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<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=description content="JSON for Modern C++ is a header-only C++11 library that makes JSON a first-class data type."><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/arbitrary_types/" rel="canonical"><link href=../ rel=prev><link href=../type_recipes/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.8.0"><title>Arbitrary Type Conversions - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.57a0d79d.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.fb2fffe2.min.css><link rel=stylesheet href=../../css/custom.css><link rel=stylesheet href=../../css/brand.css><link rel=stylesheet href=../../css/fig-sax.css><link rel=stylesheet href=../../css/fig-jsonview.css><link rel=stylesheet 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data-md-component=announce> </div> <header class=md-header data-md-component=header> <nav class="md-header__inner md-grid" aria-label=Header> <a href=../.. title="JSON for Modern C++" class="md-header__button md-logo" aria-label="JSON for Modern C++" data-md-component=logo> <img src=../../images/wordmark-header.svg alt=logo> </a> <label class="md-header__button md-icon" for=__drawer> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M3 6h18v2H3zm0 5h18v2H3zm0 5h18v2H3z"/></svg> </label> <div class=md-header__title data-md-component=header-title> <div class=md-header__ellipsis> <div class=md-header__topic> <span class=md-ellipsis> JSON for Modern C++ </span> </div> <div class=md-header__topic data-md-component=header-topic> <span class=md-ellipsis> Arbitrary Type Conversions </span> </div> </div> </div> <form class=md-header__option data-md-component=palette> <input class=md-option data-md-color-media="(prefers-color-scheme: light)" data-md-color-scheme=default 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<span class=w> </span><span class=c1>// a simple struct to model a person</span>
<span class=w> </span><span class=k>struct</span><span class=w> </span><span class=nc>person</span><span class=w> </span><span class=p>{</span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>string</span><span class=w> </span><span class=n>name</span><span class=p>;</span>
@@ -357,4 +357,4 @@
<span class=w> </span><span class=n>value</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>j</span><span class=p>.</span><span class=k>template</span><span class=w> </span><span class=n>get</span><span class=o>&lt;</span><span class=n>T</span><span class=o>&gt;</span><span class=p>();</span><span class=w> </span><span class=c1>// oops!</span>
<span class=w> </span><span class=p>}</span>
<span class=p>};</span>
</code></pre></div> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../type_recipes/ >Recipes for common types</a> - smart pointers, class hierarchies, <code>std::variant</code>, and <code>std::chrono</code></li> <li><a href=../conversions/ >Converting values</a> - the general overview of <code>get</code>/<code>get_to</code> and implicit conversions</li> <li><a href=../enum_conversion/ >Specializing enum conversion</a> - map enums to <abbr title="JavaScript Object Notation">JSON</abbr> strings instead of integers</li> <li><a href=../macros/ >Supported macros</a> - reference for <code>NLOHMANN_DEFINE_TYPE_*</code> and related macros</li> <li><a href=../../api/adl_serializer/ ><code>adl_serializer</code></a> - the default <code>JSONSerializer</code> used in the conversion dispatch</li> <li><a href=../../api/macros/nlohmann_define_type_intrusive/ ><code>NLOHMANN_DEFINE_TYPE_INTRUSIVE</code></a> - generate the conversion functions inside the type</li> <li><a href=../../api/macros/nlohmann_define_type_non_intrusive/ ><code>NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE</code></a> - generate the conversion functions outside the type</li> <li><a href=../../api/macros/nlohmann_define_type_with_names/ ><code>NLOHMANN_DEFINE_TYPE_WITH_NAMES</code></a> - use <abbr title="JavaScript Object Notation">JSON</abbr> keys that differ from the member names</li> <li><a href=../../api/macros/nlohmann_define_derived_type/ ><code>NLOHMANN_DEFINE_DERIVED_TYPE</code></a> - generate the conversion functions for derived classes</li> <li><a href=../../api/macros/nlohmann_json_serialize_enum/ ><code>NLOHMANN_JSON_SERIALIZE_ENUM</code></a> - map enum values to <abbr title="JavaScript Object Notation">JSON</abbr> values</li> <li><a href=../../api/basic_json/get/ ><code>get</code></a> - convert a <abbr title="JavaScript Object Notation">JSON</abbr> value to a C++ type</li> <li><a href=../../api/basic_json/get_to/ ><code>get_to</code></a> - convert a <abbr title="JavaScript Object Notation">JSON</abbr> value into an existing variable</li> <li><a href=../../api/basic_json/json_serializer/ ><code>json_serializer</code></a> - the <code>JSONSerializer</code> template