9223372036854775807 | int64 | 0x12 |
|
||||
| number_unsigned | 0..2147483647 | int32 | 0x10 |
|
||||
| number_unsigned | 2147483648..9223372036854775807 | int64 | 0x12 |
|
||||
| number_unsigned | 9223372036854775808..18446744073709551615 | uint64 | 0x11 |
|
||||
| number_float | *any value* | double | 0x01 |
|
||||
| string | *any value* | string | 0x02 |
|
||||
| array | *any value* | document | 0x04 |
|
||||
| object | *any value* | document | 0x03 |
|
||||
| binary | *any value* | binary | 0x05 |
|
||||
|
||||
!!! warning "Incomplete mapping"
|
||||
|
||||
The mapping is **incomplete**, since only JSON-objects (and things contained therein) can be serialized to BSON.
|
||||
Also, keys may not contain U+0000, since they are serialized a zero-terminated c-strings.
|
||||
|
||||
??? example
|
||||
|
||||
```cpp
|
||||
--8<-- "examples/to_bson.cpp"
|
||||
```
|
||||
|
||||
Output:
|
||||
|
||||
```c
|
||||
--8<-- "examples/to_bson.output"
|
||||
```
|
||||
|
||||
|
||||
## Deserialization
|
||||
|
||||
The library maps BSON record types to JSON value types as follows:
|
||||
|
||||
| BSON type | BSON marker byte | JSON value type |
|
||||
|--------------------------|------------------|-----------------|
|
||||
| double | 0x01 | number_float |
|
||||
| string | 0x02 | string |
|
||||
| document | 0x03 | object |
|
||||
| array | 0x04 | array |
|
||||
| binary | 0x05 | binary |
|
||||
| undefined | 0x06 | *unsupported* |
|
||||
| ObjectId | 0x07 | *unsupported* |
|
||||
| boolean | 0x08 | boolean |
|
||||
| UTC Date-Time | 0x09 | *unsupported* |
|
||||
| null | 0x0A | null |
|
||||
| Regular Expr. | 0x0B | *unsupported* |
|
||||
| DB Pointer | 0x0C | *unsupported* |
|
||||
| JavaScript Code | 0x0D | *unsupported* |
|
||||
| Symbol | 0x0E | *unsupported* |
|
||||
| JavaScript Code w/ scope | 0x0F | *unsupported* |
|
||||
| int32 | 0x10 | number_integer |
|
||||
| uint64(Timestamp) | 0x11 | number_unsigned |
|
||||
| int64 | 0x12 | number_integer |
|
||||
| 128-bit decimal float | 0x13 | *unsupported* |
|
||||
| Max Key | 0x7F | *unsupported* |
|
||||
| Min Key | 0xFF | *unsupported* |
|
||||
|
||||
!!! warning "Incomplete mapping"
|
||||
|
||||
The mapping is **incomplete**. The unsupported mappings are indicated in the table above.
|
||||
|
||||
!!! note "Handling of BSON type 0x11"
|
||||
|
||||
BSON type 0x11 is used to represent uint64 numbers. This library treats these values purely as uint64 numbers
|
||||
and does not parse them into date-related formats.
|
||||
|
||||
??? example
|
||||
|
||||
```cpp
|
||||
--8<-- "examples/from_bson.cpp"
|
||||
```
|
||||
|
||||
Output:
|
||||
|
||||
```json
|
||||
--8<-- "examples/from_bson.output"
|
||||
```
|
||||
@@ -46,4 +46,4 @@
|
||||
<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="July 4, 2026 09:13:25 UTC">July 4, 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> <!-- 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="July 8, 2026 18:18:24 UTC">July 8, 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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|
||||
@@ -0,0 +1,136 @@
|
||||
# BSON
|
||||
|
||||
BSON, short for Binary JSON, is a binary-encoded serialization of JSON-like documents. Like JSON, BSON supports the embedding of documents and arrays within other documents and arrays. BSON also contains extensions that allow representation of data types that are not part of the JSON spec. For example, BSON has a Date type and a BinData type.
|
||||
|
||||
References
|
||||
|
||||
- [BSON Website](http://bsonspec.org) - the main source on BSON
|
||||
- [BSON Specification](http://bsonspec.org/spec.html) - the specification
|
||||
|
||||
## Serialization
|
||||
|
||||
The library uses the following mapping from JSON values types to BSON types:
|
||||
|
||||
| JSON value type | value/range | BSON type | marker |
|
||||
| --------------- | ----------------------------------------- | --------- | ------ |
|
||||
| null | `null` | null | 0x0A |
|
||||
| boolean | `true`, `false` | boolean | 0x08 |
|
||||
| number_integer | -9223372036854775808..-2147483649 | int64 | 0x12 |
|
||||
| number_integer | -2147483648..2147483647 | int32 | 0x10 |
|
||||
| number_integer | 2147483648..9223372036854775807 | int64 | 0x12 |
|
||||
| number_unsigned | 0..2147483647 | int32 | 0x10 |
|
||||
| number_unsigned | 2147483648..9223372036854775807 | int64 | 0x12 |
|
||||
| number_unsigned | 9223372036854775808..18446744073709551615 | uint64 | 0x11 |
|
||||
| number_float | *any value* | double | 0x01 |
|
||||
| string | *any value* | string | 0x02 |
|
||||
| array | *any value* | document | 0x04 |
|
||||
| object | *any value* | document | 0x03 |
|
||||
| binary | *any value* | binary | 0x05 |
|
||||
|
||||
Incomplete mapping
|
||||
|
||||
The mapping is **incomplete**, since only JSON-objects (and things contained therein) can be serialized to BSON. Also, keys may not contain U+0000, since they are serialized a zero-terminated c-strings.
|
||||
|
||||
Example
|
||||
|
||||
```
|
||||
#include <iostream>
|
||||
#include <iomanip>
|
||||
#include <nlohmann/json.hpp>
|
||||
|
||||
using json = nlohmann::json;
|
||||
using namespace nlohmann::literals;
|
||||
|
||||
int main()
|
||||
{
|
||||
// create a JSON value
|
||||
json j = R"({"compact": true, "schema": 0})"_json;
|
||||
|
||||
// serialize it to BSON
|
||||
std::vector<std::uint8_t> v = json::to_bson(j);
|
||||
|
||||
// print the vector content
|
||||
for (auto& byte : v)
|
||||
{
|
||||
std::cout << "0x" << std::hex << std::setw(2) << std::setfill('0') << (int)byte << " ";
|
||||
}
|
||||
std::cout << std::endl;
|
||||
}
|
||||
```
|
||||
|
||||
Output:
|
||||
|
||||
```
|
||||
0x1b 0x00 0x00 0x00 0x08 0x63 0x6f 0x6d 0x70 0x61 0x63 0x74 0x00 0x01 0x10 0x73 0x63 0x68 0x65 0x6d 0x61 0x00 0x00 0x00 0x00 0x00 0x00
|
||||
```
|
||||
|
||||
## Deserialization
|
||||
|
||||
The library maps BSON record types to JSON value types as follows:
|
||||
|
||||
| BSON type | BSON marker byte | JSON value type |
|
||||
| ------------------------ | ---------------- | --------------- |
|
||||
| double | 0x01 | number_float |
|
||||
| string | 0x02 | string |
|
||||
| document | 0x03 | object |
|
||||
| array | 0x04 | array |
|
||||
| binary | 0x05 | binary |
|
||||
| undefined | 0x06 | *unsupported* |
|
||||
| ObjectId | 0x07 | *unsupported* |
|
||||
| boolean | 0x08 | boolean |
|
||||
| UTC Date-Time | 0x09 | *unsupported* |
|
||||
| null | 0x0A | null |
|
||||
| Regular Expr. | 0x0B | *unsupported* |
|
||||
| DB Pointer | 0x0C | *unsupported* |
|
||||
| JavaScript Code | 0x0D | *unsupported* |
|
||||
| Symbol | 0x0E | *unsupported* |
|
||||
| JavaScript Code w/ scope | 0x0F | *unsupported* |
|
||||
| int32 | 0x10 | number_integer |
|
||||
| uint64(Timestamp) | 0x11 | number_unsigned |
|
||||
| int64 | 0x12 | number_integer |
|
||||
| 128-bit decimal float | 0x13 | *unsupported* |
|
||||
| Max Key | 0x7F | *unsupported* |
|
||||
| Min Key | 0xFF | *unsupported* |
|
||||
|
||||
Incomplete mapping
|
||||
|
||||
The mapping is **incomplete**. The unsupported mappings are indicated in the table above.
|
||||
|
||||
Handling of BSON type 0x11
|
||||
|
||||
BSON type 0x11 is used to represent uint64 numbers. This library treats these values purely as uint64 numbers and does not parse them into date-related formats.
|
||||
|
||||
Example
|
||||
|
||||
```
|
||||
#include <iostream>
|
||||
#include <iomanip>
|
||||
#include <nlohmann/json.hpp>
|
||||
|
||||
using json = nlohmann::json;
|
||||
|
||||
int main()
|
||||
{
|
||||
// create byte vector
|
||||
std::vector<std::uint8_t> v = {0x1b, 0x00, 0x00, 0x00, 0x08, 0x63, 0x6f, 0x6d,
|
||||
0x70, 0x61, 0x63, 0x74, 0x00, 0x01, 0x10, 0x73,
|
||||
0x63, 0x68, 0x65, 0x6d, 0x61, 0x00, 0x00, 0x00,
|
||||
0x00, 0x00, 0x00
|
||||
};
|
||||
|
||||
// deserialize it with BSON
|
||||
json j = json::from_bson(v);
|
||||
|
||||
// print the deserialized JSON value
|
||||
std::cout << std::setw(2) << j << std::endl;
|
||||
}
|
||||
```
|
||||
|
||||
Output:
|
||||
|
||||
```
|
||||
{
|
||||
"compact": true,
|
||||
"schema": 0
|
||||
}
|
||||
```
|
||||
@@ -0,0 +1,181 @@
|
||||
# CBOR
|
||||
|
||||
The Concise Binary Object Representation (CBOR) is a data format whose design goals include the possibility of
|
||||
extremely small code sizes, fairly small message size, and extensibility without the need for version negotiation.
|
||||
|
||||
!!! abstract "References"
|
||||
|
||||
- [CBOR Website](http://cbor.io) - the main source on CBOR
|
||||
- [CBOR Playground](http://cbor.me) - an interactive webpage to translate between JSON and CBOR
|
||||
- [RFC 7049](https://tools.ietf.org/html/rfc7049) - the CBOR specification
|
||||
|
||||
## Serialization
|
||||
|
||||
The library uses the following mapping from JSON values types to CBOR types according to the CBOR specification
|
||||
([RFC 7049](https://www.rfc-editor.org/rfc/rfc7049.html)):
|
||||
|
||||
| JSON value type | value/range | CBOR type | first byte |
|
||||
|-----------------|--------------------------------------------|-----------------------------------|------------|
|
||||
| null | `null` | Null | 0xF6 |
|
||||
| boolean | `true` | True | 0xF5 |
|
||||
| boolean | `false` | False | 0xF4 |
|
||||
| number_integer | -9223372036854775808..-2147483649 | Negative integer (8 bytes follow) | 0x3B |
|
||||
| number_integer | -2147483648..-32769 | Negative integer (4 bytes follow) | 0x3A |
|
||||
| number_integer | -32768..-129 | Negative integer (2 bytes follow) | 0x39 |
|
||||
| number_integer | -128..-25 | Negative integer (1 byte follow) | 0x38 |
|
||||
| number_integer | -24..-1 | Negative integer | 0x20..0x37 |
|
||||
| number_integer | 0..23 | Integer | 0x00..0x17 |
|
||||
| number_integer | 24..255 | Unsigned integer (1 byte follow) | 0x18 |
|
||||
| number_integer | 256..65535 | Unsigned integer (2 bytes follow) | 0x19 |
|
||||
| number_integer | 65536..4294967295 | Unsigned integer (4 bytes follow) | 0x1A |
|
||||
| number_integer | 4294967296..18446744073709551615 | Unsigned integer (8 bytes follow) | 0x1B |
|
||||
| number_unsigned | 0..23 | Integer | 0x00..0x17 |
|
||||
| number_unsigned | 24..255 | Unsigned integer (1 byte follow) | 0x18 |
|
||||
| number_unsigned | 256..65535 | Unsigned integer (2 bytes follow) | 0x19 |
|
||||
| number_unsigned | 65536..4294967295 | Unsigned integer (4 bytes follow) | 0x1A |
|
||||
| number_unsigned | 4294967296..18446744073709551615 | Unsigned integer (8 bytes follow) | 0x1B |
|
||||
| number_float | *any value representable by a float* | Single-Precision Float | 0xFA |
|
||||
| number_float | *any value NOT representable by a float* | Double-Precision Float | 0xFB |
|
||||
| string | *length*: 0..23 | UTF-8 string | 0x60..0x77 |
|
||||
| string | *length*: 24..255 | UTF-8 string (1 byte follow) | 0x78 |
|
||||
| string | *length*: 256..65535 | UTF-8 string (2 bytes follow) | 0x79 |
|
||||
| string | *length*: 65536..4294967295 | UTF-8 string (4 bytes follow) | 0x7A |
|
||||
| string | *length*: 4294967296..18446744073709551615 | UTF-8 string (8 bytes follow) | 0x7B |
|
||||
| array | *size*: 0..23 | array | 0x80..0x97 |
|
||||
| array | *size*: 24..255 | array (1 byte follow) | 0x98 |
|
||||
| array | *size*: 256..65535 | array (2 bytes follow) | 0x99 |
|
||||
| array | *size*: 65536..4294967295 | array (4 bytes follow) | 0x9A |
|
||||
| array | *size*: 4294967296..18446744073709551615 | array (8 bytes follow) | 0x9B |
|
||||
| object | *size*: 0..23 | map | 0xA0..0xB7 |
|
||||
| object | *size*: 24..255 | map (1 byte follow) | 0xB8 |
|
||||
| object | *size*: 256..65535 | map (2 bytes follow) | 0xB9 |
|
||||
| object | *size*: 65536..4294967295 | map (4 bytes follow) | 0xBA |
|
||||
| object | *size*: 4294967296..18446744073709551615 | map (8 bytes follow) | 0xBB |
|
||||
| binary | *size*: 0..23 | byte string | 0x40..0x57 |
|
||||
| binary | *size*: 24..255 | byte string (1 byte follow) | 0x58 |
|
||||
| binary | *size*: 256..65535 | byte string (2 bytes follow) | 0x59 |
|
||||
| binary | *size*: 65536..4294967295 | byte string (4 bytes follow) | 0x5A |
|
||||
| binary | *size*: 4294967296..18446744073709551615 | byte string (8 bytes follow) | 0x5B |
|
||||
|
||||
Binary values with subtype are mapped to tagged values (0xD8..0xDB) depending on the subtype, followed by a byte string,
|
||||
see "binary" cells in the table above.
|
||||
|
||||
!!! success "Complete mapping"
|
||||
|
||||
The mapping is **complete** in the sense that any JSON value type can be converted to a CBOR value.
|
||||
|
||||
!!! info "NaN/infinity handling"
|
||||
|
||||
If NaN or Infinity are stored inside a JSON number, they are serialized properly. This behavior differs from the normal JSON serialization which serializes NaN or Infinity to `null`.
|
||||
|
||||
!!! info "Unused CBOR types"
|
||||
|
||||
The following CBOR types are not used in the conversion:
|
||||
|
||||
- UTF-8 strings terminated by "break" (0x7F)
|
||||
- arrays terminated by "break" (0x9F)
|
||||
- maps terminated by "break" (0xBF)
|
||||
- byte strings terminated by "break" (0x5F)
|
||||
- date/time (0xC0..0xC1)
|
||||
- bignum (0xC2..0xC3)
|
||||
- decimal fraction (0xC4)
|
||||
- bigfloat (0xC5)
|
||||
- expected conversions (0xD5..0xD7)
|
||||
- simple values (0xE0..0xF3, 0xF8)
|
||||
- undefined (0xF7)
|
||||
- half-precision floats (0xF9)
|
||||
- break (0xFF)
|
||||
|
||||
!!! info "Tagged items"
|
||||
|
||||
Binary subtypes will be serialized as tagged items. See [binary values](../binary_values.md#cbor) for an example.
|
||||
|
||||
??? example
|
||||
|
||||
```cpp
|
||||
--8<-- "examples/to_cbor.cpp"
|
||||
```
|
||||
|
||||
Output:
|
||||
|
||||
```c
|
||||
--8<-- "examples/to_cbor.output"
|
||||
```
|
||||
|
||||
## Deserialization
|
||||
|
||||
The library maps CBOR types to JSON value types as follows:
|
||||
|
||||
| CBOR type | JSON value type | first byte |
|
||||
|------------------------|-----------------|------------|
|
||||
| Integer | number_unsigned | 0x00..0x17 |
|
||||
| Unsigned integer | number_unsigned | 0x18 |
|
||||
| Unsigned integer | number_unsigned | 0x19 |
|
||||
| Unsigned integer | number_unsigned | 0x1A |
|
||||
| Unsigned integer | number_unsigned | 0x1B |
|
||||
| Negative integer | number_integer | 0x20..0x37 |
|
||||
| Negative integer | number_integer | 0x38 |
|
||||
| Negative integer | number_integer | 0x39 |
|
||||
| Negative integer | number_integer | 0x3A |
|
||||
| Negative integer | number_integer | 0x3B |
|
||||
| Byte string | binary | 0x40..0x57 |
|
||||
| Byte string | binary | 0x58 |
|
||||
| Byte string | binary | 0x59 |
|
||||
| Byte string | binary | 0x5A |
|
||||
| Byte string | binary | 0x5B |
|
||||
| UTF-8 string | string | 0x60..0x77 |
|
||||
| UTF-8 string | string | 0x78 |
|
||||
| UTF-8 string | string | 0x79 |
|
||||
| UTF-8 string | string | 0x7A |
|
||||
| UTF-8 string | string | 0x7B |
|
||||
| UTF-8 string | string | 0x7F |
|
||||
| array | array | 0x80..0x97 |
|
||||
| array | array | 0x98 |
|
||||
| array | array | 0x99 |
|
||||
| array | array | 0x9A |
|
||||
| array | array | 0x9B |
|
||||
| array | array | 0x9F |
|
||||
| map | object | 0xA0..0xB7 |
|
||||
| map | object | 0xB8 |
|
||||
| map | object | 0xB9 |
|
||||
| map | object | 0xBA |
|
||||
| map | object | 0xBB |
|
||||
| map | object | 0xBF |
|
||||
| False | `false` | 0xF4 |
|
||||
| True | `true` | 0xF5 |
|
||||
| Null | `null` | 0xF6 |
|
||||
| Half-Precision Float | number_float | 0xF9 |
|
||||
| Single-Precision Float | number_float | 0xFA |
|
||||
| Double-Precision Float | number_float | 0xFB |
|
||||
|
||||
!!! warning "Incomplete mapping"
|
||||
|
||||
The mapping is **incomplete** in the sense that not all CBOR types can be converted to a JSON value. The following CBOR types are not supported and will yield parse errors:
|
||||
|
||||
- date/time (0xC0..0xC1)
|
||||
- bignum (0xC2..0xC3)
|
||||
- decimal fraction (0xC4)
|
||||
- bigfloat (0xC5)
|
||||
- expected conversions (0xD5..0xD7)
|
||||
- simple values (0xE0..0xF3, 0xF8)
|
||||
- undefined (0xF7)
|
||||
|
||||
!!! warning "Object keys"
|
||||
|
||||
CBOR allows map keys of any type, whereas JSON only allows strings as keys in object values. Therefore, CBOR maps with keys other than UTF-8 strings are rejected.
|
||||
|
||||
!!! warning "Tagged items"
|
||||
|
||||
Tagged items will throw a parse error by default. They can be ignored by passing `cbor_tag_handler_t::ignore` to function `from_cbor`. They can be stored by passing `cbor_tag_handler_t::store` to function `from_cbor`.
|
||||
|
||||
??? example
|
||||
|
||||
```cpp
|
||||
--8<-- "examples/from_cbor.cpp"
|
||||
```
|
||||
|
||||
Output:
|
||||
|
||||
```json
|
||||
--8<-- "examples/from_cbor.output"
|
||||
```
|
||||
@@ -45,4 +45,4 @@
|
||||
<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="July 4, 2026 09:13:25 UTC">July 4, 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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|
||||
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|
||||
@@ -0,0 +1,221 @@
|
||||
# CBOR
|
||||
|
||||
The Concise Binary Object Representation (CBOR) is a data format whose design goals include the possibility of extremely small code sizes, fairly small message size, and extensibility without the need for version negotiation.
|
||||
|
||||
References
|
||||
|
||||
- [CBOR Website](http://cbor.io) - the main source on CBOR
|
||||
- [CBOR Playground](http://cbor.me) - an interactive webpage to translate between JSON and CBOR
|
||||
- [RFC 7049](https://tools.ietf.org/html/rfc7049) - the CBOR specification
|
||||
|
||||
## Serialization
|
||||
|
||||
The library uses the following mapping from JSON values types to CBOR types according to the CBOR specification ([RFC 7049](https://www.rfc-editor.org/rfc/rfc7049.html)):
|
||||
|
||||
| JSON value type | value/range | CBOR type | first byte |
|
||||
| --------------- | ------------------------------------------ | --------------------------------- | ---------- |
|
||||
| null | `null` | Null | 0xF6 |
|
||||
| boolean | `true` | True | 0xF5 |
|
||||
| boolean | `false` | False | 0xF4 |
|
||||
| number_integer | -9223372036854775808..-2147483649 | Negative integer (8 bytes follow) | 0x3B |
|
||||
| number_integer | -2147483648..-32769 | Negative integer (4 bytes follow) | 0x3A |
|
||||
| number_integer | -32768..-129 | Negative integer (2 bytes follow) | 0x39 |
|
||||
| number_integer | -128..-25 | Negative integer (1 byte follow) | 0x38 |
|
||||
| number_integer | -24..-1 | Negative integer | 0x20..0x37 |
|
||||
| number_integer | 0..23 | Integer | 0x00..0x17 |
|
||||
| number_integer | 24..255 | Unsigned integer (1 byte follow) | 0x18 |
|
||||
| number_integer | 256..65535 | Unsigned integer (2 bytes follow) | 0x19 |
|
||||
| number_integer | 65536..4294967295 | Unsigned integer (4 bytes follow) | 0x1A |
|
||||
| number_integer | 4294967296..18446744073709551615 | Unsigned integer (8 bytes follow) | 0x1B |
|
||||
| number_unsigned | 0..23 | Integer | 0x00..0x17 |
|
||||
| number_unsigned | 24..255 | Unsigned integer (1 byte follow) | 0x18 |
|
||||
| number_unsigned | 256..65535 | Unsigned integer (2 bytes follow) | 0x19 |
|
||||
| number_unsigned | 65536..4294967295 | Unsigned integer (4 bytes follow) | 0x1A |
|
||||
| number_unsigned | 4294967296..18446744073709551615 | Unsigned integer (8 bytes follow) | 0x1B |
|
||||
| number_float | *any value representable by a float* | Single-Precision Float | 0xFA |
|
||||
| number_float | *any value NOT representable by a float* | Double-Precision Float | 0xFB |
|
||||
| string | *length*: 0..23 | UTF-8 string | 0x60..0x77 |
|
||||
| string | *length*: 24..255 | UTF-8 string (1 byte follow) | 0x78 |
|
||||
| string | *length*: 256..65535 | UTF-8 string (2 bytes follow) | 0x79 |
|
||||
| string | *length*: 65536..4294967295 | UTF-8 string (4 bytes follow) | 0x7A |
|
||||
| string | *length*: 4294967296..18446744073709551615 | UTF-8 string (8 bytes follow) | 0x7B |
|
||||
| array | *size*: 0..23 | array | 0x80..0x97 |
|
||||
| array | *size*: 24..255 | array (1 byte follow) | 0x98 |
|
||||
| array | *size*: 256..65535 | array (2 bytes follow) | 0x99 |
|
||||
| array | *size*: 65536..4294967295 | array (4 bytes follow) | 0x9A |
|
||||
| array | *size*: 4294967296..18446744073709551615 | array (8 bytes follow) | 0x9B |
|
||||
| object | *size*: 0..23 | map | 0xA0..0xB7 |
|
||||
| object | *size*: 24..255 | map (1 byte follow) | 0xB8 |
|
||||
| object | *size*: 256..65535 | map (2 bytes follow) | 0xB9 |
|
||||
| object | *size*: 65536..4294967295 | map (4 bytes follow) | 0xBA |
|
||||
| object | *size*: 4294967296..18446744073709551615 | map (8 bytes follow) | 0xBB |
|
||||
| binary | *size*: 0..23 | byte string | 0x40..0x57 |
|
||||
| binary | *size*: 24..255 | byte string (1 byte follow) | 0x58 |
|
||||
| binary | *size*: 256..65535 | byte string (2 bytes follow) | 0x59 |
|
||||
| binary | *size*: 65536..4294967295 | byte string (4 bytes follow) | 0x5A |
|
||||
| binary | *size*: 4294967296..18446744073709551615 | byte string (8 bytes follow) | 0x5B |
|
||||
|
||||
Binary values with subtype are mapped to tagged values (0xD8..0xDB) depending on the subtype, followed by a byte string, see "binary" cells in the table above.
|
||||
|
||||
Complete mapping
|
||||
|
||||
The mapping is **complete** in the sense that any JSON value type can be converted to a CBOR value.
|
||||
|
||||
NaN/infinity handling
|
||||
|
||||
If NaN or Infinity are stored inside a JSON number, they are serialized properly. This behavior differs from the normal JSON serialization which serializes NaN or Infinity to `null`.
|
||||
|
||||
Unused CBOR types
|
||||
|
||||
The following CBOR types are not used in the conversion:
|
||||
|
||||
- UTF-8 strings terminated by "break" (0x7F)
|
||||
- arrays terminated by "break" (0x9F)
|
||||
- maps terminated by "break" (0xBF)
|
||||
- byte strings terminated by "break" (0x5F)
|
||||
- date/time (0xC0..0xC1)
|
||||
- bignum (0xC2..0xC3)
|
||||
- decimal fraction (0xC4)
|
||||
- bigfloat (0xC5)
|
||||
- expected conversions (0xD5..0xD7)
|
||||
- simple values (0xE0..0xF3, 0xF8)
|
||||
- undefined (0xF7)
|
||||
- half-precision floats (0xF9)
|
||||
- break (0xFF)
|
||||
|
||||
Tagged items
|
||||
|
||||
Binary subtypes will be serialized as tagged items. See [binary values](https://json.nlohmann.me/features/binary_values/#cbor) for an example.
|
||||
|
||||
Example
|
||||
|
||||
```
|
||||
#include <iostream>
|
||||
#include <iomanip>
|
||||
#include <nlohmann/json.hpp>
|
||||
|
||||
using json = nlohmann::json;
|
||||
using namespace nlohmann::literals;
|
||||
|
||||
int main()
|
||||
{
|
||||
// create a JSON value
|
||||
json j = R"({"compact": true, "schema": 0})"_json;
|
||||
|
||||
// serialize it to CBOR
|
||||
std::vector<std::uint8_t> v = json::to_cbor(j);
|
||||
|
||||
// print the vector content
|
||||
for (auto& byte : v)
|
||||
{
|
||||
std::cout << "0x" << std::hex << std::setw(2) << std::setfill('0') << (int)byte << " ";
|
||||
}
|
||||
std::cout << std::endl;
|
||||
}
|
||||
```
|
||||
|
||||
Output:
|
||||
|
||||
```
|
||||
0xa2 0x67 0x63 0x6f 0x6d 0x70 0x61 0x63 0x74 0xf5 0x66 0x73 0x63 0x68 0x65 0x6d 0x61 0x00
|
||||
```
|
||||
|
||||
## Deserialization
|
||||
|
||||
The library maps CBOR types to JSON value types as follows:
|
||||
|
||||
| CBOR type | JSON value type | first byte |
|
||||
| ---------------------- | --------------- | ---------- |
|
||||
| Integer | number_unsigned | 0x00..0x17 |
|
||||
| Unsigned integer | number_unsigned | 0x18 |
|
||||
| Unsigned integer | number_unsigned | 0x19 |
|
||||
| Unsigned integer | number_unsigned | 0x1A |
|
||||
| Unsigned integer | number_unsigned | 0x1B |
|
||||
| Negative integer | number_integer | 0x20..0x37 |
|
||||
| Negative integer | number_integer | 0x38 |
|
||||
| Negative integer | number_integer | 0x39 |
|
||||
| Negative integer | number_integer | 0x3A |
|
||||
| Negative integer | number_integer | 0x3B |
|
||||
| Byte string | binary | 0x40..0x57 |
|
||||
| Byte string | binary | 0x58 |
|
||||
| Byte string | binary | 0x59 |
|
||||
| Byte string | binary | 0x5A |
|
||||
| Byte string | binary | 0x5B |
|
||||
| UTF-8 string | string | 0x60..0x77 |
|
||||
| UTF-8 string | string | 0x78 |
|
||||
| UTF-8 string | string | 0x79 |
|
||||
| UTF-8 string | string | 0x7A |
|
||||
| UTF-8 string | string | 0x7B |
|
||||
| UTF-8 string | string | 0x7F |
|
||||
| array | array | 0x80..0x97 |
|
||||
| array | array | 0x98 |
|
||||
| array | array | 0x99 |
|
||||
| array | array | 0x9A |
|
||||
| array | array | 0x9B |
|
||||
| array | array | 0x9F |
|
||||
| map | object | 0xA0..0xB7 |
|
||||
| map | object | 0xB8 |
|
||||
| map | object | 0xB9 |
|
||||
| map | object | 0xBA |
|
||||
| map | object | 0xBB |
|
||||
| map | object | 0xBF |
|
||||
| False | `false` | 0xF4 |
|
||||
| True | `true` | 0xF5 |
|
||||
| Null | `null` | 0xF6 |
|
||||
| Half-Precision Float | number_float | 0xF9 |
|
||||
| Single-Precision Float | number_float | 0xFA |
|
||||
| Double-Precision Float | number_float | 0xFB |
|
||||
|
||||
Incomplete mapping
|
||||
|
||||
The mapping is **incomplete** in the sense that not all CBOR types can be converted to a JSON value. The following CBOR types are not supported and will yield parse errors:
|
||||
|
||||
- date/time (0xC0..0xC1)
|
||||
- bignum (0xC2..0xC3)
|
||||
- decimal fraction (0xC4)
|
||||
- bigfloat (0xC5)
|
||||
- expected conversions (0xD5..0xD7)
|
||||
- simple values (0xE0..0xF3, 0xF8)
|
||||
- undefined (0xF7)
|
||||
|
||||
Object keys
|
||||
|
||||
CBOR allows map keys of any type, whereas JSON only allows strings as keys in object values. Therefore, CBOR maps with keys other than UTF-8 strings are rejected.
|
||||
|
||||
Tagged items
|
||||
|
||||
Tagged items will throw a parse error by default. They can be ignored by passing `cbor_tag_handler_t::ignore` to function `from_cbor`. They can be stored by passing `cbor_tag_handler_t::store` to function `from_cbor`.
|
||||
|
||||
Example
|
||||
|
||||
```
|
||||
#include <iostream>
|
||||
#include <iomanip>
|
||||
#include <nlohmann/json.hpp>
|
||||
|
||||
using json = nlohmann::json;
|
||||
|
||||
int main()
|
||||
{
|
||||
// create byte vector
|
||||
std::vector<std::uint8_t> v = {0xa2, 0x67, 0x63, 0x6f, 0x6d, 0x70, 0x61, 0x63,
|
||||
0x74, 0xf5, 0x66, 0x73, 0x63, 0x68, 0x65, 0x6d,
|
||||
0x61, 0x00
|
||||
};
|
||||
|
||||
// deserialize it with CBOR
|
||||
json j = json::from_cbor(v);
|
||||
|
||||
// print the deserialized JSON value
|
||||
std::cout << std::setw(2) << j << std::endl;
|
||||
}
|
||||
```
|
||||
|
||||
Output:
|
||||
|
||||
```
|
||||
{
|
||||
"compact": true,
|
||||
"schema": 0
|
||||
}
|
||||
```
|
||||
@@ -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=../arbitrary_types/ 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-9.7.6"><title>Binary Formats - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.484c7ddc.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 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=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/binary_formats/" rel="canonical"><link href=../arbitrary_types/ 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-9.7.6"><title>Binary Formats - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.484c7ddc.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> 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=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
|
||||
@@ -0,0 +1,51 @@
|
||||
# Binary Formats
|
||||
|
||||
Though JSON is a ubiquitous data format, it is not a very compact format suitable for data exchange, for instance, over a network. Hence, the library supports
|
||||
|
||||
- [BJData](https://json.nlohmann.me/features/binary_formats/bjdata/index.md) (Binary JData),
|
||||
- [BSON](https://json.nlohmann.me/features/binary_formats/bson/index.md) (Binary JSON),
|
||||
- [CBOR](https://json.nlohmann.me/features/binary_formats/cbor/index.md) (Concise Binary Object Representation),
|
||||
- [MessagePack](https://json.nlohmann.me/features/binary_formats/messagepack/index.md), and
|
||||
- [UBJSON](https://json.nlohmann.me/features/binary_formats/ubjson/index.md) (Universal Binary JSON)
|
||||
|
||||
to efficiently encode JSON values to byte vectors and to decode such vectors.
|
||||
|
||||
## Comparison
|
||||
|
||||
### Completeness
|
||||
|
||||
| Format | Serialization | Deserialization |
|
||||
| ----------- | --------------------------------------------- | -------------------------------------------- |
|
||||
| BJData | complete | complete |
|
||||
| BSON | incomplete: top-level value must be an object | incomplete, but all JSON types are supported |
|
||||
| CBOR | complete | incomplete, but all JSON types are supported |
|
||||
| MessagePack | complete | complete |
|
||||
| UBJSON | complete | complete |
|
||||
|
||||
### Binary values
|
||||
|
||||
| Format | Binary values | Binary subtypes |
|
||||
| ----------- | ------------- | --------------- |
|
||||
| BJData | not supported | not supported |
|
||||
| BSON | supported | supported |
|
||||
| CBOR | supported | supported |
|
||||
| MessagePack | supported | supported |
|
||||
| UBJSON | not supported | not supported |
|
||||
|
||||
See [binary values](https://json.nlohmann.me/features/binary_values/index.md) for more information.
|
||||
|
||||
### Sizes
|
||||
|
||||
| Format | canada.json | twitter.json | citm_catalog.json | jeopardy.json |
|
||||
| ------------------ | ----------- | ------------ | ----------------- | ------------- |
|
||||
| BJData | 53.2 % | 91.1 % | 78.1 % | 96.6 % |
|
||||
| BJData (size) | 58.6 % | 92.1 % | 86.7 % | 97.4 % |
|
||||
| BJData (size+type) | 58.6 % | 92.1 % | 86.5 % | 97.4 % |
|
||||
| BSON | 85.8 % | 95.2 % | 95.8 % | 106.7 % |
|
||||
| CBOR | 50.5 % | 86.3 % | 68.4 % | 88.0 % |
|
||||
| MessagePack | 50.5 % | 86.0 % | 68.5 % | 87.9 % |
|
||||
| UBJSON | 53.2 % | 91.3 % | 78.2 % | 96.6 % |
|
||||
| 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.
|
||||
@@ -0,0 +1,143 @@
|
||||
# MessagePack
|
||||
|
||||
MessagePack is an efficient binary serialization format. It lets you exchange data among multiple languages like JSON.
|
||||
But it's faster and smaller. Small integers are encoded into a single byte, and typical short strings require only one
|
||||
extra byte in addition to the strings themselves.
|
||||
|
||||
!!! abstract "References"
|
||||
|
||||
- [MessagePack website](https://msgpack.org)
|
||||
- [MessagePack specification](https://github.com/msgpack/msgpack/blob/master/spec.md)
|
||||
|
||||
## Serialization
|
||||
|
||||
The library uses the following mapping from JSON values types to MessagePack types according to the MessagePack
|
||||
specification:
|
||||
|
||||
| JSON value type | value/range | MessagePack type | first byte |
|
||||
|-----------------|------------------------------------------|------------------|------------|
|
||||
| null | `null` | nil | 0xC0 |
|
||||
| boolean | `true` | true | 0xC3 |
|
||||
| boolean | `false` | false | 0xC2 |
|
||||
| number_integer | -9223372036854775808..-2147483649 | int64 | 0xD3 |
|
||||
| number_integer | -2147483648..-32769 | int32 | 0xD2 |
|
||||
| number_integer | -32768..-129 | int16 | 0xD1 |
|
||||
| number_integer | -128..-33 | int8 | 0xD0 |
|
||||
| number_integer | -32..-1 | negative fixint | 0xE0..0xFF |
|
||||
| number_integer | 0..127 | positive fixint | 0x00..0x7F |
|
||||
| number_integer | 128..255 | uint 8 | 0xCC |
|
||||
| number_integer | 256..65535 | uint 16 | 0xCD |
|
||||
| number_integer | 65536..4294967295 | uint 32 | 0xCE |
|
||||
| number_integer | 4294967296..18446744073709551615 | uint 64 | 0xCF |
|
||||
| number_unsigned | 0..127 | positive fixint | 0x00..0x7F |
|
||||
| number_unsigned | 128..255 | uint 8 | 0xCC |
|
||||
| number_unsigned | 256..65535 | uint 16 | 0xCD |
|
||||
| number_unsigned | 65536..4294967295 | uint 32 | 0xCE |
|
||||
| number_unsigned | 4294967296..18446744073709551615 | uint 64 | 0xCF |
|
||||
| number_float | *any value representable by a float* | float 32 | 0xCA |
|
||||
| number_float | *any value NOT representable by a float* | float 64 | 0xCB |
|
||||
| string | *length*: 0..31 | fixstr | 0xA0..0xBF |
|
||||
| string | *length*: 32..255 | str 8 | 0xD9 |
|
||||
| string | *length*: 256..65535 | str 16 | 0xDA |
|
||||
| string | *length*: 65536..4294967295 | str 32 | 0xDB |
|
||||
| array | *size*: 0..15 | fixarray | 0x90..0x9F |
|
||||
| array | *size*: 16..65535 | array 16 | 0xDC |
|
||||
| array | *size*: 65536..4294967295 | array 32 | 0xDD |
|
||||
| object | *size*: 0..15 | fix map | 0x80..0x8F |
|
||||
| object | *size*: 16..65535 | map 16 | 0xDE |
|
||||
| object | *size*: 65536..4294967295 | map 32 | 0xDF |
|
||||
| binary | *size*: 0..255 | bin 8 | 0xC4 |
|
||||
| binary | *size*: 256..65535 | bin 16 | 0xC5 |
|
||||
| binary | *size*: 65536..4294967295 | bin 32 | 0xC6 |
|
||||
|
||||
!!! success "Complete mapping"
|
||||
|
||||
The mapping is **complete** in the sense that any JSON value type can be converted to a MessagePack value.
|
||||
|
||||
Any MessagePack output created by `to_msgpack` can be successfully parsed by `from_msgpack`.
|
||||
|
||||
!!! warning "Size constraints"
|
||||
|
||||
The following values can **not** be converted to a MessagePack value:
|
||||
|
||||
- strings with more than 4294967295 bytes
|
||||
- byte strings with more than 4294967295 bytes
|
||||
- arrays with more than 4294967295 elements
|
||||
- objects with more than 4294967295 elements
|
||||
|
||||
!!! info "NaN/infinity handling"
|
||||
|
||||
If NaN or Infinity are stored inside a JSON number, they are serialized properly in contrast to the
|
||||
[dump](../../api/basic_json/dump.md) function which serializes NaN or Infinity to `null`.
