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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/types/" rel="canonical"><link href=../trailing_commas/ rel=prev><link href=number_handling/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>Types - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.ab4e12ef.min.css><link rel="stylesheet" href="../../assets/external/fonts.googleapis.com/css.61a430c9.css"><style>:root{--md-text-font:"Roboto";--md-code-font:"JetBrains Mono"}</style><link rel=stylesheet href=../../css/custom.css><script>__md_scope=new 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xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 8a3 3 0 0 0 3-3 3 3 0 0 0-3-3 3 3 0 0 0-3 3 3 3 0 0 0 3 3m0 3.54C9.64 9.35 6.5 8 3 8v11c3.5 0 6.64 1.35 9 3.54 2.36-2.19 5.5-3.54 9-3.54V8c-3.5 0-6.64 1.35-9 3.54"/></svg> </a> <label class="md-header__button md-icon" for=__drawer> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M3 6h18v2H3zm0 5h18v2H3zm0 5h18v2H3z"/></svg> </label> <div class=md-header__title data-md-component=header-title> <div class=md-header__ellipsis> <div class=md-header__topic> <span class=md-ellipsis> JSON for Modern C++ </span> </div> <div class=md-header__topic data-md-component=header-topic> <span class=md-ellipsis> Types </span> </div> </div> </div> <form class=md-header__option data-md-component=palette> <input class=md-option data-md-color-media="(prefers-color-scheme: light)" data-md-color-scheme=default data-md-color-primary=indigo data-md-color-accent=indigo aria-label="Switch to dark mode" type=radio name=__palette 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<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/types/" rel="canonical"><link href=../trailing_commas/ rel=prev><link href=number_handling/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>Types - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.ab4e12ef.min.css><link rel="stylesheet" href="../../assets/external/fonts.googleapis.com/css.61a430c9.css"><style>:root{--md-text-font:"Roboto";--md-code-font:"JetBrains Mono"}</style><link rel=stylesheet href=../../css/custom.css><script>__md_scope=new URL("../..",location),__md_hash=e=>[...e].reduce(((e,_)=>(e<<5)-e+_.charCodeAt(0)),0),__md_get=(e,_=localStorage,t=__md_scope)=>JSON.parse(_.getItem(t.pathname+"."+e)),__md_set=(e,_,t=localStorage,a=__md_scope)=>{try{t.setItem(a.pathname+"."+e,JSON.stringify(_))}catch(e){}}</script></head> <body dir=ltr data-md-color-scheme=default data-md-color-primary=indigo data-md-color-accent=indigo> <input class=md-toggle data-md-toggle=drawer type=checkbox id=__drawer autocomplete=off> <input class=md-toggle data-md-toggle=search type=checkbox id=__search autocomplete=off> <label class=md-overlay for=__drawer></label> <div data-md-component=skip> <a href=#types 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> Types </span> </div> </div> </div> <form class=md-header__option data-md-component=palette> <input class=md-option data-md-color-media="(prefers-color-scheme: light)" data-md-color-scheme=default data-md-color-primary=indigo data-md-color-accent=indigo aria-label="Switch to dark mode" type=radio name=__palette 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23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> </form> <script>var palette=__md_get("__palette");if(palette&&palette.color){if("(prefers-color-scheme)"===palette.color.media){var media=matchMedia("(prefers-color-scheme: light)"),input=document.querySelector(media.matches?"[data-md-color-media='(prefers-color-scheme: light)']":"[data-md-color-media='(prefers-color-scheme: dark)']");palette.color.media=input.getAttribute("data-md-color-media"),palette.color.scheme=input.getAttribute("data-md-color-scheme"),palette.color.primary=input.getAttribute("data-md-color-primary"),palette.color.accent=input.getAttribute("data-md-color-accent")}for(var[key,value]of Object.entries(palette.color))document.body.setAttribute("data-md-color-"+key,value)}</script> <label class="md-header__button md-icon" for=__search> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M9.5 3A6.5 6.5 0 0 1 16 9.5c0 1.61-.59 3.09-1.56 4.23l.27.27h.79l5 5-1.5 1.5-5-5v-.79l-.27-.27A6.52 6.52 0 0 1 9.5 16 6.5 6.5 0 0 1 3 9.5 6.5 6.5 0 0 1 9.5 3m0 2C7 5 5 7 5 9.5S7 14 9.5 14 14 12 14 9.5 12 5 9.5 5"/></svg> </label> <div class=md-search data-md-componLine truncated
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class value_t {
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<<enumeration>>
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@@ -81,4 +81,4 @@ basic_json .. value_t</code></pre> <h2 id=template-arguments>Template arguments<
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<span class=w> </span><span class=n>basic_json</span><span class=p>,</span><span class=w> </span><span class=c1>// value_type</span>
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<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>allocator</span><span class=o><</span><span class=n>basic_json</span><span class=o>></span><span class=w> </span><span class=c1>// allocator_type</span>
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<span class=o>></span>
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</code></pre></div> <h3 id=limits_1>Limits<a class=headerlink href=#limits_1 title="Permanent link">¶</a></h3> <p><a href="https://tools.ietf.org/html/rfc8259"><abbr title="Request for Comments">RFC</abbr> 8259</a> specifies:</p> <blockquote> <p>An implementation may set limits on the maximum depth of nesting.</p> </blockquote> <p>In this class, the array's limit of nesting is not explicitly constrained. However, a maximum depth of nesting may be introduced by the compiler or runtime environment. A theoretical limit can be queried by calling the <code>max_size</code> function of a <abbr title="JavaScript Object Notation">JSON</abbr> array.</p> <h3 id=storage_2>Storage<a class=headerlink href=#storage_2 title="Permanent link">¶</a></h3> <p>Arrays are stored as pointers in a <code>basic_json</code> type. That is, for any access to array values, a pointer of type <code>array_t*</code> must be dereferenced.</p> <h2 id=strings>Strings<a class=headerlink href=#strings title="Permanent link">¶</a></h2> <p><a href="https://tools.ietf.org/html/rfc8259"><abbr title="Request for Comments">RFC</abbr> 8259</a> describes <abbr title="JavaScript Object Notation">JSON</abbr> strings as follows:</p> <blockquote> <p>A string is a sequence of zero or more Unicode characters.</p> </blockquote> <p>Unicode values are split by the <abbr title="JavaScript Object Notation">JSON</abbr> class into byte-sized characters during deserialization.</p> <h3 id=default-type_2>Default type<a class=headerlink href=#default-type_2 title="Permanent link">¶</a></h3> <p>With the default values for <em>StringType</em> (<code>std::string</code>), the default value for <code>string_t</code> is <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>string</span></code>.</p> <h3 id=encoding>Encoding<a class=headerlink href=#encoding title="Permanent link">¶</a></h3> <p>Strings are stored in <abbr title="Unicode Transformation Format">UTF</abbr>-8 encoding. Therefore, functions like <code>std::string::size()</code> or <code>std::string::length()</code> return the number of <strong>bytes</strong> in the string rather than the number of characters or glyphs.</p> <h3 id=string-comparison>String comparison<a class=headerlink href=#string-comparison title="Permanent link">¶</a></h3> <p><a href="https://tools.ietf.org/html/rfc8259"><abbr title="Request for Comments">RFC</abbr> 8259</a> states:</p> <blockquote> <p>Software implementations are typically required to test names of object members for equality. Implementations that transform the textual representation into sequences of Unicode code units and then perform the comparison numerically, code unit by code unit are interoperable in the sense that implementations will agree in all cases on equality or inequality of two strings. For example, implementations that compare strings with escaped characters unconverted may incorrectly find that <code>"a\\b"</code> and <code>"a\u005Cb"</code> are not equal.</p> </blockquote> <p>This implementation is interoperable as it does compare strings code unit by code unit.</p> <h3 id=storage_3>Storage<a class=headerlink href=#storage_3 title="Permanent link">¶</a></h3> <p>String values are stored as pointers in a <code>basic_json</code> type. That is, for any access to string values, a pointer of type <code>string_t*</code> must be dereferenced.</p> <h2 id=booleans>Booleans<a class=headerlink href=#booleans title="Permanent link">¶</a></h2> <p><a href="https://tools.ietf.org/html/rfc8259"><abbr title="Request for Comments">RFC</abbr> 8259</a> implicitly describes a boolean as a type which differentiates the two literals <code>true</code> and <code>false</code>.</p> <h3 id=default-type_3>Default type<a class=headerlink href=#default-type_3 title="Permanent link">¶</a></h3> <p>With the default values for <em>BooleanType</em> (<code class=highlight><span class=kt>bool</span></code>), the default value for <code>boolean_t</code> is <code class=highlight><span class=kt>bool</span></code>.</p> <h3 id=storage_4>Storage<a class=headerlink href=#storage_4 title="Permanent link">¶</a></h3> <p>Boolean values are stored directly inside a <code>basic_json</code> type.</p> <h2 id=numbers>Numbers<a class=headerlink href=#numbers title="Permanent link">¶</a></h2> <p>See the <a href=number_handling/ >number handling</a> article for a detailed discussion on how numbers are handled by this library.</p> <p><a href="https://tools.ietf.org/html/rfc8259"><abbr title="Request for Comments">RFC</abbr> 8259</a> describes numbers as follows:</p> <blockquote> <p>The representation of numbers is similar to that used in most programming languages. A number is represented in base 10 using decimal digits. It contains an integer component that may be prefixed with an optional minus sign, which may be followed by a fraction part and/or an exponent part. Leading zeros are not allowed. (...) Numeric values that cannot be reprLine truncated
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</code></pre></div> <h3 id=limits_1>Limits<a class=headerlink href=#limits_1 title="Permanent link">¶</a></h3> <p><a href="https://tools.ietf.org/html/rfc8259"><abbr title="Request for Comments">RFC</abbr> 8259</a> specifies:</p> <blockquote> <p>An implementation may set limits on the maximum depth of nesting.</p> </blockquote> <p>In this class, the array's limit of nesting is not explicitly constrained. However, a maximum depth of nesting may be introduced by the compiler or runtime environment. A theoretical limit can be queried by calling the <code>max_size</code> function of a <abbr title="JavaScript Object Notation">JSON</abbr> array.</p> <h3 id=storage_2>Storage<a class=headerlink href=#storage_2 title="Permanent link">¶</a></h3> <p>Arrays are stored as pointers in a <code>basic_json</code> type. That is, for any access to array values, a pointer of type <code>array_t*</code> must be dereferenced.</p> <h2 id=strings>Strings<a class=headerlink href=#strings title="Permanent link">¶</a></h2> <p><a href="https://tools.ietf.org/html/rfc8259"><abbr title="Request for Comments">RFC</abbr> 8259</a> describes <abbr title="JavaScript Object Notation">JSON</abbr> strings as follows:</p> <blockquote> <p>A string is a sequence of zero or more Unicode characters.</p> </blockquote> <p>Unicode values are split by the <abbr title="JavaScript Object Notation">JSON</abbr> class into byte-sized characters during deserialization.</p> <h3 id=default-type_2>Default type<a class=headerlink href=#default-type_2 title="Permanent link">¶</a></h3> <p>With the default values for <em>StringType</em> (<code>std::string</code>), the default value for <code>string_t</code> is <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>string</span></code>.</p> <h3 id=encoding>Encoding<a class=headerlink href=#encoding title="Permanent link">¶</a></h3> <p>Strings are stored in <abbr title="Unicode Transformation Format">UTF</abbr>-8 encoding. Therefore, functions like <code>std::string::size()</code> or <code>std::string::length()</code> return the number of <strong>bytes</strong> in the string rather than the number of characters or glyphs.</p> <h3 id=string-comparison>String comparison<a class=headerlink href=#string-comparison title="Permanent link">¶</a></h3> <p><a href="https://tools.ietf.org/html/rfc8259"><abbr title="Request for Comments">RFC</abbr> 8259</a> states:</p> <blockquote> <p>Software implementations are typically required to test names of object members for equality. Implementations that transform the textual representation into sequences of Unicode code units and then perform the comparison numerically, code unit by code unit are interoperable in the sense that implementations will agree in all cases on equality or inequality of two strings. For example, implementations that compare strings with escaped characters unconverted may incorrectly find that <code>"a\\b"</code> and <code>"a\u005Cb"</code> are not equal.</p> </blockquote> <p>This implementation is interoperable as it does compare strings code unit by code unit.</p> <h3 id=storage_3>Storage<a class=headerlink href=#storage_3 title="Permanent link">¶</a></h3> <p>String values are stored as pointers in a <code>basic_json</code> type. That is, for any access to string values, a pointer of type <code>string_t*</code> must be dereferenced.</p> <h2 id=booleans>Booleans<a class=headerlink href=#booleans title="Permanent link">¶</a></h2> <p><a href="https://tools.ietf.org/html/rfc8259"><abbr title="Request for Comments">RFC</abbr> 8259</a> implicitly describes a boolean as a type which differentiates the two literals <code>true</code> and <code>false</code>.</p> <h3 id=default-type_3>Default type<a class=headerlink href=#default-type_3 title="Permanent link">¶</a></h3> <p>With the default values for <em>BooleanType</em> (<code class=highlight><span class=kt>bool</span></code>), the default value for <code>boolean_t</code> is <code class=highlight><span class=kt>bool</span></code>.</p> <h3 id=storage_4>Storage<a class=headerlink href=#storage_4 title="Permanent link">¶</a></h3> <p>Boolean values are stored directly inside a <code>basic_json</code> type.</p> <h2 id=numbers>Numbers<a class=headerlink href=#numbers title="Permanent link">¶</a></h2> <p>See the <a href=number_handling/ >number handling</a> article for a detailed discussion on how numbers are handled by this library.</p> <p><a href="https://tools.ietf.org/html/rfc8259"><abbr title="Request for Comments">RFC</abbr> 8259</a> describes numbers as follows:</p> <blockquote> <p>The representation of numbers is similar to that used in most programming languages. A number is represented in base 10 using decimal digits. It contains an integer component that may be prefixed with an optional minus sign, which may be followed by a fraction part and/or an exponent part. Leading zeros are not allowed. (...) Numeric values that cannot be reprLine truncated
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@@ -67,14 +67,29 @@ Positive integers are stored as `#!c std::uint64_t`, while negative integers are
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distinction is determined at parse time: if the JSON number has a leading minus sign, it uses signed integer storage;
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otherwise, it uses unsigned integer storage.