parameter</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../ class="md-footer__link md-footer__link--prev" aria-label="Previous: Features"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> Features </div> </div> </a> <a href=../type_recipes/ class="md-footer__link md-footer__link--next" aria-label="Next: Recipes for Common Types"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> Recipes for Common Types </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 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</code></pre></div> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../type_recipes/ >Recipes for common types</a> - smart pointers, class hierarchies, <code>std::variant</code>, and <code>std::chrono</code></li> <li><a href=../conversions/ >Converting values</a> - the general overview of <code>get</code>/<code>get_to</code> and implicit conversions</li> <li><a href=../enum_conversion/ >Specializing enum conversion</a> - map enums to <abbr title="JavaScript Object Notation">JSON</abbr> strings instead of integers</li> <li><a href=../macros/ >Supported macros</a> - reference for <code>NLOHMANN_DEFINE_TYPE_*</code> and related macros</li> <li><a href=../../api/adl_serializer/ ><code>adl_serializer</code></a> - the default <code>JSONSerializer</code> used in the conversion dispatch</li> <li><a href=../../api/macros/nlohmann_define_type_intrusive/ ><code>NLOHMANN_DEFINE_TYPE_INTRUSIVE</code></a> - generate the conversion functions inside the type</li> <li><a href=../../api/macros/nlohmann_define_type_non_intrusive/ ><code>NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE</code></a> - generate the conversion functions outside the type</li> <li><a href=../../api/macros/nlohmann_define_type_with_names/ ><code>NLOHMANN_DEFINE_TYPE_WITH_NAMES</code></a> - use <abbr title="JavaScript Object Notation">JSON</abbr> keys that differ from the member names</li> <li><a href=../../api/macros/nlohmann_define_derived_type/ ><code>NLOHMANN_DEFINE_DERIVED_TYPE</code></a> - generate the conversion functions for derived classes</li> <li><a href=../../api/macros/nlohmann_json_serialize_enum/ ><code>NLOHMANN_JSON_SERIALIZE_ENUM</code></a> - map enum values to <abbr title="JavaScript Object Notation">JSON</abbr> values</li> <li><a href=../../api/basic_json/get/ ><code>get</code></a> - convert a <abbr title="JavaScript Object Notation">JSON</abbr> value to a C++ type</li> <li><a href=../../api/basic_json/get_to/ ><code>get_to</code></a> - convert a <abbr title="JavaScript Object Notation">JSON</abbr> value into an existing variable</li> <li><a href=../../api/basic_json/json_serializer/ ><code>json_serializer</code></a> - the <code>JSONSerializer</code> template parameter</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../ class="md-footer__link md-footer__link--prev" aria-label="Previous: Features"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> Features </div> </div> </a> <a href=../type_recipes/ class="md-footer__link md-footer__link--next" aria-label="Next: Recipes for Common Types"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> Recipes for Common Types </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 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<span class=k>using</span><span class=w> </span><span class=n>nlohmann</span><span class=o>::</span><span class=n>json</span><span class=p>;</span>
@@ -54,4 +54,4 @@
<span class=w> </span><span class=p>}</span>
<span class=p>}</span>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code>[json.exception.parse_error.101] parse error: attempting to parse an empty input; check that your input string or stream contains the expected JSON
</code></pre></div> </details> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../../api/macros/json_assert/ ><code>JSON_ASSERT</code></a> - control behavior of runtime assertions</li> <li><a href=../macros/ >Supported Macros</a> - all supported macros</li> <li><a href=../no_exceptions/ >Working Without Exceptions</a> - using the library without exceptions</li> <li><a href=../../home/exceptions/ >Exceptions</a> - all exceptions and their error codes</li> <li><a href=../../api/macros/ >Macros</a> - reference of all macros</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../parsing/untrusted_input/ class="md-footer__link md-footer__link--prev" aria-label="Previous: Parsing Untrusted Input"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> Parsing Untrusted Input </div> </div> </a> <a href=../serialization/ class="md-footer__link md-footer__link--next" aria-label="Next: Serialization"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> Serialization </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 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</code></pre></div> </details> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../../api/macros/json_assert/ ><code>JSON_ASSERT</code></a> - control