|
||||
|
||||
??? example
|
||||
|
||||
```cpp
|
||||
--8<-- "examples/to_msgpack.cpp"
|
||||
```
|
||||
|
||||
Output:
|
||||
|
||||
```c
|
||||
--8<-- "examples/to_msgpack.output"
|
||||
```
|
||||
|
||||
## Deserialization
|
||||
|
||||
The library maps MessagePack types to JSON value types as follows:
|
||||
|
||||
| MessagePack type | JSON value type | first byte |
|
||||
|------------------|-----------------|------------|
|
||||
| positive fixint | number_unsigned | 0x00..0x7F |
|
||||
| fixmap | object | 0x80..0x8F |
|
||||
| fixarray | array | 0x90..0x9F |
|
||||
| fixstr | string | 0xA0..0xBF |
|
||||
| nil | `null` | 0xC0 |
|
||||
| false | `false` | 0xC2 |
|
||||
| true | `true` | 0xC3 |
|
||||
| float 32 | number_float | 0xCA |
|
||||
| float 64 | number_float | 0xCB |
|
||||
| uint 8 | number_unsigned | 0xCC |
|
||||
| uint 16 | number_unsigned | 0xCD |
|
||||
| uint 32 | number_unsigned | 0xCE |
|
||||
| uint 64 | number_unsigned | 0xCF |
|
||||
| int 8 | number_integer | 0xD0 |
|
||||
| int 16 | number_integer | 0xD1 |
|
||||
| int 32 | number_integer | 0xD2 |
|
||||
| int 64 | number_integer | 0xD3 |
|
||||
| str 8 | string | 0xD9 |
|
||||
| str 16 | string | 0xDA |
|
||||
| str 32 | string | 0xDB |
|
||||
| array 16 | array | 0xDC |
|
||||
| array 32 | array | 0xDD |
|
||||
| map 16 | object | 0xDE |
|
||||
| map 32 | object | 0xDF |
|
||||
| bin 8 | binary | 0xC4 |
|
||||
| bin 16 | binary | 0xC5 |
|
||||
| bin 32 | binary | 0xC6 |
|
||||
| ext 8 | binary | 0xC7 |
|
||||
| ext 16 | binary | 0xC8 |
|
||||
| ext 32 | binary | 0xC9 |
|
||||
| fixext 1 | binary | 0xD4 |
|
||||
| fixext 2 | binary | 0xD5 |
|
||||
| fixext 4 | binary | 0xD6 |
|
||||
| fixext 8 | binary | 0xD7 |
|
||||
| fixext 16 | binary | 0xD8 |
|
||||
| negative fixint | number_integer | 0xE0-0xFF |
|
||||
|
||||
!!! info
|
||||
|
||||
Any MessagePack output created by `to_msgpack` can be successfully parsed by `from_msgpack`.
|
||||
|
||||
|
||||
??? example
|
||||
|
||||
```cpp
|
||||
--8<-- "examples/from_msgpack.cpp"
|
||||
```
|
||||
|
||||
Output:
|
||||
|
||||
```json
|
||||
--8<-- "examples/from_msgpack.output"
|
||||
```
|
||||
@@ -45,4 +45,4 @@
|
||||
<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="July 4, 2026 09:13:25 UTC">July 4, 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> <!-- 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="July 8, 2026 18:18:24 UTC">July 8, 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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|
||||
@@ -0,0 +1,181 @@
|
||||
# MessagePack
|
||||
|
||||
MessagePack is an efficient binary serialization format. It lets you exchange data among multiple languages like JSON. But it's faster and smaller. Small integers are encoded into a single byte, and typical short strings require only one extra byte in addition to the strings themselves.
|
||||
|
||||
References
|
||||
|
||||
- [MessagePack website](https://msgpack.org)
|
||||
- [MessagePack specification](https://github.com/msgpack/msgpack/blob/master/spec.md)
|
||||
|
||||
## Serialization
|
||||
|
||||
The library uses the following mapping from JSON values types to MessagePack types according to the MessagePack specification:
|
||||
|
||||
| JSON value type | value/range | MessagePack type | first byte |
|
||||
| --------------- | ---------------------------------------- | ---------------- | ---------- |
|
||||
| null | `null` | nil | 0xC0 |
|
||||
| boolean | `true` | true | 0xC3 |
|
||||
| boolean | `false` | false | 0xC2 |
|
||||
| number_integer | -9223372036854775808..-2147483649 | int64 | 0xD3 |
|
||||
| number_integer | -2147483648..-32769 | int32 | 0xD2 |
|
||||
| number_integer | -32768..-129 | int16 | 0xD1 |
|
||||
| number_integer | -128..-33 | int8 | 0xD0 |
|
||||
| number_integer | -32..-1 | negative fixint | 0xE0..0xFF |
|
||||
| number_integer | 0..127 | positive fixint | 0x00..0x7F |
|
||||
| number_integer | 128..255 | uint 8 | 0xCC |
|
||||
| number_integer | 256..65535 | uint 16 | 0xCD |
|
||||
| number_integer | 65536..4294967295 | uint 32 | 0xCE |
|
||||
| number_integer | 4294967296..18446744073709551615 | uint 64 | 0xCF |
|
||||
| number_unsigned | 0..127 | positive fixint | 0x00..0x7F |
|
||||
| number_unsigned | 128..255 | uint 8 | 0xCC |
|
||||
| number_unsigned | 256..65535 | uint 16 | 0xCD |
|
||||
| number_unsigned | 65536..4294967295 | uint 32 | 0xCE |
|
||||
| number_unsigned | 4294967296..18446744073709551615 | uint 64 | 0xCF |
|
||||
| number_float | *any value representable by a float* | float 32 | 0xCA |
|
||||
| number_float | *any value NOT representable by a float* | float 64 | 0xCB |
|
||||
| string | *length*: 0..31 | fixstr | 0xA0..0xBF |
|
||||
| string | *length*: 32..255 | str 8 | 0xD9 |
|
||||
| string | *length*: 256..65535 | str 16 | 0xDA |
|
||||
| string | *length*: 65536..4294967295 | str 32 | 0xDB |
|
||||
| array | *size*: 0..15 | fixarray | 0x90..0x9F |
|
||||
| array | *size*: 16..65535 | array 16 | 0xDC |
|
||||
| array | *size*: 65536..4294967295 | array 32 | 0xDD |
|
||||
| object | *size*: 0..15 | fix map | 0x80..0x8F |
|
||||
| object | *size*: 16..65535 | map 16 | 0xDE |
|
||||
| object | *size*: 65536..4294967295 | map 32 | 0xDF |
|
||||
| binary | *size*: 0..255 | bin 8 | 0xC4 |
|
||||
| binary | *size*: 256..65535 | bin 16 | 0xC5 |
|
||||
| binary | *size*: 65536..4294967295 | bin 32 | 0xC6 |
|
||||
|
||||
Complete mapping
|
||||
|
||||
The mapping is **complete** in the sense that any JSON value type can be converted to a MessagePack value.
|
||||
|
||||
Any MessagePack output created by `to_msgpack` can be successfully parsed by `from_msgpack`.
|
||||
|
||||
Size constraints
|
||||
|
||||
The following values can **not** be converted to a MessagePack value:
|
||||
|
||||
- strings with more than 4294967295 bytes
|
||||
- byte strings with more than 4294967295 bytes
|
||||
- arrays with more than 4294967295 elements
|
||||
- objects with more than 4294967295 elements
|
||||
|
||||
NaN/infinity handling
|
||||
|
||||
If NaN or Infinity are stored inside a JSON number, they are serialized properly in contrast to the [dump](https://json.nlohmann.me/api/basic_json/dump/index.md) function which serializes NaN or Infinity to `null`.
|
||||
|
||||
Example
|
||||
|
||||
```
|
||||
#include <iostream>
|
||||
#include <iomanip>
|
||||
#include <nlohmann/json.hpp>
|
||||
|
||||
using json = nlohmann::json;
|
||||
using namespace nlohmann::literals;
|
||||
|
||||
int main()
|
||||
{
|
||||
// create a JSON value
|
||||
json j = R"({"compact": true, "schema": 0})"_json;
|
||||
|
||||
// serialize it to MessagePack
|
||||
std::vector<std::uint8_t> v = json::to_msgpack(j);
|
||||
|
||||
// print the vector content
|
||||
for (auto& byte : v)
|
||||
{
|
||||
std::cout << "0x" << std::hex << std::setw(2) << std::setfill('0') << (int)byte << " ";
|
||||
}
|
||||
std::cout << std::endl;
|
||||
}
|
||||
```
|
||||
|
||||
Output:
|
||||
|
||||
```
|
||||
0x82 0xa7 0x63 0x6f 0x6d 0x70 0x61 0x63 0x74 0xc3 0xa6 0x73 0x63 0x68 0x65 0x6d 0x61 0x00
|
||||
```
|
||||
|
||||
## Deserialization
|
||||
|
||||
The library maps MessagePack types to JSON value types as follows:
|
||||
|
||||
| MessagePack type | JSON value type | first byte |
|
||||
| ---------------- | --------------- | ---------- |
|
||||
| positive fixint | number_unsigned | 0x00..0x7F |
|
||||
| fixmap | object | 0x80..0x8F |
|
||||
| fixarray | array | 0x90..0x9F |
|
||||
| fixstr | string | 0xA0..0xBF |
|
||||
| nil | `null` | 0xC0 |
|
||||
| false | `false` | 0xC2 |
|
||||
| true | `true` | 0xC3 |
|
||||
| float 32 | number_float | 0xCA |
|
||||
| float 64 | number_float | 0xCB |
|
||||
| uint 8 | number_unsigned | 0xCC |
|
||||
| uint 16 | number_unsigned | 0xCD |
|
||||
| uint 32 | number_unsigned | 0xCE |
|
||||
| uint 64 | number_unsigned | 0xCF |
|
||||
| int 8 | number_integer | 0xD0 |
|
||||
| int 16 | number_integer | 0xD1 |
|
||||
| int 32 | number_integer | 0xD2 |
|
||||
| int 64 | number_integer | 0xD3 |
|
||||
| str 8 | string | 0xD9 |
|
||||
| str 16 | string | 0xDA |
|
||||
| str 32 | string | 0xDB |
|
||||
| array 16 | array | 0xDC |
|
||||
| array 32 | array | 0xDD |
|
||||
| map 16 | object | 0xDE |
|
||||
| map 32 | object | 0xDF |
|
||||
| bin 8 | binary | 0xC4 |
|
||||
| bin 16 | binary | 0xC5 |
|
||||
| bin 32 | binary | 0xC6 |
|
||||
| ext 8 | binary | 0xC7 |
|
||||
| ext 16 | binary | 0xC8 |
|
||||
| ext 32 | binary | 0xC9 |
|
||||
| fixext 1 | binary | 0xD4 |
|
||||
| fixext 2 | binary | 0xD5 |
|
||||
| fixext 4 | binary | 0xD6 |
|
||||
| fixext 8 | binary | 0xD7 |
|
||||
| fixext 16 | binary | 0xD8 |
|
||||
| negative fixint | number_integer | 0xE0-0xFF |
|
||||
|
||||
Info
|
||||
|
||||
Any MessagePack output created by `to_msgpack` can be successfully parsed by `from_msgpack`.
|
||||
|
||||
Example
|
||||
|
||||
```
|
||||
#include <iostream>
|
||||
#include <iomanip>
|
||||
#include <nlohmann/json.hpp>
|
||||
|
||||
using json = nlohmann::json;
|
||||
|
||||
int main()
|
||||
{
|
||||
// create byte vector
|
||||
std::vector<std::uint8_t> v = {0x82, 0xa7, 0x63, 0x6f, 0x6d, 0x70, 0x61, 0x63,
|
||||
0x74, 0xc3, 0xa6, 0x73, 0x63, 0x68, 0x65, 0x6d,
|
||||
0x61, 0x00
|
||||
};
|
||||
|
||||
// deserialize it with MessagePack
|
||||
json j = json::from_msgpack(v);
|
||||
|
||||
// print the deserialized JSON value
|
||||
std::cout << std::setw(2) << j << std::endl;
|
||||
}
|
||||
```
|
||||
|
||||
Output:
|
||||
|
||||
```
|
||||
{
|
||||
"compact": true,
|
||||
"schema": 0
|
||||
}
|
||||
```
|
||||
@@ -0,0 +1,126 @@
|
||||
# UBJSON
|
||||
|
||||
Universal Binary JSON (UBJSON) is a binary form directly imitating JSON, but requiring fewer bytes of data. It aims to
|
||||
achieve the generality of JSON, combined with being much easier to process than JSON.
|
||||
|
||||
!!! abstract "References"
|
||||
|
||||
- [UBJSON Website](http://ubjson.org)
|
||||
|
||||
## Serialization
|
||||
|
||||
The library uses the following mapping from JSON values types to UBJSON types according to the UBJSON specification:
|
||||
|
||||
| JSON value type | value/range | UBJSON type | marker |
|
||||
|-----------------|-----------------------------------|----------------|--------|
|
||||
| null | `null` | null | `Z` |
|
||||
| boolean | `true` | true | `T` |
|
||||
| boolean | `false` | false | `F` |
|
||||
| number_integer | -9223372036854775808..-2147483649 | int64 | `L` |
|
||||
| number_integer | -2147483648..-32769 | int32 | `l` |
|
||||
| number_integer | -32768..-129 | int16 | `I` |
|
||||
| number_integer | -128..127 | int8 | `i` |
|
||||
| number_integer | 128..255 | uint8 | `U` |
|
||||
| number_integer | 256..32767 | int16 | `I` |
|
||||
| number_integer | 32768..2147483647 | int32 | `l` |
|
||||
| number_integer | 2147483648..9223372036854775807 | int64 | `L` |
|
||||
| number_unsigned | 0..127 | int8 | `i` |
|
||||
| number_unsigned | 128..255 | uint8 | `U` |
|
||||
| number_unsigned | 256..32767 | int16 | `I` |
|
||||
| number_unsigned | 32768..2147483647 | int32 | `l` |
|
||||
| number_unsigned | 2147483648..9223372036854775807 | int64 | `L` |
|
||||
| number_unsigned | 2147483649..18446744073709551615 | high-precision | `H` |
|
||||
| number_float | *any value* | float64 | `D` |
|
||||
| string | *with shortest length indicator* | string | `S` |
|
||||
| array | *see notes on optimized format* | array | `[` |
|
||||
| object | *see notes on optimized format* | map | `{` |
|
||||
|
||||
!!! success "Complete mapping"
|
||||
|
||||
The mapping is **complete** in the sense that any JSON value type can be converted to a UBJSON value.
|
||||
|
||||
Any UBJSON output created by `to_ubjson` can be successfully parsed by `from_ubjson`.
|
||||
|
||||
!!! warning "Size constraints"
|
||||
|
||||
The following values can **not** be converted to a UBJSON value:
|
||||
|
||||
- strings with more than 9223372036854775807 bytes (theoretical)
|
||||
|
||||
!!! info "Unused UBJSON markers"
|
||||
|
||||
The following markers are not used in the conversion:
|
||||
|
||||
- `Z`: no-op values are not created.
|
||||
- `C`: single-byte strings are serialized with `S` markers.
|
||||
|
||||
!!! info "NaN/infinity handling"
|
||||
|
||||
If NaN or Infinity are stored inside a JSON number, they are serialized properly. This behavior differs from the
|
||||
`dump()` function which serializes NaN or Infinity to `null`.
|
||||
|
||||
!!! info "Optimized formats"
|
||||
|
||||
The optimized formats for containers are supported: Parameter `use_size` adds size information to the beginning of a
|
||||
container and removes the closing marker. Parameter `use_type` further checks whether all elements of a container
|
||||
have the same type and adds the type marker to the beginning of the container. The `use_type` parameter must only be
|
||||
used together with `use_size = true`.
|
||||
|
||||
Note that `use_size = true` alone may result in larger representations - the benefit of this parameter is that the
|
||||
receiving side is immediately informed on the number of elements of the container.
|
||||
|
||||
!!! info "Binary values"
|
||||
|
||||
If the JSON data contains the binary type, the value stored is a list of integers, as suggested by the UBJSON
|
||||
documentation. In particular, this means that serialization and the deserialization of a JSON containing binary
|
||||
values into UBJSON and back will result in a different JSON object.
|
||||
|
||||
??? example
|
||||
|
||||
```cpp
|
||||
--8<-- "examples/to_ubjson.cpp"
|
||||
```
|
||||
|
||||
Output:
|
||||
|
||||
```c
|
||||
--8<-- "examples/to_ubjson.output"
|
||||
```
|
||||
|
||||
## Deserialization
|
||||
|
||||
The library maps UBJSON types to JSON value types as follows:
|
||||
|
||||
| UBJSON type | JSON value type | marker |
|
||||
|-------------|-----------------------------------------|--------|
|
||||
| no-op | *no value, next value is read* | `N` |
|
||||
| null | `null` | `Z` |
|
||||
| false | `false` | `F` |
|
||||
| true | `true` | `T` |
|
||||
| float32 | number_float | `d` |
|
||||
| float64 | number_float | `D` |
|
||||
| uint8 | number_unsigned | `U` |
|
||||
| int8 | number_integer | `i` |
|
||||
| int16 | number_integer | `I` |
|
||||
| int32 | number_integer | `l` |
|
||||
| int64 | number_integer | `L` |
|
||||
| string | string | `S` |
|
||||
| char | string | `C` |
|
||||
| array | array (optimized values are supported) | `[` |
|
||||
| object | object (optimized values are supported) | `{` |
|
||||
|
||||
!!! success "Complete mapping"
|
||||
|
||||
The mapping is **complete** in the sense that any UBJSON value can be converted to a JSON value.
|
||||
|
||||
??? example
|
||||
|
||||
```cpp
|
||||
--8<-- "examples/from_ubjson.cpp"
|
||||
```
|
||||
|
||||
Output:
|
||||
|
||||
```json
|
||||
--8<-- "examples/from_ubjson.output"
|
||||
```
|
||||
@@ -90,4 +90,4 @@
|
||||
<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="July 4, 2026 09:13:25 UTC">July 4, 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> <!-- 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="July 8, 2026 18:18:24 UTC">July 8, 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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|
||||
@@ -0,0 +1,206 @@
|
||||
# UBJSON
|
||||
|
||||
Universal Binary JSON (UBJSON) is a binary form directly imitating JSON, but requiring fewer bytes of data. It aims to achieve the generality of JSON, combined with being much easier to process than JSON.
|
||||
|
||||
References
|
||||
|
||||
- [UBJSON Website](http://ubjson.org)
|
||||
|
||||
## Serialization
|
||||
|
||||
The library uses the following mapping from JSON values types to UBJSON types according to the UBJSON specification:
|
||||
|
||||
| JSON value type | value/range | UBJSON type | marker |
|
||||
| --------------- | --------------------------------- | -------------- | ------ |
|
||||
| null | `null` | null | `Z` |
|
||||
| boolean | `true` | true | `T` |
|
||||
| boolean | `false` | false | `F` |
|
||||
| number_integer | -9223372036854775808..-2147483649 | int64 | `L` |
|
||||
| number_integer | -2147483648..-32769 | int32 | `l` |
|
||||
| number_integer | -32768..-129 | int16 | `I` |
|
||||
| number_integer | -128..127 | int8 | `i` |
|
||||
| number_integer | 128..255 | uint8 | `U` |
|
||||
| number_integer | 256..32767 | int16 | `I` |
|
||||
| number_integer | 32768..2147483647 | int32 | `l` |
|
||||
| number_integer | 2147483648..9223372036854775807 | int64 | `L` |
|
||||
| number_unsigned | 0..127 | int8 | `i` |
|
||||
| number_unsigned | 128..255 | uint8 | `U` |
|
||||
| number_unsigned | 256..32767 | int16 | `I` |
|
||||
| number_unsigned | 32768..2147483647 | int32 | `l` |
|
||||
| number_unsigned | 2147483648..9223372036854775807 | int64 | `L` |
|
||||
| number_unsigned | 2147483649..18446744073709551615 | high-precision | `H` |
|
||||
| number_float | *any value* | float64 | `D` |
|
||||
| string | *with shortest length indicator* | string | `S` |
|
||||
| array | *see notes on optimized format* | array | `[` |
|
||||
| object | *see notes on optimized format* | map | `{` |
|
||||
|
||||
Complete mapping
|
||||
|
||||
The mapping is **complete** in the sense that any JSON value type can be converted to a UBJSON value.
|
||||
|
||||
Any UBJSON output created by `to_ubjson` can be successfully parsed by `from_ubjson`.
|
||||
|
||||
Size constraints
|
||||
|
||||
The following values can **not** be converted to a UBJSON value:
|
||||
|
||||
- strings with more than 9223372036854775807 bytes (theoretical)
|
||||
|
||||
Unused UBJSON markers
|
||||
|
||||
The following markers are not used in the conversion:
|
||||
|
||||
- `Z`: no-op values are not created.
|
||||
- `C`: single-byte strings are serialized with `S` markers.
|
||||
|
||||
NaN/infinity handling
|
||||
|
||||
If NaN or Infinity are stored inside a JSON number, they are serialized properly. This behavior differs from the `dump()` function which serializes NaN or Infinity to `null`.
|
||||
|
||||
Optimized formats
|
||||
|
||||
The optimized formats for containers are supported: Parameter `use_size` adds size information to the beginning of a container and removes the closing marker. Parameter `use_type` further checks whether all elements of a container have the same type and adds the type marker to the beginning of the container. The `use_type` parameter must only be used together with `use_size = true`.
|
||||
|
||||
Note that `use_size = true` alone may result in larger representations - the benefit of this parameter is that the receiving side is immediately informed on the number of elements of the container.
|
||||
|
||||
Binary values
|
||||
|
||||
If the JSON data contains the binary type, the value stored is a list of integers, as suggested by the UBJSON documentation. In particular, this means that serialization and the deserialization of a JSON containing binary values into UBJSON and back will result in a different JSON object.
|
||||
|
||||
Example
|
||||
|
||||
```
|
||||
#include <iostream>
|
||||
#include <iomanip>
|
||||
#include <nlohmann/json.hpp>
|
||||
|
||||
using json = nlohmann::json;
|
||||
using namespace nlohmann::literals;
|
||||
|
||||
// function to print UBJSON's diagnostic format
|
||||
void print_byte(uint8_t byte)
|
||||
{
|
||||
if (32 < byte and byte < 128)
|
||||
{
|
||||
std::cout << (char)byte;
|
||||
}
|
||||
else
|
||||
{
|
||||
std::cout << (int)byte;
|
||||
}
|
||||
}
|
||||
|
||||
int main()
|
||||
{
|
||||
// create a JSON value
|
||||
json j = R"({"compact": true, "schema": false})"_json;
|
||||
|
||||
// serialize it to UBJSON
|
||||
std::vector<std::uint8_t> v = json::to_ubjson(j);
|
||||
|
||||
// print the vector content
|
||||
for (auto& byte : v)
|
||||
{
|
||||
print_byte(byte);
|
||||
}
|
||||
std::cout << std::endl;
|
||||
|
||||
// create an array of numbers
|
||||
json array = {1, 2, 3, 4, 5, 6, 7, 8};
|
||||
|
||||
// serialize it to UBJSON using default representation
|
||||
std::vector<std::uint8_t> v_array = json::to_ubjson(array);
|
||||
// serialize it to UBJSON using size optimization
|
||||
std::vector<std::uint8_t> v_array_size = json::to_ubjson(array, true);
|
||||
// serialize it to UBJSON using type optimization
|
||||
std::vector<std::uint8_t> v_array_size_and_type = json::to_ubjson(array, true, true);
|
||||
|
||||
// print the vector contents
|
||||
for (auto& byte : v_array)
|
||||
{
|
||||
print_byte(byte);
|
||||
}
|
||||
std::cout << std::endl;
|
||||
|
||||
for (auto& byte : v_array_size)
|
||||
{
|
||||
print_byte(byte);
|
||||
}
|
||||
std::cout << std::endl;
|
||||
|
||||
for (auto& byte : v_array_size_and_type)
|
||||
{
|
||||
print_byte(byte);
|
||||
}
|
||||
std::cout << std::endl;
|
||||
}
|
||||
```
|
||||
|
||||
Output:
|
||||
|
||||
```
|
||||
{i7compactTi6schemaF}
|
||||
[i1i2i3i4i5i6i7i8]
|
||||
[#i8i1i2i3i4i5i6i7i8
|
||||
[$i#i812345678
|
||||
```
|
||||
|
||||
## Deserialization
|
||||
|
||||
The library maps UBJSON types to JSON value types as follows:
|
||||
|
||||
| UBJSON type | JSON value type | marker |
|
||||
| ----------- | --------------------------------------- | ------ |
|
||||
| no-op | *no value, next value is read* | `N` |
|
||||
| null | `null` | `Z` |
|
||||
| false | `false` | `F` |
|
||||
| true | `true` | `T` |
|
||||
| float32 | number_float | `d` |
|
||||
| float64 | number_float | `D` |
|
||||
| uint8 | number_unsigned | `U` |
|
||||
| int8 | number_integer | `i` |
|
||||
| int16 | number_integer | `I` |
|
||||
| int32 | number_integer | `l` |
|
||||
| int64 | number_integer | `L` |
|
||||
| string | string | `S` |
|
||||
| char | string | `C` |
|
||||
| array | array (optimized values are supported) | `[` |
|
||||
| object | object (optimized values are supported) | `{` |
|
||||
|
||||
Complete mapping
|
||||
|
||||
The mapping is **complete** in the sense that any UBJSON value can be converted to a JSON value.
|
||||
|
||||
Example
|
||||
|
||||
```
|
||||
#include <iostream>
|
||||
#include <iomanip>
|
||||
#include <nlohmann/json.hpp>
|
||||
|
||||
using json = nlohmann::json;
|
||||
|
||||
int main()
|
||||
{
|
||||
// create byte vector
|
||||
std::vector<std::uint8_t> v = {0x7B, 0x69, 0x07, 0x63, 0x6F, 0x6D, 0x70, 0x61,
|
||||
0x63, 0x74, 0x54, 0x69, 0x06, 0x73, 0x63, 0x68,
|
||||
0x65, 0x6D, 0x61, 0x69, 0x00, 0x7D
|
||||
};
|
||||
|
||||
// deserialize it with UBJSON
|
||||
json j = json::from_ubjson(v);
|
||||
|
||||
// print the deserialized JSON value
|
||||
std::cout << std::setw(2) << j << std::endl;
|
||||
}
|
||||
```
|
||||
|
||||
Output:
|
||||
|
||||
```
|
||||
{
|
||||
"compact": true,
|
||||
"schema": 0
|
||||
}
|
||||
```
|
||||
@@ -0,0 +1,374 @@
|
||||
# Binary Values
|
||||
|
||||
The library implements several [binary formats](binary_formats/index.md) that encode JSON in an efficient way. Most of
|
||||
these formats support binary values; that is, values that have semantics defined outside the library and only define a
|
||||
sequence of bytes to be stored.
|
||||
|
||||
JSON itself does not have a binary value. As such, binary values are an extension that this library implements to store
|
||||
values received by a binary format. Binary values are never created by the JSON parser and are only part of a
|
||||
serialized JSON text if they have been created manually or via a binary format.
|
||||
|
||||
## API for binary values
|
||||
|
||||
```mermaid
|
||||
classDiagram
|
||||
|
||||
class binary_t ["json::binary_t"] {
|
||||
+void set_subtype(std::uint64_t subtype)
|
||||
+void clear_subtype()
|
||||
+std::uint64_t subtype() const
|
||||
+bool has_subtype() const
|
||||
}
|
||||
|
||||
class vector ["std::vector<uint8_t>"]
|
||||
|
||||
vector <|-- binary_t
|
||||
```
|
||||
|
||||
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`:
|
||||
|
||||
```cpp
|
||||
auto binary = json::binary_t({0xCA, 0xFE, 0xBA, 0xBE});
|
||||
auto binary_with_subtype = json::binary_t({0xCA, 0xFE, 0xBA, 0xBE}, 42);
|
||||
```
|
||||
|
||||
There are several convenience functions to check and set the subtype:
|
||||
|
||||
```cpp
|
||||
binary.has_subtype(); // returns false
|
||||
binary_with_subtype.has_subtype(); // returns true
|
||||
|
||||
binary_with_subtype.clear_subtype();
|
||||
binary_with_subtype.has_subtype(); // returns false
|
||||
|
||||
binary_with_subtype.set_subtype(42);
|
||||
binary.set_subtype(23);
|
||||
|
||||
binary.subtype(); // returns 23
|
||||
```
|
||||
|
||||
As `json::binary_t` is subclassing `std::vector<std::uint8_t>`, all member functions are available:
|
||||
|
||||
```cpp
|
||||
binary.size(); // returns 4
|
||||
binary[1]; // returns 0xFE
|
||||
```
|
||||
|
||||
JSON values can be constructed from `json::binary_t`:
|
||||
|
||||
```cpp
|
||||
json j = binary;
|
||||
```
|
||||
|
||||
Binary values are primitive values just like numbers or strings:
|
||||
|
||||
```cpp
|
||||
j.is_binary(); // returns true
|
||||
j.is_primitive(); // returns true
|
||||
```
|
||||
|
||||
Given a binary JSON value, the `binary_t` can be accessed by reference as via `get_binary()`:
|
||||
|
||||
```cpp
|
||||
j.get_binary().has_subtype(); // returns true
|
||||
j.get_binary().size(); // returns 4
|
||||
```
|
||||
|
||||
For convenience, binary JSON values can be constructed via `json::binary`:
|
||||
|
||||
```cpp
|
||||
auto j2 = json::binary({0xCA, 0xFE, 0xBA, 0xBE}, 23);
|
||||
auto j3 = json::binary({0xCA, 0xFE, 0xBA, 0xBE});
|
||||
|
||||
j2 == j; // returns true
|
||||
j3.get_binary().has_subtype(); // returns false
|
||||
j3.get_binary().subtype(); // returns std::uint64_t(-1) as j3 has no subtype
|
||||
```
|
||||
|
||||
|
||||
|
||||
## Serialization
|
||||
|
||||
Binary values are serialized differently according to the formats.
|
||||
|
||||
### JSON
|
||||
|
||||
JSON does not have a binary type, and this library does not introduce a new type as this would break conformance.
|
||||
Instead, binary values are serialized as an object with two keys: `bytes` holds an array of integers, and `subtype`
|
||||
is an integer or `null`.
|
||||
|
||||
??? example
|
||||
|
||||
Code:
|
||||
|
||||
```cpp
|
||||
// create a binary value of subtype 42
|
||||
json j;
|
||||
j["binary"] = json::binary({0xCA, 0xFE, 0xBA, 0xBE}, 42);
|
||||
|
||||
// serialize to standard output
|
||||
std::cout << j.dump(2) << std::endl;
|
||||
```
|
||||
|
||||
Output:
|
||||
|
||||
```json
|
||||
{
|
||||
"binary": {
|
||||
"bytes": [202, 254, 186, 190],
|
||||
"subtype": 42
|
||||
}
|
||||
}
|
||||
```
|
||||
|
||||
!!! warning "No roundtrip for binary values"
|
||||
|
||||
The JSON parser will not parse the objects generated by binary values back to binary values. This is by design to
|
||||
remain standards compliant. Serializing binary values to JSON is only implemented for debugging purposes.
|
||||
|
||||
### BJData
|
||||
|
||||
[BJData](binary_formats/bjdata.md) neither supports binary values nor subtypes and proposes to serialize binary values
|
||||
as an array of uint8 values. The library implements this translation.
|
||||
|
||||
??? example
|
||||
|
||||
Code:
|
||||
|
||||
```cpp
|
||||
// create a binary value of subtype 42 (will be ignored in BJData)
|
||||
json j;
|
||||
j["binary"] = json::binary({0xCA, 0xFE, 0xBA, 0xBE}, 42);
|
||||
|
||||
// convert to BJData
|
||||
auto v = json::to_bjdata(j);
|
||||
```
|
||||
|
||||
`v` is a `std::vector<std::uint8_t>` with the following 20 elements:
|
||||
|
||||
```c
|
||||
0x7B // '{'
|
||||
0x69 0x06 // i 6 (length of the key)
|
||||
0x62 0x69 0x6E 0x61 0x72 0x79 // "binary"
|
||||
0x5B // '['
|
||||
0x55 0xCA 0x55 0xFE 0x55 0xBA 0x55 0xBE // content (each byte prefixed with 'U')
|
||||
0x5D // ']'
|
||||
0x7D // '}'
|
||||
```
|
||||
|
||||
The following code uses the type and size optimization for BJData:
|
||||
|
||||
```cpp
|
||||
// convert to BJData using the size and type optimization
|
||||
auto v = json::to_bjdata(j, true, true);
|
||||
```
|
||||
|
||||
The resulting vector has 22 elements; the optimization is not effective for examples with few values:
|
||||
|
||||
```c
|
||||
0x7B // '{'
|
||||
0x23 0x69 0x01 // '#' 'i' type of the array elements: unsigned integers
|
||||
0x69 0x06 // i 6 (length of the key)
|
||||
0x62 0x69 0x6E 0x61 0x72 0x79 // "binary"
|
||||
0x5B // '[' array
|
||||
0x24 0x55 // '$' 'U' type of the array elements: unsigned integers
|
||||
0x23 0x69 0x04 // '#' i 4 number of array elements
|
||||
0xCA 0xFE 0xBA 0xBE // content
|
||||
```
|
||||
|
||||
Note that subtype (42) is **not** serialized and that BJData has **no binary type**, and deserializing `v` would
|
||||
yield the following value:
|
||||
|
||||
```json
|
||||
{
|
||||
"binary": [202, 254, 186, 190]
|
||||
}
|
||||
```
|
||||
|
||||
### BSON
|
||||
|
||||
[BSON](binary_formats/bson.md) supports binary values and subtypes. If a subtype is given, it is used and added as an
|
||||
unsigned 8-bit integer. If no subtype is given, the generic binary subtype 0x00 is used.
|
||||
|
||||
??? example
|
||||
|
||||
Code:
|
||||
|
||||
```cpp
|
||||
// create a binary value of subtype 42
|
||||
json j;
|
||||
j["binary"] = json::binary({0xCA, 0xFE, 0xBA, 0xBE}, 42);
|
||||
|
||||
// convert to BSON
|
||||
auto v = json::to_bson(j);
|
||||
```
|
||||
|
||||
`v` is a `std::vector<std::uint8_t>` with the following 22 elements:
|
||||
|
||||
```c
|
||||
0x16 0x00 0x00 0x00 // number of bytes in the document
|
||||
0x05 // binary value
|
||||
0x62 0x69 0x6E 0x61 0x72 0x79 0x00 // key "binary" + null byte
|
||||
0x04 0x00 0x00 0x00 // number of bytes
|
||||
0x2a // subtype
|
||||
0xCA 0xFE 0xBA 0xBE // content
|
||||
0x00 // end of the document
|
||||
```
|
||||
|
||||
Note that the serialization preserves the subtype, and deserializing `v` would yield the following value:
|
||||
|
||||
```json
|
||||
{
|
||||
"binary": {
|
||||
"bytes": [202, 254, 186, 190],
|
||||
"subtype": 42
|
||||
}
|
||||
}
|
||||
```
|
||||
|
||||
### CBOR
|
||||
|
||||
[CBOR](binary_formats/cbor.md) supports binary values, but no subtypes. Subtypes will be serialized as tags. Any binary
|
||||
value will be serialized as byte strings. The library will choose the smallest representation using the length of the
|
||||
byte array.
|
||||
|
||||
??? example
|
||||
|
||||
Code:
|
||||
|
||||
```cpp
|
||||
// create a binary value of subtype 42
|
||||
json j;
|
||||
j["binary"] = json::binary({0xCA, 0xFE, 0xBA, 0xBE}, 42);
|
||||
|
||||
// convert to CBOR
|
||||
auto v = json::to_cbor(j);
|
||||
```
|
||||
|
||||
`v` is a `std::vector<std::uint8_t>` with the following 15 elements:
|
||||
|
||||
```c
|
||||
0xA1 // map(1)
|
||||
0x66 // text(6)
|
||||
0x62 0x69 0x6E 0x61 0x72 0x79 // "binary"
|
||||
0xD8 0x2A // tag(42)
|
||||
0x44 // bytes(4)
|
||||
0xCA 0xFE 0xBA 0xBE // content
|
||||
```
|
||||
|
||||
Note that the subtype is serialized as tag. However, parsing tagged values yield a parse error unless
|
||||
`json::cbor_tag_handler_t::ignore` or `json::cbor_tag_handler_t::store` is passed to `json::from_cbor`.
|
||||
|
||||
```json
|
||||
{
|
||||
"binary": {
|
||||
"bytes": [202, 254, 186, 190],
|
||||
"subtype": null
|
||||
}
|
||||
}
|
||||
```
|
||||
|
||||
### MessagePack
|
||||
|
||||
[MessagePack](binary_formats/messagepack.md) supports binary values and subtypes. If a subtype is given, the ext family
|
||||
is used. The library will choose the smallest representation among fixext1, fixext2, fixext4, fixext8, ext8, ext16, and
|
||||
ext32. The subtype is then added as a signed 8-bit integer.
|
||||
|
||||
If no subtype is given, the bin family (bin8, bin16, bin32) is used.
|
||||
|
||||
??? example
|
||||
|
||||
Code:
|
||||
|
||||
```cpp
|
||||
// create a binary value of subtype 42
|
||||
json j;
|
||||
j["binary"] = json::binary({0xCA, 0xFE, 0xBA, 0xBE}, 42);
|
||||
|
||||
// convert to MessagePack
|
||||
auto v = json::to_msgpack(j);
|
||||
```
|
||||
|
||||
`v` is a `std::vector<std::uint8_t>` with the following 14 elements:
|
||||
|
||||
```c
|
||||
0x81 // fixmap1
|
||||
0xA6 // fixstr6
|
||||
0x62 0x69 0x6E 0x61 0x72 0x79 // "binary"
|
||||
0xD6 // fixext4
|
||||
0x2A // subtype
|
||||
0xCA 0xFE 0xBA 0xBE // content
|
||||
```
|
||||
|
||||
Note that the serialization preserves the subtype, and deserializing `v` would yield the following value:
|
||||
|
||||
```json
|
||||
{
|
||||
"binary": {
|
||||
"bytes": [202, 254, 186, 190],
|
||||
"subtype": 42
|
||||
}
|
||||
}
|
||||
```
|
||||
|
||||
### UBJSON
|
||||
|
||||
[UBJSON](binary_formats/ubjson.md) neither supports binary values nor subtypes and proposes to serialize binary values
|
||||
as an array of uint8 values. The library implements this translation.
|
||||
|
||||
??? example
|
||||
|
||||
Code:
|
||||
|
||||
```cpp
|
||||
// create a binary value of subtype 42 (will be ignored in UBJSON)
|
||||
json j;
|
||||
j["binary"] = json::binary({0xCA, 0xFE, 0xBA, 0xBE}, 42);
|
||||
|
||||
// convert to UBJSON
|
||||
auto v = json::to_ubjson(j);
|
||||
```
|
||||
|
||||
`v` is a `std::vector<std::uint8_t>` with the following 20 elements:
|
||||
|
||||
```c
|
||||
0x7B // '{'
|
||||
0x69 0x06 // i 6 (length of the key)
|
||||
0x62 0x69 0x6E 0x61 0x72 0x79 // "binary"
|
||||
0x5B // '['
|
||||
0x55 0xCA 0x55 0xFE 0x55 0xBA 0x55 0xBE // content (each byte prefixed with 'U')
|
||||
0x5D // ']'
|
||||
0x7D // '}'
|
||||
```
|
||||
|
||||
The following code uses the type and size optimization for UBJSON:
|
||||
|
||||
```cpp
|
||||
// convert to UBJSON using the size and type optimization
|
||||
auto v = json::to_ubjson(j, true, true);
|
||||
```
|
||||
|
||||
The resulting vector has 23 elements; the optimization is not effective for examples with few values:
|
||||
|
||||
```c
|
||||
0x7B // '{'
|
||||
0x24 // '$' type of the object elements
|
||||
0x5B // '[' array
|
||||
0x23 0x69 0x01 // '#' i 1 number of object elements
|
||||
0x69 0x06 // i 6 (length of the key)
|
||||
0x62 0x69 0x6E 0x61 0x72 0x79 // "binary"
|
||||
0x24 0x55 // '$' 'U' type of the array elements: unsigned integers
|
||||
0x23 0x69 0x04 // '#' i 4 number of array elements
|
||||
0xCA 0xFE 0xBA 0xBE // content
|
||||
```
|
||||
|
||||
Note that subtype (42) is **not** serialized and that UBJSON has **no binary type**, and deserializing `v` would
|
||||
yield the following value:
|
||||
|
||||
```json
|
||||
{
|
||||
"binary": [202, 254, 186, 190]
|
||||
}
|
||||
```
|
||||
@@ -154,4 +154,4 @@ 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="July 4, 2026 09:13:25 UTC">July 4, 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> <!-- 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="July 8, 2026 18:18:24 UTC">July 8, 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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|
||||
@@ -0,0 +1,353 @@
|
||||
# Binary Values
|
||||
|
||||
The library implements several [binary formats](https://json.nlohmann.me/features/binary_formats/index.md) that encode JSON in an efficient way. Most of these formats support binary values; that is, values that have semantics defined outside the library and only define a sequence of bytes to be stored.