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```mermaid
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flowchart TD
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A["number literal"] --> B{"has a fraction (.) or exponent (e/E)?"}
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B -->|"yes"| F["number_float_t"]
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B -->|"no"| C{"has a leading minus sign?"}
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C -->|"yes"| D["try number_integer_t"]
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C -->|"no"| E["try number_unsigned_t"]
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D -->|"overflow"| F
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E -->|"overflow"| F
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```
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!!! info "Notes"
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- Numbers with a decimal digit or scientific notation are always stored as `#!c double`.
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- The number types can be changed, see [Template number types](#template-number-types).
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- As of version 3.9.1, the conversion is realized by
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- Integers are converted by the library's own digit parser. Floating-point numbers are converted with
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[`std::from_chars`](https://en.cppreference.com/w/cpp/utility/from_chars) if the library is compiled with C++17
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and the standard library supports it, then with an exact fast path for `#!c double` values with few significant
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digits, and otherwise with the locale-aware
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[`std::strtod`](https://en.cppreference.com/w/cpp/string/byte/strtof) (`std::strtof`/`std::strtold` for the
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other floating-point types). Before version 3.13.0, the conversion was realized by
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[`std::strtoull`](https://en.cppreference.com/w/cpp/string/byte/strtoul),
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[`std::strtoll`](https://en.cppreference.com/w/cpp/string/byte/strtol), and
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[`std::strtod`](https://en.cppreference.com/w/cpp/string/byte/strtof), respectively.
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[`std::strtoll`](https://en.cppreference.com/w/cpp/string/byte/strtol), and `std::strtod`, respectively.
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!!! example "Examples"
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@@ -1,4 +1,11 @@
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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/types/number_handling/" rel="canonical"><link href=../ rel=prev><link href=../template_parameters/ rel=next><link rel=icon href=../../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>Number Handling - JSON for Modern C++</title><link rel=stylesheet href=../../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../../assets/stylesheets/palette.ab4e12ef.min.css><link rel="stylesheet" href="../../../assets/external/fonts.googleapis.com/css.61a430c9.css"><style>:root{--md-text-font:"Roboto";--md-code-font:"JetBrains Mono"}</style><link rel=stylesheet href=../../../css/custom.css><script>__md_scope=new 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<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> Number Handling </span> </div> </div> </div> <form class=md-header__option data-md-component=palette> <input class=md-option data-md-color-media="(prefers-color-scheme: light)" data-md-color-scheme=default data-md-color-primary=indigo data-md-color-accent=indigo aria-label="Switch to dark mode" type=radio 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6.52 0 0 1 9.5 16 6.5 6.5 0 0 1 3 9.5 6.5 6.5 0 0 1 9.5 3m0 2C7 5 5 7 5 9.5S7 14 9.5 14 14 12 14 9.5 12 5 9Line truncated
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A["number literal"] --> B{"has a fraction (.) or exponent (e/E)?"}
|
||||
B -->|"yes"| F["number_float_t"]
|
||||
B -->|"no"| C{"has a leading minus sign?"}
|
||||
C -->|"yes"| D["try number_integer_t"]
|
||||
C -->|"no"| E["try number_unsigned_t"]
|
||||
D -->|"overflow"| F
|
||||
E -->|"overflow"| F</code></pre> <div class="admonition info"> <p class=admonition-title>Notes</p> <ul> <li>Numbers with a decimal digit or scientific notation are always stored as <code class=highlight><span class=kt>double</span></code>.</li> <li>The number types can be changed, see <a href=#template-number-types>Template number types</a>. </li> <li>Integers are converted by the library's own digit parser. Floating-point numbers are converted with <a href="https://en.cppreference.com/w/cpp/utility/from_chars"><code>std::from_chars</code></a> if the library is compiled with C++17 and the standard library supports it, then with an exact fast path for <code class=highlight><span class=kt>double</span></code> values with few significant digits, and otherwise with the locale-aware <a href="https://en.cppreference.com/w/cpp/string/byte/strtof"><code>std::strtod</code></a> (<code>std::strtof</code>/<code>std::strtold</code> for the other floating-point types). Before version 3.13.0 <span class=unreleased-version title="Not part of a release yet; the latest release is 3.12.0.">unreleased</span>, the conversion was realized by <a href="https://en.cppreference.com/w/cpp/string/byte/strtoul"><code>std::strtoull</code></a>, <a href="https://en.cppreference.com/w/cpp/string/byte/strtol"><code>std::strtoll</code></a>, and <code>std::strtod</code>, respectively.</li> </ul> </div> <div class="admonition example"> <p class=admonition-title>Examples</p> <ul> <li>Integer <code class=highlight><span class=mi>-12345678912345789123456789</span></code> is smaller than <code class=highlight><span class=n>INT64_MIN</span></code> and will be stored as floating-point number <code class=highlight><span class=mf>-1.2345678912345788e+25</span></code>.</li> <li>Integer <code class=highlight><span class=mf>1E3</span></code> will be stored as floating-point number <code class=highlight><span class=mf>1000.0</span></code>.</li> </ul> </div> <h3 id=number-limits>Number limits<a class=headerlink href=#number-limits title="Permanent link">¶</a></h3> <ul> <li>Any 64-bit signed or unsigned integer can be stored without loss of precision.</li> <li>Numbers exceeding the limits of <code class=highlight><span class=kt>double</span></code> (i.e., numbers that after conversion via <a href="https://en.cppreference.com/w/cpp/string/byte/strtof"><code>std::strtod</code></a> are not satisfying <a href="https://en.cppreference.com/w/cpp/numeric/math/isfinite"><code>std::isfinite</code></a> such as <code class=highlight><span class=mf>1E400</span></code>) will throw exception <a href=../../../home/exceptions/#jsonexceptionout_of_range406><code>json.exception.out_of_range.406</code></a> during parsing.</li> <li>Floating-point numbers are rounded to the next number representable as <code>double</code>. For instance <code class=highlight><span class=mf>3.141592653589793238462643383279</span></code> is stored as <a href="https://float.exposed/0x400921fb54442d18"><code>0x400921fb54442d18</code></a>. This is the same behavior as the code <code class=highlight><span class=kt>double</span><span class=w> </span><span class=n>x</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=mf>3.141592653589793238462643383279</span><span class=p>;</span></code>.</li> </ul> <div class="admonition success"> <p class=admonition-title>Interoperability</p> <ul> <li>The library is interoperable with respect to the specification, because its supported range [-2<sup>63</sup>, 2<sup>64</sup>-1] is larger than the described range [-2<sup>53</sup>+1, 2<sup>53</sup>-1].</li> <li>All integers outside the range [-2<sup>63</sup>, 2<sup>64</sup>-1], as well as floating-point numbers are stored as <code>double</code>. This also concurs with the specification above.</li> </ul> </div> <h3 id=zeros>Zeros<a class=headerlink href=#zeros title="Permanent link">¶</a></h3> <p>The <abbr title="JavaScript Object Notation">JSON</abbr> number grammar allows for different ways to express zero, and this library will store zeros differently:</p> <table> <thead> <tr> <th>Literal</th> <th>Stored value and type</th> <th>Serialization</th> </tr> </thead> <tbody> <tr> <td><code>0</code></td> <td><code class=highlight><span class=n>std</span><span class=o>::</span><span class=kt>uint64_t</span><span class=p>(</span><span class=mi>0</span><span class=p>)</span></code></td> <td><code>0</code></td> </tr> <tr> <td><code>-0</code></td> <td><code class=highlight><span class=n>std</span><span class=o>::</span><span class=kt>int64_t</span><span class=p>(</span><span class=mi>0</span><span class=p>)</span></code></td> <td><code>0</code></td> </tr> <tr> <td><code>0.0</code></td> <td><code class=highlight><span class=kt>double</span><span class=p>(</span><span class=mf>0.0</span><span class=p>)</span></code></td> <td><code>0.0</code></td> </tr> <tr> <td><code>-0.0</code></td> <td><code class=highlight><span class=kt>double</span><span class=p>(</span><span class=mf>-0.0</spanLine truncated
|
||||
<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>f</span><span class=p>;</span>
|
||||
<span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=n>j</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=sc>'\n'</span><span class=p>;</span>
|
||||
</code></pre></div> yields <code class=highlight><span class=mf>0.30000001192092896</span></code>.</li> </ul> <p>All examples here can be reproduced by passing the original double value to</p> <div class=highlight><pre><span></span><code><span class=n>std</span><span class=o>::</span><span class=n>printf</span><span class=p>(</span><span class=s>"%.*g</span><span class=se>\n</span><span class=s>"</span><span class=p>,</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>numeric_limits</span><span class=o><</span><span class=kt>double</span><span class=o>>::</span><span class=n>max_digits10</span><span class=p>,</span><span class=w> </span><span class=n>double_value</span><span class=p>);</span>
|
||||
@@ -54,4 +61,4 @@ val=nan
|
||||
<span class=p>}</span>
|
||||