behavior of runtime assertions</li> <li><a href=../macros/ >Supported Macros</a> - all supported macros</li> <li><a href=../no_exceptions/ >Working Without Exceptions</a> - using the library without exceptions</li> <li><a href=../../home/exceptions/ >Exceptions</a> - all exceptions and their error codes</li> <li><a href=../../api/macros/ >Macros</a> - reference of all macros</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../parsing/untrusted_input/ class="md-footer__link md-footer__link--prev" aria-label="Previous: Parsing Untrusted Input"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> Parsing Untrusted Input </div> </div> </a> <a href=../serialization/ class="md-footer__link md-footer__link--next" aria-label="Next: Serialization"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> Serialization </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 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12s-1.44-4.5-3.5-5.45C10.26 6.2 11.11 6 12 6a6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> </form> <script>var palette=__md_get("__palette");if(palette&&palette.color){if("(prefers-color-scheme)"===palette.color.media){var media=matchMedia("(prefers-color-scheme: light)"),input=document.querySelector(media.matches?"[data-md-color-media='(prefers-color-scheme: light)']":"[data-md-color-media='(prefers-color-scheme: dark)']");palette.color.media=input.getAttribute("data-md-color-media"),palette.color.scheme=input.getAttribute("data-md-color-scheme"),palette.color.primary=input.getAttribute("data-md-color-primary"),palette.color.accent=input.getAttribute("data-md-color-accent")}for(var[key,value]of Object.entries(palette.color))document.body.setAttribute("data-md-color-"+key,value)}</script> <label class="md-header__button md-icon" for=__search> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M9.5 3A6.5 6.5 0 0 1 16 9.5c0 1.61-.59 3.09-1.Line truncated
<span class=w> </span><span class=nt>"id"</span><span class=p>:</span><span class=w> </span><span class=mi>300</span><span class=p>,</span>
<span class=w> </span><span class=nt>"name"</span><span class=p>:</span><span class=w> </span><span class=s2>"json"</span><span class=p>,</span>
<span class=w> </span><span class=nt>"pi"</span><span class=p>:</span><span class=w> </span><span class=mf>3.14</span><span class=p>,</span>
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<span class=w> </span><span class=nt>"compact"</span><span class=p>:</span><span class=w> </span><span class=kc>true</span><span class=p>,</span>
<span class=w> </span><span class=nt>"schema"</span><span class=p>:</span><span class=w> </span><span class=mi>0</span>
<span class=p>}</span>
</code></pre></div> </details> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../ >Binary formats</a> - overview and comparison of the supported binary formats</li> <li><a href=../../binary_values/ >Binary values</a> - how binary values and subtypes map to <abbr title="Binary JData">BJData</abbr></li> <li><a href=../../../api/basic_json/to_bjdata/ ><code>to_bjdata</code></a> - serialize a <abbr title="JavaScript Object Notation">JSON</abbr> value to <abbr title="Binary JData">BJData</abbr></li> <li><a href=../../../api/basic_json/from_bjdata/ ><code>from_bjdata</code></a> - parse a <abbr title="JavaScript Object Notation">JSON</abbr> value from <abbr title="Binary JData">BJData</abbr></li> <li><a href=../../../api/macros/json_strict_binary_utf8/ ><code>JSON_STRICT_BINARY_UTF8</code></a> - reject ill-formed <abbr title="Unicode Transformation Format">UTF</abbr>-8 in strings</li> <li><a href=../../parsing/sax_interface/ ><abbr title="Simple API for XML">SAX</abbr> interface</a> - decode binary formats without building a value</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../ class="md-footer__link md-footer__link--prev" aria-label="Previous: Binary Formats"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> Binary Formats </div> </div> </a> <a href=../bon8/ class="md-footer__link md-footer__link--next" aria-label="Next: BON8"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> BON8 </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 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</code></pre></div> </details> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../ >Binary formats</a> - overview and comparison of the supported binary formats</li> <li><a href=../../binary_values/ >Binary values</a> - how binary values and subtypes map to <abbr title="Binary JData">BJData</abbr></li> <li><a href=../../../api/basic_json/to_bjdata/ ><code>to_bjdata</code></a> - serialize a <abbr title="JavaScript Object Notation">JSON</abbr> value to <abbr title="Binary JData">BJData</abbr></li> <li><a href=../../../api/basic_json/from_bjdata/ ><code>from_bjdata</code></a> - parse a <abbr title="JavaScript Object Notation">JSON</abbr> value from <abbr