|
||||
|
||||
JSON itself does not have a binary value. As such, binary values are an extension that this library implements to store values received by a binary format. Binary values are never created by the JSON parser and are only part of a serialized JSON text if they have been created manually or via a binary format.
|
||||
|
||||
## API for binary values
|
||||
|
||||
```
|
||||
classDiagram
|
||||
|
||||
class binary_t ["json::binary_t"] {
|
||||
+void set_subtype(std::uint64_t subtype)
|
||||
+void clear_subtype()
|
||||
+std::uint64_t subtype() const
|
||||
+bool has_subtype() const
|
||||
}
|
||||
|
||||
class vector ["std::vector<uint8_t>"]
|
||||
|
||||
vector <|-- binary_t
|
||||
```
|
||||
|
||||
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`:
|
||||
|
||||
```
|
||||
auto binary = json::binary_t({0xCA, 0xFE, 0xBA, 0xBE});
|
||||
auto binary_with_subtype = json::binary_t({0xCA, 0xFE, 0xBA, 0xBE}, 42);
|
||||
```
|
||||
|
||||
There are several convenience functions to check and set the subtype:
|
||||
|
||||
```
|
||||
binary.has_subtype(); // returns false
|
||||
binary_with_subtype.has_subtype(); // returns true
|
||||
|
||||
binary_with_subtype.clear_subtype();
|
||||
binary_with_subtype.has_subtype(); // returns false
|
||||
|
||||
binary_with_subtype.set_subtype(42);
|
||||
binary.set_subtype(23);
|
||||
|
||||
binary.subtype(); // returns 23
|
||||
```
|
||||
|
||||
As `json::binary_t` is subclassing `std::vector<std::uint8_t>`, all member functions are available:
|
||||
|
||||
```
|
||||
binary.size(); // returns 4
|
||||
binary[1]; // returns 0xFE
|
||||
```
|
||||
|
||||
JSON values can be constructed from `json::binary_t`:
|
||||
|
||||
```
|
||||
json j = binary;
|
||||
```
|
||||
|
||||
Binary values are primitive values just like numbers or strings:
|
||||
|
||||
```
|
||||
j.is_binary(); // returns true
|
||||
j.is_primitive(); // returns true
|
||||
```
|
||||
|
||||
Given a binary JSON value, the `binary_t` can be accessed by reference as via `get_binary()`:
|
||||
|
||||
```
|
||||
j.get_binary().has_subtype(); // returns true
|
||||
j.get_binary().size(); // returns 4
|
||||
```
|
||||
|
||||
For convenience, binary JSON values can be constructed via `json::binary`:
|
||||
|
||||
```
|
||||
auto j2 = json::binary({0xCA, 0xFE, 0xBA, 0xBE}, 23);
|
||||
auto j3 = json::binary({0xCA, 0xFE, 0xBA, 0xBE});
|
||||
|
||||
j2 == j; // returns true
|
||||
j3.get_binary().has_subtype(); // returns false
|
||||
j3.get_binary().subtype(); // returns std::uint64_t(-1) as j3 has no subtype
|
||||
```
|
||||
|
||||
## Serialization
|
||||
|
||||
Binary values are serialized differently according to the formats.
|
||||
|
||||
### JSON
|
||||
|
||||
JSON does not have a binary type, and this library does not introduce a new type as this would break conformance. Instead, binary values are serialized as an object with two keys: `bytes` holds an array of integers, and `subtype` is an integer or `null`.
|
||||
|
||||
Example
|
||||
|
||||
Code:
|
||||
|
||||
```
|
||||
// create a binary value of subtype 42
|
||||
json j;
|
||||
j["binary"] = json::binary({0xCA, 0xFE, 0xBA, 0xBE}, 42);
|
||||
|
||||
// serialize to standard output
|
||||
std::cout << j.dump(2) << std::endl;
|
||||
```
|
||||
|
||||
Output:
|
||||
|
||||
```
|
||||
{
|
||||
"binary": {
|
||||
"bytes": [202, 254, 186, 190],
|
||||
"subtype": 42
|
||||
}
|
||||
}
|
||||
```
|
||||
|
||||
No roundtrip for binary values
|
||||
|
||||
The JSON parser will not parse the objects generated by binary values back to binary values. This is by design to remain standards compliant. Serializing binary values to JSON is only implemented for debugging purposes.
|
||||
|
||||
### BJData
|
||||
|
||||
[BJData](https://json.nlohmann.me/features/binary_formats/bjdata/index.md) neither supports binary values nor subtypes and proposes to serialize binary values as an array of uint8 values. The library implements this translation.
|
||||
|
||||
Example
|
||||
|
||||
Code:
|
||||
|
||||
```
|
||||
// create a binary value of subtype 42 (will be ignored in BJData)
|
||||
json j;
|
||||
j["binary"] = json::binary({0xCA, 0xFE, 0xBA, 0xBE}, 42);
|
||||
|
||||
// convert to BJData
|
||||
auto v = json::to_bjdata(j);
|
||||
```
|
||||
|
||||
`v` is a `std::vector<std::uint8_t>` with the following 20 elements:
|
||||
|
||||
```
|
||||
0x7B // '{'
|
||||
0x69 0x06 // i 6 (length of the key)
|
||||
0x62 0x69 0x6E 0x61 0x72 0x79 // "binary"
|
||||
0x5B // '['
|
||||
0x55 0xCA 0x55 0xFE 0x55 0xBA 0x55 0xBE // content (each byte prefixed with 'U')
|
||||
0x5D // ']'
|
||||
0x7D // '}'
|
||||
```
|
||||
|
||||
The following code uses the type and size optimization for BJData:
|
||||
|
||||
```
|
||||
// convert to BJData using the size and type optimization
|
||||
auto v = json::to_bjdata(j, true, true);
|
||||
```
|
||||
|
||||
The resulting vector has 22 elements; the optimization is not effective for examples with few values:
|
||||
|
||||
```
|
||||
0x7B // '{'
|
||||
0x23 0x69 0x01 // '#' 'i' type of the array elements: unsigned integers
|
||||
0x69 0x06 // i 6 (length of the key)
|
||||
0x62 0x69 0x6E 0x61 0x72 0x79 // "binary"
|
||||
0x5B // '[' array
|
||||
0x24 0x55 // '$' 'U' type of the array elements: unsigned integers
|
||||
0x23 0x69 0x04 // '#' i 4 number of array elements
|
||||
0xCA 0xFE 0xBA 0xBE // content
|
||||
```
|
||||
|
||||
Note that subtype (42) is **not** serialized and that BJData has **no binary type**, and deserializing `v` would yield the following value:
|
||||
|
||||
```
|
||||
{
|
||||
"binary": [202, 254, 186, 190]
|
||||
}
|
||||
```
|
||||
|
||||
### BSON
|
||||
|
||||
[BSON](https://json.nlohmann.me/features/binary_formats/bson/index.md) supports binary values and subtypes. If a subtype is given, it is used and added as an unsigned 8-bit integer. If no subtype is given, the generic binary subtype 0x00 is used.
|
||||
|
||||
Example
|
||||
|
||||
Code:
|
||||
|
||||
```
|
||||
// create a binary value of subtype 42
|
||||
json j;
|
||||
j["binary"] = json::binary({0xCA, 0xFE, 0xBA, 0xBE}, 42);
|
||||
|
||||
// convert to BSON
|
||||
auto v = json::to_bson(j);
|
||||
```
|
||||
|
||||
`v` is a `std::vector<std::uint8_t>` with the following 22 elements:
|
||||
|
||||
```
|
||||
0x16 0x00 0x00 0x00 // number of bytes in the document
|
||||
0x05 // binary value
|
||||
0x62 0x69 0x6E 0x61 0x72 0x79 0x00 // key "binary" + null byte
|
||||
0x04 0x00 0x00 0x00 // number of bytes
|
||||
0x2a // subtype
|
||||
0xCA 0xFE 0xBA 0xBE // content
|
||||
0x00 // end of the document
|
||||
```
|
||||
|
||||
Note that the serialization preserves the subtype, and deserializing `v` would yield the following value:
|
||||
|
||||
```
|
||||
{
|
||||
"binary": {
|
||||
"bytes": [202, 254, 186, 190],
|
||||
"subtype": 42
|
||||
}
|
||||
}
|
||||
```
|
||||
|
||||
### CBOR
|
||||
|
||||
[CBOR](https://json.nlohmann.me/features/binary_formats/cbor/index.md) supports binary values, but no subtypes. Subtypes will be serialized as tags. Any binary value will be serialized as byte strings. The library will choose the smallest representation using the length of the byte array.
|
||||
|
||||
Example
|
||||
|
||||
Code:
|
||||
|
||||
```
|
||||
// create a binary value of subtype 42
|
||||
json j;
|
||||
j["binary"] = json::binary({0xCA, 0xFE, 0xBA, 0xBE}, 42);
|
||||
|
||||
// convert to CBOR
|
||||
auto v = json::to_cbor(j);
|
||||
```
|
||||
|
||||
`v` is a `std::vector<std::uint8_t>` with the following 15 elements:
|
||||
|
||||
```
|
||||
0xA1 // map(1)
|
||||
0x66 // text(6)
|
||||
0x62 0x69 0x6E 0x61 0x72 0x79 // "binary"
|
||||
0xD8 0x2A // tag(42)
|
||||
0x44 // bytes(4)
|
||||
0xCA 0xFE 0xBA 0xBE // content
|
||||
```
|
||||
|
||||
Note that the subtype is serialized as tag. However, parsing tagged values yield a parse error unless `json::cbor_tag_handler_t::ignore` or `json::cbor_tag_handler_t::store` is passed to `json::from_cbor`.
|
||||
|
||||
```
|
||||
{
|
||||
"binary": {
|
||||
"bytes": [202, 254, 186, 190],
|
||||
"subtype": null
|
||||
}
|
||||
}
|
||||
```
|
||||
|
||||
### MessagePack
|
||||
|
||||
[MessagePack](https://json.nlohmann.me/features/binary_formats/messagepack/index.md) supports binary values and subtypes. If a subtype is given, the ext family is used. The library will choose the smallest representation among fixext1, fixext2, fixext4, fixext8, ext8, ext16, and ext32. The subtype is then added as a signed 8-bit integer.
|
||||
|
||||
If no subtype is given, the bin family (bin8, bin16, bin32) is used.
|
||||
|
||||
Example
|
||||
|
||||
Code:
|
||||
|
||||
```
|
||||
// create a binary value of subtype 42
|
||||
json j;
|
||||
j["binary"] = json::binary({0xCA, 0xFE, 0xBA, 0xBE}, 42);
|
||||
|
||||
// convert to MessagePack
|
||||
auto v = json::to_msgpack(j);
|
||||
```
|
||||
|
||||
`v` is a `std::vector<std::uint8_t>` with the following 14 elements:
|
||||
|
||||
```
|
||||
0x81 // fixmap1
|
||||
0xA6 // fixstr6
|
||||
0x62 0x69 0x6E 0x61 0x72 0x79 // "binary"
|
||||
0xD6 // fixext4
|
||||
0x2A // subtype
|
||||
0xCA 0xFE 0xBA 0xBE // content
|
||||
```
|
||||
|
||||
Note that the serialization preserves the subtype, and deserializing `v` would yield the following value:
|
||||
|
||||
```
|
||||
{
|
||||
"binary": {
|
||||
"bytes": [202, 254, 186, 190],
|
||||
"subtype": 42
|
||||
}
|
||||
}
|
||||
```
|
||||
|
||||
### UBJSON
|
||||
|
||||
[UBJSON](https://json.nlohmann.me/features/binary_formats/ubjson/index.md) neither supports binary values nor subtypes and proposes to serialize binary values as an array of uint8 values. The library implements this translation.
|
||||
|
||||
Example
|
||||
|
||||
Code:
|
||||
|
||||
```
|
||||
// create a binary value of subtype 42 (will be ignored in UBJSON)
|
||||
json j;
|
||||
j["binary"] = json::binary({0xCA, 0xFE, 0xBA, 0xBE}, 42);
|
||||
|
||||
// convert to UBJSON
|
||||
auto v = json::to_ubjson(j);
|
||||
```
|
||||
|
||||
`v` is a `std::vector<std::uint8_t>` with the following 20 elements:
|
||||
|
||||
```
|
||||
0x7B // '{'
|
||||
0x69 0x06 // i 6 (length of the key)
|
||||
0x62 0x69 0x6E 0x61 0x72 0x79 // "binary"
|
||||
0x5B // '['
|
||||
0x55 0xCA 0x55 0xFE 0x55 0xBA 0x55 0xBE // content (each byte prefixed with 'U')
|
||||
0x5D // ']'
|
||||
0x7D // '}'
|
||||
```
|
||||
|
||||
The following code uses the type and size optimization for UBJSON:
|
||||
|
||||
```
|
||||
// convert to UBJSON using the size and type optimization
|
||||
auto v = json::to_ubjson(j, true, true);
|
||||
```
|
||||
|
||||
The resulting vector has 23 elements; the optimization is not effective for examples with few values:
|
||||
|
||||
```
|
||||
0x7B // '{'
|
||||
0x24 // '$' type of the object elements
|
||||
0x5B // '[' array
|
||||
0x23 0x69 0x01 // '#' i 1 number of object elements
|
||||
0x69 0x06 // i 6 (length of the key)
|
||||
0x62 0x69 0x6E 0x61 0x72 0x79 // "binary"
|
||||
0x24 0x55 // '$' 'U' type of the array elements: unsigned integers
|
||||
0x23 0x69 0x04 // '#' i 4 number of array elements
|
||||
0xCA 0xFE 0xBA 0xBE // content
|
||||
```
|
||||
|
||||
Note that subtype (42) is **not** serialized and that UBJSON has **no binary type**, and deserializing `v` would yield the following value:
|
||||
|
||||
```
|
||||
{
|
||||
"binary": [202, 254, 186, 190]
|
||||
}
|
||||
```
|
||||
@@ -0,0 +1,40 @@
|
||||
# Comments
|
||||
|
||||
This library does not support comments *by default*. It does so for three reasons:
|
||||
|
||||
1. Comments are not part of the [JSON specification](https://tools.ietf.org/html/rfc8259). You may argue that `//` or `/* */` are allowed in JavaScript, but JSON is not JavaScript.
|
||||
2. This was not an oversight: Douglas Crockford [wrote on this](https://news.ycombinator.com/item?id=3912149) in May 2012:
|
||||
|
||||
> I removed comments from JSON because I saw people were using them to hold parsing directives, a practice which would have destroyed interoperability. I know that the lack of comments makes some people sad, but it shouldn't.
|
||||
|
||||
> Suppose you are using JSON to keep configuration files, which you would like to annotate. Go ahead and insert all the comments you like. Then pipe it through JSMin before handing it to your JSON parser.
|
||||
|
||||
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.
|
||||
|
||||
For more information, see [JSON With Commas and Comments (JWCC)](https://nigeltao.github.io/blog/2021/json-with-commas-comments.html).
|
||||
|
||||
!!! example
|
||||
|
||||
Consider the following JSON with comments.
|
||||
|
||||
```json
|
||||
{
|
||||
// update in 2006: removed Pluto
|
||||
"planets": ["Mercury", "Venus", "Earth", "Mars",
|
||||
"Jupiter", "Uranus", "Neptune" /*, "Pluto" */]
|
||||
}
|
||||
```
|
||||
|
||||
When calling `parse` without additional argument, a parse error exception is thrown. If `ignore_comments` is set to `#!cpp true`, the comments are ignored during parsing:
|
||||
|
||||
```cpp
|
||||
--8<-- "examples/comments.cpp"
|
||||
```
|
||||
|
||||
Output:
|
||||
|
||||
```
|
||||
--8<-- "examples/comments.output"
|
||||
```
|
||||
@@ -45,4 +45,4 @@
|
||||
"Neptune"
|
||||
]
|
||||
}
|
||||
</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="July 4, 2026 09:13:25 UTC">July 4, 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> <!-- 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="July 8, 2026 18:18:24 UTC">July 8, 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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|
||||
@@ -0,0 +1,81 @@
|
||||
# Comments
|
||||
|
||||
This library does not support comments *by default*. It does so for three reasons:
|
||||
|
||||
1. Comments are not part of the [JSON specification](https://tools.ietf.org/html/rfc8259). You may argue that `//` or `/* */` are allowed in JavaScript, but JSON is not JavaScript.
|
||||
|
||||
1. This was not an oversight: Douglas Crockford [wrote on this](https://news.ycombinator.com/item?id=3912149) in May 2012:
|
||||
|
||||
> I removed comments from JSON because I saw people were using them to hold parsing directives, a practice which would have destroyed interoperability. I know that the lack of comments makes some people sad, but it shouldn't.
|
||||
>
|
||||
> Suppose you are using JSON to keep configuration files, which you would like to annotate. Go ahead and insert all the comments you like. Then pipe it through JSMin before handing it to your JSON parser.
|
||||
|
||||
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.
|
||||
|
||||
For more information, see [JSON With Commas and Comments (JWCC)](https://nigeltao.github.io/blog/2021/json-with-commas-comments.html).
|
||||
|
||||
Example
|
||||
|
||||
Consider the following JSON with comments.
|
||||
|
||||
```
|
||||
{
|
||||
// update in 2006: removed Pluto
|
||||
"planets": ["Mercury", "Venus", "Earth", "Mars",
|
||||
"Jupiter", "Uranus", "Neptune" /*, "Pluto" */]
|
||||
}
|
||||
```
|
||||
|
||||
When calling `parse` without additional argument, a parse error exception is thrown. If `ignore_comments` is set to `true`, the comments are ignored during parsing:
|
||||
|
||||
```
|
||||
#include <iostream>
|
||||
#include <nlohmann/json.hpp>
|
||||
|
||||
using json = nlohmann::json;
|
||||
|
||||
int main()
|
||||
{
|
||||
std::string s = R"(
|
||||
{
|
||||
// update in 2006: removed Pluto
|
||||
"planets": ["Mercury", "Venus", "Earth", "Mars",
|
||||
"Jupiter", "Uranus", "Neptune" /*, "Pluto" */]
|
||||
}
|
||||
)";
|
||||
|
||||
try
|
||||
{
|
||||
json j = json::parse(s);
|
||||
}
|
||||
catch (json::exception& e)
|
||||
{
|
||||
std::cout << e.what() << std::endl;
|
||||
}
|
||||
|
||||
json j = json::parse(s,
|
||||
/* callback */ nullptr,
|
||||
/* allow exceptions */ true,
|
||||
/* ignore_comments */ true);
|
||||
std::cout << j.dump(2) << '\n';
|
||||
}
|
||||
```
|
||||
|
||||
Output:
|
||||
|
||||
```
|
||||
[json.exception.parse_error.101] parse error at line 3, column 9: syntax error while parsing object key - invalid literal; last read: '<U+000A> {<U+000A> /'; expected string literal
|
||||
{
|
||||
"planets": [
|
||||
"Mercury",
|
||||
"Venus",
|
||||
"Earth",
|
||||
"Mars",
|
||||
"Jupiter",
|
||||
"Uranus",
|
||||
"Neptune"
|
||||
]
|
||||
}
|
||||
```
|
||||
@@ -0,0 +1,91 @@
|
||||
# Converting values
|
||||
|
||||
A `basic_json` value stores JSON data, but most of the time you want to move that data into ordinary C++ types (an
|
||||
`#!cpp int`, a `#!cpp std::string`, a `#!cpp std::vector`, or one of your own structs) and back. This page describes how
|
||||
these conversions work.
|
||||
|
||||
## Getting values out
|
||||
|
||||
The [`get`](../api/basic_json/get.md) function template returns a copy of the stored value converted to the requested
|
||||
type:
|
||||
|
||||
```cpp
|
||||
json j = R"({"name": "Mary", "age": 42, "hobbies": ["hiking", "reading"]})"_json;
|
||||
|
||||
auto name = j["name"].get<std::string>(); // "Mary"
|
||||
auto age = j["age"].get<int>(); // 42
|
||||
auto hobbies = j["hobbies"].get<std::vector<std::string>>(); // {"hiking", "reading"}
|
||||
```
|
||||
|
||||
!!! note "Getting a string without quotes"
|
||||
|
||||
A frequent point of confusion: use [`get`](../api/basic_json/get.md), **not** [`dump`](serialization.md), to read a
|
||||
string value. `#!cpp j["name"].get<std::string>()` yields `#!cpp Mary`, whereas `#!cpp j["name"].dump()` yields the
|
||||
JSON text `#!cpp "Mary"` (**with** quotes), because `dump` always produces a JSON text.
|
||||
|
||||
Alternatively, [`get_to`](../api/basic_json/get_to.md) writes into an existing variable and deduces the target type,
|
||||
which avoids repeating it:
|
||||
|
||||
??? example
|
||||
|
||||
```cpp
|
||||
--8<-- "examples/get_to.cpp"
|
||||
```
|
||||
|
||||
Output:
|
||||
|
||||
```json
|
||||
--8<-- "examples/get_to.output"
|
||||
```
|
||||
|
||||
The library already knows how to convert to and from the scalar types and the STL containers (such as
|
||||
`#!cpp std::vector`, `#!cpp std::map`, `#!cpp std::array`, `#!cpp std::optional`, and many more). Converting a JSON
|
||||
object back to a `#!cpp std::map` or a JSON array back to a `#!cpp std::vector` therefore works without any extra code:
|
||||
|
||||
```cpp
|
||||
json j = {{"one", 1}, {"two", 2}};
|
||||
auto m = j.get<std::map<std::string, int>>(); // {{"one", 1}, {"two", 2}}
|
||||
```
|
||||
|
||||
## Implicit conversions
|
||||
|
||||
By default, a JSON value implicitly converts to a compatible C++ type, so the explicit `get` call can often be omitted:
|
||||
|
||||
```cpp
|
||||
json j = "Hello";
|
||||
std::string s = j; // implicit conversion, same as j.get<std::string>()
|
||||
```
|
||||
|
||||
Implicit conversions are convenient but can be surprising (for example, in overload resolution or with `auto`). They can
|
||||
be disabled by defining [`JSON_USE_IMPLICIT_CONVERSIONS`](../api/macros/json_use_implicit_conversions.md) to `#!cpp 0`,
|
||||
which forces the explicit `get` form and can catch unintended conversions at compile time.
|
||||
|
||||
!!! warning "Conversions do not range-check numbers"
|
||||
|
||||
Just like C++ itself, the `get` family performs numeric conversions without range checks — retrieving a
|
||||
floating-point value as an integer truncates it, and narrowing conversions may overflow. See
|
||||
[number conversion](types/number_handling.md#number-conversion) for details and how to guard against it.
|
||||
|
||||
## Putting values in
|
||||
|
||||
The reverse direction works the same way: assigning or constructing a `json` from a C++ value converts it to JSON.
|
||||
|
||||
```cpp
|
||||
std::vector<int> numbers = {1, 2, 3};
|
||||
json j = numbers; // [1,2,3]
|
||||
```
|
||||
|
||||
## Your own types
|
||||
|
||||
The conversions above are built in for standard types. To make the same syntax work for **your own** types, provide
|
||||
`to_json`/`from_json` functions (or use one of the convenience macros). This is described in detail on the
|
||||
[arbitrary types conversions](arbitrary_types.md) page. Enums can be mapped to strings as described in
|
||||
[specializing enum conversion](enum_conversion.md).
|
||||
|
||||
## See also
|
||||
|
||||
- [`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
|
||||
- [Arbitrary types conversions](arbitrary_types.md) - support your own types
|
||||
- [`JSON_USE_IMPLICIT_CONVERSIONS`](../api/macros/json_use_implicit_conversions.md) - toggle implicit conversions
|
||||
@@ -79,4 +79,4 @@
|
||||
<span class=n>std</span><span class=o>::</span><span class=n>string</span><span class=w> </span><span class=n>s</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=c1>// implicit conversion, same as j.get<std::string>()</span>
|
||||
</code></pre></div> <p>Implicit conversions are convenient but can be surprising (for example, in overload resolution or with <code>auto</code>). They can be disabled by defining <a href=../../api/macros/json_use_implicit_conversions/ ><code>JSON_USE_IMPLICIT_CONVERSIONS</code></a> to <code class=highlight><span class=mi>0</span></code>, which forces the explicit <code>get</code> form and can catch unintended conversions at compile time.</p> <div class="admonition warning"> <p class=admonition-title>Conversions do not range-check numbers</p> <p>Just like C++ itself, the <code>get</code> family performs numeric conversions without range checks — retrieving a floating-point value as an integer truncates it, and narrowing conversions may overflow. See <a href=../types/number_handling/#number-conversion>number conversion</a> for details and how to guard against it.</p> </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> <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> </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="July 4, 2026 09:13:25 UTC">July 4, 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 FreLine truncated
|
||||
</code></pre></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> </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="July 8, 2026 18:18:24 UTC">July 8, 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 FreLine truncated
|
||||
@@ -0,0 +1,144 @@
|
||||
# Converting values
|
||||
|
||||
A `basic_json` value stores JSON data, but most of the time you want to move that data into ordinary C++ types (an `int`, a `std::string`, a `std::vector`, or one of your own structs) and back. This page describes how these conversions work.
|
||||
|
||||
## Getting values out
|
||||
|
||||
The [`get`](https://json.nlohmann.me/api/basic_json/get/index.md) function template returns a copy of the stored value converted to the requested type:
|
||||
|
||||
```
|
||||
json j = R"({"name": "Mary", "age": 42, "hobbies": ["hiking", "reading"]})"_json;
|
||||
|
||||
auto name = j["name"].get<std::string>(); // "Mary"
|
||||
auto age = j["age"].get<int>(); // 42
|
||||
auto hobbies = j["hobbies"].get<std::vector<std::string>>(); // {"hiking", "reading"}
|
||||
```
|
||||
|
||||
Getting a string without quotes
|
||||
|
||||
A frequent point of confusion: use [`get`](https://json.nlohmann.me/api/basic_json/get/index.md), **not** [`dump`](https://json.nlohmann.me/features/serialization/index.md), to read a string value. `j["name"].get<std::string>()` yields `Mary`, whereas `j["name"].dump()` yields the JSON text `"Mary"` (**with** quotes), because `dump` always produces a JSON text.
|
||||
|
||||
Alternatively, [`get_to`](https://json.nlohmann.me/api/basic_json/get_to/index.md) writes into an existing variable and deduces the target type, which avoids repeating it:
|
||||
|
||||
Example
|
||||
|
||||
```
|
||||
#include <iostream>
|
||||
#include <unordered_map>
|
||||
#include <nlohmann/json.hpp>
|
||||
|
||||
using json = nlohmann::json;
|
||||
|
||||
int main()
|
||||
{
|
||||
// create a JSON value with different types
|
||||
json json_types =
|
||||
{
|
||||
{"boolean", true},
|
||||
{
|
||||
"number", {
|
||||
{"integer", 42},
|
||||
{"floating-point", 17.23}
|
||||
}
|
||||
},
|
||||
{"string", "Hello, world!"},
|
||||
{"array", {1, 2, 3, 4, 5}},
|
||||
{"null", nullptr}
|
||||
};
|
||||
|
||||
bool v1;
|
||||
int v2;
|
||||
short v3;
|
||||
float v4;
|
||||
int v5;
|
||||
std::string v6;
|
||||
std::vector<short> v7;
|
||||
std::unordered_map<std::string, json> v8;
|
||||
|
||||
// use explicit conversions
|
||||
json_types["boolean"].get_to(v1);
|
||||
json_types["number"]["integer"].get_to(v2);
|
||||
json_types["number"]["integer"].get_to(v3);
|
||||
json_types["number"]["floating-point"].get_to(v4);
|
||||
json_types["number"]["floating-point"].get_to(v5);
|
||||
json_types["string"].get_to(v6);
|
||||
json_types["array"].get_to(v7);
|
||||
json_types.get_to(v8);
|
||||
|
||||
// print the conversion results
|
||||
std::cout << v1 << '\n';
|
||||
std::cout << v2 << ' ' << v3 << '\n';
|
||||
std::cout << v4 << ' ' << v5 << '\n';
|
||||
std::cout << v6 << '\n';
|
||||
|
||||
for (auto i : v7)
|
||||
{
|
||||
std::cout << i << ' ';
|
||||
}
|
||||
std::cout << "\n\n";
|
||||
|
||||
for (auto i : v8)
|
||||
{
|
||||
std::cout << i.first << ": " << i.second << '\n';
|
||||
}
|
||||
}
|
||||
```
|
||||
|
||||
Output:
|
||||
|
||||
```
|
||||
1
|
||||
42 42
|
||||
17.23 17
|
||||
Hello, world!
|
||||
1 2 3 4 5
|
||||
|
||||
string: "Hello, world!"
|
||||
number: {"floating-point":17.23,"integer":42}
|
||||
null: null
|
||||
boolean: true
|
||||
array: [1,2,3,4,5]
|
||||
```
|
||||
|
||||
The library already knows how to convert to and from the scalar types and the STL containers (such as `std::vector`, `std::map`, `std::array`, `std::optional`, and many more). Converting a JSON object back to a `std::map` or a JSON array back to a `std::vector` therefore works without any extra code:
|
||||
|
||||
```
|
||||
json j = {{"one", 1}, {"two", 2}};
|
||||
auto m = j.get<std::map<std::string, int>>(); // {{"one", 1}, {"two", 2}}
|
||||
```
|
||||
|
||||
## Implicit conversions
|
||||
|
||||
By default, a JSON value implicitly converts to a compatible C++ type, so the explicit `get` call can often be omitted:
|
||||
|
||||
```
|
||||
json j = "Hello";
|
||||
std::string s = j; // implicit conversion, same as j.get<std::string>()
|
||||
```
|
||||
|
||||
Implicit conversions are convenient but can be surprising (for example, in overload resolution or with `auto`). They can be disabled by defining [`JSON_USE_IMPLICIT_CONVERSIONS`](https://json.nlohmann.me/api/macros/json_use_implicit_conversions/index.md) to `0`, which forces the explicit `get` form and can catch unintended conversions at compile time.
|
||||
|
||||
Conversions do not range-check numbers
|
||||
|
||||
Just like C++ itself, the `get` family performs numeric conversions without range checks — retrieving a floating-point value as an integer truncates it, and narrowing conversions may overflow. See [number conversion](https://json.nlohmann.me/features/types/number_handling/#number-conversion) for details and how to guard against it.
|
||||
|
||||
## Putting values in
|
||||
|
||||
The reverse direction works the same way: assigning or constructing a `json` from a C++ value converts it to JSON.
|
||||
|
||||
```
|
||||
std::vector<int> numbers = {1, 2, 3};
|
||||
json j = numbers; // [1,2,3]
|
||||
```
|
||||
|
||||
## Your own types
|
||||
|
||||
The conversions above are built in for standard types. To make the same syntax work for **your own** types, provide `to_json`/`from_json` functions (or use one of the convenience macros). This is described in detail on the [arbitrary types conversions](https://json.nlohmann.me/features/arbitrary_types/index.md) page. Enums can be mapped to strings as described in [specializing enum conversion](https://json.nlohmann.me/features/enum_conversion/index.md).
|
||||
|
||||
## See also
|
||||
|
||||
- [`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
|
||||
- [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
|
||||
@@ -0,0 +1,103 @@
|
||||
# Creating JSON values
|
||||
|
||||
There are several ways to create a JSON value in memory. This page gives an overview; to read a value from JSON text
|
||||
instead, see [parsing](parsing/index.md).
|
||||
|
||||
## From C++ values
|
||||
|
||||
Any value of a supported C++ type can be assigned to or used to construct a `json`:
|
||||
|
||||
```cpp
|
||||
json j_number = 42;
|
||||
json j_float = 3.141;
|
||||
json j_string = "Hello";
|
||||
json j_boolean = true;
|
||||
json j_null = nullptr;
|
||||
json j_vector = std::vector<int>{1, 2, 3}; // array
|
||||
```
|
||||
|
||||
See [converting values](conversions.md) for the full set of supported types.
|
||||
|
||||
## With initializer lists
|
||||
|
||||
Objects and arrays can be written concisely with brace-enclosed initializer lists:
|
||||
|
||||
```cpp
|
||||
// an array
|
||||
json array = {1, 2, 3, 4};
|
||||
|
||||
// an object (a list of key/value pairs)
|
||||
json object = {
|
||||
{"pi", 3.141},
|
||||
{"happy", true},
|
||||
{"name", "Niels"},
|
||||
{"nothing", nullptr},
|
||||
{"list", {1, 0, 2}},
|
||||
{"object", {{"currency", "USD"}, {"value", 42.99}}}
|
||||
};
|
||||
```
|
||||
|
||||
The library decides between an array and an object based on the content: a list whose elements are all two-element lists
|
||||
with a string as the first element is treated as an object, everything else as an array.
|
||||
|
||||
!!! warning "Ambiguous cases: `#!cpp {}` vs. `#!cpp []`"
|
||||
|
||||
Because the same `#!cpp {}` syntax is used for both arrays and objects, some cases are ambiguous. To force a
|
||||
particular type, use the explicit factory functions [`json::array`](../api/basic_json/array.md) and
|
||||
[`json::object`](../api/basic_json/object.md):
|
||||
|
||||
```cpp
|
||||
json empty_array_explicit = json::array(); // []
|
||||
json empty_object_explicit = json::object(); // {}
|
||||
|
||||
// a JSON array with one object, not an object with one member
|
||||
json array_of_objects = json::array({{"key", "value"}}); // [{"key":"value"}]
|
||||
```
|
||||
|
||||
Related to this, single-element brace initialization such as `#!cpp json j{value};` wraps the element in a
|
||||
single-element **array** by default, and its behavior even differs between compilers. See the
|
||||
[FAQ](../home/faq.md#brace-initialization-yields-arrays) for details and the opt-in
|
||||
[`JSON_BRACE_INIT_COPY_SEMANTICS`](../api/macros/json_brace_init_copy_semantics.md) macro.
|
||||
|
||||
## Building incrementally
|
||||
|
||||
A value can also be built up piece by piece. Accessing a non-existing object key or array index with
|
||||
[`operator[]`](element_access/unchecked_access.md) creates the element on the fly:
|
||||
|
||||
```cpp
|
||||
json j; // null
|
||||
j["answer"]["everything"] = 42; // becomes an object
|
||||
j["list"] = {1, 0, 2};
|
||||
j["list"].push_back(3); // [1,0,2,3]
|
||||
```
|
||||
|
||||
See [modifying values](modifying_values.md) for [`push_back`](../api/basic_json/push_back.md),
|
||||
[`emplace`](../api/basic_json/emplace.md), and related functions.
|
||||
|
||||
## With the `_json` literal
|
||||
|
||||
The `_json` [user-defined literal](../api/operator_literal_json.md) parses a string at the call site and is a
|
||||
convenient way to write a JSON value inline:
|
||||
|
||||
??? example
|
||||
|
||||
```cpp
|
||||
--8<-- "examples/operator_literal_json.cpp"
|
||||
```
|
||||
|
||||
Output:
|
||||
|
||||
```json
|
||||
--8<-- "examples/operator_literal_json.output"
|
||||
```
|
||||
|
||||
Note this **parses** the string, so `#!cpp "42"_json` is the number `#!cpp 42`, whereas `#!cpp json("42")` is the JSON
|
||||
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
|
||||
- [`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
|
||||
@@ -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>
|
||||
</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> </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="July 4, 2026 09:13:25 UTC">July 4, 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://fontawLine truncated
|
||||
</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> </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="July 8, 2026 18:18:24 UTC">July 8, 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://fontawLine truncated
|
||||
@@ -0,0 +1,107 @@
|
||||
# Creating JSON values
|
||||
|
||||
There are several ways to create a JSON value in memory. This page gives an overview; to read a value from JSON text instead, see [parsing](https://json.nlohmann.me/features/parsing/index.md).
|
||||
|
||||
## From C++ values
|
||||
|
||||
Any value of a supported C++ type can be assigned to or used to construct a `json`:
|
||||
|
||||
```
|
||||
json j_number = 42;
|
||||
json j_float = 3.141;
|
||||
json j_string = "Hello";
|
||||
json j_boolean = true;
|
||||
json j_null = nullptr;
|
||||
json j_vector = std::vector<int>{1, 2, 3}; // array
|
||||
```
|
||||
|
||||
See [converting values](https://json.nlohmann.me/features/conversions/index.md) for the full set of supported types.
|
||||
|
||||
## With initializer lists
|
||||
|
||||
Objects and arrays can be written concisely with brace-enclosed initializer lists:
|
||||
|
||||
```
|
||||
// an array
|
||||
json array = {1, 2, 3, 4};
|
||||
|
||||
// an object (a list of key/value pairs)
|
||||
json object = {
|
||||
{"pi", 3.141},
|
||||
{"happy", true},
|
||||
{"name", "Niels"},
|
||||
{"nothing", nullptr},
|
||||
{"list", {1, 0, 2}},
|
||||
{"object", {{"currency", "USD"}, {"value", 42.99}}}
|
||||
};
|
||||
```
|
||||
|
||||
The library decides between an array and an object based on the content: a list whose elements are all two-element lists with a string as the first element is treated as an object, everything else as an array.
|
||||
|
||||
Ambiguous cases: `{}` vs. `[]`
|
||||
|
||||
Because the same `{}` syntax is used for both arrays and objects, some cases are ambiguous. To force a particular type, use the explicit factory functions [`json::array`](https://json.nlohmann.me/api/basic_json/array/index.md) and [`json::object`](https://json.nlohmann.me/api/basic_json/object/index.md):
|
||||
|
||||
```
|
||||
json empty_array_explicit = json::array(); // []
|
||||
json empty_object_explicit = json::object(); // {}
|
||||
|
||||
// a JSON array with one object, not an object with one member
|
||||
json array_of_objects = json::array({{"key", "value"}}); // [{"key":"value"}]
|
||||
```
|
||||
|
||||
Related to this, single-element brace initialization such as `json j{value};` wraps the element in a single-element **array** by default, and its behavior even differs between compilers. See the [FAQ](https://json.nlohmann.me/home/faq/#brace-initialization-yields-arrays) for details and the opt-in [`JSON_BRACE_INIT_COPY_SEMANTICS`](https://json.nlohmann.me/api/macros/json_brace_init_copy_semantics/index.md) macro.
|
||||
|
||||
## Building incrementally
|
||||
|
||||
A value can also be built up piece by piece. Accessing a non-existing object key or array index with [`operator[]`](https://json.nlohmann.me/features/element_access/unchecked_access/index.md) creates the element on the fly:
|
||||
|
||||
```
|
||||
json j; // null
|
||||
j["answer"]["everything"] = 42; // becomes an object
|
||||
j["list"] = {1, 0, 2};
|
||||
j["list"].push_back(3); // [1,0,2,3]
|
||||
```
|
||||
|
||||
See [modifying values](https://json.nlohmann.me/features/modifying_values/index.md) for [`push_back`](https://json.nlohmann.me/api/basic_json/push_back/index.md), [`emplace`](https://json.nlohmann.me/api/basic_json/emplace/index.md), and related functions.
|
||||
|
||||
## With the `_json` literal
|
||||
|
||||
The `_json` [user-defined literal](https://json.nlohmann.me/api/operator_literal_json/index.md) parses a string at the call site and is a convenient way to write a JSON value inline:
|
||||
|
||||
Example
|
||||
|
||||
```
|
||||
#include <iostream>
|
||||
#include <iomanip>
|
||||
#include <nlohmann/json.hpp>
|
||||
|
||||
using json = nlohmann::json;
|
||||
using namespace nlohmann::literals;
|
||||
|
||||
int main()
|
||||
{
|
||||
json j = R"( {"hello": "world", "answer": 42} )"_json;
|
||||
|
||||
std::cout << std::setw(2) << j << '\n';
|
||||
}
|
||||
```
|
||||
|
||||
Output:
|
||||
|
||||
```
|
||||
{
|
||||
"answer": 42,
|
||||
"hello": "world"
|
||||
}
|
||||
```
|
||||
|
||||
Note this **parses** the string, so `"42"_json` is the number `42`, whereas `json("42")` is the JSON string `"42"`.