</code></pre></div> <p>Note this approach also has the advantage that it can react on non-numerical <abbr title="JavaScript Object Notation">JSON</abbr> value types such as strings.</p> <p>(Example taken from <a href="https://github.com/nlohmann/json/issues/777#issuecomment-459968458">#777</a>.)</p> </div> <h3 id=determine-number-types>Determine number types<a class=headerlink href=#determine-number-types title="Permanent link">¶</a></h3> <p>As the example in <a href=#number-conversion>Number conversion</a> shows, there are different functions to determine the type of the stored number:</p> <ul> <li><a href=../../../api/basic_json/is_number/ ><code>is_number()</code></a> returns <code class=highlight><span class=nb>true</span></code> for any number type</li> <li><a href=../../../api/basic_json/is_number_integer/ ><code>is_number_integer()</code></a> returns <code class=highlight><span class=nb>true</span></code> for signed and unsigned integers</li> <li><a href=../../../api/basic_json/is_number_unsigned/ ><code>is_number_unsigned()</code></a> returns <code class=highlight><span class=nb>true</span></code> for unsigned integers only</li> <li><a href=../../../api/basic_json/is_number_float/ ><code>is_number_float()</code></a> returns <code class=highlight><span class=nb>true</span></code> for floating-point numbers</li> <li><a href=../../../api/basic_json/type_name/ ><code>type_name()</code></a> returns <code class=highlight><span class=s>"number"</span></code> for any number type</li> <li><a href=../../../api/basic_json/type/ ><code>type()</code></a> returns a different enumerator of <a href=../../../api/basic_json/value_t/ ><code>value_t</code></a> for all number types</li> </ul> <table> <thead> <tr> <th>function</th> <th>unsigned integer</th> <th>signed integer</th> <th>floating-point</th> <th>string</th> </tr> </thead> <tbody> <tr> <td><a href=../../../api/basic_json/is_number/ ><code>is_number()</code></a></td> <td><code class=highlight><span class=nb>true</span></code></td> <td><code class=highlight><span class=nb>true</span></code></td> <td><code class=highlight><span class=nb>true</span></code></td> <td><code class=highlight><span class=nb>false</span></code></td> </tr> <tr> <td><a href=../../../api/basic_json/is_number_integer/ ><code>is_number_integer()</code></a></td> <td><code class=highlight><span class=nb>true</span></code></td> <td><code class=highlight><span class=nb>true</span></code></td> <td><code class=highlight><span class=nb>false</span></code></td> <td><code class=highlight><span class=nb>false</span></code></td> </tr> <tr> <td><a href=../../../api/basic_json/is_number_unsigned/ ><code>is_number_unsigned()</code></a></td> <td><code class=highlight><span class=nb>true</span></code></td> <td><code class=highlight><span class=nb>false</span></code></td> <td><code class=highlight><span class=nb>false</span></code></td> <td><code class=highlight><span class=nb>false</span></code></td> </tr> <tr> <td><a href=../../../api/basic_json/is_number_float/ ><code>is_number_float()</code></a></td> <td><code class=highlight><span class=nb>false</span></code></td> <td><code class=highlight><span class=nb>false</span></code></td> <td><code class=highlight><span class=nb>true</span></code></td> <td><code class=highlight><span class=nb>false</span></code></td> </tr> <tr> <td><a href=../../../api/basic_json/type_name/ ><code>type_name()</code></a></td> <td><code class=highlight><span class=s>"number"</span></code></td> <td><code class=highlight><span class=s>"number"</span></code></td> <td><code class=highlight><span class=s>"number"</span></code></td> <td><code class=highlight><span class=s>"string"</span></code></td> </tr> <tr> <td><a href=../../../api/basic_json/type/ ><code>type()</code></a></td> <td><code>number_unsigned</code></td> <td><code>number_integer</code></td> <td><code>number_float</code></td> <td><code>string</code></td> </tr> </tbody> </table> <h3 id=template-number-types>Template number types<a class=headerlink href=#template-number-types title="Permanent link">¶</a></h3> <p>The number types can be changed with template parameters.</p> <table> <thead> <tr> <th>position</th> <th>number type</th> <th>default type</th> <th>possible values</th> </tr> </thead> <tbody> <tr> <td>5</td> <td>signed integers</td> <td><code class=highlight><span class=n>std</span><span class=o>::</span><span class=kt>int64_t</span></code></td> <td><code class=highlight><span class=n>std</span><span class=o>::</span><span class=kt>int32_t</span></code>, <code class=highlight><span class=n>std</span><span class=o>::</span><span class=kt>int16_t</span></code>, etc.</td> </tr> <tr> <td>6</td> <td>unsigned integers</td> <td><code class=highlight><span class=n>std</span><span class=o>::</span><span class=kt>uint64_t</span></code></td> <td><code class=highlight><span class=n>std</span><span class=o>::</span><span class=kt>uint32_t</span></code>, <code class=highlight><span class=n>std</span><span class=o>::</spanLine truncated
|
||||
<span class=hll><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=kt>int64_t</span><span class=p>,</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=kt>uint64_t</span><span class=p>,</span><span class=w> </span><span class=kt>long</span><span class=w> </span><span class=kt>double</span><span class=o>></span><span class=p>;</span>
|
||||
</span></code></pre></div> <p>Note values should then be parsed with <code>json_ld::parse</code> rather than <code>json::parse</code> as the latter would parse floating-point values to <code class=highlight><span class=kt>double</span></code> before then converting them to <code class=highlight><span class=kt>long</span><span class=w> </span><span class=kt>double</span></code>.</p> </div> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 2, 2026 09:23:55 UTC">October 2, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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||||
</span></code></pre></div> <p>Note values should then be parsed with <code>json_ld::parse</code> rather than <code>json::parse</code> as the latter would parse floating-point values to <code class=highlight><span class=kt>double</span></code> before then converting them to <code class=highlight><span class=kt>long</span><span class=w> </span><span class=kt>double</span></code>.</p> </div> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 2, 2026 09:32:15 UTC">October 2, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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|
||||
@@ -40,11 +40,22 @@ In the default [`json`](https://json.nlohmann.me/api/json/index.md) type, number
|
||||
|
||||
Positive integers are stored as `std::uint64_t`, while negative integers are stored as `std::int64_t`. This distinction is determined at parse time: if the JSON number has a leading minus sign, it uses signed integer storage; otherwise, it uses unsigned integer storage.
|
||||
|
||||
```
|
||||
flowchart TD
|
||||
A["number literal"] --> B{"has a fraction (.) or exponent (e/E)?"}
|
||||
B -->|"yes"| F["number_float_t"]
|
||||
B -->|"no"| C{"has a leading minus sign?"}
|
||||
C -->|"yes"| D["try number_integer_t"]
|
||||
C -->|"no"| E["try number_unsigned_t"]
|
||||
D -->|"overflow"| F
|
||||
E -->|"overflow"| F
|
||||
```
|
||||
|
||||
Notes
|
||||
|
||||
- Numbers with a decimal digit or scientific notation are always stored as `double`.
|
||||
- The number types can be changed, see [Template number types](#template-number-types).
|
||||
- As of version 3.9.1, the conversion is realized by [`std::strtoull`](https://en.cppreference.com/w/cpp/string/byte/strtoul), [`std::strtoll`](https://en.cppreference.com/w/cpp/string/byte/strtol), and [`std::strtod`](https://en.cppreference.com/w/cpp/string/byte/strtof), respectively.
|
||||
- Integers are converted by the library's own digit parser. Floating-point numbers are converted with [`std::from_chars`](https://en.cppreference.com/w/cpp/utility/from_chars) if the library is compiled with C++17 and the standard library supports it, then with an exact fast path for `double` values with few significant digits, and otherwise with the locale-aware [`std::strtod`](https://en.cppreference.com/w/cpp/string/byte/strtof) (`std::strtof`/`std::strtold` for the other floating-point types). Before version 3.13.0 unreleased, the conversion was realized by [`std::strtoull`](https://en.cppreference.com/w/cpp/string/byte/strtoul), [`std::strtoll`](https://en.cppreference.com/w/cpp/string/byte/strtol), and `std::strtod`, respectively.
|
||||
|
||||
Examples
|
||||
|
||||
|
||||
@@ -26,8 +26,9 @@ Requirements are split into two groups:
|
||||
diagnosed with dedicated error messages, and violating most of them results in a compiler error somewhere inside
|
||||
the library. Four violations are not caught at compile time at all:
|
||||
|
||||
- A [`StringType`](#stringtype) whose `data()` is not null-terminated compiles and silently misparses numbers,
|
||||
because the lexer hands the buffer to `#!cpp std::strtoull`/`#!cpp std::strtoll`/`#!cpp std::strtod`.
|
||||
- A [`StringType`](#stringtype) whose `data()` is not null-terminated compiles and can silently misparse
|
||||
floating-point numbers, because the lexer may hand the buffer to `#!cpp std::strtod`, which reads up to the
|
||||
terminating null character.
|
||||
- A stateful [`AllocatorType`](#allocatortype) compiles and silently ignores its state: allocation, deallocation,
|
||||
and [`get_allocator()`](../../api/basic_json/get_allocator.md) each use a different default-constructed instance.
|
||||
- The two [cross-specialization conversions](#cross-specialization-conversions) below. These abort on an assertion
|
||||
@@ -213,7 +214,7 @@ The library does not sort or de-duplicate keys itself; the behavior described in
|
||||
--8<-- "examples/custom_object_type.hpp"
|
||||
```
|
||||
|
||||
??? example "Compiling and using it"
|
||||
??? example "Example: use the custom `ObjectType`"
|
||||
|
||||
```cpp
|
||||
--8<-- "examples/custom_object_type.cpp"
|
||||
@@ -306,7 +307,7 @@ using array_t = ArrayType<basic_json, AllocatorType<basic_json>>;
|
||||
--8<-- "examples/custom_array_type.hpp"
|
||||
```
|
||||
|
||||
??? example "Compiling and using it"
|
||||
??? example "Example: use the custom `ArrayType`"
|
||||
|
||||
```cpp
|
||||
--8<-- "examples/custom_array_type.cpp"
|
||||
@@ -348,16 +349,16 @@ using array_t = ArrayType<basic_json, AllocatorType<basic_json>>;
|
||||
### Always required
|
||||
|
||||
- A member type `value_type` that is one byte wide and `char`-compatible. The library stores and processes UTF-8
|
||||
encoded `char` data and hands `data()` to `#!cpp std::strtoull`/`#!cpp std::strtoll`.
|
||||
encoded `char` data and passes `data()` to functions that take a `#!cpp const char*`, such as `#!cpp std::strtod`.
|
||||
`#!cpp std::wstring`, `#!cpp std::u16string`, and `#!cpp std::u32string` are **not** valid choices; see the FAQ on
|
||||
[wide string handling](../../home/faq.md#wide-string-handling).
|
||||
- Constructors: default, copy, move, from `#!cpp const char*` (which must not be `#!cpp explicit`), from
|
||||
`#!cpp (const char*, size_type)`, and from `#!cpp (size_type, char)`; and copy or move assignment.
|
||||
- Member functions `size()`, `clear()`, `resize(n, c)`, `data()`, `push_back(char)`, and `operator[]`
|
||||
(const and non-const, returning references). `c_str()` and `back()` are **not** required.
|
||||
- `data()` must return a pointer to a contiguous, **null-terminated** buffer -- the parser hands it to
|
||||
`#!cpp std::strtoull`. A type whose `data()` is not null-terminated does not fail to compile; it silently
|
||||
misparses numbers.
|
||||
- `data()` must return a pointer to a contiguous, **null-terminated** buffer -- the parser may hand it to
|
||||
`#!cpp std::strtod`, which reads up to the null character. A type whose `data()` is not null-terminated does not
|
||||
fail to compile; it can silently misparse floating-point numbers.