|
||||
|
||||
## 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
|
||||
- [`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
|
||||
@@ -0,0 +1,9 @@
|
||||
# Element Access
|
||||
|
||||
There are many ways elements in a JSON value can be accessed:
|
||||
|
||||
- unchecked access via [`operator[]`](unchecked_access.md)
|
||||
- checked access via [`at`](checked_access.md)
|
||||
- access with default value via [`value`](default_value.md)
|
||||
- iterators
|
||||
- JSON pointers
|
||||
@@ -0,0 +1,91 @@
|
||||
# Checked access: at
|
||||
|
||||
## Overview
|
||||
|
||||
The [`at`](../../api/basic_json/at.md) member function performs checked access; that is, it returns a reference to the
|
||||
desired value if it exists and throws a [`basic_json::out_of_range` exception](../../home/exceptions.md#out-of-range)
|
||||
otherwise.
|
||||
|
||||
??? example "Read access"
|
||||
|
||||
Consider the following JSON value:
|
||||
|
||||
```json
|
||||
{
|
||||
"name": "Mary Smith",
|
||||
"age": 42,
|
||||
"hobbies": ["hiking", "reading"]
|
||||
}
|
||||
```
|
||||
|
||||
Assume the value is parsed to a `json` variable `j`.
|
||||
|
||||
| expression | value |
|
||||
|-------------------------------|------------------------------------------------------------------------------|
|
||||
| `#!cpp j` | `#!json {"name": "Mary Smith", "age": 42, "hobbies": ["hiking", "reading"]}` |
|
||||
| `#!cpp j.at("name")` | `#!json "Mary Smith"` |
|
||||
| `#!cpp j.at("age")` | `#!json 42` |
|
||||
| `#!cpp j.at("hobbies")` | `#!json ["hiking", "reading"]` |
|
||||
| `#!cpp j.at("hobbies").at(0)` | `#!json "hiking"` |
|
||||
| `#!cpp j.at("hobbies").at(1)` | `#!json "reading"` |
|
||||
|
||||
The return value is a reference, so it can be used to modify the original value.
|
||||
|
||||
??? example "Write access"
|
||||
|
||||
```cpp
|
||||
j.at("name") = "John Smith";
|
||||
```
|
||||
|
||||
This code produces the following JSON value:
|
||||
|
||||
```json
|
||||
{
|
||||
"name": "John Smith",
|
||||
"age": 42,
|
||||
"hobbies": ["hiking", "reading"]
|
||||
}
|
||||
```
|
||||
|
||||
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.
|
||||
|
||||
??? example "Accessing via invalid index or missing key"
|
||||
|
||||
```cpp
|
||||
j.at("hobbies").at(3) = "cooking";
|
||||
```
|
||||
|
||||
This code produces the following exception:
|
||||
|
||||
```
|
||||
[json.exception.out_of_range.401] array index 3 is out of range
|
||||
```
|
||||
|
||||
When [extended diagnostic messages](../../home/exceptions.md#extended-diagnostic-messages) are enabled by
|
||||
defining [`JSON_DIAGNOSTICS`](../../api/macros/json_diagnostics.md), the exception further gives information where
|
||||
the key or index is missing or out of range.
|
||||
|
||||
```
|
||||
[json.exception.out_of_range.401] (/hobbies) array index 3 is out of range
|
||||
```
|
||||
|
||||
## Notes
|
||||
|
||||
|
||||
!!! failure "Exceptions"
|
||||
|
||||
- [`at`](../../api/basic_json/at.md) can only be used with objects (with a string argument) or with arrays (with a
|
||||
numeric argument). For other types, a [`basic_json::type_error`](../../home/exceptions.md#jsonexceptiontype_error304)
|
||||
is thrown.
|
||||
- [`basic_json::out_of_range` exception](../../home/exceptions.md#out-of-range) exceptions are thrown if the
|
||||
provided key is not found in an object or the provided index is invalid.
|
||||
|
||||
## Summary
|
||||
|
||||
| scenario | non-const value | const value |
|
||||
|-----------------------------------|------------------------------------------------|------------------------------------------------|
|
||||
| access to existing object key | reference to existing value is returned | const reference to existing value is returned |
|
||||
| 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 |
|
||||
@@ -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>Accessing 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="July 4, 2026 09:13:25 UTC">July 4, 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.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> <!-- 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="July 8, 2026 18:18:24 UTC">July 8, 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.Line truncated
|
||||
@@ -0,0 +1,82 @@
|
||||
# Checked access: at
|
||||
|
||||
## Overview
|
||||
|
||||
The [`at`](https://json.nlohmann.me/api/basic_json/at/index.md) member function performs checked access; that is, it returns a reference to the desired value if it exists and throws a [`basic_json::out_of_range` exception](https://json.nlohmann.me/home/exceptions/#out-of-range) otherwise.
|
||||
|
||||
Read access
|
||||
|
||||
Consider the following JSON value:
|
||||
|
||||
```
|
||||
{
|
||||
"name": "Mary Smith",
|
||||
"age": 42,
|
||||
"hobbies": ["hiking", "reading"]
|
||||
}
|
||||
```
|
||||
|
||||
Assume the value is parsed to a `json` variable `j`.
|
||||
|
||||
| expression | value |
|
||||
| ----------------------- | --------------------------------------------------------------------- |
|
||||
| `j` | `{"name": "Mary Smith", "age": 42, "hobbies": ["hiking", "reading"]}` |
|
||||
| `j.at("name")` | `"Mary Smith"` |
|
||||
| `j.at("age")` | `42` |
|
||||
| `j.at("hobbies")` | `["hiking", "reading"]` |
|
||||
| `j.at("hobbies").at(0)` | `"hiking"` |
|
||||
| `j.at("hobbies").at(1)` | `"reading"` |
|
||||
|
||||
The return value is a reference, so it can be used to modify the original value.
|
||||
|
||||
Write access
|
||||
|
||||
```
|
||||
j.at("name") = "John Smith";
|
||||
```
|
||||
|
||||
This code produces the following JSON value:
|
||||
|
||||
```
|
||||
{
|
||||
"name": "John Smith",
|
||||
"age": 42,
|
||||
"hobbies": ["hiking", "reading"]
|
||||
}
|
||||
```
|
||||
|
||||
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.
|
||||
|
||||
Accessing via invalid index or missing key
|
||||
|
||||
```
|
||||
j.at("hobbies").at(3) = "cooking";
|
||||
```
|
||||
|
||||
This code produces the following exception:
|
||||
|
||||
```
|
||||
[json.exception.out_of_range.401] array index 3 is out of range
|
||||
```
|
||||
|
||||
When [extended diagnostic messages](https://json.nlohmann.me/home/exceptions/#extended-diagnostic-messages) are enabled by defining [`JSON_DIAGNOSTICS`](https://json.nlohmann.me/api/macros/json_diagnostics/index.md), the exception further gives information where the key or index is missing or out of range.
|
||||
|
||||
```
|
||||
[json.exception.out_of_range.401] (/hobbies) array index 3 is out of range
|
||||
```
|
||||
|
||||
## Notes
|
||||
|
||||
Exceptions
|
||||
|
||||
- [`at`](https://json.nlohmann.me/api/basic_json/at/index.md) can only be used with objects (with a string argument) or with arrays (with a numeric argument). For other types, a [`basic_json::type_error`](https://json.nlohmann.me/home/exceptions/#jsonexceptiontype_error304) is thrown.
|
||||
- [`basic_json::out_of_range` exception](https://json.nlohmann.me/home/exceptions/#out-of-range) exceptions are thrown if the provided key is not found in an object or the provided index is invalid.
|
||||
|
||||
## Summary
|
||||
|
||||
| scenario | non-const value | const value |
|
||||
| --------------------------------- | ---------------------------------------------- | ---------------------------------------------- |
|
||||
| access to existing object key | reference to existing value is returned | const reference to existing value is returned |
|
||||
| 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 |
|
||||
@@ -0,0 +1,64 @@
|
||||
# Access with default value: value
|
||||
|
||||
## Overview
|
||||
|
||||
In many situations, such as configuration files, missing values are not exceptional, but may be treated as if a default
|
||||
value was present. For this case, use [`value(key, default_value)`](../../api/basic_json/value.md) which takes the key
|
||||
you want to access and a default value in case there is no value stored with that key.
|
||||
|
||||
## Example
|
||||
|
||||
??? example
|
||||
|
||||
Consider the following JSON value:
|
||||
|
||||
```json
|
||||
{
|
||||
"logOutput": "result.log",
|
||||
"append": true
|
||||
}
|
||||
```
|
||||
|
||||
Assume the value is parsed to a `json` variable `j`.
|
||||
|
||||
| expression | value |
|
||||
|---------------------------------------------|------------------------------------------------------|
|
||||
| `#!cpp j` | `#!json {"logOutput": "result.log", "append": true}` |
|
||||
| `#!cpp j.value("logOutput", "logfile.log")` | `#!json "result.log"` |
|
||||
| `#!cpp j.value("append", true)` | `#!json true` |
|
||||
| `#!cpp j.value("append", false)` | `#!json true` |
|
||||
| `#!cpp j.value("logLevel", "verbose")` | `#!json "verbose"` |
|
||||
|
||||
## Notes
|
||||
|
||||
!!! failure "Exceptions"
|
||||
|
||||
- With string keys, `value` can only be used with objects. For other types, a [`basic_json::type_error`](../../home/exceptions.md#jsonexceptiontype_error306) is thrown.
|
||||
- With JSON Pointers, `value` can be used with both objects and arrays. For other types (null, boolean, number, string), a [`basic_json::type_error`](../../home/exceptions.md#jsonexceptiontype_error306) is thrown.
|
||||
|
||||
!!! warning "Return type"
|
||||
|
||||
The value function is a template, and the return type of the function is determined by the type of the provided
|
||||
default value unless otherwise specified. This can have unexpected effects. In the example below, we store a 64-bit
|
||||
unsigned integer. We get exactly that value when using [`operator[]`](../../api/basic_json/operator[].md). However,
|
||||
when we call `value` and provide `#!c 0` as default value, then `#!c -1` is returned. This occurs, because `#!c 0`
|
||||
has type `#!c int` which overflows when handling the value `#!c 18446744073709551615`.
|
||||
|
||||
To address this issue, either provide a correctly typed default value or use the template parameter to specify the
|
||||
desired return type. Note that this issue occurs even when a value is stored at the provided key, and the default
|
||||
value is not used as the return value.
|
||||
|
||||
```cpp
|
||||
--8<-- "examples/value__return_type.cpp"
|
||||
```
|
||||
|
||||
Output:
|
||||
|
||||
```json
|
||||
--8<-- "examples/value__return_type.output"
|
||||
```
|
||||
|
||||
## See also
|
||||
|
||||
- [`value`](../../api/basic_json/value.md) for access with default value
|
||||
- documentation on [checked access](checked_access.md)
|
||||
@@ -20,4 +20,4 @@
|
||||
<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> </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="July 4, 2026 09:13:25 UTC">July 4, 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>documentation on <a href=../checked_access/ >checked access</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="July 8, 2026 18:18:24 UTC">July 8, 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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|
||||
@@ -0,0 +1,72 @@
|
||||
# Access with default value: value
|
||||
|
||||
## Overview
|
||||
|
||||
In many situations, such as configuration files, missing values are not exceptional, but may be treated as if a default value was present. For this case, use [`value(key, default_value)`](https://json.nlohmann.me/api/basic_json/value/index.md) which takes the key you want to access and a default value in case there is no value stored with that key.
|
||||
|
||||
## Example
|
||||
|
||||
Example
|
||||
|
||||
Consider the following JSON value:
|
||||
|
||||
```
|
||||
{
|
||||
"logOutput": "result.log",
|
||||
"append": true
|
||||
}
|
||||
```
|
||||
|
||||
Assume the value is parsed to a `json` variable `j`.
|
||||
|
||||
| expression | value |
|
||||
| ------------------------------------- | --------------------------------------------- |
|
||||
| `j` | `{"logOutput": "result.log", "append": true}` |
|
||||
| `j.value("logOutput", "logfile.log")` | `"result.log"` |
|
||||
| `j.value("append", true)` | `true` |
|
||||
| `j.value("append", false)` | `true` |
|
||||
| `j.value("logLevel", "verbose")` | `"verbose"` |
|
||||
|
||||
## Notes
|
||||
|
||||
Exceptions
|
||||
|
||||
- With string keys, `value` can only be used with objects. For other types, a [`basic_json::type_error`](https://json.nlohmann.me/home/exceptions/#jsonexceptiontype_error306) is thrown.
|
||||
- With JSON Pointers, `value` can be used with both objects and arrays. For other types (null, boolean, number, string), a [`basic_json::type_error`](https://json.nlohmann.me/home/exceptions/#jsonexceptiontype_error306) is thrown.
|
||||
|
||||
Return type
|
||||
|
||||
The value function is a template, and the return type of the function is determined by the type of the provided default value unless otherwise specified. This can have unexpected effects. In the example below, we store a 64-bit unsigned integer. We get exactly that value when using [`operator[]`](https://json.nlohmann.me/api/basic_json/operator%5B%5D/index.md). However, when we call `value` and provide `0` as default value, then `-1` is returned. This occurs, because `0` has type `int` which overflows when handling the value `18446744073709551615`.
|
||||
|
||||
To address this issue, either provide a correctly typed default value or use the template parameter to specify the desired return type. Note that this issue occurs even when a value is stored at the provided key, and the default value is not used as the return value.
|
||||
|
||||
```
|
||||
#include <iostream>
|
||||
#include <nlohmann/json.hpp>
|
||||
|
||||
using json = nlohmann::json;
|
||||
|
||||
int main()
|
||||
{
|
||||
json j = json::parse(R"({"uint64": 18446744073709551615})");
|
||||
|
||||
std::cout << "operator[]: " << j["uint64"] << '\n'
|
||||
<< "default value (int): " << j.value("uint64", 0) << '\n'
|
||||
<< "default value (uint64_t): " << j.value("uint64", std::uint64_t(0)) << '\n'
|
||||
<< "explicit return value type: " << j.value<std::uint64_t>("uint64", 0) << '\n';
|
||||
}
|
||||
```
|
||||
|
||||
Output:
|
||||
|
||||
```
|
||||
operator[]: 18446744073709551615
|
||||
default value (int): -1
|
||||
default value (uint64_t): 18446744073709551615
|
||||
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)
|
||||
@@ -1 +1 @@
|
||||
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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/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-9.7.6"><title>Element Access - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.484c7ddc.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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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
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||||
@@ -0,0 +1,9 @@
|
||||
# Element Access
|
||||
|
||||
There are many ways elements in a JSON value can be accessed:
|
||||
|
||||
- unchecked access via [`operator[]`](https://json.nlohmann.me/features/element_access/unchecked_access/index.md)
|
||||
- checked access via [`at`](https://json.nlohmann.me/features/element_access/checked_access/index.md)
|
||||
- access with default value via [`value`](https://json.nlohmann.me/features/element_access/default_value/index.md)
|
||||
- iterators
|
||||
- JSON pointers
|
||||
@@ -0,0 +1,112 @@
|
||||
# Unchecked access: operator[]
|
||||
|
||||
## Overview
|
||||
|
||||
Elements in a JSON object and a JSON array can be accessed via [`operator[]`](../../api/basic_json/operator%5B%5D.md)
|
||||
similar to a `#!cpp std::map` and a `#!cpp std::vector`, respectively.
|
||||
|
||||
??? example "Read access"
|
||||
|
||||
Consider the following JSON value:
|
||||
|
||||
```json
|
||||
{
|
||||
"name": "Mary Smith",
|
||||
"age": 42,
|
||||
"hobbies": ["hiking", "reading"]
|
||||
}
|
||||
```
|
||||
|
||||
Assume the value is parsed to a `json` variable `j`.
|
||||
|
||||
| expression | value |
|
||||
|-------------------------|------------------------------------------------------------------------------|
|
||||
| `#!cpp j` | `#!json {"name": "Mary Smith", "age": 42, "hobbies": ["hiking", "reading"]}` |
|
||||
| `#!cpp j["name"]` | `#!json "Mary Smith"` |
|
||||
| `#!cpp j["age"]` | `#!json 42` |
|
||||
| `#!cpp j["hobbies"]` | `#!json ["hiking", "reading"]` |
|
||||
| `#!cpp j["hobbies"][0]` | `#!json "hiking"` |
|
||||
| `#!cpp j["hobbies"][1]` | `#!json "reading"` |
|
||||
|
||||
The return value is a reference, so it can modify the original value. In case the passed object key is non-existing, a
|
||||
`#!json null` value is inserted which can immediately be overwritten.
|
||||
|
||||
??? example "Write access"
|
||||
|
||||
```cpp
|
||||
j["name"] = "John Smith";
|
||||
j["maidenName"] = "Jones";
|
||||
```
|
||||
|
||||
This code produces the following JSON value:
|
||||
|
||||
```json
|
||||
{
|
||||
"name": "John Smith",
|
||||
"maidenName": "Jones",
|
||||
"age": 42,
|
||||
"hobbies": ["hiking", "reading"]
|
||||
}
|
||||
```
|
||||
|
||||
When accessing an invalid index (i.e., an index greater than or equal to the array size), the JSON array is resized such
|
||||
that the passed index is the new maximal index. Intermediate values are filled with `#!json null`.
|
||||
|
||||
??? example "Filling up arrays with `#!json null` values"
|
||||
|
||||
```cpp
|
||||
j["hobbies"][0] = "running";
|
||||
j["hobbies"][3] = "cooking";
|
||||
```
|
||||
|
||||
This code produces the following JSON value:
|
||||
|
||||
```json
|
||||
{
|
||||
"name": "John Smith",
|
||||
"maidenName": "Jones",
|
||||
"age": 42,
|
||||
"hobbies": ["running", "reading", null, "cooking"]
|
||||
}
|
||||
```
|
||||
|
||||
## Notes
|
||||
|
||||
!!! info "Design rationale"
|
||||
|
||||
The library behaves differently to `#!cpp std::vector` and `#!cpp std::map`:
|
||||
|
||||
- `#!cpp std::vector::operator[]` never inserts a new element.
|
||||
- `#!cpp std::map::operator[]` is not available for const values.
|
||||
|
||||
The type `#!cpp json` wraps all JSON value types. It would be impossible to remove
|
||||
[`operator[]`](../../api/basic_json/operator%5B%5D.md) for const objects. At the same time, inserting elements for
|
||||
non-const objects is really convenient as it avoids awkward `insert` calls. To this end, we decided to have an
|
||||
inserting non-const behavior for both arrays and objects.
|
||||
|
||||
!!! info
|
||||
|
||||
The access is unchecked. In case the passed object key does not exist or the passed array index is invalid, no
|
||||
exception is thrown.
|
||||
|
||||
!!! danger
|
||||
|
||||
- It is **undefined behavior** to access a const object with a non-existing key.
|
||||
- It is **undefined behavior** to access a const array with an invalid index.
|
||||
- In debug mode, an **assertion** will fire in both cases. You can disable assertions by defining the preprocessor
|
||||
symbol `#!cpp NDEBUG` or redefine the macro [`JSON_ASSERT(x)`](../macros.md#json_assertx). See the documentation
|
||||
on [runtime assertions](../assertions.md) for more information.
|
||||
|
||||
!!! failure "Exceptions"
|
||||
|
||||
`operator[]` can only be used with objects (with a string argument) or with arrays (with a numeric argument). For
|
||||
other types, a [`basic_json::type_error`](../../home/exceptions.md#jsonexceptiontype_error305) is thrown.
|
||||
|
||||
## Summary
|
||||
|
||||
| scenario | non-const value | const value |
|
||||
|-----------------------------------|------------------------------------------------------------------------------------------------------------------------------------------------------|-----------------------------------------------------------------------------|
|
||||
| access to existing object key | reference to existing value is returned | const reference to existing value is returned |
|
||||
| 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 |
|
||||
@@ -19,4 +19,4 @@
|
||||
<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>"running"</span><span class=p>,</span><span class=w> </span><span class=s2>"reading"</span><span class=p>,</span><span class=w> </span><span class=kc>null</span><span class=p>,</span><span class=w> </span><span class=s2>"cooking"</span><span class=p>]</span>
|
||||
<span class=p>}</span>
|
||||
</code></pre></div> </details> <h2 id=notes>Notes<a class=headerlink href=#notes title="Permanent link">¶</a></h2> <div class="admonition info"> <p class=admonition-title>Design rationale</p> <p>The library behaves differently to <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>vector</span></code> and <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>map</span></code>:</p> <ul> <li><code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>vector</span><span class=o>::</span><span class=k>operator</span><span class=p>[]</span></code> never inserts a new element.</li> <li><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> is not available for const values.</li> </ul> <p>The type <code class=highlight><span class=n>json</span></code> wraps all <abbr title="JavaScript Object Notation">JSON</abbr> value types. It would be impossible to remove <a href=../../../api/basic_json/operator%5B%5D/ ><code>operator[]</code></a> for const objects. At the same time, inserting elements for non-const objects is really convenient as it avoids awkward <code>insert</code> calls. To this end, we decided to have an inserting non-const behavior for both arrays and objects.</p> </div> <div class="admonition info"> <p class=admonition-title>Info</p> <p>The access is unchecked. In case the passed object key does not exist or the passed array index is invalid, no exception is thrown.</p> </div> <div class="admonition danger"> <p class=admonition-title>Danger</p> <ul> <li>It is <strong>undefined behavior</strong> to access a const object with a non-existing key.</li> <li>It is <strong>undefined behavior</strong> to access a const array with an invalid index.</li> <li>In debug mode, an <strong>assertion</strong> will fire in both cases. You can disable assertions by defining the preprocessor symbol <code class=highlight><span class=n>NDEBUG</span></code> or redefine the macro <a href=../../macros/#json_assertx><code>JSON_ASSERT(x)</code></a>. See the documentation on <a href=../../assertions/ >runtime assertions</a> for more information.</li> </ul> </div> <div class="admonition failure"> <p class=admonition-title>Exceptions</p> <p><code>operator[]</code> 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_error305><code>basic_json::type_error</code></a> is thrown.</p> </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="July 4, 2026 09:13:25 UTC">July 4, 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(locatLine truncated
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</code></pre></div> </details> <h2 id=notes>Notes<a class=headerlink href=#notes title="Permanent link">¶</a></h2> <div class="admonition info"> <p class=admonition-title>Design rationale</p> <p>The library behaves differently to <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>vector</span></code> and <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>map</span></code>:</p> <ul> <li><code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>vector</span><span class=o>::</span><span class=k>operator</span><span class=p>[]</span></code> never inserts a new element.</li> <li><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> is not available for const values.</li> </ul> <p>The type <code class=highlight><span class=n>json</span></code> wraps all <abbr title="JavaScript Object Notation">JSON</abbr> value types. It would be impossible to remove <a href=../../../api/basic_json/operator%5B%5D/ ><code>operator[]</code></a> for const objects. At the same time, inserting elements for non-const objects is really convenient as it avoids awkward <code>insert</code> calls. To this end, we decided to have an inserting non-const behavior for both arrays and objects.</p> </div> <div class="admonition info"> <p class=admonition-title>Info</p> <p>The access is unchecked. In case the passed object key does not exist or the passed array index is invalid, no exception is thrown.</p> </div> <div class="admonition danger"> <p class=admonition-title>Danger</p> <ul> <li>It is <strong>undefined behavior</strong> to access a const object with a non-existing key.</li> <li>It is <strong>undefined behavior</strong> to access a const array with an invalid index.</li> <li>In debug mode, an <strong>assertion</strong> will fire in both cases. You can disable assertions by defining the preprocessor symbol <code class=highlight><span class=n>NDEBUG</span></code> or redefine the macro <a href=../../macros/#json_assertx><code>JSON_ASSERT(x)</code></a>. See the documentation on <a href=../../assertions/ >runtime assertions</a> for more information.</li> </ul> </div> <div class="admonition failure"> <p class=admonition-title>Exceptions</p> <p><code>operator[]</code> 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_error305><code>basic_json::type_error</code></a> is thrown.</p> </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="July 8, 2026 18:18:24 UTC">July 8, 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(locatLine truncated
|
||||
@@ -0,0 +1,102 @@
|
||||
# Unchecked access: operator[]
|
||||
|
||||
## Overview
|
||||
|
||||
Elements in a JSON object and a JSON array can be accessed via [`operator[]`](https://json.nlohmann.me/api/basic_json/operator%5B%5D/index.md) similar to a `std::map` and a `std::vector`, respectively.
|
||||
|
||||
Read access
|
||||
|
||||
Consider the following JSON value:
|
||||
|
||||
```
|
||||
{
|
||||
"name": "Mary Smith",
|
||||
"age": 42,
|
||||
"hobbies": ["hiking", "reading"]
|
||||
}
|
||||
```
|
||||
|
||||
Assume the value is parsed to a `json` variable `j`.
|
||||
|
||||
| expression | value |
|
||||
| ----------------- | --------------------------------------------------------------------- |
|
||||
| `j` | `{"name": "Mary Smith", "age": 42, "hobbies": ["hiking", "reading"]}` |
|
||||
| `j["name"]` | `"Mary Smith"` |
|
||||
| `j["age"]` | `42` |
|
||||
| `j["hobbies"]` | `["hiking", "reading"]` |
|
||||
| `j["hobbies"][0]` | `"hiking"` |
|
||||
| `j["hobbies"][1]` | `"reading"` |
|
||||
|
||||
The return value is a reference, so it can modify the original value. In case the passed object key is non-existing, a `null` value is inserted which can immediately be overwritten.
|
||||
|
||||
Write access
|
||||
|
||||
```
|
||||
j["name"] = "John Smith";
|
||||
j["maidenName"] = "Jones";
|
||||
```
|
||||
|
||||
This code produces the following JSON value:
|
||||
|
||||
```
|
||||
{
|
||||
"name": "John Smith",
|
||||
"maidenName": "Jones",
|
||||
"age": 42,
|
||||
"hobbies": ["hiking", "reading"]
|
||||
}
|
||||
```
|
||||
|
||||
When accessing an invalid index (i.e., an index greater than or equal to the array size), the JSON array is resized such that the passed index is the new maximal index. Intermediate values are filled with `null`.
|
||||
|
||||
Filling up arrays with `null` values
|
||||
|
||||
```
|
||||
j["hobbies"][0] = "running";
|
||||
j["hobbies"][3] = "cooking";
|
||||
```
|
||||
|
||||
This code produces the following JSON value:
|
||||
|
||||
```
|
||||
{
|
||||
"name": "John Smith",
|
||||
"maidenName": "Jones",
|
||||
"age": 42,
|
||||
"hobbies": ["running", "reading", null, "cooking"]
|
||||
}
|
||||
```
|
||||
|
||||
## Notes
|
||||
|
||||
Design rationale
|
||||
|
||||
The library behaves differently to `std::vector` and `std::map`:
|
||||
|
||||
- `std::vector::operator[]` never inserts a new element.
|
||||
- `std::map::operator[]` is not available for const values.
|
||||
|
||||
The type `json` wraps all JSON value types. It would be impossible to remove [`operator[]`](https://json.nlohmann.me/api/basic_json/operator%5B%5D/index.md) for const objects. At the same time, inserting elements for non-const objects is really convenient as it avoids awkward `insert` calls. To this end, we decided to have an inserting non-const behavior for both arrays and objects.
|
||||
|
||||
Info
|
||||
|
||||
The access is unchecked. In case the passed object key does not exist or the passed array index is invalid, no exception is thrown.
|
||||
|
||||
Danger
|
||||
|
||||
- It is **undefined behavior** to access a const object with a non-existing key.
|
||||
- It is **undefined behavior** to access a const array with an invalid index.
|
||||
- In debug mode, an **assertion** will fire in both cases. You can disable assertions by defining the preprocessor symbol `NDEBUG` or redefine the macro [`JSON_ASSERT(x)`](https://json.nlohmann.me/features/macros/#json_assertx). See the documentation on [runtime assertions](https://json.nlohmann.me/features/assertions/index.md) for more information.
|
||||
|
||||
Exceptions
|
||||
|
||||
`operator[]` can only be used with objects (with a string argument) or with arrays (with a numeric argument). For other types, a [`basic_json::type_error`](https://json.nlohmann.me/home/exceptions/#jsonexceptiontype_error305) is thrown.
|
||||
|
||||
## Summary
|
||||
|
||||
| scenario | non-const value | const value |
|
||||
| --------------------------------- | -------------------------------------------------------------------------------------------------------------------------------------- | ---------------------------------------------------------------------------------------------------------------- |
|
||||
| access to existing object key | reference to existing value is returned | const reference to existing value is returned |
|
||||
| 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 |
|
||||
@@ -0,0 +1,62 @@
|
||||
# Specializing enum conversion
|
||||
|
||||
By default, enum values are serialized to JSON as integers. In some cases, this could result in undesired behavior. If
|
||||
the integer values of any enum values are changed after data using those enum values has been serialized to JSON, then
|
||||
deserializing that JSON would result in a different enum value being restored, or the value not being found at all.
|
||||
|
||||
It is possible to more precisely specify how a given enum is mapped to and from JSON as shown below:
|
||||
|
||||
```cpp
|
||||
// example enum type declaration
|
||||
enum TaskState {
|
||||
TS_STOPPED,
|
||||
TS_RUNNING,
|
||||
TS_COMPLETED,
|
||||
TS_INVALID=-1,
|
||||
};
|
||||
|
||||
// map TaskState values to JSON as strings
|
||||
NLOHMANN_JSON_SERIALIZE_ENUM( TaskState, {
|
||||
{TS_INVALID, nullptr},
|
||||
{TS_STOPPED, "stopped"},
|
||||
{TS_RUNNING, "running"},
|
||||
{TS_COMPLETED, "completed"},
|
||||
})
|
||||
```
|
||||
|
||||
The [`NLOHMANN_JSON_SERIALIZE_ENUM()` macro](../api/macros/nlohmann_json_serialize_enum.md) declares a set of
|
||||
`to_json()` / `from_json()` functions for type `TaskState` while avoiding repetition and boilerplate serialization code.
|
||||
|
||||
## Usage
|
||||
|
||||
```cpp
|
||||
// enum to JSON as string
|
||||
json j = TS_STOPPED;
|
||||
assert(j == "stopped");
|
||||
|
||||
// json string to enum
|
||||
json j3 = "running";
|
||||
assert(j3.get<TaskState>() == TS_RUNNING);
|
||||
|
||||
// undefined json value to enum (where the first map entry above is the default)
|
||||
json jPi = 3.14;
|
||||
assert(jPi.get<TaskState>() == TS_INVALID );
|
||||
```
|
||||
|
||||
## Notes
|
||||
|
||||
Just as in [Arbitrary Type Conversions](arbitrary_types.md) above,
|
||||
|
||||
- [`NLOHMANN_JSON_SERIALIZE_ENUM()`](../api/macros/nlohmann_json_serialize_enum.md) MUST be declared in your enum type's
|
||||
namespace (which can be the global namespace), or the library will not be able to locate it, and it will default to
|
||||
integer serialization.
|
||||
- It MUST be available (e.g., proper headers must be included) everywhere you use the conversions.
|
||||
|
||||
Other Important points:
|
||||
|
||||
- When using `get<ENUM_TYPE>()`, undefined JSON values will default to the first pair specified in your map. Select this
|
||||
default pair carefully. If you desire an exception in this circumstance use [`NLOHMANN_JSON_SERIALIZE_ENUM_STRICT()`](../api/macros/nlohmann_json_serialize_enum_strict.md)
|
||||
which behaves identically except for throwing an exception on unrecognized values.
|
||||
- If an enum or JSON value is specified more than once in your map, the first matching occurrence from the top of the
|
||||
map will be returned when converting to or from JSON.
|
||||
- To disable the default serialization of enumerators as integers and force a compiler error instead, see [`JSON_DISABLE_ENUM_SERIALIZATION`](../api/macros/json_disable_enum_serialization.md).
|
||||
@@ -24,4 +24,4 @@
|
||||
<span class=c1>// undefined json value to enum (where the first map entry above is the default)</span>
|
||||
<span class=n>json</span><span class=w> </span><span class=n>jPi</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=mf>3.14</span><span class=p>;</span>
|
||||
<span class=n>assert</span><span class=p>(</span><span class=n>jPi</span><span class=p>.</span><span class=n>get</span><span class=o><</span><span class=n>TaskState</span><span class=o>></span><span class=p>()</span><span class=w> </span><span class=o>==</span><span class=w> </span><span class=n>TS_INVALID</span><span class=w> </span><span class=p>);</span>
|
||||
</code></pre></div> <h2 id=notes>Notes<a class=headerlink href=#notes title="Permanent link">¶</a></h2> <p>Just as in <a href=../arbitrary_types/ >Arbitrary Type Conversions</a> above,</p> <ul> <li><a href=../../api/macros/nlohmann_json_serialize_enum/ ><code>NLOHMANN_JSON_SERIALIZE_ENUM()</code></a> MUST be declared in your enum type's namespace (which can be the global namespace), or the library will not be able to locate it, and it will default to integer serialization.</li> <li>It MUST be available (e.g., proper headers must be included) everywhere you use the conversions.</li> </ul> <p>Other Important points:</p> <ul> <li>When using <code>get<ENUM_TYPE>()</code>, undefined <abbr title="JavaScript Object Notation">JSON</abbr> values will default to the first pair specified in your map. Select this default pair carefully. If you desire an exception in this circumstance use <a href=../../api/macros/nlohmann_json_serialize_enum_strict/ ><code>NLOHMANN_JSON_SERIALIZE_ENUM_STRICT()</code></a> which behaves identically except for throwing an exception on unrecognized values.</li> <li>If an enum or <abbr title="JavaScript Object Notation">JSON</abbr> value is specified more than once in your map, the first matching occurrence from the top of the map will be returned when converting to or from <abbr title="JavaScript Object Notation">JSON</abbr>.</li> <li>To disable the default serialization of enumerators as integers and force a compiler error instead, see <a href=../../api/macros/json_disable_enum_serialization/ ><code>JSON_DISABLE_ENUM_SERIALIZATION</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="July 4, 2026 09:13:25 UTC">July 4, 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 Line truncated
|
||||
</code></pre></div> <h2 id=notes>Notes<a class=headerlink href=#notes title="Permanent link">¶</a></h2> <p>Just as in <a href=../arbitrary_types/ >Arbitrary Type Conversions</a> above,</p> <ul> <li><a href=../../api/macros/nlohmann_json_serialize_enum/ ><code>NLOHMANN_JSON_SERIALIZE_ENUM()</code></a> MUST be declared in your enum type's namespace (which can be the global namespace), or the library will not be able to locate it, and it will default to integer serialization.</li> <li>It MUST be available (e.g., proper headers must be included) everywhere you use the conversions.</li> </ul> <p>Other Important points:</p> <ul> <li>When using <code>get<ENUM_TYPE>()</code>, undefined <abbr title="JavaScript Object Notation">JSON</abbr> values will default to the first pair specified in your map. Select this default pair carefully. If you desire an exception in this circumstance use <a href=../../api/macros/nlohmann_json_serialize_enum_strict/ ><code>NLOHMANN_JSON_SERIALIZE_ENUM_STRICT()</code></a> which behaves identically except for throwing an exception on unrecognized values.</li> <li>If an enum or <abbr title="JavaScript Object Notation">JSON</abbr> value is specified more than once in your map, the first matching occurrence from the top of the map will be returned when converting to or from <abbr title="JavaScript Object Notation">JSON</abbr>.</li> <li>To disable the default serialization of enumerators as integers and force a compiler error instead, see <a href=../../api/macros/json_disable_enum_serialization/ ><code>JSON_DISABLE_ENUM_SERIALIZATION</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="July 8, 2026 18:18:24 UTC">July 8, 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 Line truncated
|
||||
@@ -0,0 +1,54 @@
|
||||
# Specializing enum conversion
|
||||
|
||||
By default, enum values are serialized to JSON as integers. In some cases, this could result in undesired behavior. If the integer values of any enum values are changed after data using those enum values has been serialized to JSON, then deserializing that JSON would result in a different enum value being restored, or the value not being found at all.
|
||||
|
||||
It is possible to more precisely specify how a given enum is mapped to and from JSON as shown below:
|
||||
|
||||
```
|
||||
// example enum type declaration
|
||||
enum TaskState {
|
||||
TS_STOPPED,
|
||||
TS_RUNNING,
|
||||
TS_COMPLETED,
|
||||
TS_INVALID=-1,
|
||||
};
|
||||
|
||||
// map TaskState values to JSON as strings
|
||||
NLOHMANN_JSON_SERIALIZE_ENUM( TaskState, {
|
||||
{TS_INVALID, nullptr},
|
||||
{TS_STOPPED, "stopped"},
|
||||
{TS_RUNNING, "running"},
|
||||
{TS_COMPLETED, "completed"},
|
||||
})
|
||||
```
|
||||
|
||||
The [`NLOHMANN_JSON_SERIALIZE_ENUM()` macro](https://json.nlohmann.me/api/macros/nlohmann_json_serialize_enum/index.md) declares a set of `to_json()` / `from_json()` functions for type `TaskState` while avoiding repetition and boilerplate serialization code.
|
||||
|
||||
## Usage
|
||||
|
||||
```
|
||||
// enum to JSON as string
|
||||
json j = TS_STOPPED;
|
||||
assert(j == "stopped");
|
||||
|
||||
// json string to enum
|
||||
json j3 = "running";
|
||||
assert(j3.get<TaskState>() == TS_RUNNING);
|
||||
|
||||
// undefined json value to enum (where the first map entry above is the default)
|
||||
json jPi = 3.14;
|
||||
assert(jPi.get<TaskState>() == TS_INVALID );
|
||||
```
|
||||
|
||||
## Notes
|
||||
|
||||
Just as in [Arbitrary Type Conversions](https://json.nlohmann.me/features/arbitrary_types/index.md) above,
|
||||
|
||||
- [`NLOHMANN_JSON_SERIALIZE_ENUM()`](https://json.nlohmann.me/api/macros/nlohmann_json_serialize_enum/index.md) MUST be declared in your enum type's namespace (which can be the global namespace), or the library will not be able to locate it, and it will default to integer serialization.
|
||||
- It MUST be available (e.g., proper headers must be included) everywhere you use the conversions.
|
||||
|
||||
Other Important points:
|
||||
|
||||
- When using `get<ENUM_TYPE>()`, undefined JSON values will default to the first pair specified in your map. Select this default pair carefully. If you desire an exception in this circumstance use [`NLOHMANN_JSON_SERIALIZE_ENUM_STRICT()`](https://json.nlohmann.me/api/macros/nlohmann_json_serialize_enum_strict/index.md) which behaves identically except for throwing an exception on unrecognized values.
|
||||
- If an enum or JSON value is specified more than once in your map, the first matching occurrence from the top of the map will be returned when converting to or from JSON.
|
||||
- To disable the default serialization of enumerators as integers and force a compiler error instead, see [`JSON_DISABLE_ENUM_SERIALIZATION`](https://json.nlohmann.me/api/macros/json_disable_enum_serialization/index.md).
|
||||
+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/sponsors/ 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-9.7.6"><title>Features - JSON for Modern C++</title><link rel=stylesheet href=../assets/stylesheets/main.484c7ddc.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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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-component=search role=dialLine 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=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/" rel="canonical"><link href=../home/sponsors/ 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-9.7.6"><title>Features - JSON for Modern C++</title><link rel=stylesheet href=../assets/stylesheets/main.484c7ddc.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=#features 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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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-component=search role=dialLine truncated
|
||||
@@ -0,0 +1,39 @@
|
||||
# Features
|
||||
|
||||
This section describes the features of the library in detail. If you are new to the library, the pages below are roughly ordered along a typical workflow: create or parse a value, access and modify it, convert it to and from your own C++ types, and finally serialize it again.
|
||||
|
||||
## 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), and [error handling](https://json.nlohmann.me/features/parsing/parse_exceptions/index.md).
|
||||
- [Comments](https://json.nlohmann.me/features/comments/index.md) and [trailing commas](https://json.nlohmann.me/features/trailing_commas/index.md) — opt-in relaxations of the JSON grammar.
|
||||
|
||||
## Accessing and modifying values
|
||||
|
||||
- [Element access](https://json.nlohmann.me/features/element_access/index.md) — unchecked ([`operator[]`](https://json.nlohmann.me/features/element_access/unchecked_access/index.md)), checked ([`at`](https://json.nlohmann.me/features/element_access/checked_access/index.md)), and access with a [default value](https://json.nlohmann.me/features/element_access/default_value/index.md).
|
||||
- [JSON Pointer](https://json.nlohmann.me/features/json_pointer/index.md) — address values deep inside a document with [RFC 6901](https://tools.ietf.org/html/rfc6901) pointers.
|
||||
- [Iterators](https://json.nlohmann.me/features/iterators/index.md) — traverse arrays and objects.
|
||||
- [Modifying values](https://json.nlohmann.me/features/modifying_values/index.md) — add, update, merge, and 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) — apply and compute structured changes.
|
||||
|
||||
## Converting to and from C++ types
|
||||
|
||||
- [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.
|
||||
- [Specializing enum conversion](https://json.nlohmann.me/features/enum_conversion/index.md) — map enums to strings instead of integers.
|
||||
|
||||
## Serializing values
|
||||
|
||||
- [Serialization](https://json.nlohmann.me/features/serialization/index.md) — turn a value back into JSON text with [`dump`](https://json.nlohmann.me/api/basic_json/dump/index.md), including pretty-printing and handling of non-ASCII and invalid UTF-8.
|
||||
- [Binary formats](https://json.nlohmann.me/features/binary_formats/index.md) — encode values more compactly as [BJData](https://json.nlohmann.me/features/binary_formats/bjdata/index.md), [BSON](https://json.nlohmann.me/features/binary_formats/bson/index.md), [CBOR](https://json.nlohmann.me/features/binary_formats/cbor/index.md), [MessagePack](https://json.nlohmann.me/features/binary_formats/messagepack/index.md), or [UBJSON](https://json.nlohmann.me/features/binary_formats/ubjson/index.md).
|
||||
- [Binary values](https://json.nlohmann.me/features/binary_values/index.md) — store and exchange raw byte sequences.
|
||||
|
||||
## How values are stored and configured
|
||||
|
||||
- [Types](https://json.nlohmann.me/features/types/index.md) and [number handling](https://json.nlohmann.me/features/types/number_handling/index.md) — how JSON types map to C++ types and how numbers are treated.
|
||||
- [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).
|
||||
- [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?
|
||||
|
||||
This section gives conceptual overviews. For the precise signature, parameters, and return value of a function, see the [API Documentation](https://json.nlohmann.me/api/basic_json/index.md).
|
||||
@@ -0,0 +1,155 @@
|
||||
# Iterators
|
||||
|
||||
## Overview
|
||||
|
||||
A `basic_json` value is a container and allows access via iterators. Depending on the value type, `basic_json` stores zero or more values.
|
||||
|
||||
As for other containers, `begin()` returns an iterator to the first value and `end()` 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()`.
|
||||
|
||||
|
||||
|
||||
### 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](types/index.md#key-order) for more information.
|
||||
|
||||
??? example
|
||||
|
||||
```cpp
|
||||
// create JSON object {"one": 1, "two": 2, "three": 3}
|
||||
json j;
|
||||
j["one"] = 1;
|
||||
j["two"] = 2;
|
||||
j["three"] = 3;
|
||||
|
||||
for (auto it = j.begin(); it != j.end(); ++it)
|
||||
{
|
||||
std::cout << *it << std::endl;
|
||||
}
|
||||
```
|
||||
|
||||
Output:
|
||||
|
||||
```json
|
||||
1
|
||||
3
|
||||
2
|
||||
```
|
||||
|
||||
The reason for the order is the lexicographic ordering of the object keys "one", "three", "two".
|
||||
|
||||
### Access object keys during iteration
|
||||
|
||||
The JSON iterators have two member functions, `key()` and `value()` to access the object key and stored value, respectively. When calling `key()` on a non-object iterator, an [invalid_iterator.207](../home/exceptions.md#jsonexceptioninvalid_iterator207) exception is thrown.
|
||||
|
||||
??? example
|
||||
|
||||
```cpp
|
||||
// create JSON object {"one": 1, "two": 2, "three": 3}
|
||||
json j;
|
||||
j["one"] = 1;
|
||||
j["two"] = 2;
|
||||
j["three"] = 3;
|
||||
|
||||
for (auto it = j.begin(); it != j.end(); ++it)
|
||||
{
|
||||
std::cout << it.key() << " : " << it.value() << std::endl;
|
||||
}
|
||||
```
|
||||
|
||||
Output:
|
||||
|
||||
```json
|
||||
one : 1
|
||||
three : 3
|
||||
two : 2
|
||||
```
|
||||
|
||||
### Range-based for loops
|
||||
|
||||
C++11 allows using range-based for loops to iterate over a container.
|
||||
|
||||
```cpp
|
||||
for (auto it : j_object)
|
||||
{
|
||||
// "it" is of type json::reference and has no key() member
|
||||
std::cout << "value: " << it << '\n';
|
||||
}
|
||||
```
|
||||
|
||||
For this reason, the `items()` function allows accessing `iterator::key()` and `iterator::value()` during range-based for loops. In these loops, a reference to the JSON values is returned, so there is no access to the underlying iterator.
|
||||
|
||||
```cpp
|
||||
for (auto& el : j_object.items())
|
||||
{
|
||||
std::cout << "key: " << el.key() << ", value:" << el.value() << '\n';
|
||||
}
|
||||
```
|
||||
|
||||
The items() function also allows using structured bindings (C++17):
|
||||
|
||||
```cpp
|
||||
for (auto& [key, val] : j_object.items())
|
||||
{
|
||||
std::cout << "key: " << key << ", value:" << val << '\n';
|
||||
}
|
||||
```
|
||||
|
||||
!!! note
|
||||
|
||||
When iterating over an array, `key()` will return the index of the element as string. For primitive types (e.g., numbers), `key()` returns an empty string.
|
||||
|
||||
!!! warning
|
||||
|
||||
Using `items()` on temporary objects is dangerous. Make sure the object's lifetime exceeds the iteration. See [#2040](https://github.com/nlohmann/json/issues/2040) for more information.
|
||||
|
||||
### Reverse iteration order
|
||||
|
||||
`rbegin()` and `rend()` return iterators in the reverse sequence.
|
||||
|
||||
|
||||
|
||||
??? example
|
||||
|
||||
```cpp
|
||||
json j = {1, 2, 3, 4};
|
||||
|
||||
for (auto it = j.rbegin(); it != j.rend(); ++it)
|
||||
{
|
||||
std::cout << *it << std::endl;
|
||||
}
|
||||
```
|
||||
|
||||
Output:
|
||||
|
||||
```json
|
||||
4
|
||||
3
|
||||
2
|
||||
1
|
||||
```
|
||||
|
||||
### Iterating strings and binary values
|
||||
|
||||
Note that "value" means a JSON value in this setting, not values stored in the underlying containers. That is, `*begin()` returns the complete string or binary array and is also safe if the underlying string or binary array is empty.
|
||||
|
||||
??? example
|
||||
|
||||
```cpp
|
||||
json j = "Hello, world";
|
||||
for (auto it = j.begin(); it != j.end(); ++it)
|
||||
{
|
||||
std::cout << *it << std::endl;
|
||||
}
|
||||
```
|
||||
|
||||
Output:
|
||||
|
||||
```json
|
||||
"Hello, world"
|
||||
```
|
||||
|
||||
## Iterator invalidation
|
||||
|
||||
| Operations | invalidated iterators |
|
||||
|------------|-----------------------|
|
||||
| `clear` | all |
|
||||
@@ -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="July 4, 2026 09:13:25 UTC">July 4, 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> <!-- 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="July 8, 2026 18:18:24 UTC">July 8, 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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|
||||
@@ -0,0 +1,151 @@
|
||||
# Iterators
|
||||
|
||||
## Overview
|
||||
|
||||
A `basic_json` value is a container and allows access via iterators. Depending on the value type, `basic_json` stores zero or more values.
|
||||
|
||||
As for other containers, `begin()` returns an iterator to the first value and `end()` 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()`.
|
||||
|
||||
### 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.
|
||||
|
||||
Example
|
||||
|
||||
```
|
||||
// create JSON object {"one": 1, "two": 2, "three": 3}
|
||||
json j;
|
||||
j["one"] = 1;
|
||||
j["two"] = 2;
|
||||
j["three"] = 3;
|
||||
|
||||
for (auto it = j.begin(); it != j.end(); ++it)
|
||||
{
|
||||
std::cout << *it << std::endl;
|
||||
}
|
||||
```
|
||||
|
||||
Output:
|
||||
|
||||
```
|
||||
1
|
||||
3
|
||||
2
|
||||
```
|
||||
|
||||
The reason for the order is the lexicographic ordering of the object keys "one", "three", "two".
|
||||
|
||||
### Access object keys during iteration
|
||||
|
||||
The JSON iterators have two member functions, `key()` and `value()` to access the object key and stored value, respectively. When calling `key()` on a non-object iterator, an [invalid_iterator.207](https://json.nlohmann.me/home/exceptions/#jsonexceptioninvalid_iterator207) exception is thrown.
|
||||
|
||||
Example
|
||||
|
||||
```
|
||||
// create JSON object {"one": 1, "two": 2, "three": 3}
|
||||
json j;
|
||||
j["one"] = 1;
|
||||
j["two"] = 2;
|
||||
j["three"] = 3;
|
||||
|
||||
for (auto it = j.begin(); it != j.end(); ++it)
|
||||
{
|
||||
std::cout << it.key() << " : " << it.value() << std::endl;
|
||||
}
|
||||
```
|
||||
|
||||
Output:
|
||||
|
||||
```
|
||||
one : 1
|
||||
three : 3
|
||||
two : 2
|
||||
```
|
||||
|
||||
### Range-based for loops
|
||||
|
||||
C++11 allows using range-based for loops to iterate over a container.
|
||||
|
||||
```
|
||||
for (auto it : j_object)
|
||||
{
|
||||
// "it" is of type json::reference and has no key() member
|
||||
std::cout << "value: " << it << '\n';
|
||||
}
|
||||
```
|
||||
|
||||
For this reason, the `items()` function allows accessing `iterator::key()` and `iterator::value()` during range-based for loops. In these loops, a reference to the JSON values is returned, so there is no access to the underlying iterator.
|
||||
|
||||
```
|
||||
for (auto& el : j_object.items())
|
||||
{
|
||||
std::cout << "key: " << el.key() << ", value:" << el.value() << '\n';
|
||||
}
|
||||
```
|
||||
|
||||
The items() function also allows using structured bindings (C++17):
|
||||
|
||||
```
|
||||
for (auto& [key, val] : j_object.items())
|
||||
{
|
||||
std::cout << "key: " << key << ", value:" << val << '\n';
|
||||
}
|
||||
```
|
||||
|
||||
Note
|
||||
|
||||
When iterating over an array, `key()` will return the index of the element as string. For primitive types (e.g., numbers), `key()` returns an empty string.
|
||||
|
||||
Warning
|
||||
|
||||
Using `items()` on temporary objects is dangerous. Make sure the object's lifetime exceeds the iteration. See [#2040](https://github.com/nlohmann/json/issues/2040) for more information.
|
||||
|
||||
### Reverse iteration order
|
||||
|
||||
`rbegin()` and `rend()` return iterators in the reverse sequence.
|
||||
|
||||
Example
|
||||
|
||||
```
|
||||
json j = {1, 2, 3, 4};
|
||||
|
||||
for (auto it = j.rbegin(); it != j.rend(); ++it)
|
||||
{
|
||||
std::cout << *it << std::endl;
|
||||
}
|
||||
```
|
||||
|
||||
Output:
|
||||
|
||||
```
|
||||
4
|
||||
3
|
||||
2
|
||||
1
|
||||
```
|
||||
|
||||
### Iterating strings and binary values
|
||||
|
||||
Note that "value" means a JSON value in this setting, not values stored in the underlying containers. That is, `*begin()` returns the complete string or binary array and is also safe if the underlying string or binary array is empty.
|
||||
|
||||
Example
|
||||
|
||||
```
|
||||
json j = "Hello, world";
|
||||
for (auto it = j.begin(); it != j.end(); ++it)
|
||||
{
|
||||
std::cout << *it << std::endl;
|
||||
}
|
||||
```
|
||||
|
||||
Output:
|
||||
|
||||
```
|
||||
"Hello, world"
|
||||
```
|
||||
|
||||
## Iterator invalidation
|
||||
|
||||
| Operations | invalidated iterators |
|
||||
| ---------- | --------------------- |
|
||||
| `clear` | all |
|
||||
@@ -0,0 +1,47 @@
|
||||
# JSON Patch and Diff
|
||||
|
||||
## Patches
|
||||
|
||||
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. With the `patch` function, a JSON Patch is applied to the current JSON value
|
||||
by executing all operations from the patch.
|
||||
|
||||
??? example
|
||||
|
||||
The following code shows how a JSON patch is applied to a value.
|
||||
|
||||
```cpp
|
||||
--8<-- "examples/patch.cpp"
|
||||
```
|
||||
|
||||
Output:
|
||||
|
||||
```json
|
||||
--8<-- "examples/patch.output"
|
||||
```
|
||||
|
||||
## Diff
|
||||
|
||||
The library can also calculate a JSON patch (i.e., a **diff**) given two JSON values.
|
||||
|
||||
!!! success "Invariant"
|
||||
|
||||
For two JSON values *source* and *target*, the following code yields always true:
|
||||
|
||||
```cpp
|
||||
source.patch(diff(source, target)) == target;
|
||||
```
|
||||
|
||||
??? example
|
||||
|
||||
The following code shows how a JSON patch is created as a diff for two JSON values.
|
||||
|
||||
```cpp
|
||||
--8<-- "examples/diff.cpp"
|
||||
```
|
||||
|
||||
Output:
|
||||
|
||||
```json
|
||||
--8<-- "examples/diff.output"
|
||||
```
|
||||
@@ -105,4 +105,4 @@
|
||||
<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> <!-- 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="July 4, 2026 09:13:25 UTC">July 4, 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 - 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|
||||
</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="July 8, 2026 18:18:24 UTC">July 8, 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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|
||||
@@ -0,0 +1,147 @@
|
||||
# JSON Patch and Diff
|
||||
|
||||
## Patches
|
||||
|
||||
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. With the `patch` function, a JSON Patch is applied to the current JSON value by executing all operations from the patch.
|
||||
|
||||
Example
|
||||
|
||||
The following code shows how a JSON patch is applied to a value.
|
||||
|
||||
```
|
||||
#include <iostream>
|
||||
#include <iomanip>
|
||||
#include <nlohmann/json.hpp>
|
||||
|
||||
using json = nlohmann::json;
|
||||
using namespace nlohmann::literals;
|
||||
|
||||
int main()
|
||||
{
|
||||
// the original document
|
||||
json doc = R"(
|
||||
{
|
||||
"baz": "qux",
|
||||
"foo": "bar"
|
||||
}
|
||||
)"_json;
|
||||
|
||||
// the patch
|
||||
json patch = R"(
|
||||
[
|
||||
{ "op": "replace", "path": "/baz", "value": "boo" },
|
||||
{ "op": "add", "path": "/hello", "value": ["world"] },
|
||||
{ "op": "remove", "path": "/foo"}
|
||||
]
|
||||
)"_json;
|
||||
|
||||
// apply the patch
|
||||
json patched_doc = doc.patch(patch);
|
||||
|
||||
// output original and patched document
|
||||
std::cout << std::setw(4) << doc << "\n\n"
|
||||
<< std::setw(4) << patched_doc << std::endl;
|
||||
}
|
||||
```
|
||||
|
||||
Output:
|
||||
|
||||
```
|
||||
{
|
||||
"baz": "qux",
|
||||
"foo": "bar"
|
||||
}
|
||||
|
||||
{
|
||||
"baz": "boo",
|
||||
"hello": [
|
||||
"world"
|
||||
]
|
||||
}
|
||||
```
|
||||
|
||||
## Diff
|
||||
|
||||
The library can also calculate a JSON patch (i.e., a **diff**) given two JSON values.
|
||||
|
||||
Invariant
|
||||
|
||||
For two JSON values *source* and *target*, the following code yields always true:
|
||||
|
||||
```
|
||||
source.patch(diff(source, target)) == target;
|
||||
```
|
||||
|
||||
Example
|
||||
|
||||
The following code shows how a JSON patch is created as a diff for two JSON values.
|
||||
|
||||
```
|
||||
#include <iostream>
|
||||
#include <iomanip>
|
||||
#include <nlohmann/json.hpp>
|
||||
|
||||
using json = nlohmann::json;
|
||||
using namespace nlohmann::literals;
|
||||
|
||||
int main()
|
||||
{
|
||||
// the source document
|
||||
json source = R"(
|
||||
{
|
||||
"baz": "qux",
|
||||
"foo": "bar"
|
||||
}
|
||||
)"_json;
|
||||
|
||||
// the target document
|
||||
json target = R"(
|
||||
{
|
||||
"baz": "boo",
|
||||
"hello": [
|
||||
"world"
|
||||
]
|
||||
}
|
||||
)"_json;
|
||||
|
||||
// create the patch
|
||||
json patch = json::diff(source, target);
|
||||
|
||||
// roundtrip
|
||||
json patched_source = source.patch(patch);
|
||||
|
||||
// output patch and roundtrip result
|
||||
std::cout << std::setw(4) << patch << "\n\n"
|
||||
<< std::setw(4) << patched_source << std::endl;
|
||||
}
|
||||
```
|
||||
|
||||
Output:
|
||||
|
||||
```
|
||||
[
|
||||
{
|
||||
"op": "replace",
|
||||
"path": "/baz",
|
||||
"value": "boo"
|
||||
},
|
||||
{
|
||||
"op": "remove",
|
||||
"path": "/foo"
|
||||
},
|
||||
{
|
||||
"op": "add",
|
||||
"path": "/hello",
|
||||
"value": [
|
||||
"world"
|
||||
]
|
||||
}
|
||||
]
|
||||
|
||||
{
|
||||
"baz": "boo",
|
||||
"hello": [
|
||||
"world"
|
||||
]
|
||||
}
|
||||
```
|
||||
@@ -0,0 +1,126 @@
|
||||
# JSON Pointer
|
||||
|
||||
## Introduction
|
||||
|
||||
The library supports **JSON Pointer** ([RFC 6901](https://tools.ietf.org/html/rfc6901)) as an alternative means to
|
||||
address structured values. A JSON Pointer is a string that identifies a specific value within a JSON document.
|
||||
|
||||
Consider the following JSON document
|
||||
|
||||
```json
|
||||
{
|
||||
"array": ["A", "B", "C"],
|
||||
"nested": {
|
||||
"one": 1,
|
||||
"two": 2,
|
||||
"three": [true, false]
|
||||
}
|
||||
}
|
||||
```
|
||||
|
||||
Then every value inside the JSON document can be identified as follows:
|
||||
|
||||
| JSON Pointer | JSON value |
|
||||
|-------------------|----------------------------------------------------------------------------------|
|
||||
| `` | `#!json {"array":["A","B","C"],"nested":{"one":1,"two":2,"three":[true,false]}}` |
|
||||
| `/array` | `#!json ["A","B","C"]` |
|
||||
| `/array/0` | `#!json A` |
|
||||
| `/array/1` | `#!json B` |
|
||||
| `/array/2` | `#!json C` |
|
||||
| `/nested` | `#!json {"one":1,"two":2,"three":[true,false]}` |
|
||||
| `/nested/one` | `#!json 1` |
|
||||
| `/nested/two` | `#!json 2` |
|
||||
| `/nested/three` | `#!json [true,false]` |
|
||||
| `/nested/three/0` | `#!json true` |
|
||||
| `/nested/three/1` | `#!json false` |
|
||||
|
||||
Note `/` does not identify the root (i.e., the whole document), but an object entry with empty key `""`. See
|
||||
[RFC 6901](https://tools.ietf.org/html/rfc6901) for more information.
|
||||
|
||||
## JSON Pointer creation
|
||||
|
||||
JSON Pointers can be created from a string:
|
||||
|
||||
```cpp
|
||||
json::json_pointer p("/nested/one");
|
||||
```
|
||||
|
||||
Furthermore, a user-defined string literal can be used to achieve the same result:
|
||||
|
||||
```cpp
|
||||
auto p = "/nested/one"_json_pointer;
|
||||
```
|
||||
|
||||
The escaping rules of [RFC 6901](https://tools.ietf.org/html/rfc6901) are implemented. See the
|
||||
[constructor documentation](../api/json_pointer/json_pointer.md) for more information.
|
||||
|
||||
## Value access
|
||||
|
||||
JSON Pointers can be used in the [`at`](../api/basic_json/at.md), [`operator[]`](../api/basic_json/operator%5B%5D.md),
|
||||
and [`value`](../api/basic_json/value.md) functions just like object keys or array indices.
|
||||
|
||||
```cpp
|
||||
// the JSON value from above
|
||||
auto j = json::parse(R"({
|
||||
"array": ["A", "B", "C"],
|
||||
"nested": {
|
||||
"one": 1,
|
||||
"two": 2,
|
||||
"three": [true, false]
|
||||
}
|
||||
})");
|
||||
|
||||
// access values
|
||||
auto val = j[""_json_pointer]; // {"array":["A","B","C"],...}
|
||||
auto val1 = j["/nested/one"_json_pointer]; // 1
|
||||
auto val2 = j.at(json::json_pointer("/nested/three/1")); // false
|
||||
auto val3 = j.value(json::json_pointer("/nested/four"), 0); // 0
|
||||
```
|
||||
|
||||
## Flatten / unflatten
|
||||
|
||||
The library implements a function [`flatten`](../api/basic_json/flatten.md) to convert any JSON document into a JSON
|
||||
object where each key is a JSON Pointer and each value is a primitive JSON value (i.e., a string, boolean, number, or
|
||||
null).
|
||||
|
||||
```cpp
|
||||
// the JSON value from above
|
||||
auto j = json::parse(R"({
|
||||
"array": ["A", "B", "C"],
|
||||
"nested": {
|
||||
"one": 1,
|
||||
"two": 2,
|
||||
"three": [true, false]
|
||||
}
|
||||
})");
|
||||
|
||||
// create flattened value
|
||||
auto j_flat = j.flatten();
|
||||
```
|
||||
|
||||
The resulting value `j_flat` is:
|
||||
|
||||
```json
|
||||
{
|
||||
"/array/0": "A",
|
||||
"/array/1": "B",
|
||||
"/array/2": "C",
|
||||
"/nested/one": 1,
|
||||
"/nested/two": 2,
|
||||
"/nested/three/0": true,
|
||||
"/nested/three/1": false
|
||||
}
|
||||
```
|
||||
|
||||
The reverse function, [`unflatten`](../api/basic_json/unflatten.md) recreates the original value.
|
||||
|
||||
```cpp
|
||||
auto j_original = j_flat.unflatten();
|
||||
```
|
||||
|
||||
## See also
|
||||
|
||||
- Class [`json_pointer`](../api/json_pointer/index.md)
|
||||
- Function [`flatten`](../api/basic_json/flatten.md)
|
||||
- Function [`unflatten`](../api/basic_json/unflatten.md)
|
||||
- [JSON Patch](json_patch.md)
|
||||
@@ -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>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></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="July 4, 2026 09:13:25 UTC">July 4, 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> <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>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></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="July 8, 2026 18:18:24 UTC">July 8, 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-.5-56.7-34.5-56.7-34.6 0-39.9 27-39.9 54.9V416h-66.4V202.2h63.7v29.2h.9c8.9-16.8 30.6-34.5 62.9-34.5 67.2 0 79.7 44.3 79.7 101.9z"/></svg> </a> <a href="https://www.xing.com/profile/Niels_Lohmann" target="_blank" rel="noopener" title="www.xing.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 384 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - https://fontawesome.com License - httLine truncated
|
||||
@@ -0,0 +1,120 @@
|
||||
# JSON Pointer
|
||||
|
||||
## Introduction
|
||||
|
||||
The library supports **JSON Pointer** ([RFC 6901](https://tools.ietf.org/html/rfc6901)) as an alternative means to address structured values. A JSON Pointer is a string that identifies a specific value within a JSON document.
|
||||
|
||||
Consider the following JSON document
|
||||
|
||||
```
|
||||
{
|
||||
"array": ["A", "B", "C"],
|
||||
"nested": {
|
||||
"one": 1,
|
||||
"two": 2,
|
||||
"three": [true, false]
|
||||
}
|
||||
}
|
||||
```
|
||||
|
||||
Then every value inside the JSON document can be identified as follows:
|
||||
|
||||
| JSON Pointer | JSON value |
|
||||
| ----------------- | ------------------------------------------------------------------------- |
|
||||
| \`\` | `{"array":["A","B","C"],"nested":{"one":1,"two":2,"three":[true,false]}}` |
|
||||
| `/array` | `["A","B","C"]` |
|
||||
| `/array/0` | `A` |
|
||||
| `/array/1` | `B` |
|
||||
| `/array/2` | `C` |
|
||||
| `/nested` | `{"one":1,"two":2,"three":[true,false]}` |
|
||||
| `/nested/one` | `1` |
|
||||
| `/nested/two` | `2` |
|
||||
| `/nested/three` | `[true,false]` |
|
||||
| `/nested/three/0` | `true` |
|
||||
| `/nested/three/1` | `false` |
|
||||
|
||||
Note `/` does not identify the root (i.e., the whole document), but an object entry with empty key `""`. See [RFC 6901](https://tools.ietf.org/html/rfc6901) for more information.
|
||||
|
||||
## JSON Pointer creation
|
||||
|
||||
JSON Pointers can be created from a string:
|
||||
|
||||
```
|
||||
json::json_pointer p("/nested/one");
|
||||
```
|
||||
|
||||
Furthermore, a user-defined string literal can be used to achieve the same result:
|
||||
|
||||
```
|
||||
auto p = "/nested/one"_json_pointer;
|
||||
```
|
||||
|
||||
The escaping rules of [RFC 6901](https://tools.ietf.org/html/rfc6901) are implemented. See the [constructor documentation](https://json.nlohmann.me/api/json_pointer/json_pointer/index.md) for more information.
|
||||
|
||||
## Value access
|
||||
|
||||
JSON Pointers can be used in the [`at`](https://json.nlohmann.me/api/basic_json/at/index.md), [`operator[]`](https://json.nlohmann.me/api/basic_json/operator%5B%5D/index.md), and [`value`](https://json.nlohmann.me/api/basic_json/value/index.md) functions just like object keys or array indices.
|
||||
|
||||
```
|
||||
// the JSON value from above
|
||||
auto j = json::parse(R"({
|
||||
"array": ["A", "B", "C"],
|
||||
"nested": {
|
||||
"one": 1,
|
||||
"two": 2,
|
||||
"three": [true, false]
|
||||
}
|
||||
})");
|
||||
|
||||
// access values
|
||||
auto val = j[""_json_pointer]; // {"array":["A","B","C"],...}
|
||||
auto val1 = j["/nested/one"_json_pointer]; // 1
|
||||
auto val2 = j.at(json::json_pointer("/nested/three/1")); // false
|
||||
auto val3 = j.value(json::json_pointer("/nested/four"), 0); // 0
|
||||
```
|
||||
|
||||
## Flatten / unflatten
|
||||
|
||||
The library implements a function [`flatten`](https://json.nlohmann.me/api/basic_json/flatten/index.md) to convert any JSON document into a JSON object where each key is a JSON Pointer and each value is a primitive JSON value (i.e., a string, boolean, number, or null).
|
||||
|
||||
```
|
||||
// the JSON value from above
|
||||
auto j = json::parse(R"({
|
||||
"array": ["A", "B", "C"],
|
||||
"nested": {
|
||||
"one": 1,
|
||||
"two": 2,
|
||||
"three": [true, false]
|
||||
}
|
||||
})");
|
||||
|
||||
// create flattened value
|
||||
auto j_flat = j.flatten();
|
||||
```
|
||||
|
||||
The resulting value `j_flat` is:
|
||||
|
||||
```
|
||||
{
|
||||
"/array/0": "A",
|
||||
"/array/1": "B",
|
||||
"/array/2": "C",
|
||||
"/nested/one": 1,
|
||||
"/nested/two": 2,
|
||||
"/nested/three/0": true,
|
||||
"/nested/three/1": false
|
||||
}
|
||||
```
|
||||
|
||||
The reverse function, [`unflatten`](https://json.nlohmann.me/api/basic_json/unflatten/index.md) recreates the original value.
|
||||
|
||||
```
|
||||
auto j_original = j_flat.unflatten();
|
||||
```
|
||||
|
||||
## See also
|
||||
|
||||
- Class [`json_pointer`](https://json.nlohmann.me/api/json_pointer/index.md)
|
||||
- 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 Patch](https://json.nlohmann.me/features/json_patch/index.md)
|
||||
@@ -0,0 +1,174 @@
|
||||
# Supported Macros
|
||||
|
||||
Some aspects of the library can be configured by defining preprocessor macros before including the `json.hpp` header.
|
||||
See also the [API documentation for macros](../api/macros/index.md) for examples and more information.
|
||||
|
||||
## `JSON_ASSERT(x)`
|
||||
|
||||
This macro controls which code is executed for [runtime assertions](assertions.md) of the library.
|
||||
|
||||
See [full documentation of `JSON_ASSERT(x)`](../api/macros/json_assert.md).
|
||||
|
||||
## `JSON_BRACE_INIT_COPY_SEMANTICS`
|
||||
|
||||
When defined to `1`, single-element brace initialization of a `basic_json` value (e.g., `#!cpp json j{value};`) is
|
||||
treated as a copy/move of the element rather than wrapping it in a single-element array. The default value is `0`, which
|
||||
preserves the existing behavior.
|
||||
|
||||
See [full documentation of `JSON_BRACE_INIT_COPY_SEMANTICS`](../api/macros/json_brace_init_copy_semantics.md).
|
||||
|
||||
## `JSON_CATCH_USER(exception)`
|
||||
|
||||
This macro overrides [`#!cpp catch`](https://en.cppreference.com/w/cpp/language/try_catch) calls inside the library.
|
||||
|
||||
See [full documentation of `JSON_CATCH_USER(exception)`](../api/macros/json_throw_user.md).
|
||||
|
||||
## `JSON_DIAGNOSTICS`
|
||||
|
||||
This macro enables extended diagnostics for exception messages. Possible values are `1` to enable or `0` to disable
|
||||
(default).
|
||||
|
||||
When enabled, exception messages contain a [JSON Pointer](json_pointer.md) to the JSON value that triggered the
|
||||
exception, see [Extended diagnostic messages](../home/exceptions.md#extended-diagnostic-messages) for an example. Note
|
||||
that enabling this macro increases the size of every JSON value by one pointer and adds some runtime overhead.
|
||||
|
||||
The diagnostics messages can also be controlled with the CMake option
|
||||
[`JSON_Diagnostics`](../integration/cmake.md#json_diagnostics) (`OFF` by default) which sets `JSON_DIAGNOSTICS`
|
||||
accordingly.
|
||||
|
||||
See [full documentation of `JSON_DIAGNOSTICS`](../api/macros/json_diagnostics.md).
|
||||
|
||||
## `JSON_DIAGNOSTIC_POSITIONS`
|
||||
|
||||
When enabled, two new member functions [`start_pos()`](../api/basic_json/start_pos.md) and
|
||||
[`end_pos()`](../api/basic_json/end_pos.md) are added to [`basic_json`](../api/basic_json/index.md) values. If the value
|
||||
was created by calling the[`parse`](../api/basic_json/parse.md) function, then these functions allow querying the byte
|
||||
positions of the value in the input it was parsed from. The byte positions are also used in exceptions to help locate
|
||||
errors.
|
||||
|
||||
The diagnostics positions can also be controlled with the CMake option
|
||||
[`JSON_Diagnostic_Positions`](../integration/cmake.md#json_diagnostic_positions) (`OFF` by default) which sets
|
||||
`JSON_DIAGNOSTIC_POSITIONS` accordingly.
|
||||
|
||||
See [full documentation of `JSON_DIAGNOSTIC_POSITIONS`](../api/macros/json_diagnostic_positions.md)
|
||||
|
||||
## `JSON_HAS_CPP_11`, `JSON_HAS_CPP_14`, `JSON_HAS_CPP_17`, `JSON_HAS_CPP_20`, `JSON_HAS_CPP_23`, `JSON_HAS_CPP_26`
|
||||
|
||||
The library targets C++11, but also supports some features introduced in later C++ versions (e.g., `std::string_view`
|
||||
support for C++17). For these new features, the library implements some preprocessor checks to determine the C++
|
||||
standard. By defining any of these symbols, the internal check is overridden and the provided C++ version is
|
||||
unconditionally assumed. This can be helpful for compilers that only implement parts of the standard and would be
|
||||
detected incorrectly.
|
||||
|
||||
See [full documentation of `JSON_HAS_CPP_11`, `JSON_HAS_CPP_14`, `JSON_HAS_CPP_17`, `JSON_HAS_CPP_20`, `JSON_HAS_CPP_23`, and `JSON_HAS_CPP_26`](../api/macros/json_has_cpp_11.md).
|
||||
|
||||
## `JSON_HAS_FILESYSTEM`, `JSON_HAS_EXPERIMENTAL_FILESYSTEM`
|
||||
|
||||
When compiling with C++17, the library provides conversions from and to `std::filesystem::path`. As compiler support
|
||||
for filesystem is limited, the library tries to detect whether `<filesystem>`/`std::filesystem` (`JSON_HAS_FILESYSTEM`)
|
||||
or `<experimental/filesystem>`/`std::experimental::filesystem` (`JSON_HAS_EXPERIMENTAL_FILESYSTEM`) should be used.
|
||||
To override the built-in check, define `JSON_HAS_FILESYSTEM` or `JSON_HAS_EXPERIMENTAL_FILESYSTEM` to `1`.
|
||||
|
||||
See [full documentation of `JSON_HAS_FILESYSTEM` and `JSON_HAS_EXPERIMENTAL_FILESYSTEM`](../api/macros/json_has_filesystem.md).
|
||||
|
||||
## `JSON_NOEXCEPTION`
|
||||
|
||||
Exceptions can be switched off by defining the symbol `JSON_NOEXCEPTION`.
|
||||
|
||||
See [full documentation of `JSON_NOEXCEPTION`](../api/macros/json_noexception.md).
|
||||
|
||||
## `JSON_DISABLE_ENUM_SERIALIZATION`
|
||||
|
||||
When defined, default parse and serialize functions for enums are excluded and have to be provided by the user, for example, using [`NLOHMANN_JSON_SERIALIZE_ENUM`](../api/macros/nlohmann_json_serialize_enum.md).
|
||||
|
||||
See [full documentation of `JSON_DISABLE_ENUM_SERIALIZATION`](../api/macros/json_disable_enum_serialization.md).
|
||||
|
||||
## `JSON_NO_IO`
|
||||
|
||||
When defined, headers `<cstdio>`, `<ios>`, `<iosfwd>`, `<istream>`, and `<ostream>` are not included and parse functions
|
||||
relying on these headers are excluded. This is relevant for environment where these I/O functions are disallowed for
|
||||
security reasons (e.g., Intel Software Guard Extensions (SGX)).
|
||||
|
||||
See [full documentation of `JSON_NO_IO`](../api/macros/json_no_io.md).
|
||||
|
||||
## `JSON_SKIP_LIBRARY_VERSION_CHECK`
|
||||
|
||||
When defined, the library will not create a compiler warning when a different version of the library was already
|
||||
included.
|
||||
|
||||
See [full documentation of `JSON_SKIP_LIBRARY_VERSION_CHECK`](../api/macros/json_skip_library_version_check.md).
|
||||
|
||||
## `JSON_SKIP_UNSUPPORTED_COMPILER_CHECK`
|
||||
|
||||
When defined, the library will not create a compile error when a known unsupported compiler is detected. This allows
|
||||
using the library with compilers that do not fully support C++11 and may only work if unsupported features are not used.
|
||||
|
||||
See [full documentation of `JSON_SKIP_UNSUPPORTED_COMPILER_CHECK`](../api/macros/json_skip_unsupported_compiler_check.md).
|
||||
|
||||
## `JSON_THROW_USER(exception)`
|
||||
|
||||
This macro overrides `#!cpp throw` calls inside the library. The argument is the exception to be thrown.
|
||||
|
||||
See [full documentation of `JSON_THROW_USER(exception)`](../api/macros/json_throw_user.md).
|
||||
|
||||
## `JSON_TRY_USER`
|
||||
|
||||
This macro overrides `#!cpp try` calls inside the library.
|
||||
|
||||
See [full documentation of `JSON_TRY_USER`](../api/macros/json_throw_user.md).
|
||||
|
||||
## `JSON_USE_IMPLICIT_CONVERSIONS`
|
||||
|
||||
When defined to `0`, implicit conversions are switched off. By default, implicit conversions are switched on.
|
||||
|
||||
See [full documentation of `JSON_USE_IMPLICIT_CONVERSIONS`](../api/macros/json_use_implicit_conversions.md).
|
||||
|
||||
## `JSON_USE_GLOBAL_UDLS`
|
||||
|
||||
When defined to `1` (default), the user-defined string literals `operator""_json` and `operator""_json_pointer` are
|
||||
placed into the global namespace instead of `nlohmann::literals::json_literals`.
|
||||
|
||||
See [full documentation of `JSON_USE_GLOBAL_UDLS`](../api/macros/json_use_global_udls.md).
|
||||
|
||||
## `JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON`
|
||||
|
||||
When defined to `1`, the library restores the legacy behavior in which a discarded value compared equal to itself. This
|
||||
behavior is deprecated and switched off (`0`) by default.
|
||||
|
||||
See [full documentation of `JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON`](../api/macros/json_use_legacy_discarded_value_comparison.md).
|
||||
|
||||
## `NLOHMANN_DEFINE_TYPE_*(...)`, `NLOHMANN_DEFINE_DERIVED_TYPE_*(...)`
|
||||
|
||||
The library defines 12 macros to simplify the serialization/deserialization of types. See the page on
|
||||
[arbitrary type conversion](arbitrary_types.md#simplify-your-life-with-macros) for a detailed discussion.
|
||||
|
||||
## `NLOHMANN_JSON_NAMESPACE`, `NLOHMANN_JSON_NAMESPACE_BEGIN`, `NLOHMANN_JSON_NAMESPACE_END`, `NLOHMANN_JSON_NAMESPACE_NO_VERSION`
|
||||
|
||||
These macros relate to the versioned, inline `nlohmann` namespace:
|
||||
|
||||
- `NLOHMANN_JSON_NAMESPACE` evaluates to the full name of the `nlohmann` namespace (including the inline ABI namespace).
|
||||
- `NLOHMANN_JSON_NAMESPACE_BEGIN` / `NLOHMANN_JSON_NAMESPACE_END` open and close the namespace (for example, to add
|
||||
specializations).
|
||||
- `NLOHMANN_JSON_NAMESPACE_NO_VERSION`, when defined to `1`, omits the version component from the inline namespace.
|
||||
|
||||
See the [`nlohmann` Namespace](namespace.md) page, and the full documentation of
|
||||
[`NLOHMANN_JSON_NAMESPACE`](../api/macros/nlohmann_json_namespace.md),
|
||||
[`NLOHMANN_JSON_NAMESPACE_BEGIN` / `NLOHMANN_JSON_NAMESPACE_END`](../api/macros/nlohmann_json_namespace_begin.md), and
|
||||
[`NLOHMANN_JSON_NAMESPACE_NO_VERSION`](../api/macros/nlohmann_json_namespace_no_version.md).
|
||||
|
||||
## `NLOHMANN_JSON_SERIALIZE_ENUM(type, ...)`
|
||||
|
||||
This macro simplifies the serialization/deserialization of enum types. See
|
||||
[Specializing enum conversion](enum_conversion.md) for more information.
|
||||
|
||||
See [full documentation of `NLOHMANN_JSON_SERIALIZE_ENUM`](../api/macros/nlohmann_json_serialize_enum.md).
|
||||
|
||||
A strict variant [`NLOHMANN_JSON_SERIALIZE_ENUM_STRICT`](../api/macros/nlohmann_json_serialize_enum_strict.md) throws an
|
||||
exception on undefined input instead of falling back to the first mapping.