|
||||
- `append(const char*, size_type)`, used by [`dump`](../../api/basic_json/dump.md), and `append(const StringType&)`,
|
||||
used by the CBOR reader for indefinite-length strings. The library's internal string concatenation additionally has
|
||||
to append a `#!cpp char` and a `#!cpp const char*`; for each it selects between `append(arg)`, `#!cpp operator+=`,
|
||||
@@ -394,6 +395,7 @@ using array_t = ArrayType<basic_json, AllocatorType<basic_json>>;
|
||||
| [`to_bson`](../../api/basic_json/to_bson.md) | `find(value_type)` and `npos` |
|
||||
| [`parse`](../../api/basic_json/parse.md) from a `string_t` | the input adapters must accept it; otherwise pass a character range |
|
||||
| `#!cpp operator<<(std::ostream&, const json_pointer&)` | streamability to `#!cpp std::ostream` |
|
||||
| [`to_string`](../../api/basic_json/to_string.md) | conversion of `StringType` to `#!cpp std::string` (the function returns a `#!cpp std::string`) |
|
||||
| exception messages | `data()` and `size()`, or `begin()` and `end()` |
|
||||
|
||||
### Compatible types
|
||||
@@ -448,7 +450,7 @@ using array_t = ArrayType<basic_json, AllocatorType<basic_json>>;
|
||||
--8<-- "examples/custom_string_type.hpp"
|
||||
```
|
||||
|
||||
??? example "Compiling and using it"
|
||||
??? example "Example: use the custom `StringType`"
|
||||
|
||||
```cpp
|
||||
--8<-- "examples/custom_string_type.cpp"
|
||||
@@ -535,8 +537,9 @@ therefore silently changes parse results rather than raising an error. See
|
||||
|
||||
`NumberFloatType` must be one of `#!cpp float`, `#!cpp double`, or `#!cpp long double`:
|
||||
|
||||
- The [parser](../parsing/index.md) converts number literals with `#!cpp std::strtof`, `#!cpp std::strtod`, or
|
||||
`#!cpp std::strtold`; the library provides overloads for exactly these three types.
|
||||
- The [parser](../parsing/index.md) converts number literals with `#!cpp std::from_chars` or, as a fallback, with
|
||||
`#!cpp std::strtof`, `#!cpp std::strtod`, or `#!cpp std::strtold`; the library provides overloads for exactly these
|
||||
three types.
|
||||
- [`dump`](../../api/basic_json/dump.md) falls back to `#!cpp std::snprintf` with the `%g` and `%Lg` conversion
|
||||
specifiers, for which the library likewise provides only `#!cpp double` and `#!cpp long double` overloads
|
||||
(`#!cpp float` is promoted to `#!cpp double`).
|
||||
@@ -669,7 +672,7 @@ such a container to a `basic_json` value.
|
||||
--8<-- "examples/custom_binary_type.hpp"
|
||||
```
|
||||
|
||||
??? example "Compiling and using it"
|
||||
??? example "Example: use the custom `BinaryType`"
|
||||
|
||||
```cpp
|
||||
--8<-- "examples/custom_binary_type.cpp"
|
||||
|
||||
@@ -1,4 +1,4 @@
|
||||
<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/types/template_parameters/" rel="canonical"><link href=../number_handling/ rel=prev><link href=../../../integration/ rel=next><link rel=icon href=../../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>Template Parameter Requirements - JSON for Modern C++</title><link rel=stylesheet href=../../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../../assets/stylesheets/palette.ab4e12ef.min.css><link rel="stylesheet" href="../../../assets/external/fonts.googleapis.com/css.61a430c9.css"><style>:root{--md-text-font:"Roboto";--md-code-font:"JetBrains Mono"}</style><link rel=stylesheet href=../../../css/custom.css><script>__md_scope=new 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data-md-component=logo> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 8a3 3 0 0 0 3-3 3 3 0 0 0-3-3 3 3 0 0 0-3 3 3 3 0 0 0 3 3m0 3.54C9.64 9.35 6.5 8 3 8v11c3.5 0 6.64 1.35 9 3.54 2.36-2.19 5.5-3.54 9-3.54V8c-3.5 0-6.64 1.35-9 3.54"/></svg> </a> <label class="md-header__button md-icon" for=__drawer> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M3 6h18v2H3zm0 5h18v2H3zm0 5h18v2H3z"/></svg> </label> <div class=md-header__title data-md-component=header-title> <div class=md-header__ellipsis> <div class=md-header__topic> <span class=md-ellipsis> JSON for Modern C++ </span> </div> <div class=md-header__topic data-md-component=header-topic> <span class=md-ellipsis> Template Parameter Requirements </span> </div> </div> </div> <form class=md-header__option data-md-component=palette> <input class=md-option data-md-color-media="(prefers-color-scheme: light)" data-md-color-scheme=default data-md-color-primary=indigo data-md-color-accent=indigo 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6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> </form> <script>var palette=__md_get("__palette");if(palette&&palette.color){if("(prefers-color-scheme)"===palette.color.media){var media=matchMedia("(prefers-color-scheme: light)"),input=document.querySelector(media.matches?"[data-md-color-media='(prefers-color-scheme: light)']":"[data-md-color-media='(prefers-color-scheme: dark)']");palette.color.media=input.getAttribute("data-md-color-media"),palette.color.scheme=input.getAttribute("data-md-color-scheme"),palette.color.primary=input.getAttribute("data-md-color-primary"),palette.color.accent=input.getAttribute("data-md-color-accent")}for(var[key,value]of Object.entries(palette.color))document.body.setAttribute("data-md-color-"+key,value)}</script> <label class="md-header__button md-icon" for=__search> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M9.5 3A6.5 6.5 0 0 1 16 9.5c0 1.61-.59 3.09-1.56 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<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/types/template_parameters/" rel="canonical"><link href=../number_handling/ rel=prev><link href=../../../integration/ rel=next><link rel=icon href=../../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>Template Parameter Requirements - JSON for Modern C++</title><link rel=stylesheet href=../../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../../assets/stylesheets/palette.ab4e12ef.min.css><link rel="stylesheet" href="../../../assets/external/fonts.googleapis.com/css.61a430c9.css"><style>:root{--md-text-font:"Roboto";--md-code-font:"JetBrains Mono"}</style><link rel=stylesheet href=../../../css/custom.css><script>__md_scope=new 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data-md-component=logo> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 8a3 3 0 0 0 3-3 3 3 0 0 0-3-3 3 3 0 0 0-3 3 3 3 0 0 0 3 3m0 3.54C9.64 9.35 6.5 8 3 8v11c3.5 0 6.64 1.35 9 3.54 2.36-2.19 5.5-3.54 9-3.54V8c-3.5 0-6.64 1.35-9 3.54"/></svg> </a> <label class="md-header__button md-icon" for=__drawer> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M3 6h18v2H3zm0 5h18v2H3zm0 5h18v2H3z"/></svg> </label> <div class=md-header__title data-md-component=header-title> <div class=md-header__ellipsis> <div class=md-header__topic> <span class=md-ellipsis> JSON for Modern C++ </span> </div> <div class=md-header__topic data-md-component=header-topic> <span class=md-ellipsis> Template Parameter Requirements </span> </div> </div> </div> <form class=md-header__option data-md-component=palette> <input class=md-option data-md-color-media="(prefers-color-scheme: light)" data-md-color-scheme=default data-md-color-primary=indigo data-md-color-accent=indigo 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6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> </form> <script>var palette=__md_get("__palette");if(palette&&palette.color){if("(prefers-color-scheme)"===palette.color.media){var media=matchMedia("(prefers-color-scheme: light)"),input=document.querySelector(media.matches?"[data-md-color-media='(prefers-color-scheme: light)']":"[data-md-color-media='(prefers-color-scheme: dark)']");palette.color.media=input.getAttribute("data-md-color-media"),palette.color.scheme=input.getAttribute("data-md-color-scheme"),palette.color.primary=input.getAttribute("data-md-color-primary"),palette.color.accent=input.getAttribute("data-md-color-accent")}for(var[key,value]of Object.entries(palette.color))document.body.setAttribute("data-md-color-"+key,value)}</script> <label class="md-header__button md-icon" for=__search> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M9.5 3A6.5 6.5 0 0 1 16 9.5c0 1.61-.59 3.09-1.56 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|
||||
<span class=w> </span><span class=n>basic_json</span><span class=p>,</span><span class=w> </span><span class=c1>// mapped_type</span>
|
||||
<span class=w> </span><span class=n>default_object_comparator_t</span><span class=p>,</span><span class=w> </span><span class=c1>// key_compare</span>
|
||||
<span class=w> </span><span class=n>AllocatorType</span><span class=o><</span><span class=n>std</span><span class=o>::</span><span class=n>pair</span><span class=o><</span><span class=k>const</span><span class=w> </span><span class=n>StringType</span><span class=p>,</span>
|
||||
@@ -165,7 +165,7 @@
|
||||
<span class=w> </span><span class=k>return</span><span class=w> </span><span class=n>lhs</span><span class=p>.</span><span class=n>data_</span><span class=w> </span><span class=o><</span><span class=w> </span><span class=n>rhs</span><span class=p>.</span><span class=n>data_</span><span class=p>;</span>
|
||||
<span class=w> </span><span class=p>}</span>
|
||||
<span class=p>};</span>
|
||||
</code></pre></div> </div> <details class=example> <summary>Compiling and using it</summary> <div class=highlight><pre><span></span><code><span class=cp>#include</span><span class=w> </span><span class=cpf><iostream></span>
|
||||
</code></pre></div> </div> <details class=example> <summary>Example: use the custom <code>ObjectType</code></summary> <div class=highlight><pre><span></span><code><span class=cp>#include</span><span class=w> </span><span class=cpf><iostream></span>
|
||||
<span class=cp>#include</span><span class=w> </span><span class=cpf><type_traits></span>
|
||||
<span class=cp>#include</span><span class=w> </span><span class=cpf><vector></span>
|
||||
|
||||
@@ -355,7 +355,7 @@
|
||||
<span class=w> </span><span class=k>return</span><span class=w> </span><span class=n>lhs</span><span class=p>.</span><span class=n>data_</span><span class=w> </span><span class=o><</span><span class=w> </span><span class=n>rhs</span><span class=p>.</span><span class=n>data_</span><span class=p>;</span>
|
||||
<span class=w> </span><span class=p>}</span>
|
||||
<span class=p>};</span>
|
||||
</code></pre></div> </div> <details class=example> <summary>Compiling and using it</summary> <div class=highlight><pre><span></span><code><span class=cp>#include</span><span class=w> </span><span class=cpf><iostream></span>
|
||||
</code></pre></div> </div> <details class=example> <summary>Example: use the custom <code>ArrayType</code></summary> <div class=highlight><pre><span></span><code><span class=cp>#include</span><span class=w> </span><span class=cpf><iostream></span>
|
||||
<span class=cp>#include</span><span class=w> </span><span class=cpf><map></span>
|
||||
|
||||
<span class=cp>#include</span><span class=w> </span><span class=cpf><nlohmann/json.hpp></span>
|
||||
@@ -376,8 +376,9 @@
|
||||
<span class=p>}</span>
|
||||
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code><span class=p>[</span><span class=mi>1</span><span class=p>,</span><span class=mi>2</span><span class=p>,</span><span class=mi>3</span><span class=p>]</span>
|
||||
<span class=kc>true</span>
|
||||
</code></pre></div> </details> <h3 id=compatible-containers_1>Compatible containers<a class=headerlink href=#compatible-containers_1 title="Permanent link">¶</a></h3> <table> <thead> <tr> <th>Container</th> <th>Notes</th> </tr> </thead> <tbody> <tr> <td><code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>vector</span></code> (default)</td> <td></td> </tr> <tr> <td><code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>deque</span></code></td> <td>references survive appends, but not insertions elsewhere; see the <code>capacity()</code> note above</td> </tr> <tr> <td><code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>pmr</span><span class=o>::</span><span class=n>vector</span></code></td> <td>through an alias, as the allocator comes from <code>AllocatorType</code> instead</td> </tr> <tr> <td><code>boost::container::vector</code>, <code>deque</code>, <code>devector</code></td> <td></td> </tr> <tr> <td><code>boost::container::stable_vector</code></td> <td>the only one tried that keeps references valid across <em>every</em> insertion</td> </tr> <tr> <td><code>boost::container::small_vector</code>, <code>folly::small_vector</code></td> <td>through an alias that fixes the inline capacity</td> </tr> <tr> <td><code>boost::container::static_vector</code></td> <td>through the same kind of alias, for arrays that stay within the fixed capacity</td> </tr> <tr> <td><code>folly::fbvector</code></td> <td>requires C++20, see the note above</td> </tr> </tbody> </table> <h3 id=containers-that-cannot-be-used_1>Containers that cannot be used<a class=headerlink href=#containers-that-cannot-be-used_1 title="Permanent link">¶</a></h3> <table> <thead> <tr> <th>Container</th> <th>Reason</th> </tr> </thead> <tbody> <tr> <td><code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>list</span></code></td> <td>no <code>operator[]</code>, and no random-access iterators</td> </tr> <tr> <td><code>eastl::vector</code>, <code>QList</code>, <code>QVector</code></td> <td>no <code>max_size()</code>; they handle the incomplete value type fine</td> </tr> <tr> <td><code>absl::InlinedVector</code></td> <td>requires a complete value type, see the note above</td> </tr> <tr> <td><code>absl::FixedArray</code></td> <td>the size is fixed at construction, so <code>resize</code>, <code>push_back</code>, <code>insert</code> and <code>erase</code> are missing</td> </tr> </tbody> </table> <h2 id=stringtype><code>StringType</code><a class=headerlink href=#stringtype title="Permanent link">¶</a></h2> <p><code>StringType</code> is used <strong>both</strong> for <abbr title="JavaScript Object Notation">JSON</abbr> string values and for the keys of <abbr title="JavaScript Object Notation">JSON</abbr> objects (<code>string_t</code> and <code>object_t::key_type</code>).</p> <h3 id=always-required_2>Always required<a class=headerlink href=#always-required_2 title="Permanent link">¶</a></h3> <ul> <li>A member type <code>value_type</code> that is one byte wide and <code>char</code>-compatible. The library stores and processes <abbr title="Unicode Transformation Format">UTF</abbr>-8 encoded <code>char</code> data and hands <code>data()</code> to <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>strtoull</span></code>/<code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>strtoll</span></code>. <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>wstring</span></code>, <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>u16string</span></code>, and <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>u32string</span></code> are <strong>not</strong> valid choices; see the <abbr title="Frequently Asked Questions">FAQ</abbr> on <a href=../../../home/faq/#wide-string-handling>wide string handling</a>.</li> <li>Constructors: default, copy, move, from <code class=highlight><span class=k>const</span><span class=w> </span><span class=kt>char</span><span class=o>*</span></code> (which must not be <code class=highlight><span class=k>explicit</span></code>), from <code class=highlight><span class=p>(</span><span class=k>const</span><span class=w> </span><span class=kt>char</span><span class=o>*</span><span class=p>,</span><span class=w> </span><span class=n>size_type</span><span class=p>)</span></code>, and from <code class=highlight><span class=p>(</span><span class=n>size_type</span><span class=p>,</span><span class=w> </span><span class=kt>char</span><span class=p>)</span></code>; and copy or move assignment.</li> <li>Member functions <code>size()</code>, <code>clear()</code>, <code>resize(n, c)</code>, <code>data()</code>, <code>push_back(char)</code>, and <code>operator[]</code> (const and non-const, returning references). <code>c_str()</code> and <code>back()</code> are <strong>not</strong> rLine truncated
|
||||
</code></pre></div> </details> <h3 id=compatible-containers_1>Compatible containers<a class=headerlink href=#compatible-containers_1 title="Permanent link">¶</a></h3> <table> <thead> <tr> <th>Container</th> <th>Notes</th> </tr> </thead> <tbody> <tr> <td><code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>vector</span></code> (default)</td> <td></td> </tr> <tr> <td><code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>deque</span></code></td> <td>references survive appends, but not insertions elsewhere; see the <code>capacity()</code> note above</td> </tr> <tr> <td><code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>pmr</span><span class=o>::</span><span class=n>vector</span></code></td> <td>through an alias, as the allocator comes from <code>AllocatorType</code> instead</td> </tr> <tr> <td><code>boost::container::vector</code>, <code>deque</code>, <code>devector</code></td> <td></td> </tr> <tr> <td><code>boost::container::stable_vector</code></td> <td>the only one tried that keeps references valid across <em>every</em> insertion</td> </tr> <tr> <td><code>boost::container::small_vector</code>, <code>folly::small_vector</code></td> <td>through an alias that fixes the inline capacity</td> </tr> <tr> <td><code>boost::container::static_vector</code></td> <td>through the same kind of alias, for arrays that stay within the fixed capacity</td> </tr> <tr> <td><code>folly::fbvector</code></td> <td>requires C++20, see the note above</td> </tr> </tbody> </table> <h3 id=containers-that-cannot-be-used_1>Containers that cannot be used<a class=headerlink href=#containers-that-cannot-be-used_1 title="Permanent link">¶</a></h3> <table> <thead> <tr> <th>Container</th> <th>Reason</th> </tr> </thead> <tbody> <tr> <td><code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>list</span></code></td> <td>no <code>operator[]</code>, and no random-access iterators</td> </tr> <tr> <td><code>eastl::vector</code>, <code>QList</code>, <code>QVector</code></td> <td>no <code>max_size()</code>; they handle the incomplete value type fine</td> </tr> <tr> <td><code>absl::InlinedVector</code></td> <td>requires a complete value type, see the note above</td> </tr> <tr> <td><code>absl::FixedArray</code></td> <td>the size is fixed at construction, so <code>resize</code>, <code>push_back</code>, <code>insert</code> and <code>erase</code> are missing</td> </tr> </tbody> </table> <h2 id=stringtype><code>StringType</code><a class=headerlink href=#stringtype title="Permanent link">¶</a></h2> <p><code>StringType</code> is used <strong>both</strong> for <abbr title="JavaScript Object Notation">JSON</abbr> string values and for the keys of <abbr title="JavaScript Object Notation">JSON</abbr> objects (<code>string_t</code> and <code>object_t::key_type</code>).</p> <h3 id=always-required_2>Always required<a class=headerlink href=#always-required_2 title="Permanent link">¶</a></h3> <ul> <li>A member type <code>value_type</code> that is one byte wide and <code>char</code>-compatible. The library stores and processes <abbr title="Unicode Transformation Format">UTF</abbr>-8 encoded <code>char</code> data and passes <code>data()</code> to functions that take a <code class=highlight><span class=k>const</span><span class=w> </span><span class=kt>char</span><span class=o>*</span></code>, such as <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>strtod</span></code>. <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>wstring</span></code>, <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>u16string</span></code>, and <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>u32string</span></code> are <strong>not</strong> valid choices; see the <abbr title="Frequently Asked Questions">FAQ</abbr> on <a href=../../../home/faq/#wide-string-handling>wide string handling</a>.</li> <li>Constructors: default, copy, move, from <code class=highlight><span class=k>const</span><span class=w> </span><span class=kt>char</span><span class=o>*</span></code> (which must not be <code class=highlight><span class=k>explicit</span></code>), from <code class=highlight><span class=p>(</span><span class=k>const</span><span class=w> </span><span class=kt>char</span><span class=o>*</span><span class=p>,</span><span class=w> </span><span class=n>size_type</span><span class=p>)</span></code>, and from <code class=highlight><span class=p>(</span><span class=n>size_type</span><span class=p>,</span><span class=w> </span><span class=kt>char</span><span class=p>)</span></code>; and copy or move assignment.</li> <li>Member functions <code>size()</code>, <code>clear()</code>, <code>resize(n, c)</code>, <code>data()</code>, <code>push_back(char)</code>, and <code>operator[]</code> (const and non-const, returning references). <code>c_str()</code>Line truncated
|
||||
|
||||
<span class=cp>#include</span><span class=w> </span><span class=cpf><cstddef></span>
|
||||
<span class=cp>#include</span><span class=w> </span><span class=cpf><ostream></span>
|
||||
<span class=cp>#include</span><span class=w> </span><span class=cpf><string></span>
|
||||
|
||||
@@ -386,10 +387,10 @@
|
||||
<span class=c1>// and nothing more of std::string's interface.</span>
|
||||
<span class=c1>//</span>
|
||||
<span class=c1>// Covers the "Always required" members, the extras needed for the binary</span>
|
||||
<span class=c1>// formats, and the extras needed for JSON Pointer / flatten / unflatten /</span>
|
||||
<span class=c1>// diff. Extending it further (e.g. for std::hash<basic_json> or to_bson) is</span>
|
||||
<span class=c1>// a matter of adding the extra members listed in the "Required for other</span>
|
||||
<span class=c1>// functionality" table.</span>
|
||||
<span class=c1>// formats, JSON Pointer / flatten / unflatten, and the int_to_string overload</span>
|
||||
<span class=c1>// needed for diff and items. Extending it further (e.g. for</span>
|
||||
<span class=c1>// std::hash<basic_json> or to_bson) is a matter of adding the extra members</span>
|
||||
<span class=c1>// listed in the "Required for other functionality" table.</span>
|
||||
<span class=c1>//</span>
|
||||
<span class=c1>// See https://json.nlohmann.me/features/types/template_parameters/#stringtype</span>
|
||||
<span class=k>class</span><span class=w> </span><span class=nc>custom_string_type</span>
|
||||
@@ -471,6 +472,11 @@
|
||||
<span class=w> </span><span class=n>data_</span><span class=p>.</span><span class=n>append</span><span class=p>(</span><span class=n>other</span><span class=p>.</span><span class=n>data_</span><span class=p>);</span>
|
||||
<span class=w> </span><span class=k>return</span><span class=w> </span><span class=o>*</span><span class=k>this</span><span class=p>;</span>
|
||||
<span class=w> </span><span class=p>}</span>
|
||||
<span class=w> </span><span class=n>custom_string_type</span><span class=o>&</span><span class=w> </span><span class=k>operator</span><span class=o>+=</span><span class=p>(</span><span class=kt>char</span><span class=w> </span><span class=n>c</span><span class=p>)</span>
|
||||
<span class=w> </span><span class=p>{</span>
|
||||
<span class=w> </span><span class=n>data_</span><span class=p>.</span><span class=n>push_back</span><span class=p>(</span><span class=n>c</span><span class=p>);</span>
|
||||
<span class=w> </span><span class=k>return</span><span class=w> </span><span class=o>*</span><span class=k>this</span><span class=p>;</span>
|
||||
<span class=w> </span><span class=p>}</span>
|
||||
|
||||
<span class=w> </span><span class=n>size_type</span><span class=w> </span><span class=n>find_first_of</span><span class=p>(</span><span class=kt>char</span><span class=w> </span><span class=n>c</span><span class=p>,</span><span class=w> </span><span class=n>size_type</span><span class=w> </span><span class=n>pos</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=mi>0</span><span class=p>)</span><span class=w> </span><span class=k>const</span>
|
||||
<span class=w> </span><span class=p>{</span>
|
||||
@@ -494,6 +500,12 @@
|
||||
<span class=w> </span><span class=k>return</span><span class=w> </span><span class=n>data_</span><span class=p>.</span><span class=n>end</span><span class=p>();</span>
|
||||
<span class=w> </span><span class=p>}</span>
|
||||
|
||||