|
||||
|
||||
## `NLOHMANN_JSON_VERSION_MAJOR`, `NLOHMANN_JSON_VERSION_MINOR`, `NLOHMANN_JSON_VERSION_PATCH`
|
||||
|
||||
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`](../api/macros/nlohmann_json_version_major.md).
|
||||
@@ -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-9.7.6"><title>Supported Macros - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.484c7ddc.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=#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> <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> 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 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
|
||||
<!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-9.7.6"><title>Supported Macros - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.484c7ddc.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=#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> <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> 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 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
|
||||
@@ -0,0 +1,139 @@
|
||||
# Supported Macros
|
||||
|
||||
Some aspects of the library can be configured by defining preprocessor macros before including the `json.hpp` header. See also the [API documentation for macros](https://json.nlohmann.me/api/macros/index.md) for examples and more information.
|
||||
|
||||
## `JSON_ASSERT(x)`
|
||||
|
||||
This macro controls which code is executed for [runtime assertions](https://json.nlohmann.me/features/assertions/index.md) of the library.
|
||||
|
||||
See [full documentation of `JSON_ASSERT(x)`](https://json.nlohmann.me/api/macros/json_assert/index.md).
|
||||
|
||||
## `JSON_BRACE_INIT_COPY_SEMANTICS`
|
||||
|
||||
When defined to `1`, single-element brace initialization of a `basic_json` value (e.g., `json j{value};`) is treated as a copy/move of the element rather than wrapping it in a single-element array. The default value is `0`, which preserves the existing behavior.
|
||||
|
||||
See [full documentation of `JSON_BRACE_INIT_COPY_SEMANTICS`](https://json.nlohmann.me/api/macros/json_brace_init_copy_semantics/index.md).
|
||||
|
||||
## `JSON_CATCH_USER(exception)`
|
||||
|
||||
This macro overrides [`catch`](https://en.cppreference.com/w/cpp/language/try_catch) calls inside the library.
|
||||
|
||||
See [full documentation of `JSON_CATCH_USER(exception)`](https://json.nlohmann.me/api/macros/json_throw_user/index.md).
|
||||
|
||||
## `JSON_DIAGNOSTICS`
|
||||
|
||||
This macro enables extended diagnostics for exception messages. Possible values are `1` to enable or `0` to disable (default).
|
||||
|
||||
When enabled, exception messages contain a [JSON Pointer](https://json.nlohmann.me/features/json_pointer/index.md) to the JSON value that triggered the exception, see [Extended diagnostic messages](https://json.nlohmann.me/home/exceptions/#extended-diagnostic-messages) for an example. Note that enabling this macro increases the size of every JSON value by one pointer and adds some runtime overhead.
|
||||
|
||||
The diagnostics messages can also be controlled with the CMake option [`JSON_Diagnostics`](https://json.nlohmann.me/integration/cmake/#json_diagnostics) (`OFF` by default) which sets `JSON_DIAGNOSTICS` accordingly.
|
||||
|
||||
See [full documentation of `JSON_DIAGNOSTICS`](https://json.nlohmann.me/api/macros/json_diagnostics/index.md).
|
||||
|
||||
## `JSON_DIAGNOSTIC_POSITIONS`
|
||||
|
||||
When enabled, two new member functions [`start_pos()`](https://json.nlohmann.me/api/basic_json/start_pos/index.md) and [`end_pos()`](https://json.nlohmann.me/api/basic_json/end_pos/index.md) are added to [`basic_json`](https://json.nlohmann.me/api/basic_json/index.md) values. If the value was created by calling the[`parse`](https://json.nlohmann.me/api/basic_json/parse/index.md) function, then these functions allow querying the byte positions of the value in the input it was parsed from. The byte positions are also used in exceptions to help locate errors.
|
||||
|
||||
The diagnostics positions can also be controlled with the CMake option [`JSON_Diagnostic_Positions`](https://json.nlohmann.me/integration/cmake/#json_diagnostic_positions) (`OFF` by default) which sets `JSON_DIAGNOSTIC_POSITIONS` accordingly.
|
||||
|
||||
See [full documentation of `JSON_DIAGNOSTIC_POSITIONS`](https://json.nlohmann.me/api/macros/json_diagnostic_positions/index.md)
|
||||
|
||||
## `JSON_HAS_CPP_11`, `JSON_HAS_CPP_14`, `JSON_HAS_CPP_17`, `JSON_HAS_CPP_20`, `JSON_HAS_CPP_23`, `JSON_HAS_CPP_26`
|
||||
|
||||
The library targets C++11, but also supports some features introduced in later C++ versions (e.g., `std::string_view` support for C++17). For these new features, the library implements some preprocessor checks to determine the C++ standard. By defining any of these symbols, the internal check is overridden and the provided C++ version is unconditionally assumed. This can be helpful for compilers that only implement parts of the standard and would be detected incorrectly.
|
||||
|
||||
See [full documentation of `JSON_HAS_CPP_11`, `JSON_HAS_CPP_14`, `JSON_HAS_CPP_17`, `JSON_HAS_CPP_20`, `JSON_HAS_CPP_23`, and `JSON_HAS_CPP_26`](https://json.nlohmann.me/api/macros/json_has_cpp_11/index.md).
|
||||
|
||||
## `JSON_HAS_FILESYSTEM`, `JSON_HAS_EXPERIMENTAL_FILESYSTEM`
|
||||
|
||||
When compiling with C++17, the library provides conversions from and to `std::filesystem::path`. As compiler support for filesystem is limited, the library tries to detect whether `<filesystem>`/`std::filesystem` (`JSON_HAS_FILESYSTEM`) or `<experimental/filesystem>`/`std::experimental::filesystem` (`JSON_HAS_EXPERIMENTAL_FILESYSTEM`) should be used. To override the built-in check, define `JSON_HAS_FILESYSTEM` or `JSON_HAS_EXPERIMENTAL_FILESYSTEM` to `1`.
|
||||
|
||||
See [full documentation of `JSON_HAS_FILESYSTEM` and `JSON_HAS_EXPERIMENTAL_FILESYSTEM`](https://json.nlohmann.me/api/macros/json_has_filesystem/index.md).
|
||||
|
||||
## `JSON_NOEXCEPTION`
|
||||
|
||||
Exceptions can be switched off by defining the symbol `JSON_NOEXCEPTION`.
|
||||
|
||||
See [full documentation of `JSON_NOEXCEPTION`](https://json.nlohmann.me/api/macros/json_noexception/index.md).
|
||||
|
||||
## `JSON_DISABLE_ENUM_SERIALIZATION`
|
||||
|
||||
When defined, default parse and serialize functions for enums are excluded and have to be provided by the user, for example, using [`NLOHMANN_JSON_SERIALIZE_ENUM`](https://json.nlohmann.me/api/macros/nlohmann_json_serialize_enum/index.md).
|
||||
|
||||
See [full documentation of `JSON_DISABLE_ENUM_SERIALIZATION`](https://json.nlohmann.me/api/macros/json_disable_enum_serialization/index.md).
|
||||
|
||||
## `JSON_NO_IO`
|
||||
|
||||
When defined, headers `<cstdio>`, `<ios>`, `<iosfwd>`, `<istream>`, and `<ostream>` are not included and parse functions relying on these headers are excluded. This is relevant for environment where these I/O functions are disallowed for security reasons (e.g., Intel Software Guard Extensions (SGX)).
|
||||
|
||||
See [full documentation of `JSON_NO_IO`](https://json.nlohmann.me/api/macros/json_no_io/index.md).
|
||||
|
||||
## `JSON_SKIP_LIBRARY_VERSION_CHECK`
|
||||
|
||||
When defined, the library will not create a compiler warning when a different version of the library was already included.
|
||||
|
||||
See [full documentation of `JSON_SKIP_LIBRARY_VERSION_CHECK`](https://json.nlohmann.me/api/macros/json_skip_library_version_check/index.md).
|
||||
|
||||
## `JSON_SKIP_UNSUPPORTED_COMPILER_CHECK`
|
||||
|
||||
When defined, the library will not create a compile error when a known unsupported compiler is detected. This allows using the library with compilers that do not fully support C++11 and may only work if unsupported features are not used.
|
||||
|
||||
See [full documentation of `JSON_SKIP_UNSUPPORTED_COMPILER_CHECK`](https://json.nlohmann.me/api/macros/json_skip_unsupported_compiler_check/index.md).
|
||||
|
||||
## `JSON_THROW_USER(exception)`
|
||||
|
||||
This macro overrides `throw` calls inside the library. The argument is the exception to be thrown.
|
||||
|
||||
See [full documentation of `JSON_THROW_USER(exception)`](https://json.nlohmann.me/api/macros/json_throw_user/index.md).
|
||||
|
||||
## `JSON_TRY_USER`
|
||||
|
||||
This macro overrides `try` calls inside the library.
|
||||
|
||||
See [full documentation of `JSON_TRY_USER`](https://json.nlohmann.me/api/macros/json_throw_user/index.md).
|
||||
|
||||
## `JSON_USE_IMPLICIT_CONVERSIONS`
|
||||
|
||||
When defined to `0`, implicit conversions are switched off. By default, implicit conversions are switched on.
|
||||
|
||||
See [full documentation of `JSON_USE_IMPLICIT_CONVERSIONS`](https://json.nlohmann.me/api/macros/json_use_implicit_conversions/index.md).
|
||||
|
||||
## `JSON_USE_GLOBAL_UDLS`
|
||||
|
||||
When defined to `1` (default), the user-defined string literals `operator""_json` and `operator""_json_pointer` are placed into the global namespace instead of `nlohmann::literals::json_literals`.
|
||||
|
||||
See [full documentation of `JSON_USE_GLOBAL_UDLS`](https://json.nlohmann.me/api/macros/json_use_global_udls/index.md).
|
||||
|
||||
## `JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON`
|
||||
|
||||
When defined to `1`, the library restores the legacy behavior in which a discarded value compared equal to itself. This behavior is deprecated and switched off (`0`) by default.
|
||||
|
||||
See [full documentation of `JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON`](https://json.nlohmann.me/api/macros/json_use_legacy_discarded_value_comparison/index.md).
|
||||
|
||||
## `NLOHMANN_DEFINE_TYPE_*(...)`, `NLOHMANN_DEFINE_DERIVED_TYPE_*(...)`
|
||||
|
||||
The library defines 12 macros to simplify the serialization/deserialization of types. See the page on [arbitrary type conversion](https://json.nlohmann.me/features/arbitrary_types/#simplify-your-life-with-macros) for a detailed discussion.
|
||||
|
||||
## `NLOHMANN_JSON_NAMESPACE`, `NLOHMANN_JSON_NAMESPACE_BEGIN`, `NLOHMANN_JSON_NAMESPACE_END`, `NLOHMANN_JSON_NAMESPACE_NO_VERSION`
|
||||
|
||||
These macros relate to the versioned, inline `nlohmann` namespace:
|
||||
|
||||
- `NLOHMANN_JSON_NAMESPACE` evaluates to the full name of the `nlohmann` namespace (including the inline ABI namespace).
|
||||
- `NLOHMANN_JSON_NAMESPACE_BEGIN` / `NLOHMANN_JSON_NAMESPACE_END` open and close the namespace (for example, to add specializations).
|
||||
- `NLOHMANN_JSON_NAMESPACE_NO_VERSION`, when defined to `1`, omits the version component from the inline namespace.
|
||||
|
||||
See the [`nlohmann` Namespace](https://json.nlohmann.me/features/namespace/index.md) page, and the full documentation of [`NLOHMANN_JSON_NAMESPACE`](https://json.nlohmann.me/api/macros/nlohmann_json_namespace/index.md), [`NLOHMANN_JSON_NAMESPACE_BEGIN` / `NLOHMANN_JSON_NAMESPACE_END`](https://json.nlohmann.me/api/macros/nlohmann_json_namespace_begin/index.md), and [`NLOHMANN_JSON_NAMESPACE_NO_VERSION`](https://json.nlohmann.me/api/macros/nlohmann_json_namespace_no_version/index.md).
|
||||
|
||||
## `NLOHMANN_JSON_SERIALIZE_ENUM(type, ...)`
|
||||
|
||||
This macro simplifies the serialization/deserialization of enum types. See [Specializing enum conversion](https://json.nlohmann.me/features/enum_conversion/index.md) for more information.
|
||||
|
||||
See [full documentation of `NLOHMANN_JSON_SERIALIZE_ENUM`](https://json.nlohmann.me/api/macros/nlohmann_json_serialize_enum/index.md).
|
||||
|
||||
A strict variant [`NLOHMANN_JSON_SERIALIZE_ENUM_STRICT`](https://json.nlohmann.me/api/macros/nlohmann_json_serialize_enum_strict/index.md) throws an exception on undefined input instead of falling back to the first mapping.
|
||||
|
||||
## `NLOHMANN_JSON_VERSION_MAJOR`, `NLOHMANN_JSON_VERSION_MINOR`, `NLOHMANN_JSON_VERSION_PATCH`
|
||||
|
||||
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).
|
||||
@@ -0,0 +1,20 @@
|
||||
# JSON Merge Patch
|
||||
|
||||
The library supports JSON Merge Patch ([RFC 7386](https://tools.ietf.org/html/rfc7386)) as a patch format.
|
||||
The merge patch format is primarily intended for use with the HTTP PATCH method as a means of describing a set of modifications to a target resource's content. This function applies a merge patch to the current JSON value.
|
||||
|
||||
Instead of using [JSON Pointer](json_pointer.md) to specify values to be manipulated, it describes the changes using a syntax that closely mimics the document being modified.
|
||||
|
||||
??? example
|
||||
|
||||
The following code shows how a JSON Merge Patch is applied to a JSON document.
|
||||
|
||||
```cpp
|
||||
--8<-- "examples/merge_patch.cpp"
|
||||
```
|
||||
|
||||
Output:
|
||||
|
||||
```json
|
||||
--8<-- "examples/merge_patch.output"
|
||||
```
|
||||
@@ -50,4 +50,4 @@
|
||||
<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> <!-- 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="July 4, 2026 09:13:25 UTC">July 4, 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> <!-- 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="July 8, 2026 18:18:24 UTC">July 8, 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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|
||||
@@ -0,0 +1,69 @@
|
||||
# JSON Merge Patch
|
||||
|
||||
The library supports JSON Merge Patch ([RFC 7386](https://tools.ietf.org/html/rfc7386)) as a patch format. The merge patch format is primarily intended for use with the HTTP PATCH method as a means of describing a set of modifications to a target resource's content. This function applies a merge patch to the current JSON value.
|
||||
|
||||
Instead of using [JSON Pointer](https://json.nlohmann.me/features/json_pointer/index.md) to specify values to be manipulated, it describes the changes using a syntax that closely mimics the document being modified.
|
||||
|
||||
Example
|
||||
|
||||
The following code shows how a JSON Merge Patch is applied to a JSON document.
|
||||
|
||||
```
|
||||
#include <iostream>
|
||||
#include <nlohmann/json.hpp>
|
||||
#include <iomanip> // for std::setw
|
||||
|
||||
using json = nlohmann::json;
|
||||
using namespace nlohmann::literals;
|
||||
|
||||
int main()
|
||||
{
|
||||
// the original document
|
||||
json document = R"({
|
||||
"title": "Goodbye!",
|
||||
"author": {
|
||||
"givenName": "John",
|
||||
"familyName": "Doe"
|
||||
},
|
||||
"tags": [
|
||||
"example",
|
||||
"sample"
|
||||
],
|
||||
"content": "This will be unchanged"
|
||||
})"_json;
|
||||
|
||||
// the patch
|
||||
json patch = R"({
|
||||
"title": "Hello!",
|
||||
"phoneNumber": "+01-123-456-7890",
|
||||
"author": {
|
||||
"familyName": null
|
||||
},
|
||||
"tags": [
|
||||
"example"
|
||||
]
|
||||
})"_json;
|
||||
|
||||
// apply the patch
|
||||
document.merge_patch(patch);
|
||||
|
||||
// output original and patched document
|
||||
std::cout << std::setw(4) << document << std::endl;
|
||||
}
|
||||
```
|
||||
|
||||
Output:
|
||||
|
||||
```
|
||||
{
|
||||
"author": {
|
||||
"givenName": "John"
|
||||
},
|
||||
"content": "This will be unchanged",
|
||||
"phoneNumber": "+01-123-456-7890",
|
||||
"tags": [
|
||||
"example"
|
||||
],
|
||||
"title": "Hello!"
|
||||
}
|
||||
```
|
||||
@@ -0,0 +1,77 @@
|
||||
# Modifying values
|
||||
|
||||
Once a JSON value exists, its content can be changed: elements can be added, replaced, merged, and removed. This page
|
||||
gives an overview of the available operations. For read access, see [element access](element_access/index.md).
|
||||
|
||||
## Adding to arrays
|
||||
|
||||
New elements are appended to an array with [`push_back`](../api/basic_json/push_back.md) or constructed in place with
|
||||
[`emplace_back`](../api/basic_json/emplace_back.md). If the value is `#!json null`, it is converted to an array first, so
|
||||
these functions can also be used to build an array from scratch.
|
||||
|
||||
```cpp
|
||||
json j; // null
|
||||
j.push_back(1); // [1]
|
||||
j.push_back(2); // [1,2]
|
||||
j.emplace_back(3); // [1,2,3]
|
||||
|
||||
// operator+= is a shorthand for push_back
|
||||
j += 4; // [1,2,3,4]
|
||||
```
|
||||
|
||||
## Adding to objects
|
||||
|
||||
The most common way to add or replace a member is [`operator[]`](element_access/unchecked_access.md), which inserts the
|
||||
key if it does not exist yet:
|
||||
|
||||
```cpp
|
||||
json j;
|
||||
j["name"] = "Mary"; // {"name":"Mary"}
|
||||
j["name"] = "John"; // {"name":"John"} (replaced)
|
||||
```
|
||||
|
||||
[`emplace`](../api/basic_json/emplace.md) inserts a member only if the key is not already present, and reports whether
|
||||
the insertion happened — useful for "add if absent" semantics.
|
||||
|
||||
## Merging objects
|
||||
|
||||
To merge one object into another, [`update`](../api/basic_json/update.md) copies all members from another object,
|
||||
overwriting existing keys (similar to Python's `dict.update`). This is the idiomatic way to combine two objects.
|
||||
|
||||
??? example
|
||||
|
||||
```cpp
|
||||
--8<-- "examples/update.cpp"
|
||||
```
|
||||
|
||||
Output:
|
||||
|
||||
```json
|
||||
--8<-- "examples/update.output"
|
||||
```
|
||||
|
||||
For a recursive merge that follows [RFC 7386](https://tools.ietf.org/html/rfc7386), see
|
||||
[JSON Merge Patch](merge_patch.md). To apply a sequence of well-defined edit operations, see
|
||||
[JSON Patch](json_patch.md).
|
||||
|
||||
## Removing elements
|
||||
|
||||
Elements are removed with [`erase`](../api/basic_json/erase.md), which accepts an object key, an array index, or an
|
||||
iterator. [`clear`](../api/basic_json/clear.md) empties a value while keeping its type, and
|
||||
[`operator[]`](element_access/unchecked_access.md) combined with assignment can overwrite a value entirely.
|
||||
|
||||
```cpp
|
||||
json j = {{"a", 1}, {"b", 2}, {"c", 3}};
|
||||
j.erase("b"); // {"a":1,"c":3}
|
||||
|
||||
json a = {1, 2, 3, 4};
|
||||
a.erase(1); // [1,3,4] (erase by index)
|
||||
```
|
||||
|
||||
## See also
|
||||
|
||||
- [`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
|
||||
- [`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
|
||||
@@ -54,4 +54,4 @@
|
||||
|
||||
<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>
|
||||
<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>
|
||||
</code></pre></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/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</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> </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="July 4, 2026 09:13:25 UTC">July 4, 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> <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</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> </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="July 8, 2026 18:18:24 UTC">July 8, 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-.5Line truncated
|
||||
@@ -0,0 +1,106 @@
|
||||
# Modifying values
|
||||
|
||||
Once a JSON value exists, its content can be changed: elements can be added, replaced, merged, and removed. This page gives an overview of the available operations. For read access, see [element access](https://json.nlohmann.me/features/element_access/index.md).
|
||||
|
||||
## Adding to arrays
|
||||
|
||||
New elements are appended to an array with [`push_back`](https://json.nlohmann.me/api/basic_json/push_back/index.md) or constructed in place with [`emplace_back`](https://json.nlohmann.me/api/basic_json/emplace_back/index.md). If the value is `null`, it is converted to an array first, so these functions can also be used to build an array from scratch.
|
||||
|
||||
```
|
||||
json j; // null
|
||||
j.push_back(1); // [1]
|
||||
j.push_back(2); // [1,2]
|
||||
j.emplace_back(3); // [1,2,3]
|
||||
|
||||
// operator+= is a shorthand for push_back
|
||||
j += 4; // [1,2,3,4]
|
||||
```
|
||||
|
||||
## Adding to objects
|
||||
|
||||
The most common way to add or replace a member is [`operator[]`](https://json.nlohmann.me/features/element_access/unchecked_access/index.md), which inserts the key if it does not exist yet:
|
||||
|
||||
```
|
||||
json j;
|
||||
j["name"] = "Mary"; // {"name":"Mary"}
|
||||
j["name"] = "John"; // {"name":"John"} (replaced)
|
||||
```
|
||||
|
||||
[`emplace`](https://json.nlohmann.me/api/basic_json/emplace/index.md) inserts a member only if the key is not already present, and reports whether the insertion happened — useful for "add if absent" semantics.
|
||||
|
||||
## Merging objects
|
||||
|
||||
To merge one object into another, [`update`](https://json.nlohmann.me/api/basic_json/update/index.md) copies all members from another object, overwriting existing keys (similar to Python's `dict.update`). This is the idiomatic way to combine two objects.
|
||||
|
||||
Example
|
||||
|
||||
```
|
||||
#include <iostream>
|
||||
#include <iomanip>
|
||||
#include <nlohmann/json.hpp>
|
||||
|
||||
using json = nlohmann::json;
|
||||
using namespace nlohmann::literals;
|
||||
|
||||
int main()
|
||||
{
|
||||
// create two JSON objects
|
||||
json o1 = R"( {"color": "red", "price": 17.99, "names": {"de": "Flugzeug"}} )"_json;
|
||||
json o2 = R"( {"color": "blue", "speed": 100, "names": {"en": "plane"}} )"_json;
|
||||
json o3 = o1;
|
||||
|
||||
// add all keys from o2 to o1 (updating "color", replacing "names")
|
||||
o1.update(o2);
|
||||
|
||||
// add all keys from o2 to o1 (updating "color", merging "names")
|
||||
o3.update(o2, true);
|
||||
|
||||
// output updated object o1 and o3
|
||||
std::cout << std::setw(2) << o1 << '\n';
|
||||
std::cout << std::setw(2) << o3 << '\n';
|
||||
}
|
||||
```
|
||||
|
||||
Output:
|
||||
|
||||
```
|
||||
{
|
||||
"color": "blue",
|
||||
"names": {
|
||||
"en": "plane"
|
||||
},
|
||||
"price": 17.99,
|
||||
"speed": 100
|
||||
}
|
||||
{
|
||||
"color": "blue",
|
||||
"names": {
|
||||
"de": "Flugzeug",
|
||||
"en": "plane"
|
||||
},
|
||||
"price": 17.99,
|
||||
"speed": 100
|
||||
}
|
||||
```
|
||||
|
||||
For a recursive merge that follows [RFC 7386](https://tools.ietf.org/html/rfc7386), see [JSON Merge Patch](https://json.nlohmann.me/features/merge_patch/index.md). To apply a sequence of well-defined edit operations, see [JSON Patch](https://json.nlohmann.me/features/json_patch/index.md).
|
||||
|
||||
## Removing elements
|
||||
|
||||
Elements are removed with [`erase`](https://json.nlohmann.me/api/basic_json/erase/index.md), which accepts an object key, an array index, or an iterator. [`clear`](https://json.nlohmann.me/api/basic_json/clear/index.md) empties a value while keeping its type, and [`operator[]`](https://json.nlohmann.me/features/element_access/unchecked_access/index.md) combined with assignment can overwrite a value entirely.
|
||||
|
||||
```
|
||||
json j = {{"a", 1}, {"b", 2}, {"c", 3}};
|
||||
j.erase("b"); // {"a":1,"c":3}
|
||||
|
||||
json a = {1, 2, 3, 4};
|
||||
a.erase(1); // [1,3,4] (erase by index)
|
||||
```
|
||||
|
||||
## 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
|
||||
- [`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
|
||||
- [`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
|
||||
@@ -0,0 +1,40 @@
|
||||
# Modules
|
||||
|
||||
This library has experimental support for C++ modules, introduced in C++20. The library can be imported by writing `import nlohmann.json;` instead of `#include <nlohmann/json.hpp>`.
|
||||
|
||||
Please be aware that the module is experimental and a full test is outstanding, and the exported symbols are subject to change.
|
||||
|
||||
## 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.
|
||||
|
||||
To enable building the `nlohmann.json` module (which is not done by default), the macro `NLOHMANN_JSON_BUILD_MODULES` must be passed to the build system.
|
||||
|
||||
## Example
|
||||
When using modules rather than headers, the previous example for creating a `json` object through a JSON file, would instead be:
|
||||
```cpp
|
||||
import std;
|
||||
import nlohmann.json;
|
||||
|
||||
using json = nlohmann::json;
|
||||
|
||||
// ...
|
||||
|
||||
std::ifstream f("example.json");
|
||||
json data = json::parse(f);
|
||||
```
|
||||
|
||||
## Modules do not export macros
|
||||
It should be noted that as modules do not export macros, the `nlohmann.json` module will not export any macros.
|
||||
|
||||
## Exported symbols
|
||||
Only the following symbols are exported from `nlohmann.json`:
|
||||
|
||||
- `nlohmann::adl_serializer`
|
||||
- `nlohmann::basic_json`
|
||||
- `nlohmann::json`
|
||||
- `nlohmann::json_pointer`
|
||||
- `nlohmann::ordered_json`
|
||||
- `nlohmann::ordered_map`
|
||||
- `nlohmann::to_string`
|
||||
- `nlohmann::literals::json_literals::operator""_json`
|
||||
- `nlohmann::literals::json_literals::operator""_json_pointer`
|
||||
@@ -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::json</code></li> <li><code>nlohmann::json_pointer</code></li> <li><code>nlohmann::ordered_json</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> <!-- 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="July 4, 2026 09:13:25 UTC">July 4, 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 416H69V2Line 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::json</code></li> <li><code>nlohmann::json_pointer</code></li> <li><code>nlohmann::ordered_json</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> <!-- 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="July 8, 2026 18:18:24 UTC">July 8, 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 416H69V2Line truncated
|
||||
@@ -0,0 +1,45 @@
|
||||
# Modules
|
||||
|
||||
This library has experimental support for C++ modules, introduced in C++20. The library can be imported by writing `import nlohmann.json;` instead of `#include <nlohmann/json.hpp>`.
|
||||
|
||||
Please be aware that the module is experimental and a full test is outstanding, and the exported symbols are subject to change.
|
||||
|
||||
## 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.
|
||||
|
||||
To enable building the `nlohmann.json` module (which is not done by default), the macro `NLOHMANN_JSON_BUILD_MODULES` must be passed to the build system.
|
||||
|
||||
## Example
|
||||
|
||||
When using modules rather than headers, the previous example for creating a `json` object through a JSON file, would instead be:
|
||||
|
||||
```
|
||||
import std;
|
||||
import nlohmann.json;
|
||||
|
||||
using json = nlohmann::json;
|
||||
|
||||
// ...
|
||||
|
||||
std::ifstream f("example.json");
|
||||
json data = json::parse(f);
|
||||
```
|
||||
|
||||
## Modules do not export macros
|
||||
|
||||
It should be noted that as modules do not export macros, the `nlohmann.json` module will not export any macros.
|
||||
|
||||
## Exported symbols
|
||||
|
||||
Only the following symbols are exported from `nlohmann.json`:
|
||||
|
||||
- `nlohmann::adl_serializer`
|
||||
- `nlohmann::basic_json`
|
||||
- `nlohmann::json`
|
||||
- `nlohmann::json_pointer`
|
||||
- `nlohmann::ordered_json`
|
||||
- `nlohmann::ordered_map`
|
||||
- `nlohmann::to_string`
|
||||
- `nlohmann::literals::json_literals::operator""_json`
|
||||
- `nlohmann::literals::json_literals::operator""_json_pointer`
|
||||
@@ -0,0 +1,95 @@
|
||||
# `nlohmann` Namespace
|
||||
|
||||
The 3.11.0 release introduced an
|
||||
[inline namespace](https://en.cppreference.com/w/cpp/language/namespace#Inline_namespaces) to allow different parts of
|
||||
a codebase to safely use different versions of the JSON library as long as they never exchange instances of library
|
||||
types.
|
||||
|
||||
## Structure
|
||||
|
||||
The complete default namespace name is derived as follows:
|
||||
|
||||
- The root namespace is always `nlohmann`.
|
||||
- The inline namespace starts with `json_abi` and is followed by several optional ABI tags according to the value of
|
||||
these ABI-affecting macros, in order:
|
||||
- [`JSON_DIAGNOSTICS`](../api/macros/json_diagnostics.md) defined non-zero appends `_diag`.
|
||||
- [`JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON`](../api/macros/json_use_legacy_discarded_value_comparison.md)
|
||||
defined non-zero appends `_ldvcmp`.
|
||||
- The inline namespace ends with the suffix `_v` followed by the 3 components of the version number separated by
|
||||
underscores. To omit the version component, see [Disabling the version component](#disabling-the-version-component)
|
||||
below.
|
||||
|
||||
For example, the namespace name for version 3.11.2 with `JSON_DIAGNOSTICS` defined to `1` is:
|
||||
|
||||
```cpp
|
||||
nlohmann::json_abi_diag_v3_11_2
|
||||
```
|
||||
|
||||
## Purpose
|
||||
|
||||
Several incompatibilities have been observed. Amongst the most common ones is linking code compiled with different
|
||||
definitions of [`JSON_DIAGNOSTICS`](../api/macros/json_diagnostics.md). This is illustrated in the diagram below.
|
||||
|
||||
```mermaid
|
||||
graph
|
||||
json["<strong>nlohmann_json (v3.10.5)</strong><br>JSON_DIAGNOSTICS=0"]
|
||||
json_diag["<strong>nlohmann_json (v3.10.5)</strong><br>JSON_DIAGNOSTICS=1"]
|
||||
library["<strong>some library</strong>"]
|
||||
app["<strong>application</strong>"]
|
||||
|
||||
library --> json
|
||||
app --> json_diag
|
||||
app --> library
|
||||
```
|
||||
|
||||
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
|
||||
|
||||
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 JSON library – exchange and use library
|
||||
types.
|
||||
|
||||
There is an exception when forward declarations are used (i.e., when including `json_fwd.hpp`) in which case the linker
|
||||
may complain about undefined references.
|
||||
|
||||
## Disabling the version component
|
||||
|
||||
Different versions are not necessarily ABI-incompatible, but the project does not actively track changes in the ABI and
|
||||
recommends that all parts of a codebase exchanging library types be built with the same version. Users can, **at their
|
||||
own risk**, 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.
|
||||
|
||||
To do so, define [`NLOHMANN_JSON_NAMESPACE_NO_VERSION`](../api/macros/nlohmann_json_namespace_no_version.md) to `1`.
|
||||
|
||||
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 `NLOHMANN_JSON_NAMESPACE_NO_VERSION` macro.
|
||||
|
||||
!!! danger "Use at your own risk"
|
||||
|
||||
Disabling the namespace version component and mixing ABI-incompatible versions will result in crashes or incorrect
|
||||
behavior. You have been warned!
|
||||
|
||||
## Disabling the inline namespace completely
|
||||
|
||||
When interoperability with code using a pre-3.11.0 version of the library is required, users can, **at their own risk**
|
||||
restore the old namespace layout by redefining
|
||||
[`NLOHMANN_JSON_NAMESPACE_BEGIN, NLOHMANN_JSON_NAMESPACE_END`](../api/macros/nlohmann_json_namespace_begin.md) as
|
||||
follows:
|
||||
|
||||
```cpp
|
||||
#define NLOHMANN_JSON_NAMESPACE_BEGIN namespace nlohmann {
|
||||
#define NLOHMANN_JSON_NAMESPACE_END }
|
||||
```
|
||||
|
||||
!!! danger "Use at your own risk"
|
||||
|
||||
Overriding the namespace and mixing ABI-incompatible versions will result in crashes or incorrect behavior. You
|
||||
have been warned!
|
||||
|
||||
## Version history
|
||||
|
||||
- Introduced inline namespace (`json_v3_11_0[_abi-tag]*`) in version 3.11.0.
|
||||
- Changed structure of inline namespace in version 3.11.2.
|
||||
@@ -9,4 +9,4 @@
|
||||
app --> json_diag
|
||||
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 ABI-incompatible, but the project does not actively track changes in the ABI 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 ABI-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>
|
||||
<span class=cp>#define NLOHMANN_JSON_NAMESPACE_END }</span>
|
||||
</code></pre></div> <div class="admonition danger"> <p class=admonition-title>Use at your own risk</p> <p>Overriding the namespace and mixing ABI-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> </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="July 4, 2026 09:13:25 UTC">July 4, 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-66.4V202.2h63.7v29.2h.9c8.9-16.8 30.6-34.5 62.9-34.5 67.2 0 79.7 44.3 79.7 101.9z"/></svg> </a> <a href="https://www.xing.com/profile/Niels_Lohmann" target="_blank" rel="noopener" title="www.xing.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 384 512"><!-- Font AwesomeLine truncated
|
||||
</code></pre></div> <div class="admonition danger"> <p class=admonition-title>Use at your own risk</p> <p>Overriding the namespace and mixing ABI-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> </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="July 8, 2026 18:18:24 UTC">July 8, 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-66.4V202.2h63.7v29.2h.9c8.9-16.8 30.6-34.5 62.9-34.5 67.2 0 79.7 44.3 79.7 101.9z"/></svg> </a> <a href="https://www.xing.com/profile/Niels_Lohmann" target="_blank" rel="noopener" title="www.xing.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 384 512"><!-- Font AwesomeLine truncated
|
||||
@@ -0,0 +1,73 @@
|
||||
# `nlohmann` Namespace
|
||||
|
||||
The 3.11.0 release introduced an [inline namespace](https://en.cppreference.com/w/cpp/language/namespace#Inline_namespaces) to allow different parts of a codebase to safely use different versions of the JSON library as long as they never exchange instances of library types.
|
||||
|
||||
## Structure
|
||||
|
||||
The complete default namespace name is derived as follows:
|
||||
|
||||
- The root namespace is always `nlohmann`.
|
||||
- The inline namespace starts with `json_abi` and is followed by several optional ABI tags according to the value of these ABI-affecting macros, in order:
|
||||
- [`JSON_DIAGNOSTICS`](https://json.nlohmann.me/api/macros/json_diagnostics/index.md) defined non-zero appends `_diag`.
|
||||
- [`JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON`](https://json.nlohmann.me/api/macros/json_use_legacy_discarded_value_comparison/index.md) defined non-zero appends `_ldvcmp`.
|
||||
- The inline namespace ends with the suffix `_v` followed by the 3 components of the version number separated by underscores. To omit the version component, see [Disabling the version component](#disabling-the-version-component) below.
|
||||
|
||||
For example, the namespace name for version 3.11.2 with `JSON_DIAGNOSTICS` defined to `1` is:
|
||||
|
||||
```
|
||||
nlohmann::json_abi_diag_v3_11_2
|
||||
```
|
||||
|
||||
## 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.
|
||||
|
||||
```
|
||||
graph
|
||||
json["<strong>nlohmann_json (v3.10.5)</strong><br>JSON_DIAGNOSTICS=0"]
|
||||
json_diag["<strong>nlohmann_json (v3.10.5)</strong><br>JSON_DIAGNOSTICS=1"]
|
||||
library["<strong>some library</strong>"]
|
||||
app["<strong>application</strong>"]
|
||||
|
||||
library --> json
|
||||
app --> json_diag
|
||||
app --> library
|
||||
```
|
||||
|
||||
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
|
||||
|
||||
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 JSON library – exchange and use library types.
|
||||
|
||||
There is an exception when forward declarations are used (i.e., when including `json_fwd.hpp`) in which case the linker may complain about undefined references.
|
||||
|
||||
## Disabling the version component
|
||||
|
||||
Different versions are not necessarily ABI-incompatible, but the project does not actively track changes in the ABI and recommends that all parts of a codebase exchanging library types be built with the same version. Users can, **at their own risk**, 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.
|
||||
|
||||
To do so, define [`NLOHMANN_JSON_NAMESPACE_NO_VERSION`](https://json.nlohmann.me/api/macros/nlohmann_json_namespace_no_version/index.md) to `1`.
|
||||
|
||||
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 `NLOHMANN_JSON_NAMESPACE_NO_VERSION` macro.
|
||||
|
||||
Use at your own risk
|
||||
|
||||
Disabling the namespace version component and mixing ABI-incompatible versions will result in crashes or incorrect behavior. You have been warned!
|
||||
|
||||
## Disabling the inline namespace completely
|
||||
|
||||
When interoperability with code using a pre-3.11.0 version of the library is required, users can, **at their own risk** restore the old namespace layout by redefining [`NLOHMANN_JSON_NAMESPACE_BEGIN, NLOHMANN_JSON_NAMESPACE_END`](https://json.nlohmann.me/api/macros/nlohmann_json_namespace_begin/index.md) as follows:
|
||||
|
||||
```
|
||||
#define NLOHMANN_JSON_NAMESPACE_BEGIN namespace nlohmann {
|
||||
#define NLOHMANN_JSON_NAMESPACE_END }
|
||||
```
|
||||
|
||||
Use at your own risk
|
||||
|
||||
Overriding the namespace and mixing ABI-incompatible versions will result in crashes or incorrect behavior. You have been warned!
|
||||
|
||||
## Version history
|
||||
|
||||
- Introduced inline namespace (`json_v3_11_0[_abi-tag]*`) in version 3.11.0.
|
||||
- Changed structure of inline namespace in version 3.11.2.
|
||||
@@ -0,0 +1,109 @@
|
||||
# Object Order
|
||||
|
||||
The [JSON standard](https://tools.ietf.org/html/rfc8259.html) defines objects as "an unordered collection of zero or more name/value pairs". As such, an implementation does not need to preserve any specific order of object keys.
|
||||
|
||||
## Default behavior: sort keys
|
||||
|
||||
The default type `nlohmann::json` uses a `std::map` to store JSON objects, and thus stores object keys **sorted alphabetically**.
|
||||
|
||||
??? example
|
||||
|
||||
```cpp
|
||||
#include <iostream>
|
||||
#include <nlohmann/json.hpp>
|
||||
|
||||
using json = nlohmann::json;
|
||||
|
||||
int main()
|
||||
{
|
||||
json j;
|
||||
j["one"] = 1;
|
||||
j["two"] = 2;
|
||||
j["three"] = 3;
|
||||
|
||||
std::cout << j.dump(2) << '\n';
|
||||
}
|
||||
```
|
||||
|
||||
Output:
|
||||
|
||||
```json
|
||||
{
|
||||
"one": 1,
|
||||
"three": 3,
|
||||
"two": 2
|
||||
}
|
||||
```
|
||||
|
||||
## Alternative behavior: preserve insertion order
|
||||
|
||||
If you do want to preserve the **insertion order**, you can use the type [`nlohmann::ordered_json`](../api/ordered_json.md).
|
||||
|
||||
??? example
|
||||
|
||||
```cpp
|
||||
--8<-- "examples/ordered_json.cpp"
|
||||
```
|
||||
|
||||
Output:
|
||||
|
||||
```json
|
||||
--8<-- "examples/ordered_json.output"
|
||||
```
|
||||
|
||||
Alternatively, you can use a more sophisticated ordered map like [`tsl::ordered_map`](https://github.com/Tessil/ordered-map) ([integration](https://github.com/nlohmann/json/issues/546#issuecomment-304447518)) or [`nlohmann::fifo_map`](https://github.com/nlohmann/fifo_map) ([integration](https://github.com/nlohmann/json/issues/485#issuecomment-333652309)).
|
||||
|
||||
### Notes on parsing
|
||||
|
||||
Note that you also need to call the right [`parse`](../api/basic_json/parse.md) function when reading from a file.