<span class=w> </span><span class=c1>// found by ADL; converts array indices to keys in diff and items</span>
|
||||
<span class=w> </span><span class=k>friend</span><span class=w> </span><span class=kt>void</span><span class=w> </span><span class=n>int_to_string</span><span class=p>(</span><span class=n>custom_string_type</span><span class=o>&</span><span class=w> </span><span class=n>target</span><span class=p>,</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=kt>size_t</span><span class=w> </span><span class=n>value</span><span class=p>)</span>
|
||||
<span class=w> </span><span class=p>{</span>
|
||||
<span class=w> </span><span class=n>target</span><span class=p>.</span><span class=n>data_</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>to_string</span><span class=p>(</span><span class=n>value</span><span class=p>);</span>
|
||||
<span class=w> </span><span class=p>}</span>
|
||||
|
||||
<span class=w> </span><span class=k>friend</span><span class=w> </span><span class=kt>bool</span><span class=w> </span><span class=k>operator</span><span class=o>==</span><span class=p>(</span><span class=k>const</span><span class=w> </span><span class=n>custom_string_type</span><span class=o>&</span><span class=w> </span><span class=n>lhs</span><span class=p>,</span><span class=w> </span><span class=k>const</span><span class=w> </span><span class=n>custom_string_type</span><span class=o>&</span><span class=w> </span><span class=n>rhs</span><span class=p>)</span>
|
||||
<span class=w> </span><span class=p>{</span>
|
||||
<span class=w> </span><span class=k>return</span><span class=w> </span><span class=n>lhs</span><span class=p>.</span><span class=n>data_</span><span class=w> </span><span class=o>==</span><span class=w> </span><span class=n>rhs</span><span class=p>.</span><span class=n>data_</span><span class=p>;</span>
|
||||
@@ -510,7 +522,7 @@
|
||||
<span class=w> </span><span class=k>return</span><span class=w> </span><span class=n>os</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=n>s</span><span class=p>.</span><span class=n>data_</span><span class=p>;</span>
|
||||
<span class=w> </span><span class=p>}</span>
|
||||
<span class=p>};</span>
|
||||
</code></pre></div> </div> <details class=example> <summary>Compiling and using it</summary> <div class=highlight><pre><span></span><code><span class=cp>#include</span><span class=w> </span><span class=cpf><iostream></span>
|
||||
</code></pre></div> </div> <details class=example> <summary>Example: use the custom <code>StringType</code></summary> <div class=highlight><pre><span></span><code><span class=cp>#include</span><span class=w> </span><span class=cpf><iostream></span>
|
||||
<span class=cp>#include</span><span class=w> </span><span class=cpf><map></span>
|
||||
<span class=cp>#include</span><span class=w> </span><span class=cpf><vector></span>
|
||||
|
||||
@@ -540,7 +552,7 @@
|
||||
<span class=w> </span><span class=nt>"pi"</span><span class=p>:</span><span class=w> </span><span class=mf>3.141</span>
|
||||
<span class=p>}</span>
|
||||
<span class=kc>true</span>
|
||||
</code></pre></div> </details> <h2 id=booleantype><code>BooleanType</code><a class=headerlink href=#booleantype title="Permanent link">¶</a></h2> <p><code>boolean_t</code> is stored <strong>directly</strong> inside <code>basic_json</code>, as a member of an anonymous union.</p> <h3 id=always-required_3>Always required<a class=headerlink href=#always-required_3 title="Permanent link">¶</a></h3> <ul> <li>A literal type that is trivially default-constructible, trivially copyable, and trivially destructible; otherwise the union's special member functions are deleted.</li> <li><strong>Implicitly</strong> convertible from <code class=highlight><span class=kt>bool</span></code> -- an <code class=highlight><span class=k>explicit</span></code> constructor is not enough, because the <code>to_json</code> overload for a custom <code>BooleanType</code> is constrained on <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>is_convertible</span></code> -- and contextually convertible to <code class=highlight><span class=kt>bool</span></code> (here an <code class=highlight><span class=k>explicit</span><span class=w> </span><span class=k>operator</span><span class=w> </span><span class=kt>bool</span></code> is fine).</li> <li>Comparison operators <code>==</code>, <code>!=</code>, <code><</code>, <code><=</code>, <code>></code>, <code>>=</code> (or <code><=></code>).</li> <li>Convertible from and to <code class=highlight><span class=kt>bool</span></code> through the serializer, because <a href=../../../api/basic_json/get/ ><code>get<bool>()</code></a> is used internally.</li> </ul> <p>There is little reason to use anything other than <code class=highlight><span class=kt>bool</span></code> here.</p> <h3 id=compatible-types_1>Compatible types<a class=headerlink href=#compatible-types_1 title="Permanent link">¶</a></h3> <p><code class=highlight><span class=kt>bool</span></code> is the only usable choice. Another trivially copyable type that is implicitly convertible to and from <code class=highlight><span class=kt>bool</span></code> -- <code class=highlight><span class=n>std</span><span class=o>::</span><span class=kt>uint8_t</span></code>, say -- does compile, and <abbr title="JavaScript Object Notation">JSON</abbr> booleans still round-trip, but the type then serves as both <code>boolean_t</code> and an ordinary integer: <code>basic_json</code> can no longer be constructed or assigned from a <code class=highlight><span class=n>std</span><span class=o>::</span><span class=kt>uint8_t</span></code> at all (the boolean and unsigned-integer <code>to_json</code> overloads become ambiguous), and <a href=../../../api/basic_json/get/ ><code>get<std::uint8_t>()</code></a> on a number throws <a href=../../../home/exceptions/#jsonexceptiontype_error302><code>type_error.302</code></a> instead of returning the value.</p> <h2 id=numberintegertype-and-numberunsignedtype><code>NumberIntegerType</code> and <code>NumberUnsignedType</code><a class=headerlink href=#numberintegertype-and-numberunsignedtype title="Permanent link">¶</a></h2> <p>Both types are stored <strong>directly</strong> inside <code>basic_json</code>'s union.</p> <h3 id=always-required_4>Always required<a class=headerlink href=#always-required_4 title="Permanent link">¶</a></h3> <ul> <li><code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>is_integral</span></code> must be satisfied: <code>NumberIntegerType</code> must be a <strong>signed</strong> integer type, <code>NumberUnsignedType</code> an <strong>unsigned</strong> integer type. Class types are not supported -- among others, the constructors taking integer values are constrained on <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>is_integral</span></code>.</li> <li>Trivially default-constructible, trivially copyable, and trivially destructible (union member).</li> <li><code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>numeric_limits</span></code> must be specialized for both types.</li> <li><code>NumberUnsignedType</code> must be able to represent the absolute value of every <code>NumberIntegerType</code> value; serialization of negative numbers converts the value to <code>NumberUnsignedType</code>. A <code class=highlight><span class=k>static_assert</span></code> requires it to be at least as wide as <code>NumberIntegerType</code>, which is what that amounts to for the standard integer types.</li> <li>Both types must fit into the internal 64-character number buffer used by <a href=../../../api/basic_json/dump/ ><code>dump</code></a>, which is the case for all standard integer types.</li> <li><a href=../../../api/basic_json/std_hash/ ><code>std::hash<basic_json></code></a> additionally requires <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>hash</span></code> specializations.</li> </ul> <h3 id=notes_1>Line truncated
|
||||
</code></pre></div> </details> <h2 id=booleantype><code>BooleanType</code><a class=headerlink href=#booleantype title="Permanent link">¶</a></h2> <p><code>boolean_t</code> is stored <strong>directly</strong> inside <code>basic_json</code>, as a member of an anonymous union.</p> <h3 id=always-required_3>Always required<a class=headerlink href=#always-required_3 title="Permanent link">¶</a></h3> <ul> <li>A literal type that is trivially default-constructible, trivially copyable, and trivially destructible; otherwise the union's special member functions are deleted.</li> <li><strong>Implicitly</strong> convertible from <code class=highlight><span class=kt>bool</span></code> -- an <code class=highlight><span class=k>explicit</span></code> constructor is not enough, because the <code>to_json</code> overload for a custom <code>BooleanType</code> is constrained on <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>is_convertible</span></code> -- and contextually convertible to <code class=highlight><span class=kt>bool</span></code> (here an <code class=highlight><span class=k>explicit</span><span class=w> </span><span class=k>operator</span><span class=w> </span><span class=kt>bool</span></code> is fine).</li> <li>Comparison operators <code>==</code>, <code>!=</code>, <code><</code>, <code><=</code>, <code>></code>, <code>>=</code> (or <code><=></code>).</li> <li>Convertible from and to <code class=highlight><span class=kt>bool</span></code> through the serializer, because <a href=../../../api/basic_json/get/ ><code>get<bool>()</code></a> is used internally.</li> </ul> <p>There is little reason to use anything other than <code class=highlight><span class=kt>bool</span></code> here.</p> <h3 id=compatible-types_1>Compatible types<a class=headerlink href=#compatible-types_1 title="Permanent link">¶</a></h3> <p><code class=highlight><span class=kt>bool</span></code> is the only usable choice. Another trivially copyable type that is implicitly convertible to and from <code class=highlight><span class=kt>bool</span></code> -- <code class=highlight><span class=n>std</span><span class=o>::</span><span class=kt>uint8_t</span></code>, say -- does compile, and <abbr title="JavaScript Object Notation">JSON</abbr> booleans still round-trip, but the type then serves as both <code>boolean_t</code> and an ordinary integer: <code>basic_json</code> can no longer be constructed or assigned from a <code class=highlight><span class=n>std</span><span class=o>::</span><span class=kt>uint8_t</span></code> at all (the boolean and unsigned-integer <code>to_json</code> overloads become ambiguous), and <a href=../../../api/basic_json/get/ ><code>get<std::uint8_t>()</code></a> on a number throws <a href=../../../home/exceptions/#jsonexceptiontype_error302><code>type_error.302</code></a> instead of returning the value.</p> <h2 id=numberintegertype-and-numberunsignedtype><code>NumberIntegerType</code> and <code>NumberUnsignedType</code><a class=headerlink href=#numberintegertype-and-numberunsignedtype title="Permanent link">¶</a></h2> <p>Both types are stored <strong>directly</strong> inside <code>basic_json</code>'s union.</p> <h3 id=always-required_4>Always required<a class=headerlink href=#always-required_4 title="Permanent link">¶</a></h3> <ul> <li><code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>is_integral</span></code> must be satisfied: <code>NumberIntegerType</code> must be a <strong>signed</strong> integer type, <code>NumberUnsignedType</code> an <strong>unsigned</strong> integer type. Class types are not supported -- among others, the constructors taking integer values are constrained on <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>is_integral</span></code>.</li> <li>Trivially default-constructible, trivially copyable, and trivially destructible (union member).</li> <li><code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>numeric_limits</span></code> must be specialized for both types.</li> <li><code>NumberUnsignedType</code> must be able to represent the absolute value of every <code>NumberIntegerType</code> value; serialization of negative numbers converts the value to <code>NumberUnsignedType</code>. A <code class=highlight><span class=k>static_assert</span></code> requires it to be at least as wide as <code>NumberIntegerType</code>, which is what that amounts to for the standard integer types.</li> <li>Both types must fit into the internal 64-character number buffer used by <a href=../../../api/basic_json/dump/ ><code>dump</code></a>, which is the case for all standard integer types.</li> <li><a href=../../../api/basic_json/std_hash/ ><code>std::hash<basic_json></code></a> additionally requires <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>hash</span></code> specializations.</li> </ul> <h3 id=notes_1>Line truncated