|
||||
Assume file `input.json` contains the JSON object above:
|
||||
|
||||
```json
|
||||
{
|
||||
"one": 1,
|
||||
"two": 2,
|
||||
"three": 3
|
||||
}
|
||||
```
|
||||
|
||||
!!! success "Right way"
|
||||
|
||||
The following code correctly calls the `parse` function from `nlohmann::ordered_json`:
|
||||
|
||||
```cpp
|
||||
std::ifstream i("input.json");
|
||||
auto j = nlohmann::ordered_json::parse(i);
|
||||
std::cout << j.dump(2) << std::endl;
|
||||
```
|
||||
|
||||
The output will be:
|
||||
|
||||
```json
|
||||
{
|
||||
"one": 1,
|
||||
"two": 2,
|
||||
"three": 3
|
||||
}
|
||||
```
|
||||
|
||||
??? failure "Wrong way"
|
||||
|
||||
The following code incorrectly calls the `parse` function from `nlohmann::json` which does not preserve the
|
||||
insertion order, but sorts object keys. Assigning the result to `nlohmann::ordered_json` compiles, but does not
|
||||
restore the order from the input file.
|
||||
|
||||
```cpp
|
||||
std::ifstream i("input.json");
|
||||
nlohmann::ordered_json j = nlohmann::json::parse(i);
|
||||
std::cout << j.dump(2) << std::endl;
|
||||
```
|
||||
|
||||
The output will be:
|
||||
|
||||
```json
|
||||
{
|
||||
"one": 1,
|
||||
"three": 3,
|
||||
"two": 2
|
||||
}
|
||||
```
|
||||
@@ -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="July 4, 2026 09:13:25 UTC">July 4, 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> <!-- 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="July 8, 2026 18:18:24 UTC">July 8, 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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|
||||
@@ -0,0 +1,123 @@
|
||||
# Object Order
|
||||
|
||||
The [JSON standard](https://tools.ietf.org/html/rfc8259.html) defines objects as "an unordered collection of zero or more name/value pairs". As such, an implementation does not need to preserve any specific order of object keys.
|
||||
|
||||
## Default behavior: sort keys
|
||||
|
||||
The default type `nlohmann::json` uses a `std::map` to store JSON objects, and thus stores object keys **sorted alphabetically**.
|
||||
|
||||
Example
|
||||
|
||||
```
|
||||
#include <iostream>
|
||||
#include <nlohmann/json.hpp>
|
||||
|
||||
using json = nlohmann::json;
|
||||
|
||||
int main()
|
||||
{
|
||||
json j;
|
||||
j["one"] = 1;
|
||||
j["two"] = 2;
|
||||
j["three"] = 3;
|
||||
|
||||
std::cout << j.dump(2) << '\n';
|
||||
}
|
||||
```
|
||||
|
||||
Output:
|
||||
|
||||
```
|
||||
{
|
||||
"one": 1,
|
||||
"three": 3,
|
||||
"two": 2
|
||||
}
|
||||
```
|
||||
|
||||
## Alternative behavior: preserve insertion order
|
||||
|
||||
If you do want to preserve the **insertion order**, you can use the type [`nlohmann::ordered_json`](https://json.nlohmann.me/api/ordered_json/index.md).
|
||||
|
||||
Example
|
||||
|
||||
```
|
||||
#include <iostream>
|
||||
#include <nlohmann/json.hpp>
|
||||
|
||||
using ordered_json = nlohmann::ordered_json;
|
||||
|
||||
int main()
|
||||
{
|
||||
ordered_json j;
|
||||
j["one"] = 1;
|
||||
j["two"] = 2;
|
||||
j["three"] = 3;
|
||||
|
||||
std::cout << j.dump(2) << '\n';
|
||||
}
|
||||
```
|
||||
|
||||
Output:
|
||||
|
||||
```
|
||||
{
|
||||
"one": 1,
|
||||
"two": 2,
|
||||
"three": 3
|
||||
}
|
||||
```
|
||||
|
||||
Alternatively, you can use a more sophisticated ordered map like [`tsl::ordered_map`](https://github.com/Tessil/ordered-map) ([integration](https://github.com/nlohmann/json/issues/546#issuecomment-304447518)) or [`nlohmann::fifo_map`](https://github.com/nlohmann/fifo_map) ([integration](https://github.com/nlohmann/json/issues/485#issuecomment-333652309)).
|
||||
|
||||
### Notes on parsing
|
||||
|
||||
Note that you also need to call the right [`parse`](https://json.nlohmann.me/api/basic_json/parse/index.md) function when reading from a file. Assume file `input.json` contains the JSON object above:
|
||||
|
||||
```
|
||||
{
|
||||
"one": 1,
|
||||
"two": 2,
|
||||
"three": 3
|
||||
}
|
||||
```
|
||||
|
||||
Right way
|
||||
|
||||
The following code correctly calls the `parse` function from `nlohmann::ordered_json`:
|
||||
|
||||
```
|
||||
std::ifstream i("input.json");
|
||||
auto j = nlohmann::ordered_json::parse(i);
|
||||
std::cout << j.dump(2) << std::endl;
|
||||
```
|
||||
|
||||
The output will be:
|
||||
|
||||
```
|
||||
{
|
||||
"one": 1,
|
||||
"two": 2,
|
||||
"three": 3
|
||||
}
|
||||
```
|
||||
|
||||
Wrong way
|
||||
|
||||
The following code incorrectly calls the `parse` function from `nlohmann::json` which does not preserve the insertion order, but sorts object keys. Assigning the result to `nlohmann::ordered_json` compiles, but does not restore the order from the input file.
|
||||
|
||||
```
|
||||
std::ifstream i("input.json");
|
||||
nlohmann::ordered_json j = nlohmann::json::parse(i);
|
||||
std::cout << j.dump(2) << std::endl;
|
||||
```
|
||||
|
||||
The output will be:
|
||||
|
||||
```
|
||||
{
|
||||
"one": 1,
|
||||
"three": 3,
|
||||
"two": 2
|
||||
}
|
||||
```
|
||||
@@ -0,0 +1,61 @@
|
||||
# Parsing
|
||||
|
||||
This library can create a JSON value from a wide range of inputs. This page gives an overview of the available parsing
|
||||
functions and how they behave; the linked pages go into more detail.
|
||||
|
||||
## Input
|
||||
|
||||
The [`parse`](../../api/basic_json/parse.md) function reads a JSON value from an input. The input can be
|
||||
|
||||
- a string (`#!cpp std::string`, C string, or string literal),
|
||||
- a `#!cpp std::istream` (e.g., an `#!cpp std::ifstream` reading from a file),
|
||||
- a `#!cpp FILE*` pointer,
|
||||
- a pair of iterators over a contiguous range (e.g., a `#!cpp std::vector<std::uint8_t>`), or
|
||||
- a contiguous container.
|
||||
|
||||
```cpp
|
||||
// parse from a string
|
||||
json j = json::parse(R"({"happy": true, "pi": 3.141})");
|
||||
|
||||
// parse from a file
|
||||
std::ifstream f("example.json");
|
||||
json data = json::parse(f);
|
||||
```
|
||||
|
||||
The input must be encoded in UTF-8; other encodings are not supported. 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.
|
||||
|
||||
By default, the library rejects comments and trailing commas. Both can be enabled with parameters of the `parse`
|
||||
function — see [comments](../comments.md) and [trailing commas](../trailing_commas.md).
|
||||
|
||||
## SAX vs. DOM parsing
|
||||
|
||||
The library offers two parsing models:
|
||||
|
||||
- **DOM parsing** (the default): the complete input is read and stored as an in-memory `basic_json` value that can be
|
||||
traversed and modified freely. This is what [`parse`](../../api/basic_json/parse.md) does, and it is the right choice
|
||||
for most use cases.
|
||||
- **SAX parsing**: instead of building a value, the parser reports events (such as "a string was read" or "an object
|
||||
started") to a handler that you implement. This avoids building the full value in memory and is useful for very large
|
||||
inputs or when you only need to extract parts of the input. See the [SAX interface](sax_interface.md) for details and
|
||||
[`sax_parse`](../../api/basic_json/sax_parse.md) for the API.
|
||||
|
||||
You can influence a DOM parse without switching to the SAX interface by passing a
|
||||
[parser callback](parser_callbacks.md), which is called during parsing and can, for example, discard parts of the input.
|
||||
|
||||
## Exceptions
|
||||
|
||||
When the input is not valid JSON, the `parse` function throws an exception by default. If exceptions are undesired or
|
||||
unavailable, the parser can instead return a discarded value, or [`accept`](../../api/basic_json/accept.md) can be used
|
||||
to only check whether an input is valid JSON. See [parsing and exceptions](parse_exceptions.md) for the available
|
||||
options.
|
||||
|
||||
## 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
|
||||
- [JSON Lines](json_lines.md) - parse newline-delimited JSON
|
||||
- [parser callbacks](parser_callbacks.md) - influence the parsing by a callback function
|
||||
- [SAX interface](sax_interface.md) - implement a custom SAX handler
|
||||
- [parsing and exceptions](parse_exceptions.md) - control error handling
|
||||
@@ -4,4 +4,4 @@
|
||||
<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. 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.</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> <h2 id=sax-vs-dom-parsing><abbr title="Simple API for XML">SAX</abbr> vs. <abbr title="Document Object Model">DOM</abbr> parsing<a class=headerlink href=#sax-vs-dom-parsing title="Permanent link">¶</a></h2> <p>The library offers two parsing models:</p> <ul> <li><strong><abbr title="Document Object Model">DOM</abbr> parsing</strong> (the default): the complete input is read and stored as an in-memory <code>basic_json</code> value that can be traversed and modified freely. This is what <a href=../../api/basic_json/parse/ ><code>parse</code></a> does, and it is the right choice for most use cases.</li> <li><strong><abbr title="Simple API for XML">SAX</abbr> parsing</strong>: instead of building a value, the parser reports events (such as "a string was read" or "an object started") to a handler that you implement. This avoids building the full value in memory and is useful for very large inputs or when you only need to extract parts of the input. See the <a href=sax_interface/ ><abbr title="Simple API for XML">SAX</abbr> interface</a> for details and <a href=../../api/basic_json/sax_parse/ ><code>sax_parse</code></a> for the <abbr title="Application Programming Interfaces">API</abbr>.</li> </ul> <p>You can influence a <abbr title="Document Object Model">DOM</abbr> parse without switching to the <abbr title="Simple API for XML">SAX</abbr> interface by passing a <a href=parser_callbacks/ >parser callback</a>, which is called during parsing and can, for example, discard parts of the input.</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/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=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=parse_exceptions/ >parsing and exceptions</a> - control error handling</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="July 4, 2026 09:13:25 UTC">July 4, 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.sliceLine truncated
|
||||
</code></pre></div> <p>The input must be encoded in <abbr title="Unicode Transformation Format">UTF</abbr>-8; other encodings are not supported. 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.</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> <h2 id=sax-vs-dom-parsing><abbr title="Simple API for XML">SAX</abbr> vs. <abbr title="Document Object Model">DOM</abbr> parsing<a class=headerlink href=#sax-vs-dom-parsing title="Permanent link">¶</a></h2> <p>The library offers two parsing models:</p> <ul> <li><strong><abbr title="Document Object Model">DOM</abbr> parsing</strong> (the default): the complete input is read and stored as an in-memory <code>basic_json</code> value that can be traversed and modified freely. This is what <a href=../../api/basic_json/parse/ ><code>parse</code></a> does, and it is the right choice for most use cases.</li> <li><strong><abbr title="Simple API for XML">SAX</abbr> parsing</strong>: instead of building a value, the parser reports events (such as "a string was read" or "an object started") to a handler that you implement. This avoids building the full value in memory and is useful for very large inputs or when you only need to extract parts of the input. See the <a href=sax_interface/ ><abbr title="Simple API for XML">SAX</abbr> interface</a> for details and <a href=../../api/basic_json/sax_parse/ ><code>sax_parse</code></a> for the <abbr title="Application Programming Interfaces">API</abbr>.</li> </ul> <p>You can influence a <abbr title="Document Object Model">DOM</abbr> parse without switching to the <abbr title="Simple API for XML">SAX</abbr> interface by passing a <a href=parser_callbacks/ >parser callback</a>, which is called during parsing and can, for example, discard parts of the input.</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/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=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=parse_exceptions/ >parsing and exceptions</a> - control error handling</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="July 8, 2026 18:18:24 UTC">July 8, 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.sliceLine truncated
|
||||
@@ -0,0 +1,49 @@
|
||||
# Parsing
|
||||
|
||||
This library can create a JSON value from a wide range of inputs. This page gives an overview of the available parsing functions and how they behave; the linked pages go into more detail.
|
||||
|
||||
## 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
|
||||
|
||||
- a string (`std::string`, C string, or string literal),
|
||||
- a `std::istream` (e.g., an `std::ifstream` reading from a file),
|
||||
- a `FILE*` pointer,
|
||||
- a pair of iterators over a contiguous range (e.g., a `std::vector<std::uint8_t>`), or
|
||||
- a contiguous container.
|
||||
|
||||
```
|
||||
// parse from a string
|
||||
json j = json::parse(R"({"happy": true, "pi": 3.141})");
|
||||
|
||||
// parse from a file
|
||||
std::ifstream f("example.json");
|
||||
json data = json::parse(f);
|
||||
```
|
||||
|
||||
The input must be encoded in UTF-8; other encodings are not supported. 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.
|
||||
|
||||
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).
|
||||
|
||||
## SAX vs. DOM parsing
|
||||
|
||||
The library offers two parsing models:
|
||||
|
||||
- **DOM parsing** (the default): the complete input is read and stored as an in-memory `basic_json` value that can be traversed and modified freely. This is what [`parse`](https://json.nlohmann.me/api/basic_json/parse/index.md) does, and it is the right choice for most use cases.
|
||||
- **SAX parsing**: instead of building a value, the parser reports events (such as "a string was read" or "an object started") to a handler that you implement. This avoids building the full value in memory and is useful for very large inputs or when you only need to extract parts of the input. See the [SAX interface](https://json.nlohmann.me/features/parsing/sax_interface/index.md) for details and [`sax_parse`](https://json.nlohmann.me/api/basic_json/sax_parse/index.md) for the API.
|
||||
|
||||
You can influence a DOM parse without switching to the SAX interface by passing a [parser callback](https://json.nlohmann.me/features/parsing/parser_callbacks/index.md), which is called during parsing and can, for example, discard parts of the input.
|
||||
|
||||
## Exceptions
|
||||
|
||||
When the input is not valid JSON, the `parse` function throws an exception by default. If exceptions are undesired or unavailable, the parser can instead return a discarded value, or [`accept`](https://json.nlohmann.me/api/basic_json/accept/index.md) can be used to only check whether an input is valid JSON. See [parsing and exceptions](https://json.nlohmann.me/features/parsing/parse_exceptions/index.md) for the available options.
|
||||
|
||||
## See also
|
||||
|
||||
- [`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 if the input is valid JSON
|
||||
- [`sax_parse`](https://json.nlohmann.me/api/basic_json/sax_parse/index.md) - generate SAX events
|
||||
- [JSON Lines](https://json.nlohmann.me/features/parsing/json_lines/index.md) - parse newline-delimited JSON
|
||||
- [parser callbacks](https://json.nlohmann.me/features/parsing/parser_callbacks/index.md) - influence the parsing by a callback function
|
||||
- [SAX interface](https://json.nlohmann.me/features/parsing/sax_interface/index.md) - implement a custom SAX handler
|
||||
- [parsing and exceptions](https://json.nlohmann.me/features/parsing/parse_exceptions/index.md) - control error handling
|
||||
@@ -0,0 +1,49 @@
|
||||
# JSON Lines
|
||||
|
||||
The [JSON Lines](https://jsonlines.org) format is a text format of newline-delimited JSON. In particular:
|
||||
|
||||
1. The input must be UTF-8 encoded.
|
||||
2. Every line must be a valid JSON value.
|
||||
3. The line separator must be `\n`. As `\r` is silently ignored, `\r\n` is also supported.
|
||||
4. The final character may be `\n`, but is not required to be one.
|
||||
|
||||
!!! example "JSON Text example"
|
||||
|
||||
```json
|
||||
{"name": "Gilbert", "wins": [["straight", "7♣"], ["one pair", "10♥"]]}
|
||||
{"name": "Alexa", "wins": [["two pair", "4♠"], ["two pair", "9♠"]]}
|
||||
{"name": "May", "wins": []}
|
||||
{"name": "Deloise", "wins": [["three of a kind", "5♣"]]}
|
||||
```
|
||||
|
||||
JSON Lines input with more than one value is treated as invalid JSON by the [`parse`](../../api/basic_json/parse.md) or
|
||||
[`accept`](../../api/basic_json/accept.md) functions. To process it line by line, functions like
|
||||
[`std::getline`](https://en.cppreference.com/w/cpp/string/basic_string/getline) can be used:
|
||||
|
||||
!!! example "Example: Parse JSON Text input line by line"
|
||||
|
||||
The example below demonstrates how JSON Lines can be processed.
|
||||
|
||||
```cpp
|
||||
--8<-- "examples/json_lines.cpp"
|
||||
```
|
||||
|
||||
Output:
|
||||
|
||||
```json
|
||||
--8<-- "examples/json_lines.output"
|
||||
```
|
||||
|
||||
!!! warning "Note"
|
||||
|
||||
Using [`operator>>`](../../api/operator_gtgt.md) like
|
||||
|
||||
```cpp
|
||||
json j;
|
||||
while (input >> j)
|
||||
{
|
||||
std::cout << j << std::endl;
|
||||
}
|
||||
```
|
||||
|
||||
with a JSON Lines input does not work, because the parser will try to parse one value after the last one.
|
||||
@@ -33,4 +33,4 @@
|
||||
<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>with a <abbr title="JavaScript Object Notation">JSON</abbr> Lines input does not work, because the parser will try to parse one value after the last 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="July 4, 2026 09:13:25 UTC">July 4, 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>with a <abbr title="JavaScript Object Notation">JSON</abbr> Lines input does not work, because the parser will try to parse one value after the last 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="July 8, 2026 18:18:24 UTC">July 8, 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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|
||||
@@ -0,0 +1,71 @@
|
||||
# JSON Lines
|
||||
|
||||
The [JSON Lines](https://jsonlines.org) format is a text format of newline-delimited JSON. In particular:
|
||||
|
||||
1. The input must be UTF-8 encoded.
|
||||
1. Every line must be a valid JSON value.
|
||||
1. The line separator must be `\n`. As `\r` is silently ignored, `\r\n` is also supported.
|
||||
1. The final character may be `\n`, but is not required to be one.
|
||||
|
||||
JSON Text example
|
||||
|
||||
```
|
||||
{"name": "Gilbert", "wins": [["straight", "7♣"], ["one pair", "10♥"]]}
|
||||
{"name": "Alexa", "wins": [["two pair", "4♠"], ["two pair", "9♠"]]}
|
||||
{"name": "May", "wins": []}
|
||||
{"name": "Deloise", "wins": [["three of a kind", "5♣"]]}
|
||||
```
|
||||
|
||||
JSON Lines input with more than one value is treated as invalid JSON by the [`parse`](https://json.nlohmann.me/api/basic_json/parse/index.md) or [`accept`](https://json.nlohmann.me/api/basic_json/accept/index.md) functions. To process it line by line, functions like [`std::getline`](https://en.cppreference.com/w/cpp/string/basic_string/getline) can be used:
|
||||
|
||||
Example: Parse JSON Text input line by line
|
||||
|
||||
The example below demonstrates how JSON Lines can be processed.
|
||||
|
||||
```
|
||||
#include <sstream>
|
||||
#include <iostream>
|
||||
#include <nlohmann/json.hpp>
|
||||
|
||||
using json = nlohmann::json;
|
||||
|
||||
int main()
|
||||
{
|
||||
// JSON Lines (see https://jsonlines.org)
|
||||
std::stringstream input;
|
||||
input << R"({"name": "Gilbert", "wins": [["straight", "7♣"], ["one pair", "10♥"]]}
|
||||
{"name": "Alexa", "wins": [["two pair", "4♠"], ["two pair", "9♠"]]}
|
||||
{"name": "May", "wins": []}
|
||||
{"name": "Deloise", "wins": [["three of a kind", "5♣"]]}
|
||||
)";
|
||||
|
||||
std::string line;
|
||||
while (std::getline(input, line))
|
||||
{
|
||||
std::cout << json::parse(line) << std::endl;
|
||||
}
|
||||
}
|
||||
```
|
||||
|
||||
Output:
|
||||
|
||||
```
|
||||
{"name":"Gilbert","wins":[["straight","7♣"],["one pair","10♥"]]}
|
||||
{"name":"Alexa","wins":[["two pair","4♠"],["two pair","9♠"]]}
|
||||
{"name":"May","wins":[]}
|
||||
{"name":"Deloise","wins":[["three of a kind","5♣"]]}
|
||||
```
|
||||
|
||||
Note
|
||||
|
||||
Using [`operator>>`](https://json.nlohmann.me/api/operator_gtgt/index.md) like
|
||||
|
||||
```
|
||||
json j;
|
||||
while (input >> j)
|
||||
{
|
||||
std::cout << j << std::endl;
|
||||
}
|
||||
```
|
||||
|
||||
with a JSON Lines input does not work, because the parser will try to parse one value after the last one.
|
||||
@@ -0,0 +1,121 @@
|
||||
# Parsing and Exceptions
|
||||
|
||||
When the input is not valid JSON, an exception of type [`parse_error`](../../home/exceptions.md#parse-errors) is thrown.
|
||||
This exception contains the position in the input where the error occurred, together with a diagnostic message and the
|
||||
last read input token. The exceptions page contains a
|
||||
[list of examples for parse error exceptions](../../home/exceptions.md#parse-errors). In case you process untrusted
|
||||
input, always enclose your code with a `#!cpp try`/`#!cpp catch` block, like
|
||||
|
||||
```cpp
|
||||
json j;
|
||||
try
|
||||
{
|
||||
j = json::parse(my_input);
|
||||
}
|
||||
catch (json::parse_error& ex)
|
||||
{
|
||||
std::cerr << "parse error at byte " << ex.byte << std::endl;
|
||||
}
|
||||
```
|
||||
|
||||
In case exceptions are undesired or not supported by the environment, there are different ways to proceed:
|
||||
|
||||
|
||||
## Switch off exceptions
|
||||
|
||||
The `parse()` function accepts a `#!cpp bool` parameter `allow_exceptions` which controls whether an exception is
|
||||
thrown when a parse error occurs (`#!cpp true`, default) or whether a discarded value should be returned
|
||||
(`#!cpp false`).
|
||||
|
||||
```cpp
|
||||
json j = json::parse(my_input, nullptr, false);
|
||||
if (j.is_discarded())
|
||||
{
|
||||
std::cerr << "parse error" << std::endl;
|
||||
}
|
||||
```
|
||||
|
||||
Note there is no diagnostic information available in this scenario.
|
||||
|
||||
## Use accept() function
|
||||
|
||||
Alternatively, function `accept()` can be used which does not return a `json` value, but a `#!cpp bool` indicating
|
||||
whether the input is valid JSON.
|
||||
|
||||
```cpp
|
||||
if (!json::accept(my_input))
|
||||
{
|
||||
std::cerr << "parse error" << std::endl;
|
||||
}
|
||||
```
|
||||
|
||||
Again, there is no diagnostic information available.
|
||||
|
||||
|
||||
## User-defined SAX interface
|
||||
|
||||
Finally, you can implement the [SAX interface](sax_interface.md) and decide what should happen in case of a parse error.
|
||||
|
||||
This function has the following interface:
|
||||
|
||||
```cpp
|
||||
bool parse_error(std::size_t position,
|
||||
const std::string& last_token,
|
||||
const json::exception& ex);
|
||||
```
|
||||
|
||||
The return value indicates whether the parsing should continue, so the function should usually return `#!cpp false`.
|
||||
|
||||
??? example
|
||||
|
||||
```cpp
|
||||
#include <iostream>
|
||||
#include <nlohmann/json.hpp>
|
||||
|
||||
using json = nlohmann::json;
|
||||
|
||||
class sax_no_exception : public nlohmann::detail::json_sax_dom_parser<json>
|
||||
{
|
||||
public:
|
||||
sax_no_exception(json& j)
|
||||
: nlohmann::detail::json_sax_dom_parser<json>(j, false)
|
||||
{}
|
||||
|
||||
bool parse_error(std::size_t position,
|
||||
const std::string& last_token,
|
||||
const json::exception& ex)
|
||||
{
|
||||
std::cerr << "parse error at input byte " << position << "\n"
|
||||
<< ex.what() << "\n"
|
||||
<< "last read: \"" << last_token << "\""
|
||||
<< std::endl;
|
||||
return false;
|
||||
}
|
||||
};
|
||||
|
||||
int main()
|
||||
{
|
||||
std::string myinput = "[1,2,3,]";
|
||||
|
||||
json result;
|
||||
sax_no_exception sax(result);
|
||||
|
||||
bool parse_result = json::sax_parse(myinput, &sax);
|
||||
if (!parse_result)
|
||||
{
|
||||
std::cerr << "parsing unsuccessful!" << std::endl;
|
||||
}
|
||||
|
||||
std::cout << "parsed value: " << result << std::endl;
|
||||
}
|
||||
```
|
||||
|
||||
Output:
|
||||
|
||||
```
|
||||
parse error at input byte 8
|
||||
[json.exception.parse_error.101] parse error at line 1, column 8: syntax error while parsing value - unexpected ']'; expected '[', '{', or a literal
|
||||
last read: "3,]"
|
||||
parsing unsuccessful!
|
||||
parsed value: [1,2,3]
|
||||
```
|
||||
@@ -63,4 +63,4 @@
|
||||
last read: "3,]"
|
||||
parsing unsuccessful!
|
||||
parsed value: [1,2,3]
|
||||
</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="July 4, 2026 09:13:25 UTC">July 4, 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> <!-- 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="July 8, 2026 18:18:24 UTC">July 8, 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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|
||||
@@ -0,0 +1,112 @@
|
||||
# Parsing and Exceptions
|
||||
|
||||
When the input is not valid JSON, an exception of type [`parse_error`](https://json.nlohmann.me/home/exceptions/#parse-errors) is thrown. This exception contains the position in the input where the error occurred, together with a diagnostic message and the last read input token. The exceptions page contains a [list of examples for parse error exceptions](https://json.nlohmann.me/home/exceptions/#parse-errors). In case you process untrusted input, always enclose your code with a `try`/`catch` block, like
|
||||
|
||||
```
|
||||
json j;
|
||||
try
|
||||
{
|
||||
j = json::parse(my_input);
|
||||
}
|
||||
catch (json::parse_error& ex)
|
||||
{
|
||||
std::cerr << "parse error at byte " << ex.byte << std::endl;
|
||||
}
|
||||
```
|
||||
|
||||
In case exceptions are undesired or not supported by the environment, there are different ways to proceed:
|
||||
|
||||
## Switch off exceptions
|
||||
|
||||
The `parse()` function accepts a `bool` parameter `allow_exceptions` which controls whether an exception is thrown when a parse error occurs (`true`, default) or whether a discarded value should be returned (`false`).
|
||||
|
||||
```
|
||||
json j = json::parse(my_input, nullptr, false);
|
||||
if (j.is_discarded())
|
||||
{
|
||||
std::cerr << "parse error" << std::endl;
|
||||
}
|
||||
```
|
||||
|
||||
Note there is no diagnostic information available in this scenario.
|
||||
|
||||
## Use accept() function
|
||||
|
||||
Alternatively, function `accept()` can be used which does not return a `json` value, but a `bool` indicating whether the input is valid JSON.
|
||||
|
||||
```
|
||||
if (!json::accept(my_input))
|
||||
{
|
||||
std::cerr << "parse error" << std::endl;
|
||||
}
|
||||
```
|
||||
|
||||
Again, there is no diagnostic information available.
|
||||
|
||||
## User-defined SAX interface
|
||||
|
||||
Finally, you can implement the [SAX interface](https://json.nlohmann.me/features/parsing/sax_interface/index.md) and decide what should happen in case of a parse error.
|
||||
|
||||
This function has the following interface:
|
||||
|
||||
```
|
||||
bool parse_error(std::size_t position,
|
||||
const std::string& last_token,
|
||||
const json::exception& ex);
|
||||
```
|
||||
|
||||
The return value indicates whether the parsing should continue, so the function should usually return `false`.
|
||||
|
||||
Example
|
||||
|
||||
```
|
||||
#include <iostream>
|
||||
#include <nlohmann/json.hpp>
|
||||
|
||||
using json = nlohmann::json;
|
||||
|
||||
class sax_no_exception : public nlohmann::detail::json_sax_dom_parser<json>
|
||||
{
|
||||
public:
|
||||
sax_no_exception(json& j)
|
||||
: nlohmann::detail::json_sax_dom_parser<json>(j, false)
|
||||
{}
|
||||
|
||||
bool parse_error(std::size_t position,
|
||||
const std::string& last_token,
|
||||
const json::exception& ex)
|
||||
{
|
||||
std::cerr << "parse error at input byte " << position << "\n"
|
||||
<< ex.what() << "\n"
|
||||
<< "last read: \"" << last_token << "\""
|
||||
<< std::endl;
|
||||
return false;
|
||||
}
|
||||
};
|
||||
|
||||
int main()
|
||||
{
|
||||
std::string myinput = "[1,2,3,]";
|
||||
|
||||
json result;
|
||||
sax_no_exception sax(result);
|
||||
|
||||
bool parse_result = json::sax_parse(myinput, &sax);
|
||||
if (!parse_result)
|
||||
{
|
||||
std::cerr << "parsing unsuccessful!" << std::endl;
|
||||
}
|
||||
|
||||
std::cout << "parsed value: " << result << std::endl;
|
||||
}
|
||||
```
|
||||
|
||||
Output:
|
||||
|
||||
```
|
||||
parse error at input byte 8
|
||||
[json.exception.parse_error.101] parse error at line 1, column 8: syntax error while parsing value - unexpected ']'; expected '[', '{', or a literal
|
||||
last read: "3,]"
|
||||
parsing unsuccessful!
|
||||
parsed value: [1,2,3]
|
||||
```
|
||||
@@ -0,0 +1,83 @@
|
||||
# Parser Callbacks
|
||||
|
||||
## Overview
|
||||
|
||||
With a parser callback function, the result of parsing a JSON text can be influenced. When passed to `parse`, it is
|
||||
called on certain events (passed as `parse_event_t` via parameter `event`) with a set recursion depth `depth` and
|
||||
context JSON value `parsed`. The return value of the callback function is a boolean indicating whether the element that
|
||||
emitted the callback shall be kept or not.
|
||||
|
||||
The type of the callback function is:
|
||||
|
||||
```cpp
|
||||
template<typename BasicJsonType>
|
||||
using parser_callback_t =
|
||||
std::function<bool(int depth, parse_event_t event, BasicJsonType& parsed)>;
|
||||
```
|
||||
|
||||
|
||||
## Callback event types
|
||||
|
||||
We distinguish six scenarios (determined by the event type) in which the callback function can be called. The following
|
||||
table describes the values of the parameters `depth`, `event`, and `parsed`.
|
||||
|
||||
| parameter `event` | description | parameter `depth` | parameter `parsed` |
|
||||
|-------------------------------|-----------------------------------------------------------|-------------------------------------------|----------------------------------|
|
||||
| `parse_event_t::object_start` | the parser read `{` and started to process a JSON object | depth of the parent of the JSON object | a JSON value with type discarded |
|
||||
| `parse_event_t::key` | the parser read a key of a value in an object | depth of the currently parsed JSON object | a JSON string containing the key |
|
||||
| `parse_event_t::object_end` | the parser read `}` and finished processing a JSON object | depth of the parent of the JSON object | the parsed JSON object |
|
||||
| `parse_event_t::array_start` | the parser read `[` and started to process a JSON array | depth of the parent of the JSON array | a JSON value with type discarded |
|
||||
| `parse_event_t::array_end` | the parser read `]` and finished processing a JSON array | depth of the parent of the JSON array | the parsed JSON array |
|
||||
| `parse_event_t::value` | the parser finished reading a JSON value | depth of the value | the parsed JSON value |
|
||||
|
||||
??? example
|
||||
|
||||
When parsing the following JSON text,
|
||||
|
||||
```json
|
||||
{
|
||||
"name": "Berlin",
|
||||
"location": [
|
||||
52.519444,
|
||||
13.406667
|
||||
]
|
||||
}
|
||||
```
|
||||
|
||||
these calls are made to the callback function:
|
||||
|
||||
| event | depth | parsed |
|
||||
| -------------- | ----- | ------ |
|
||||
| `object_start` | 0 | *discarded* |
|
||||
| `key` | 1 | `#!json "name"` |
|
||||
| `value` | 1 | `#!json "Berlin"` |
|
||||
| `key` | 1 | `#!json "location"` |
|
||||
| `array_start` | 1 | *discarded* |
|
||||
| `value` | 2 | `#!json 52.519444` |
|
||||
| `value` | 2 | `#!json 13.406667` |
|
||||
| `array_end` | 1 | `#!json [52.519444,13.406667]` |
|
||||
| `object_end` | 0 | `#!json {"location":[52.519444,13.406667],"name":"Berlin"}` |
|
||||
|
||||
## Return value
|
||||
|
||||
Discarding a value (i.e., returning `#!c false`) has different effects depending on the context in which the function
|
||||
was called:
|
||||
|
||||
- Discarded values in structured types are skipped. That is, the parser will behave as if the discarded value was never
|
||||
read.
|
||||
- In case a value outside a structured type is skipped, it is replaced with `#!json null`. This case happens if the
|
||||
top-level element is skipped.
|
||||
|
||||
??? example
|
||||
|
||||
The example below demonstrates the `parse()` function with and without callback function.
|
||||
|
||||
```cpp
|
||||
--8<-- "examples/parse__string__parser_callback_t.cpp"
|
||||
```
|
||||
|
||||
Output:
|
||||
|
||||
```json
|
||||
--8<-- "examples/parse__string__parser_callback_t.output"
|
||||
```
|
||||
@@ -90,4 +90,4 @@
|
||||
<span class=w> </span><span class=nt>"Width"</span><span class=p>:</span><span class=w> </span><span class=mi>800</span>
|
||||
<span class=w> </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="July 4, 2026 09:13:25 UTC">July 4, 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> <!-- 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="July 8, 2026 18:18:24 UTC">July 8, 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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|
||||
@@ -0,0 +1,155 @@
|
||||
# Parser Callbacks
|
||||
|
||||
## Overview
|
||||
|
||||
With a parser callback function, the result of parsing a JSON text can be influenced. When passed to `parse`, it is called on certain events (passed as `parse_event_t` via parameter `event`) with a set recursion depth `depth` and context JSON value `parsed`. The return value of the callback function is a boolean indicating whether the element that emitted the callback shall be kept or not.
|
||||
|
||||
The type of the callback function is:
|
||||
|
||||
```
|
||||
template<typename BasicJsonType>
|
||||
using parser_callback_t =
|
||||
std::function<bool(int depth, parse_event_t event, BasicJsonType& parsed)>;
|
||||
```
|
||||
|
||||
## Callback event types
|
||||
|
||||
We distinguish six scenarios (determined by the event type) in which the callback function can be called. The following table describes the values of the parameters `depth`, `event`, and `parsed`.
|
||||
|
||||
| parameter `event` | description | parameter `depth` | parameter `parsed` |
|
||||
| ----------------------------- | --------------------------------------------------------- | ----------------------------------------- | -------------------------------- |
|
||||
| `parse_event_t::object_start` | the parser read `{` and started to process a JSON object | depth of the parent of the JSON object | a JSON value with type discarded |
|
||||
| `parse_event_t::key` | the parser read a key of a value in an object | depth of the currently parsed JSON object | a JSON string containing the key |
|
||||
| `parse_event_t::object_end` | the parser read `}` and finished processing a JSON object | depth of the parent of the JSON object | the parsed JSON object |
|
||||
| `parse_event_t::array_start` | the parser read `[` and started to process a JSON array | depth of the parent of the JSON array | a JSON value with type discarded |
|
||||
| `parse_event_t::array_end` | the parser read `]` and finished processing a JSON array | depth of the parent of the JSON array | the parsed JSON array |
|
||||
| `parse_event_t::value` | the parser finished reading a JSON value | depth of the value | the parsed JSON value |
|
||||
|
||||
Example
|
||||
|
||||
When parsing the following JSON text,
|
||||
|
||||
```
|
||||
{
|
||||
"name": "Berlin",
|
||||
"location": [
|
||||
52.519444,
|
||||
13.406667
|
||||
]
|
||||
}
|
||||
```
|
||||
|
||||
these calls are made to the callback function:
|
||||
|
||||
| event | depth | parsed |
|
||||
| -------------- | ----- | ---------------------------------------------------- |
|
||||
| `object_start` | 0 | *discarded* |
|
||||
| `key` | 1 | `"name"` |
|
||||
| `value` | 1 | `"Berlin"` |
|
||||
| `key` | 1 | `"location"` |
|
||||
| `array_start` | 1 | *discarded* |
|
||||
| `value` | 2 | `52.519444` |
|
||||
| `value` | 2 | `13.406667` |
|
||||
| `array_end` | 1 | `[52.519444,13.406667]` |
|
||||
| `object_end` | 0 | `{"location":[52.519444,13.406667],"name":"Berlin"}` |
|
||||
|
||||
## Return value
|
||||
|
||||
Discarding a value (i.e., returning `false`) has different effects depending on the context in which the function was called:
|
||||
|
||||
- Discarded values in structured types are skipped. That is, the parser will behave as if the discarded value was never read.
|
||||
- In case a value outside a structured type is skipped, it is replaced with `null`. This case happens if the top-level element is skipped.
|
||||
|
||||
Example
|
||||
|
||||
The example below demonstrates the `parse()` function with and without callback function.
|
||||
|
||||
```
|
||||
#include <iostream>
|
||||
#include <iomanip>
|
||||
#include <nlohmann/json.hpp>
|
||||
|
||||
using json = nlohmann::json;
|
||||
|
||||
int main()
|
||||
{
|
||||
// a JSON text
|
||||
auto text = R"(
|
||||
{
|
||||
"Image": {
|
||||
"Width": 800,
|
||||
"Height": 600,
|
||||
"Title": "View from 15th Floor",
|
||||
"Thumbnail": {
|
||||
"Url": "http://www.example.com/image/481989943",
|
||||
"Height": 125,
|
||||
"Width": 100
|
||||
},
|
||||
"Animated" : false,
|
||||
"IDs": [116, 943, 234, 38793]
|
||||
}
|
||||
}
|
||||
)";
|
||||
|
||||
// parse and serialize JSON
|
||||
json j_complete = json::parse(text);
|
||||
std::cout << std::setw(4) << j_complete << "\n\n";
|
||||
|
||||
// define parser callback
|
||||
json::parser_callback_t cb = [](int depth, json::parse_event_t event, json & parsed)
|
||||
{
|
||||
// skip object elements with key "Thumbnail"
|
||||
if (event == json::parse_event_t::key and parsed == json("Thumbnail"))
|
||||
{
|
||||
return false;
|
||||
}
|
||||
else
|
||||
{
|
||||
return true;
|
||||
}
|
||||
};
|
||||
|
||||
// parse (with callback) and serialize JSON
|
||||
json j_filtered = json::parse(text, cb);
|
||||
std::cout << std::setw(4) << j_filtered << '\n';
|
||||
}
|
||||
```
|
||||
|
||||
Output:
|
||||
|
||||
```
|
||||
{
|
||||
"Image": {
|
||||
"Animated": false,
|
||||
"Height": 600,
|
||||
"IDs": [
|
||||
116,
|
||||
943,
|
||||
234,
|
||||
38793
|
||||
],
|
||||
"Thumbnail": {
|
||||
"Height": 125,
|
||||
"Url": "http://www.example.com/image/481989943",
|
||||
"Width": 100
|
||||
},
|
||||
"Title": "View from 15th Floor",
|
||||
"Width": 800
|
||||
}
|
||||
}
|
||||
|
||||
{
|
||||
"Image": {
|
||||
"Animated": false,
|
||||
"Height": 600,
|
||||
"IDs": [
|
||||
116,
|
||||
943,
|
||||
234,
|
||||
38793
|
||||
],
|
||||
"Title": "View from 15th Floor",
|
||||
"Width": 800
|
||||
}
|
||||
}
|
||||
```
|
||||
@@ -0,0 +1,76 @@
|
||||
# SAX Interface
|
||||
|
||||
The library uses a SAX-like interface with the following functions:
|
||||
|
||||
```mermaid
|
||||
classDiagram
|
||||
|
||||
class sax_t ["json::sax_t"] {
|
||||
<<interface>>
|
||||
+bool null()*
|
||||
|
||||
+bool boolean(bool val)*
|
||||
|
||||
+bool number_integer(number_integer_t val)*
|
||||
+bool number_unsigned(number_unsigned_t val)*
|
||||
|
||||
+bool number_float(number_float_t val, const string_t& s)*
|
||||
|
||||
+bool string(string_t& val)*
|
||||
+bool binary(binary_t& val)*
|
||||
|
||||
+bool start_object(std::size_t elements)*
|
||||
+bool end_object()*
|
||||
+bool start_array(std::size_t elements)*
|
||||
+bool end_array()*
|
||||
+bool key(string_t& val)*
|
||||
|
||||
+bool parse_error(std::size_t position, const std::string& last_token, const json::exception& ex)*
|
||||
}
|
||||
```
|
||||
|
||||
```cpp
|
||||
// called when null is parsed
|
||||
bool null();
|
||||
|
||||
// called when a boolean is parsed; value is passed
|
||||
bool boolean(bool val);
|
||||
|
||||
// called when a signed or unsigned integer number is parsed; value is passed
|
||||
bool number_integer(number_integer_t val);
|
||||
bool number_unsigned(number_unsigned_t val);
|
||||
|
||||
// called when a floating-point number is parsed; value and original string is passed
|
||||
bool number_float(number_float_t val, const string_t& s);
|
||||
|
||||
// called when a string is parsed; value is passed and can be safely moved away
|
||||
bool string(string_t& val);
|
||||
// called when a binary value is parsed; value is passed and can be safely moved away
|
||||
bool binary(binary_t& val);
|
||||
|
||||
// called when an object or array begins or ends, resp. The number of elements is passed (or -1 if not known)
|
||||
bool start_object(std::size_t elements);
|
||||
bool end_object();
|
||||
bool start_array(std::size_t elements);
|
||||
bool end_array();
|
||||
// called when an object key is parsed; value is passed and can be safely moved away
|
||||
bool key(string_t& val);
|
||||
|
||||
// called when a parse error occurs; byte position, the last token, and an exception is passed
|
||||
bool parse_error(std::size_t position, const std::string& last_token, const json::exception& ex);
|
||||
```
|
||||
|
||||
The return value of each function determines whether parsing should proceed.
|
||||
|
||||
To implement your own SAX handler, proceed as follows:
|
||||
|
||||
1. Implement the SAX interface in a class. You can use class `nlohmann::json_sax<json>` as base class, but you can also use any class where the functions described above are implemented and public.
|
||||
2. Create an object of your SAX interface class, e.g. `my_sax`.
|
||||
3. Call `#!cpp bool json::sax_parse(input, &my_sax);` where the first parameter can be any input like a string or an input stream and the second parameter is a pointer to your SAX interface.
|
||||
|
||||
Note the `sax_parse` function only returns a `#!cpp bool` indicating the result of the last executed SAX event. It does not return `json` value - it is up to you to decide what to do with the SAX events. Furthermore, no exceptions are thrown in case of a parse error - it is up to you what to do with the exception object passed to your `parse_error` implementation. Internally, the SAX interface is used for the DOM parser (class `json_sax_dom_parser`) as well as the acceptor (`json_sax_acceptor`), see file `json_sax.hpp`.
|
||||
|
||||
## See also
|
||||
|
||||
- [json_sax](../../api/json_sax/index.md) - documentation of the SAX interface
|
||||
- [sax_parse](../../api/basic_json/sax_parse.md) - SAX parser
|
||||
@@ -49,4 +49,4 @@ class sax_t ["json::sax_t"] {
|
||||
|
||||
<span class=c1>// called when a parse error occurs; byte position, the last token, and an exception is passed</span>
|
||||
<span class=kt>bool</span><span class=w> </span><span class=nf>parse_error</span><span class=p>(</span><span class=n>std</span><span class=o>::</span><span class=kt>size_t</span><span class=w> </span><span class=n>position</span><span class=p>,</span><span class=w> </span><span class=k>const</span><span class=w> </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>last_token</span><span class=p>,</span><span class=w> </span><span class=k>const</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>exception</span><span class=o>&</span><span class=w> </span><span class=n>ex</span><span class=p>);</span>
|
||||
</code></pre></div> <p>The return value of each function determines whether parsing should proceed.</p> <p>To implement your own <abbr title="Simple API for XML">SAX</abbr> handler, proceed as follows:</p> <ol> <li>Implement the <abbr title="Simple API for XML">SAX</abbr> interface in a class. You can use class <code>nlohmann::json_sax<json></code> as base class, but you can also use any class where the functions described above are implemented and public.</li> <li>Create an object of your <abbr title="Simple API for XML">SAX</abbr> interface class, e.g. <code>my_sax</code>.</li> <li>Call <code class=highlight><span class=kt>bool</span><span class=w> </span><span class=nf>json::sax_parse</span><span class=p>(</span><span class=n>input</span><span class=p>,</span><span class=w> </span><span class=o>&</span><span class=n>my_sax</span><span class=p>);</span></code> where the first parameter can be any input like a string or an input stream and the second parameter is a pointer to your <abbr title="Simple API for XML">SAX</abbr> interface.</li> </ol> <p>Note the <code>sax_parse</code> function only returns a <code class=highlight><span class=kt>bool</span></code> indicating the result of the last executed <abbr title="Simple API for XML">SAX</abbr> event. It does not return <code>json</code> value - it is up to you to decide what to do with the <abbr title="Simple API for XML">SAX</abbr> events. Furthermore, no exceptions are thrown in case of a parse error - it is up to you what to do with the exception object passed to your <code>parse_error</code> implementation. Internally, the <abbr title="Simple API for XML">SAX</abbr> interface is used for the <abbr title="Document Object Model">DOM</abbr> parser (class <code>json_sax_dom_parser</code>) as well as the acceptor (<code>json_sax_acceptor</code>), see file <code>json_sax.hpp</code>.</p> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">¶</a></h2> <ul> <li><a href=../../../api/json_sax/ >json_sax</a> - documentation of the <abbr title="Simple API for XML">SAX</abbr> interface</li> <li><a href=../../../api/basic_json/sax_parse/ >sax_parse</a> - <abbr title="Simple API for XML">SAX</abbr> parser</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="July 4, 2026 09:13:25 UTC">July 4, 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 23.6-58.9-2.6-6Line truncated
|
||||
</code></pre></div> <p>The return value of each function determines whether parsing should proceed.</p> <p>To implement your own <abbr title="Simple API for XML">SAX</abbr> handler, proceed as follows:</p> <ol> <li>Implement the <abbr title="Simple API for XML">SAX</abbr> interface in a class. You can use class <code>nlohmann::json_sax<json></code> as base class, but you can also use any class where the functions described above are implemented and public.</li> <li>Create an object of your <abbr title="Simple API for XML">SAX</abbr> interface class, e.g. <code>my_sax</code>.</li> <li>Call <code class=highlight><span class=kt>bool</span><span class=w> </span><span class=nf>json::sax_parse</span><span class=p>(</span><span class=n>input</span><span class=p>,</span><span class=w> </span><span class=o>&</span><span class=n>my_sax</span><span class=p>);</span></code> where the first parameter can be any input like a string or an input stream and the second parameter is a pointer to your <abbr title="Simple API for XML">SAX</abbr> interface.</li> </ol> <p>Note the <code>sax_parse</code> function only returns a <code class=highlight><span class=kt>bool</span></code> indicating the result of the last executed <abbr title="Simple API for XML">SAX</abbr> event. It does not return <code>json</code> value - it is up to you to decide what to do with the <abbr title="Simple API for XML">SAX</abbr> events. Furthermore, no exceptions are thrown in case of a parse error - it is up to you what to do with the exception object passed to your <code>parse_error</code> implementation. Internally, the <abbr title="Simple API for XML">SAX</abbr> interface is used for the <abbr title="Document Object Model">DOM</abbr> parser (class <code>json_sax_dom_parser</code>) as well as the acceptor (<code>json_sax_acceptor</code>), see file <code>json_sax.hpp</code>.</p> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">¶</a></h2> <ul> <li><a href=../../../api/json_sax/ >json_sax</a> - documentation of the <abbr title="Simple API for XML">SAX</abbr> interface</li> <li><a href=../../../api/basic_json/sax_parse/ >sax_parse</a> - <abbr title="Simple API for XML">SAX</abbr> parser</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="July 8, 2026 18:18:24 UTC">July 8, 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 23.6-58.9-2.6-6Line truncated
|
||||
@@ -0,0 +1,76 @@
|
||||
# SAX Interface
|
||||
|
||||
The library uses a SAX-like interface with the following functions:
|
||||
|
||||
```
|
||||
classDiagram
|
||||
|
||||
class sax_t ["json::sax_t"] {
|
||||
<<interface>>
|
||||
+bool null()*
|
||||
|
||||
+bool boolean(bool val)*
|
||||
|
||||
+bool number_integer(number_integer_t val)*
|
||||
+bool number_unsigned(number_unsigned_t val)*
|
||||
|
||||
+bool number_float(number_float_t val, const string_t& s)*
|
||||
|
||||
+bool string(string_t& val)*
|
||||
+bool binary(binary_t& val)*
|
||||
|
||||
+bool start_object(std::size_t elements)*
|
||||
+bool end_object()*
|
||||
+bool start_array(std::size_t elements)*
|
||||
+bool end_array()*
|
||||
+bool key(string_t& val)*
|
||||
|
||||
+bool parse_error(std::size_t position, const std::string& last_token, const json::exception& ex)*
|
||||
}
|
||||
```
|
||||
|
||||
```
|
||||
// called when null is parsed
|
||||
bool null();
|
||||
|
||||
// called when a boolean is parsed; value is passed
|
||||
bool boolean(bool val);
|
||||
|
||||
// called when a signed or unsigned integer number is parsed; value is passed
|
||||
bool number_integer(number_integer_t val);
|
||||
bool number_unsigned(number_unsigned_t val);
|
||||
|
||||
// called when a floating-point number is parsed; value and original string is passed
|
||||
bool number_float(number_float_t val, const string_t& s);
|
||||
|
||||
// called when a string is parsed; value is passed and can be safely moved away
|
||||
bool string(string_t& val);
|
||||
// called when a binary value is parsed; value is passed and can be safely moved away
|
||||
bool binary(binary_t& val);
|
||||
|
||||
// called when an object or array begins or ends, resp. The number of elements is passed (or -1 if not known)
|
||||
bool start_object(std::size_t elements);
|
||||
bool end_object();
|
||||
bool start_array(std::size_t elements);
|
||||
bool end_array();
|
||||
// called when an object key is parsed; value is passed and can be safely moved away
|
||||
bool key(string_t& val);
|
||||
|
||||
// called when a parse error occurs; byte position, the last token, and an exception is passed
|
||||
bool parse_error(std::size_t position, const std::string& last_token, const json::exception& ex);
|
||||
```
|
||||
|
||||
The return value of each function determines whether parsing should proceed.
|
||||
|
||||
To implement your own SAX handler, proceed as follows:
|
||||
|
||||
1. Implement the SAX interface in a class. You can use class `nlohmann::json_sax<json>` as base class, but you can also use any class where the functions described above are implemented and public.
|
||||
1. Create an object of your SAX interface class, e.g. `my_sax`.
|
||||
1. Call `bool json::sax_parse(input, &my_sax);` where the first parameter can be any input like a string or an input stream and the second parameter is a pointer to your SAX interface.
|
||||
|
||||
Note the `sax_parse` function only returns a `bool` indicating the result of the last executed SAX event. It does not return `json` value - it is up to you to decide what to do with the SAX events. Furthermore, no exceptions are thrown in case of a parse error - it is up to you what to do with the exception object passed to your `parse_error` implementation. Internally, the SAX interface is used for the DOM parser (class `json_sax_dom_parser`) as well as the acceptor (`json_sax_acceptor`), see file `json_sax.hpp`.
|
||||
|
||||
## See also
|
||||
|
||||
- [json_sax](https://json.nlohmann.me/api/json_sax/index.md) - documentation of the SAX interface
|
||||
- [sax_parse](https://json.nlohmann.me/api/basic_json/sax_parse/index.md) - SAX parser
|
||||
@@ -0,0 +1,129 @@
|
||||
# Serialization
|
||||
|
||||
Serialization is the process of turning a JSON value back into JSON text. It is the counterpart to
|
||||
[parsing](parsing/index.md). The central function is [`dump`](../api/basic_json/dump.md), which returns the JSON text as
|
||||
a string.
|
||||
|
||||
```cpp
|
||||
json j = {{"pi", 3.141}, {"happy", true}};
|
||||
|
||||
std::string s = j.dump(); // {"happy":true,"pi":3.141}
|
||||
```
|
||||
|
||||
To write a value directly to a stream (for example, a file or `#!cpp std::cout`), the
|
||||
[`operator<<`](../api/operator_ltlt.md) is provided:
|
||||
|
||||
```cpp
|
||||
std::cout << j << std::endl;
|
||||
```
|
||||
|
||||
!!! note "String, not raw value"
|
||||
|
||||
`dump` always returns a **JSON text**. Serializing a JSON string therefore includes the surrounding quotes and
|
||||
escapes special characters. To obtain the *contained* string value without quotes, use
|
||||
[`get<std::string>()`](conversions.md) instead of `dump`. See the [converting values](conversions.md) page.
|
||||
|
||||
## Pretty-printing
|
||||
|
||||
By default, `dump` produces the most compact representation without any superfluous whitespace. Passing a non-negative
|
||||
`indent` argument pretty-prints the output with the given number of spaces per level:
|
||||
|
||||
??? example
|
||||
|
||||
```cpp
|
||||
--8<-- "examples/dump.cpp"
|
||||
```
|
||||
|
||||
Output:
|
||||
|
||||
```json
|
||||
--8<-- "examples/dump.output"
|
||||
```
|
||||
|
||||
The indentation character can be changed with the second argument (e.g., a tab `#!cpp '\t'`). An `indent` of `0` inserts
|
||||
newlines but no leading spaces, and the default of `#!cpp -1` selects the compact single-line form.
|
||||
|
||||
## Non-ASCII characters
|
||||
|
||||
Strings are stored and serialized as UTF-8 (see [types](types/index.md#strings)). By default, `dump` copies valid
|
||||
non-ASCII characters as-is. Setting the third argument `ensure_ascii` to `#!cpp true` escapes all non-ASCII characters
|
||||
with `\uXXXX` sequences, so that the output contains only ASCII characters:
|
||||
|
||||
```cpp
|
||||
json j = "苹果";
|
||||
j.dump(); // "苹果"
|
||||
j.dump(-1, ' ', true); // "苹果"
|
||||
```
|
||||
|
||||
## Handling invalid UTF-8
|
||||
|
||||
If a string contains invalid UTF-8 sequences (for example, because it holds data in another encoding such as Latin-1),
|
||||
serialization fails by default. The fourth argument of `dump` selects an
|
||||
[`error_handler`](../api/basic_json/error_handler_t.md):
|
||||
|
||||
- `strict` (default) — throw a [`type_error.316`](../home/exceptions.md#jsonexceptiontype_error316) exception.
|
||||
- `replace` — replace invalid bytes with the Unicode replacement character U+FFFD (`�`).
|
||||
- `ignore` — silently drop invalid bytes.
|
||||
|
||||
??? example
|
||||
|
||||
```cpp
|
||||
--8<-- "examples/error_handler_t.cpp"
|
||||
```
|
||||
|
||||
Output:
|
||||
|
||||
```json
|
||||
--8<-- "examples/error_handler_t.output"
|
||||
```
|
||||
|
||||
!!! tip "Avoiding invalid UTF-8"
|
||||
|
||||
The best fix is to ensure that all strings are UTF-8 encoded before storing them. See the
|
||||
[FAQ on non-ASCII characters](../home/faq.md#parse-errors-reading-non-ascii-characters) for how to convert wide or
|
||||
Latin-1 strings.
|
||||
|
||||
## Numbers, NaN, and binary values
|
||||
|
||||
- **Numbers** are serialized with enough precision to round-trip; see [number serialization](types/number_handling.md#number-serialization).
|
||||
- **NaN and infinity** cannot be represented in JSON and are serialized as `#!json null`; see
|
||||
[NaN handling](types/number_handling.md#nan-handling). The [binary formats](binary_formats/index.md) can preserve
|
||||
them.
|
||||
- **Binary values** have no JSON representation and are serialized as a helper object for debugging only; see
|
||||
[binary values](binary_values.md#serialization).
|
||||
|
||||
## Using `std::format`, `std::print`, and `fmt`
|
||||
|
||||
Since version 3.12.0, JSON values can be formatted directly with C++20's
|
||||
[`std::format`](https://en.cppreference.com/w/cpp/utility/format/format) whenever the standard library provides the
|
||||
`<format>` header (controlled by [`JSON_HAS_STD_FORMAT`](../api/macros/json_has_std_format.md)). This is enabled by the
|
||||
[`std::formatter<basic_json>`](../api/basic_json/std_formatter.md) specialization, which also makes JSON values work with
|
||||
`std::format_to` and with C++23's `std::print`/`std::println`:
|
||||
|
||||
```cpp
|
||||
std::print("{}", j); // compact, like j.dump()
|
||||
std::print("{:2}", j); // pretty-printed with indent 2 (like j.dump(2))
|
||||
std::println("{:#}", j); // pretty-printed with the default indent
|
||||
```
|
||||
|
||||
The format spec mirrors the `dump` parameters: `#!cpp "{:#}"` pretty-prints, a width such as `#!cpp "{:2}"` sets the
|
||||
indent, and a fill-and-align prefix such as `#!cpp "{:.>#}"` sets the indent character.
|
||||
|
||||
For the [{fmt}](https://github.com/fmtlib/fmt) library, the library ships a
|
||||
[`format_as`](../api/basic_json/format_as.md) helper. Note its behavior depends on the `fmt` version; see the
|
||||
[FAQ entry](../home/faq.md#using-json-values-with-stdformat-or-fmt) for the details and a recipe for a full
|
||||
`fmt::formatter` specialization.
|
||||
|
||||
## Serializing to other formats
|
||||
|
||||
Besides JSON text, a value can also be serialized to the more compact [binary formats](binary_formats/index.md)
|
||||
(BJData, BSON, CBOR, MessagePack, UBJSON).
|
||||
|
||||
## See also
|
||||
|
||||
- [`dump`](../api/basic_json/dump.md) - serialize to a JSON-formatted string
|
||||
- [`operator<<`](../api/operator_ltlt.md) - serialize to a stream
|
||||
- [`to_string`](../api/basic_json/to_string.md) - user-defined-conversion helper
|
||||
- [`std::formatter<basic_json>`](../api/basic_json/std_formatter.md) - use JSON values with `std::format` and `std::print`
|
||||
- [`format_as`](../api/basic_json/format_as.md) - use JSON values with the {fmt} library
|
||||
- [Parsing](parsing/index.md) - the reverse operation
|
||||
@@ -138,4 +138,4 @@
|
||||
</code></pre></div> </details> <div class="admonition tip"> <p class=admonition-title>Avoiding invalid <abbr title="Unicode Transformation Format">UTF</abbr>-8</p> <p>The best fix is to ensure that all strings are <abbr title="Unicode Transformation Format">UTF</abbr>-8 encoded before storing them. See the <a href=../../home/faq/#parse-errors-reading-non-ascii-characters><abbr title="Frequently Asked Questions">FAQ</abbr> on non-<abbr title="American Standard Code for Information Interchange">ASCII</abbr> characters</a> for how to convert wide or Latin-1 strings.</p> </div> <h2 id=numbers-nan-and-binary-values>Numbers, <abbr title="Not a Number">NaN</abbr>, and binary values<a class=headerlink href=#numbers-nan-and-binary-values title="Permanent link">¶</a></h2> <ul> <li><strong>Numbers</strong> are serialized with enough precision to round-trip; see <a href=../types/number_handling/#number-serialization>number serialization</a>.</li> <li><strong><abbr title="Not a Number">NaN</abbr> and infinity</strong> cannot be represented in <abbr title="JavaScript Object Notation">JSON</abbr> and are serialized as <code class=highlight><span class=kc>null</span></code>; see <a href=../types/number_handling/#nan-handling><abbr title="Not a Number">NaN</abbr> handling</a>. The <a href=../binary_formats/ >binary formats</a> can preserve them.</li> <li><strong>Binary values</strong> have no <abbr title="JavaScript Object Notation">JSON</abbr> representation and are serialized as a helper object for debugging only; see <a href=../binary_values/#serialization>binary values</a>.</li> </ul> <h2 id=using-stdformat-stdprint-and-fmt>Using <code>std::format</code>, <code>std::print</code>, and <code>fmt</code><a class=headerlink href=#using-stdformat-stdprint-and-fmt title="Permanent link">¶</a></h2> <p>Since version 3.12.0, <abbr title="JavaScript Object Notation">JSON</abbr> values can be formatted directly with C++20's <a href="https://en.cppreference.com/w/cpp/utility/format/format"><code>std::format</code></a> whenever the standard library provides the <code><format></code> header (controlled by <a href=../../api/macros/json_has_std_format/ ><code>JSON_HAS_STD_FORMAT</code></a>). This is enabled by the <a href=../../api/basic_json/std_formatter/ ><code>std::formatter<basic_json></code></a> specialization, which also makes <abbr title="JavaScript Object Notation">JSON</abbr> values work with <code>std::format_to</code> and with C++23's <code>std::print</code>/<code>std::println</code>:</p> <div class=highlight><pre><span></span><code><span class=n>std</span><span class=o>::</span><span class=n>print</span><span class=p>(</span><span class=s>"{}"</span><span class=p>,</span><span class=w> </span><span class=n>j</span><span class=p>);</span><span class=w> </span><span class=c1>// compact, like j.dump()</span>
|
||||
<span class=n>std</span><span class=o>::</span><span class=n>print</span><span class=p>(</span><span class=s>"{:2}"</span><span class=p>,</span><span class=w> </span><span class=n>j</span><span class=p>);</span><span class=w> </span><span class=c1>// pretty-printed with indent 2 (like j.dump(2))</span>
|
||||
<span class=n>std</span><span class=o>::</span><span class=n>println</span><span class=p>(</span><span class=s>"{:#}"</span><span class=p>,</span><span class=w> </span><span class=n>j</span><span class=p>);</span><span class=w> </span><span class=c1>// pretty-printed with the default indent</span>
|
||||
</code></pre></div> <p>The format spec mirrors the <code>dump</code> parameters: <code class=highlight><span class=s>"{:#}"</span></code> pretty-prints, a width such as <code class=highlight><span class=s>"{:2}"</span></code> sets the indent, and a fill-and-align prefix such as <code class=highlight><span class=s>"{:.>#}"</span></code> sets the indent character.</p> <p>For the <a href="https://github.com/fmtlib/fmt">{fmt}</a> library, the library ships a <a href=../../api/basic_json/format_as/ ><code>format_as</code></a> helper. Note its behavior depends on the <code>fmt</code> version; see the <a href=../../home/faq/#using-json-values-with-stdformat-or-fmt><abbr title="Frequently Asked Questions">FAQ</abbr> entry</a> for the details and a recipe for a full <code>fmt::formatter</code> specialization.</p> <h2 id=serializing-to-other-formats>Serializing to other formats<a class=headerlink href=#serializing-to-other-formats title="Permanent link">¶</a></h2> <p>Besides <abbr title="JavaScript Object Notation">JSON</abbr> text, a value can also be serialized to the more compact <a href=../binary_formats/ >binary formats</a> (<abbr title="Binary JData">BJData</abbr>, <abbr title="Binary JSON">BSON</abbr>, <abbr title="Concise Binary Object Representation">CBOR</abbr>, MessagePack, <abbr title="Universal Binary JSON">UBJSON</abbr>).</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/dump/ ><code>dump</code></a> - serialize to a <abbr title="JavaScript Object Notation">JSON</abbr>-formatted string</li> <li><a href=../../api/operator_ltlt/ ><code>operator<<</code></a> - serialize to a stream</li> <li><a href=../../api/basic_json/to_string/ ><code>to_string</code></a> - user-defined-conversion helper</li> <li><a href=../../api/basic_json/std_formatter/ ><code>std::formatter<basic_json></code></a> - use <abbr title="JavaScript Object Notation">JSON</abbr> values with <code>std::format</code> and <code>std::print</code></li> <li><a href=../../api/basic_json/format_as/ ><code>format_as</code></a> - use <abbr title="JavaScript Object Notation">JSON</abbr> values with the {fmt} library</li> <li><a href=../parsing/ >Parsing</a> - the reverse operation</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="July 4, 2026 09:13:25 UTC">July 4, 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 2Line truncated
|
||||
</code></pre></div> <p>The format spec mirrors the <code>dump</code> parameters: <code class=highlight><span class=s>"{:#}"</span></code> pretty-prints, a width such as <code class=highlight><span class=s>"{:2}"</span></code> sets the indent, and a fill-and-align prefix such as <code class=highlight><span class=s>"{:.>#}"</span></code> sets the indent character.</p> <p>For the <a href="https://github.com/fmtlib/fmt">{fmt}</a> library, the library ships a <a href=../../api/basic_json/format_as/ ><code>format_as</code></a> helper. Note its behavior depends on the <code>fmt</code> version; see the <a href=../../home/faq/#using-json-values-with-stdformat-or-fmt><abbr title="Frequently Asked Questions">FAQ</abbr> entry</a> for the details and a recipe for a full <code>fmt::formatter</code> specialization.</p> <h2 id=serializing-to-other-formats>Serializing to other formats<a class=headerlink href=#serializing-to-other-formats title="Permanent link">¶</a></h2> <p>Besides <abbr title="JavaScript Object Notation">JSON</abbr> text, a value can also be serialized to the more compact <a href=../binary_formats/ >binary formats</a> (<abbr title="Binary JData">BJData</abbr>, <abbr title="Binary JSON">BSON</abbr>, <abbr title="Concise Binary Object Representation">CBOR</abbr>, MessagePack, <abbr title="Universal Binary JSON">UBJSON</abbr>).</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/dump/ ><code>dump</code></a> - serialize to a <abbr title="JavaScript Object Notation">JSON</abbr>-formatted string</li> <li><a href=../../api/operator_ltlt/ ><code>operator<<</code></a> - serialize to a stream</li> <li><a href=../../api/basic_json/to_string/ ><code>to_string</code></a> - user-defined-conversion helper</li> <li><a href=../../api/basic_json/std_formatter/ ><code>std::formatter<basic_json></code></a> - use <abbr title="JavaScript Object Notation">JSON</abbr> values with <code>std::format</code> and <code>std::print</code></li> <li><a href=../../api/basic_json/format_as/ ><code>format_as</code></a> - use <abbr title="JavaScript Object Notation">JSON</abbr> values with the {fmt} library</li> <li><a href=../parsing/ >Parsing</a> - the reverse operation</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="July 8, 2026 18:18:24 UTC">July 8, 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 2Line truncated
|
||||
@@ -0,0 +1,230 @@
|
||||
# Serialization
|
||||
|
||||
Serialization is the process of turning a JSON value back into JSON text. It is the counterpart to [parsing](https://json.nlohmann.me/features/parsing/index.md). The central function is [`dump`](https://json.nlohmann.me/api/basic_json/dump/index.md), which returns the JSON text as a string.
|
||||
|
||||
```
|
||||
json j = {{"pi", 3.141}, {"happy", true}};
|
||||
|
||||
std::string s = j.dump(); // {"happy":true,"pi":3.141}
|
||||
```
|
||||
|
||||
To write a value directly to a stream (for example, a file or `std::cout`), the [`operator<<`](https://json.nlohmann.me/api/operator_ltlt/index.md) is provided:
|
||||
|
||||
```
|
||||
std::cout << j << std::endl;
|
||||
```
|
||||
|
||||
String, not raw value
|
||||
|
||||
`dump` always returns a **JSON text**. Serializing a JSON string therefore includes the surrounding quotes and escapes special characters. To obtain the *contained* string value without quotes, use [`get<std::string>()`](https://json.nlohmann.me/features/conversions/index.md) instead of `dump`. See the [converting values](https://json.nlohmann.me/features/conversions/index.md) page.
|
||||
|
||||
## Pretty-printing
|
||||
|
||||
By default, `dump` produces the most compact representation without any superfluous whitespace. Passing a non-negative `indent` argument pretty-prints the output with the given number of spaces per level:
|
||||
|
||||
Example
|
||||
|
||||
```
|
||||
#include <iostream>
|
||||
#include <nlohmann/json.hpp>
|
||||
|
||||
using json = nlohmann::json;
|
||||
|
||||
int main()
|
||||
{
|
||||
// create JSON values
|
||||
json j_object = {{"one", 1}, {"two", 2}};
|
||||
json j_array = {1, 2, 4, 8, 16};
|
||||
json j_string = "Hellö 😀!";
|
||||
|
||||
// call dump()
|
||||
std::cout << "objects:" << '\n'
|
||||
<< j_object.dump() << "\n\n"
|
||||
<< j_object.dump(-1) << "\n\n"
|
||||
<< j_object.dump(0) << "\n\n"
|
||||
<< j_object.dump(4) << "\n\n"
|
||||
<< j_object.dump(1, '\t') << "\n\n";
|
||||
|
||||
std::cout << "arrays:" << '\n'
|
||||
<< j_array.dump() << "\n\n"
|
||||
<< j_array.dump(-1) << "\n\n"
|
||||
<< j_array.dump(0) << "\n\n"
|
||||
<< j_array.dump(4) << "\n\n"
|
||||
<< j_array.dump(1, '\t') << "\n\n";
|
||||
|
||||
std::cout << "strings:" << '\n'
|
||||
<< j_string.dump() << '\n'
|
||||
<< j_string.dump(-1, ' ', true) << '\n';
|
||||
|
||||
// create JSON value with invalid UTF-8 byte sequence
|
||||
json j_invalid = "ä\xA9ü";
|
||||
try
|
||||
{
|
||||
std::cout << j_invalid.dump() << std::endl;
|
||||
}
|
||||
catch (const json::type_error& e)
|
||||
{
|
||||
std::cout << e.what() << std::endl;
|
||||
}
|
||||
|
||||
std::cout << "string with replaced invalid characters: "
|
||||
<< j_invalid.dump(-1, ' ', false, json::error_handler_t::replace)
|
||||
<< "\nstring with ignored invalid characters: "
|
||||
<< j_invalid.dump(-1, ' ', false, json::error_handler_t::ignore)
|
||||
<< '\n';
|
||||
}
|
||||
```
|
||||
|
||||
Output:
|
||||
|
||||
```
|
||||
objects:
|
||||
{"one":1,"two":2}
|
||||
|
||||
{"one":1,"two":2}
|
||||
|
||||
{
|
||||
"one": 1,
|
||||
"two": 2
|
||||
}
|
||||
|
||||
{
|
||||
"one": 1,
|
||||
"two": 2
|
||||
}
|
||||
|
||||
{
|
||||
"one": 1,
|
||||
"two": 2
|
||||
}
|
||||
|
||||
arrays:
|
||||
[1,2,4,8,16]
|
||||
|
||||
[1,2,4,8,16]
|
||||
|
||||
[
|
||||
1,
|
||||
2,
|
||||
4,
|
||||
8,
|
||||
16
|
||||
]
|
||||
|
||||
[
|
||||
1,
|
||||
2,
|
||||
4,
|
||||
8,
|
||||
16
|
||||
]
|
||||
|
||||
[
|
||||
1,
|
||||
2,
|
||||
4,
|
||||
8,
|
||||
16
|
||||
]
|
||||
|
||||
strings:
|
||||
"Hellö 😀!"
|
||||
"Hell\u00f6 \ud83d\ude00!"
|
||||
[json.exception.type_error.316] invalid UTF-8 byte at index 2: 0xA9
|
||||
string with replaced invalid characters: "ä�ü"
|
||||
string with ignored invalid characters: "äü"
|
||||
```
|
||||
|
||||
The indentation character can be changed with the second argument (e.g., a tab `'\t'`). An `indent` of `0` inserts newlines but no leading spaces, and the default of `-1` selects the compact single-line form.
|
||||
|
||||
## Non-ASCII characters
|
||||
|
||||
Strings are stored and serialized as UTF-8 (see [types](https://json.nlohmann.me/features/types/#strings)). By default, `dump` copies valid non-ASCII characters as-is. Setting the third argument `ensure_ascii` to `true` escapes all non-ASCII characters with `\uXXXX` sequences, so that the output contains only ASCII characters:
|
||||
|
||||
```
|
||||
json j = "苹果";
|
||||
j.dump(); // "苹果"
|
||||
j.dump(-1, ' ', true); // "苹果"
|
||||
```
|
||||
|
||||
## Handling invalid UTF-8
|
||||
|
||||
If a string contains invalid UTF-8 sequences (for example, because it holds data in another encoding such as Latin-1), serialization fails by default. The fourth argument of `dump` selects an [`error_handler`](https://json.nlohmann.me/api/basic_json/error_handler_t/index.md):
|
||||
|
||||
- `strict` (default) — throw a [`type_error.316`](https://json.nlohmann.me/home/exceptions/#jsonexceptiontype_error316) exception.
|
||||
- `replace` — replace invalid bytes with the Unicode replacement character U+FFFD (`�`).
|
||||
- `ignore` — silently drop invalid bytes.
|
||||
|
||||
Example
|
||||
|
||||
```
|
||||
#include <iostream>
|
||||
#include <nlohmann/json.hpp>
|
||||
|
||||
using json = nlohmann::json;
|
||||
|
||||
int main()
|
||||
{
|
||||
// create JSON value with invalid UTF-8 byte sequence
|
||||
json j_invalid = "ä\xA9ü";
|
||||
try
|
||||
{
|
||||
std::cout << j_invalid.dump() << std::endl;
|
||||
}
|
||||
catch (const json::type_error& e)
|
||||
{
|
||||
std::cout << e.what() << std::endl;
|
||||
}
|
||||
|
||||
std::cout << "string with replaced invalid characters: "
|
||||
<< j_invalid.dump(-1, ' ', false, json::error_handler_t::replace)
|
||||
<< "\nstring with ignored invalid characters: "
|
||||
<< j_invalid.dump(-1, ' ', false, json::error_handler_t::ignore)
|
||||
<< '\n';
|
||||
}
|
||||
```
|
||||
|
||||
Output:
|
||||
|
||||
```
|
||||
[json.exception.type_error.316] invalid UTF-8 byte at index 2: 0xA9
|
||||
string with replaced invalid characters: "ä�ü"
|
||||
string with ignored invalid characters: "äü"
|
||||
```
|
||||
|
||||
Avoiding invalid UTF-8
|
||||
|
||||
The best fix is to ensure that all strings are UTF-8 encoded before storing them. See the [FAQ on non-ASCII characters](https://json.nlohmann.me/home/faq/#parse-errors-reading-non-ascii-characters) for how to convert wide or Latin-1 strings.
|
||||
|
||||
## Numbers, NaN, and binary values
|
||||
|
||||
- **Numbers** are serialized with enough precision to round-trip; see [number serialization](https://json.nlohmann.me/features/types/number_handling/#number-serialization).
|
||||
- **NaN and infinity** cannot be represented in JSON and are serialized as `null`; see [NaN handling](https://json.nlohmann.me/features/types/number_handling/#nan-handling). The [binary formats](https://json.nlohmann.me/features/binary_formats/index.md) can preserve them.
|
||||
- **Binary values** have no JSON representation and are serialized as a helper object for debugging only; see [binary values](https://json.nlohmann.me/features/binary_values/#serialization).
|
||||
|
||||
## Using `std::format`, `std::print`, and `fmt`
|
||||
|
||||
Since version 3.12.0, JSON values can be formatted directly with C++20's [`std::format`](https://en.cppreference.com/w/cpp/utility/format/format) whenever the standard library provides the `<format>` header (controlled by [`JSON_HAS_STD_FORMAT`](https://json.nlohmann.me/api/macros/json_has_std_format/index.md)). This is enabled by the [`std::formatter<basic_json>`](https://json.nlohmann.me/api/basic_json/std_formatter/index.md) specialization, which also makes JSON values work with `std::format_to` and with C++23's `std::print`/`std::println`:
|
||||
|
||||
```
|
||||
std::print("{}", j); // compact, like j.dump()
|
||||
std::print("{:2}", j); // pretty-printed with indent 2 (like j.dump(2))
|
||||
std::println("{:#}", j); // pretty-printed with the default indent
|
||||
```
|
||||
|
||||
The format spec mirrors the `dump` parameters: `"{:#}"` pretty-prints, a width such as `"{:2}"` sets the indent, and a fill-and-align prefix such as `"{:.>#}"` sets the indent character.
|
||||
|
||||
For the [{fmt}](https://github.com/fmtlib/fmt) library, the library ships a [`format_as`](https://json.nlohmann.me/api/basic_json/format_as/index.md) helper. Note its behavior depends on the `fmt` version; see the [FAQ entry](https://json.nlohmann.me/home/faq/#using-json-values-with-stdformat-or-fmt) for the details and a recipe for a full `fmt::formatter` specialization.
|
||||
|
||||
## Serializing to other formats
|
||||
|
||||
Besides JSON text, a value can also be serialized to the more compact [binary formats](https://json.nlohmann.me/features/binary_formats/index.md) (BJData, BSON, CBOR, MessagePack, UBJSON).
|
||||
|
||||
## See also
|
||||
|
||||
- [`dump`](https://json.nlohmann.me/api/basic_json/dump/index.md) - serialize to a JSON-formatted string
|
||||
- [`operator<<`](https://json.nlohmann.me/api/operator_ltlt/index.md) - serialize to a stream
|
||||
- [`to_string`](https://json.nlohmann.me/api/basic_json/to_string/index.md) - user-defined-conversion helper
|
||||
- [`std::formatter<basic_json>`](https://json.nlohmann.me/api/basic_json/std_formatter/index.md) - use JSON values with `std::format` and `std::print`
|
||||
- [`format_as`](https://json.nlohmann.me/api/basic_json/format_as/index.md) - use JSON values with the {fmt} library
|
||||
- [Parsing](https://json.nlohmann.me/features/parsing/index.md) - the reverse operation
|
||||
@@ -0,0 +1,39 @@
|
||||
# Trailing Commas
|
||||
|
||||
Like [comments](comments.md), this library does not support trailing commas in arrays and objects *by default*.
|
||||
|
||||
You can set parameter `ignore_trailing_commas` to `#!cpp true` in the [`parse`](../api/basic_json/parse.md) function to allow trailing commas in arrays and objects. Note that a single comma as the only content of the array or object (`[,]` or `{,}`) is not allowed, and multiple trailing commas (`[1,,]`) are not allowed either.
|
||||
|
||||
This library does not add trailing commas when serializing JSON data.
|
||||
|
||||
For more information, see [JSON With Commas and Comments (JWCC)](https://nigeltao.github.io/blog/2021/json-with-commas-comments.html).
|
||||
|
||||
!!! example
|
||||
|
||||
Consider the following JSON with trailing commas.
|
||||
|
||||
```json
|
||||
{
|
||||
"planets": [
|
||||
"Mercury",
|
||||
"Venus",
|
||||
"Earth",
|
||||
"Mars",
|
||||
"Jupiter",
|
||||
"Uranus",
|
||||
"Neptune",
|
||||
]
|
||||
}
|
||||
```
|
||||
|
||||
When calling `parse` without additional argument, a parse error exception is thrown. If `ignore_trailing_commas` is set to `#!cpp true`, the trailing commas are ignored during parsing:
|
||||
|
||||
```cpp
|
||||
--8<-- "examples/trailing_commas.cpp"
|
||||
```
|
||||
|
||||
Output:
|
||||
|
||||
```
|
||||
--8<-- "examples/trailing_commas.output"
|
||||
```
|
||||
@@ -58,4 +58,4 @@
|
||||
"Neptune"
|
||||
]
|
||||
}
|
||||
</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="July 4, 2026 09:13:25 UTC">July 4, 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 - 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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="July 8, 2026 18:18:24 UTC">July 8, 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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