|
||||
</code></pre></div> <h3 id=always-required_8>Always required<a class=headerlink href=#always-required_8 title="Permanent link">¶</a></h3> <ul> <li>A non-<code>final</code> class type -- <a href=../../../api/byte_container_with_subtype/ ><code>byte_container_with_subtype</code></a> derives from it publicly.</li> <li>A member type <code>value_type</code> that is <strong>exactly one byte</strong> wide (e.g., <code class=highlight><span class=n>std</span><span class=o>::</span><span class=kt>uint8_t</span></code>, <code class=highlight><span class=kt>char</span></code>, or <code class=highlight><span class=n>std</span><span class=o>::</span><span class=kt>byte</span></code>). Readers and writers reinterpret the container's storage as raw bytes, so a wider <code>value_type</code> is rejected with a <code class=highlight><span class=k>static_assert</span></code>.</li> <li>Contiguous storage: the binary readers <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>memcpy</span></code> into <code class=highlight><span class=o>&</span><span class=n>binary</span><span class=p>[</span><span class=n>n</span><span class=p>]</span></code>, the writers <code>reinterpret_cast</code> <code>data()</code>. <code class=highlight><span class=n>data</span><span class=p>()</span><span class=w> </span><span class=o>+</span><span class=w> </span><span class=n>n</span></code> would do for the readers too, but they share one helper with <a href=#stringtype><code>StringType</code></a>, whose non-<code class=highlight><span class=k>const</span></code> <code>data()</code> is C++17 and later only.</li> <li>Default-constructible, copy-constructible, and move-constructible.</li> <li>Member functions <code>size()</code>, <code>empty()</code>, <code>data()</code>, <code>resize()</code>, <code>operator[]</code>, <code>back()</code>, <code>begin()</code>, <code>end()</code>, <code>cbegin()</code>, and <code>cend()</code> with random-access iterators, and <code>insert(pos, first, last)</code>, which the <abbr title="Concise Binary Object Representation">CBOR</abbr> reader uses to join the chunks of an indefinite-length byte string. <code>push_back()</code> is <strong>not</strong> required.</li> <li>Comparison operators: <code>==</code> is used by <a href=../../../api/byte_container_with_subtype/ ><code>byte_container_with_subtype</code></a>, the relational operators by <a href=../../../api/basic_json/operator_le/ ><code>basic_json</code>'s comparison operators</a>.</li> </ul> <h3 id=required-for-individual-functions_1>Required for individual functions<a class=headerlink href=#required-for-individual-functions_1 title="Permanent link">¶</a></h3> <ul> <li><code>clear()</code>, for <a href=../../../api/basic_json/clear/ ><code>basic_json::clear()</code></a>.</li> </ul> <p><code>max_size()</code>, <code>at()</code>, <code>reserve()</code>, <code>erase()</code>, <code>pop_back()</code>, and <code>emplace_back()</code> are <strong>not</strong> used at all.</p> <p>See <a href=../../../api/basic_json/binary_t/ ><code>binary_t</code></a> for how a non-default <code>BinaryType</code> changes the meaning of assigning such a container to a <code>basic_json</code> value.</p> <div class="admonition tip"> <p class=admonition-title>Reference implementation</p> <p><code>docs/mkdocs/docs/examples/custom_binary_type.hpp</code> wraps a private <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>vector</span><span class=o><</span><span class=n>std</span><span class=o>::</span><span class=kt>uint8_t</span><span class=o>></span></code> and satisfies every requirement above -- a good starting point for a custom <code>BinaryType</code>.</p> <div class=highlight><pre><span></span><code><span class=cp>#pragma once</span>
|
||||
|
||||
<span class=cp>#include</span><span class=w> </span><span class=cpf><cstdint></span>
|
||||
@@ -653,7 +665,7 @@
|
||||
<span class=w> </span><span class=k>return</span><span class=w> </span><span class=n>lhs</span><span class=p>.</span><span class=n>data_</span><span class=w> </span><span class=o><</span><span class=w> </span><span class=n>rhs</span><span class=p>.</span><span class=n>data_</span><span class=p>;</span>
|
||||
<span class=w> </span><span class=p>}</span>
|
||||
<span class=p>};</span>
|
||||
</code></pre></div> </div> <details class=example> <summary>Compiling and using it</summary> <div class=highlight><pre><span></span><code><span class=cp>#include</span><span class=w> </span><span class=cpf><cstdint></span>
|
||||
</code></pre></div> </div> <details class=example> <summary>Example: use the custom <code>BinaryType</code></summary> <div class=highlight><pre><span></span><code><span class=cp>#include</span><span class=w> </span><span class=cpf><cstdint></span>
|
||||
<span class=cp>#include</span><span class=w> </span><span class=cpf><iostream></span>
|
||||
<span class=cp>#include</span><span class=w> </span><span class=cpf><map></span>
|
||||
<span class=cp>#include</span><span class=w> </span><span class=cpf><string></span>
|
||||
@@ -676,4 +688,4 @@
|
||||
<span class=p>}</span>
|
||||
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code><span class=p>{</span><span class=nt>"bytes"</span><span class=p>:[</span><span class=mi>1</span><span class=p>,</span><span class=mi>2</span><span class=p>,</span><span class=mi>3</span><span class=p>],</span><span class=nt>"subtype"</span><span class=p>:</span><span class=kc>null</span><span class=p>}</span>
|
||||
<span class=kc>true</span>
|
||||
</code></pre></div> </details> <h3 id=compatible-containers_2>Compatible containers<a class=headerlink href=#compatible-containers_2 title="Permanent link">¶</a></h3> <table> <thead> <tr> <th>Container</th> <th>Notes</th> </tr> </thead> <tbody> <tr> <td><code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>vector</span><span class=o><</span><span class=n>std</span><span class=o>::</span><span class=kt>uint8_t</span><span class=o>></span></code> (default)</td> <td></td> </tr> <tr> <td><code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>vector</span><span class=o><</span><span class=kt>char</span><span class=o>></span></code>, <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>vector</span><span class=o><</span><span class=n>std</span><span class=o>::</span><span class=kt>byte</span><span class=o>></span></code></td> <td><code>dump()</code> writes the bytes as 0..255 whichever is used</td> </tr> <tr> <td><code>boost::container::vector<std::uint8_t></code>, <code>boost::container::small_vector<std::uint8_t, N></code></td> <td></td> </tr> <tr> <td><code>absl::InlinedVector<std::uint8_t, N></code></td> <td>usable here, unlike as an <code>ArrayType</code>, because the value type is complete</td> </tr> <tr> <td><code>eastl::vector<std::uint8_t></code></td> <td>usable here, unlike as an <code>ArrayType</code>, because <code>max_size()</code> is not needed</td> </tr> <tr> <td><code>folly::fbvector<std::uint8_t></code></td> <td>requires C++20, see the note above</td> </tr> </tbody> </table> <h3 id=containers-that-cannot-be-used_2>Containers that cannot be used<a class=headerlink href=#containers-that-cannot-be-used_2 title="Permanent link">¶</a></h3> <table> <thead> <tr> <th>Container</th> <th>Reason</th> </tr> </thead> <tbody> <tr> <td><code>QByteArray</code></td> <td>no <code>empty()</code> (it spells that <code>isEmpty()</code>); its <code>insert</code> takes an index rather than an iterator; and it converts to <code>string_t</code>, which makes <code>to_json</code> ambiguous between a string and a binary value</td> </tr> <tr> <td><code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>string</span></code></td> <td><code>binary_t::container_type</code> and <code>string_t</code> would be the same type, so the two <a href=../../../api/basic_json/swap/ ><code>swap</code></a> overloads collide and <code>basic_json</code> cannot be instantiated at all</td> </tr> <tr> <td><code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>deque</span><span class=o><</span><span class=n>std</span><span class=o>::</span><span class=kt>uint8_t</span><span class=o>></span></code></td> <td>storage is not contiguous, so there is no <code>data()</code></td> </tr> <tr> <td>containers whose <code>value_type</code> is wider than one byte</td> <td>see above -- accepted by the compiler, wrong at runtime</td> </tr> </tbody> </table> <h2 id=custombaseclass><code>CustomBaseClass</code><a class=headerlink href=#custombaseclass title="Permanent link">¶</a></h2> <p><code>CustomBaseClass</code> is an extension point: unless it is <code class=highlight><span class=kt>void</span></code> (the default, which selects the empty <code>nlohmann::json_default_base</code>), <code>basic_json</code> publicly derives from it.</p> <h3 id=always-required_9>Always required<a class=headerlink href=#always-required_9 title="Permanent link">¶</a></h3> <ul> <li>A non-<code>final</code>, default-constructible class type.</li> <li><code>basic_json</code> is copy-/move-constructible and copy-/move-assignable only if <code>CustomBaseClass</code> is.</li> </ul> <h3 id=notes_2>Notes<a class=headerlink href=#notes_2 title="Permanent link">¶</a></h3> <p><code>basic_json</code> is documented to be a <a href="https://en.cppreference.com/w/cpp/named_req/StandardLayoutType">StandardLayoutType</a>. Because <code>basic_json</code> has non-static data members of its own, a <code>CustomBaseClass</code> with non-static data members forfeits this guarantee.</p> <p>Note the namespace of <code>CustomBaseClass</code> becomes an associated namespace of <code>basic_json</code> for the purpose of argument-dependent lookup.</p> <p>See <a href=../../../api/basic_json/json_base_class_t/ ><code>json_base_class_t</code></a> for an example.</p> <h3 id=compatible-types_6>Compatible types<a class=headerlink href=#compatible-types_6 title="Permanent link">¶</a></h3> <table> <thead> <tr> <th>Type</th> <th>Support</th> </tr> </thead> <tbody> <tr> <td><code class=highlight><span class=kt>void</span></code> (default)</td> <td>an empty base class is used; no effect on <code>basic_json</code></td> </tr> <tr> <td>any default-constructible, non-<code>final</code> class</td> <td>full; see <a href=../../../api/basic_json/json_base_class_t/ ><code>json_base_class_t</code></a></td> </tr>Line truncated
|
||||
</code></pre></div> </details> <h3 id=compatible-containers_2>Compatible containers<a class=headerlink href=#compatible-containers_2 title="Permanent link">¶</a></h3> <table> <thead> <tr> <th>Container</th> <th>Notes</th> </tr> </thead> <tbody> <tr> <td><code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>vector</span><span class=o><</span><span class=n>std</span><span class=o>::</span><span class=kt>uint8_t</span><span class=o>></span></code> (default)</td> <td></td> </tr> <tr> <td><code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>vector</span><span class=o><</span><span class=kt>char</span><span class=o>></span></code>, <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>vector</span><span class=o><</span><span class=n>std</span><span class=o>::</span><span class=kt>byte</span><span class=o>></span></code></td> <td><code>dump()</code> writes the bytes as 0..255 whichever is used</td> </tr> <tr> <td><code>boost::container::vector<std::uint8_t></code>, <code>boost::container::small_vector<std::uint8_t, N></code></td> <td></td> </tr> <tr> <td><code>absl::InlinedVector<std::uint8_t, N></code></td> <td>usable here, unlike as an <code>ArrayType</code>, because the value type is complete</td> </tr> <tr> <td><code>eastl::vector<std::uint8_t></code></td> <td>usable here, unlike as an <code>ArrayType</code>, because <code>max_size()</code> is not needed</td> </tr> <tr> <td><code>folly::fbvector<std::uint8_t></code></td> <td>requires C++20, see the note above</td> </tr> </tbody> </table> <h3 id=containers-that-cannot-be-used_2>Containers that cannot be used<a class=headerlink href=#containers-that-cannot-be-used_2 title="Permanent link">¶</a></h3> <table> <thead> <tr> <th>Container</th> <th>Reason</th> </tr> </thead> <tbody> <tr> <td><code>QByteArray</code></td> <td>no <code>empty()</code> (it spells that <code>isEmpty()</code>); its <code>insert</code> takes an index rather than an iterator; and it converts to <code>string_t</code>, which makes <code>to_json</code> ambiguous between a string and a binary value</td> </tr> <tr> <td><code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>string</span></code></td> <td><code>binary_t::container_type</code> and <code>string_t</code> would be the same type, so the two <a href=../../../api/basic_json/swap/ ><code>swap</code></a> overloads collide and <code>basic_json</code> cannot be instantiated at all</td> </tr> <tr> <td><code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>deque</span><span class=o><</span><span class=n>std</span><span class=o>::</span><span class=kt>uint8_t</span><span class=o>></span></code></td> <td>storage is not contiguous, so there is no <code>data()</code></td> </tr> <tr> <td>containers whose <code>value_type</code> is wider than one byte</td> <td>see above -- accepted by the compiler, wrong at runtime</td> </tr> </tbody> </table> <h2 id=custombaseclass><code>CustomBaseClass</code><a class=headerlink href=#custombaseclass title="Permanent link">¶</a></h2> <p><code>CustomBaseClass</code> is an extension point: unless it is <code class=highlight><span class=kt>void</span></code> (the default, which selects the empty <code>nlohmann::json_default_base</code>), <code>basic_json</code> publicly derives from it.</p> <h3 id=always-required_9>Always required<a class=headerlink href=#always-required_9 title="Permanent link">¶</a></h3> <ul> <li>A non-<code>final</code>, default-constructible class type.</li> <li><code>basic_json</code> is copy-/move-constructible and copy-/move-assignable only if <code>CustomBaseClass</code> is.</li> </ul> <h3 id=notes_2>Notes<a class=headerlink href=#notes_2 title="Permanent link">¶</a></h3> <p><code>basic_json</code> is documented to be a <a href="https://en.cppreference.com/w/cpp/named_req/StandardLayoutType">StandardLayoutType</a>. Because <code>basic_json</code> has non-static data members of its own, a <code>CustomBaseClass</code> with non-static data members forfeits this guarantee.</p> <p>Note the namespace of <code>CustomBaseClass</code> becomes an associated namespace of <code>basic_json</code> for the purpose of argument-dependent lookup.</p> <p>See <a href=../../../api/basic_json/json_base_class_t/ ><code>json_base_class_t</code></a> for an example.</p> <h3 id=compatible-types_6>Compatible types<a class=headerlink href=#compatible-types_6 title="Permanent link">¶</a></h3> <table> <thead> <tr> <th>Type</th> <th>Support</th> </tr> </thead> <tbody> <tr> <td><code class=highlight><span class=kt>void</span></code> (default)</td> <td>an empty base class is used; no effect on <code>basic_json</code></td> </tr> <tr> <td>any default-constructible, non-<code>final</code> class</td> <td>full; see <a href=../../../api/basic_json/json_base_class_t/ ><code>json_base_class_t</code></a></td> </tr>Line truncated
|
||||
@@ -13,7 +13,7 @@ Requirements are not checked
|
||||
|
||||
Three requirements are checked with a `static_assert`: the array iterator category, the width of [`BinaryType`](#binarytype)'s `value_type`, and [`NumberUnsignedType`](#numberintegertype-and-numberunsignedtype) being at least as wide as [`NumberIntegerType`](#numberintegertype-and-numberunsignedtype). The rest are not diagnosed with dedicated error messages, and violating most of them results in a compiler error somewhere inside the library. Four violations are not caught at compile time at all:
|
||||
|
||||
- A [`StringType`](#stringtype) whose `data()` is not null-terminated compiles and silently misparses numbers, because the lexer hands the buffer to `std::strtoull`/`std::strtoll`/`std::strtod`.
|
||||
- A [`StringType`](#stringtype) whose `data()` is not null-terminated compiles and can silently misparse floating-point numbers, because the lexer may hand the buffer to `std::strtod`, which reads up to the terminating null character.
|
||||
- A stateful [`AllocatorType`](#allocatortype) compiles and silently ignores its state: allocation, deallocation, and [`get_allocator()`](https://json.nlohmann.me/api/basic_json/get_allocator/index.md) each use a different default-constructed instance.
|
||||
- The two [cross-specialization conversions](#cross-specialization-conversions) below. These abort on an assertion in a normal build, and only fail silently under `NDEBUG`.
|
||||
|
||||
@@ -287,7 +287,7 @@ class custom_object_type
|
||||
};
|
||||
```
|
||||
|
||||
Compiling and using it
|
||||
Example: use the custom `ObjectType`
|
||||
|
||||
```
|
||||
#include <iostream>
|
||||
@@ -549,7 +549,7 @@ class custom_array_type
|
||||
};
|
||||
```
|
||||
|
||||
Compiling and using it
|
||||
Example: use the custom `ArrayType`
|
||||
|
||||
```
|
||||
#include <iostream>
|
||||
@@ -608,10 +608,10 @@ true
|
||||
|
||||
### Always required
|
||||
|
||||
- A member type `value_type` that is one byte wide and `char`-compatible. The library stores and processes UTF-8 encoded `char` data and hands `data()` to `std::strtoull`/`std::strtoll`. `std::wstring`, `std::u16string`, and `std::u32string` are **not** valid choices; see the FAQ on [wide string handling](https://json.nlohmann.me/home/faq/#wide-string-handling).
|
||||
- A member type `value_type` that is one byte wide and `char`-compatible. The library stores and processes UTF-8 encoded `char` data and passes `data()` to functions that take a `const char*`, such as `std::strtod`. `std::wstring`, `std::u16string`, and `std::u32string` are **not** valid choices; see the FAQ on [wide string handling](https://json.nlohmann.me/home/faq/#wide-string-handling).
|
||||
- Constructors: default, copy, move, from `const char*` (which must not be `explicit`), from `(const char*, size_type)`, and from `(size_type, char)`; and copy or move assignment.
|
||||
- Member functions `size()`, `clear()`, `resize(n, c)`, `data()`, `push_back(char)`, and `operator[]` (const and non-const, returning references). `c_str()` and `back()` are **not** required.
|
||||
- `data()` must return a pointer to a contiguous, **null-terminated** buffer -- the parser hands it to `std::strtoull`. A type whose `data()` is not null-terminated does not fail to compile; it silently misparses numbers.
|
||||
- `data()` must return a pointer to a contiguous, **null-terminated** buffer -- the parser may hand it to `std::strtod`, which reads up to the null character. A type whose `data()` is not null-terminated does not fail to compile; it can silently misparse floating-point numbers.
|
||||
- `append(const char*, size_type)`, used by [`dump`](https://json.nlohmann.me/api/basic_json/dump/index.md), and `append(const StringType&)`, used by the CBOR reader for indefinite-length strings. The library's internal string concatenation additionally has to append a `char` and a `const char*`; for each it selects between `append(arg)`, `operator+=`, `append(first, last)`, and `append(data, size)`.
|
||||
- The comparison operator `==` against another `StringType`, and `<` for use as a key of the chosen [`ObjectType`](#objecttype) (with the default comparator, `std::less<>` must be able to compare two `StringType` values, and a `StringType` with the key types used for lookup). `!=` is never applied to a `StringType`, and `==` against `const char*` is resolved by the implicit `const char*` constructor.
|
||||
|
||||
@@ -636,6 +636,7 @@ true
|
||||
| [`to_bson`](https://json.nlohmann.me/api/basic_json/to_bson/index.md) | `find(value_type)` and `npos` |
|
||||
| [`parse`](https://json.nlohmann.me/api/basic_json/parse/index.md) from a `string_t` | the input adapters must accept it; otherwise pass a character range |
|
||||
| `operator<<(std::ostream&, const json_pointer&)` | streamability to `std::ostream` |
|
||||
| [`to_string`](https://json.nlohmann.me/api/basic_json/to_string/index.md) | conversion of `StringType` to `std::string` (the function returns a `std::string`) |
|
||||
| exception messages | `data()` and `size()`, or `begin()` and `end()` |
|
||||
|
||||
### Compatible types
|
||||
@@ -677,6 +678,7 @@ Reference implementation
|
||||
```
|
||||
#pragma once
|
||||
|
||||
#include <cstddef>
|
||||
#include <ostream>
|
||||
#include <string>
|
||||
|
||||
@@ -685,10 +687,10 @@ Reference implementation
|
||||
// and nothing more of std::string's interface.
|
||||
//
|
||||
// Covers the "Always required" members, the extras needed for the binary
|
||||
// formats, and the extras needed for JSON Pointer / flatten / unflatten /
|
||||
// diff. Extending it further (e.g. for std::hash<basic_json> or to_bson) is
|
||||
// a matter of adding the extra members listed in the "Required for other
|
||||
// functionality" table.
|
||||
// formats, JSON Pointer / flatten / unflatten, and the int_to_string overload
|
||||
// needed for diff and items. Extending it further (e.g. for
|
||||
// std::hash<basic_json> or to_bson) is a matter of adding the extra members
|
||||
// listed in the "Required for other functionality" table.
|
||||
//
|
||||
// See https://json.nlohmann.me/features/types/template_parameters/#stringtype
|
||||
class custom_string_type
|
||||
@@ -770,6 +772,11 @@ class custom_string_type
|
||||
data_.append(other.data_);
|
||||
return *this;
|
||||
}
|
||||
custom_string_type& operator+=(char c)
|
||||
{
|
||||
data_.push_back(c);
|
||||
return *this;
|
||||
}
|
||||
|
||||
size_type find_first_of(char c, size_type pos = 0) const
|
||||
{
|
||||
@@ -793,6 +800,12 @@ class custom_string_type
|
||||
return data_.end();
|
||||
}
|
||||
|
||||
// found by ADL; converts array indices to keys in diff and items
|
||||
friend void int_to_string(custom_string_type& target, std::size_t value)
|
||||
{
|
||||
target.data_ = std::to_string(value);
|
||||
}
|
||||
|
||||
friend bool operator==(const custom_string_type& lhs, const custom_string_type& rhs)
|
||||
{
|
||||
return lhs.data_ == rhs.data_;
|
||||
@@ -811,7 +824,7 @@ class custom_string_type
|
||||
};
|
||||
```
|
||||
|
||||
Compiling and using it
|
||||
Example: use the custom `StringType`
|
||||
|
||||
```
|
||||
#include <iostream>
|
||||
@@ -909,7 +922,7 @@ The number types influence what the parser accepts: an integer literal that does
|
||||
|
||||
`NumberFloatType` must be one of `float`, `double`, or `long double`:
|
||||
|
||||
- The [parser](https://json.nlohmann.me/features/parsing/index.md) converts number literals with `std::strtof`, `std::strtod`, or `std::strtold`; the library provides overloads for exactly these three types.
|
||||
- The [parser](https://json.nlohmann.me/features/parsing/index.md) converts number literals with `std::from_chars` or, as a fallback, with `std::strtof`, `std::strtod`, or `std::strtold`; the library provides overloads for exactly these three types.
|
||||
- [`dump`](https://json.nlohmann.me/api/basic_json/dump/index.md) falls back to `std::snprintf` with the `%g` and `%Lg` conversion specifiers, for which the library likewise provides only `double` and `long double` overloads (`float` is promoted to `double`).
|
||||
|
||||
If `std::numeric_limits<NumberFloatType>` describes an IEEE 754 binary32 or binary64 number, `dump` uses the Grisu2 algorithm, which produces the shortest representation that round-trips. Otherwise the `snprintf` fallback with `max_digits10` digits is used.
|
||||
@@ -1112,7 +1125,7 @@ class custom_binary_type
|
||||
};
|
||||
```
|
||||
|
||||
Compiling and using it
|
||||
Example: use the custom `BinaryType`
|
||||
|
||||
```
|
||||
#include <cstdint>
|
||||
|
||||
Reference in new issue
Block a user