title="Binary JData">BJData</abbr></li> <li><a href=../../../api/macros/json_strict_binary_utf8/ ><code>JSON_STRICT_BINARY_UTF8</code></a> - reject ill-formed <abbr title="Unicode Transformation Format">UTF</abbr>-8 in strings</li> <li><a href=../../parsing/sax_interface/ ><abbr title="Simple API for XML">SAX</abbr> interface</a> - decode binary formats without building a value</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../ class="md-footer__link md-footer__link--prev" aria-label="Previous: Binary Formats"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> Binary Formats </div> </div> </a> <a href=../bon8/ class="md-footer__link md-footer__link--next" aria-label="Next: BON8"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> BON8 </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 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<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=description content="JSON for Modern C++ is a header-only C++11 library that makes JSON a first-class data type."><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/binary_formats/bon8/" rel="canonical"><link href=../bjdata/ rel=prev><link href=../bson/ rel=next><link rel=icon href=../../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>BON8 - JSON for Modern C++</title><link rel=stylesheet href=../../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../../assets/stylesheets/palette.ab4e12ef.min.css><link rel=stylesheet href=../../../css/custom.css><link rel=stylesheet href=../../../css/brand.css><link rel=stylesheet href=../../../css/fig-sax.css><link rel=stylesheet href=../../../css/fig-jsonview.css><link rel=stylesheet 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data-md-component=announce> </div> <header class=md-header data-md-component=header> <nav class="md-header__inner md-grid" aria-label=Header> <a href=../../.. title="JSON for Modern C++" class="md-header__button md-logo" aria-label="JSON for Modern C++" data-md-component=logo> <img src=../../../images/wordmark-header.svg alt=logo> </a> <label class="md-header__button md-icon" for=__drawer> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M3 6h18v2H3zm0 5h18v2H3zm0 5h18v2H3z"/></svg> </label> <div class=md-header__title data-md-component=header-title> <div class=md-header__ellipsis> <div class=md-header__topic> <span class=md-ellipsis> JSON for Modern C++ </span> </div> <div class=md-header__topic data-md-component=header-topic> <span class=md-ellipsis> BON8 </span> </div> </div> </div> <form class=md-header__option data-md-component=palette> <input class=md-option data-md-color-media="(prefers-color-scheme: light)" data-md-color-scheme=default 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12s-1.44-4.5-3.5-5.45C10.26 6.2 11.11 6 12 6a6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> </form> <script>var palette=__md_get("__palette");if(palette&&palette.color){if("(prefers-color-scheme)"===palette.color.media){var media=matchMedia("(prefers-color-scheme: light)"),input=document.querySelector(media.matches?"[data-md-color-media='(prefers-color-scheme: light)']":"[data-md-color-media='(prefers-color-scheme: dark)']");palette.color.media=input.getAttribute("data-md-color-media"),palette.color.scheme=input.getAttribute("data-md-color-scheme"),palette.color.primary=input.getAttribute("data-md-color-primary"),palette.color.accent=input.getAttribute("data-md-color-accent")}for(var[key,value]of Object.entries(palette.color))document.body.setAttribute("data-md-color-"+key,value)}</script> <label class="md-header__button md-icon" for=__search> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M9.5 3A6.5 6.5 0 0 1 16 9.5c0 1.61-.59 3.09-1.5Line truncated
<span class=w> </span><span class=nt>"id"</span><span class=p>:</span><span class=w> </span><span class=mi>300</span><span class=p>,</span>
<span class=w> </span><span class=nt>"name"</span><span class=p>:</span><span class=w> </span><span class=s2>"json"</span><span class=p>,</span>
<span class=w> </span><span class=nt>"pi"</span><span class=p>:</span><span class=w> </span><span class=mf>3.14</span><span class=p>,</span>
@@ -56,4 +56,4 @@
<span class=w> </span><span class=nt>"format"</span><span class=p>:</span><span class=w> </span><span class=s2>"BON8"</span><span class=p>,</span>
<span class=w> </span><span class=nt>"schema"</span><span class=p>:</span><span class=w> </span><span class=mi>0</span>
<span class=p>}</span>
</code></pre></div> </details> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../ >Binary formats</a> - overview and comparison of the supported binary formats</li> <li><a href=../../binary_values/ >Binary values</a> - how binary values and subtypes map to <abbr title="Binary Object Notation 8">BON8</abbr></li> <li><a href=../../../api/basic_json/to_bon8/ ><code>to_bon8</code></a> - serialize a <abbr title="JavaScript Object Notation">JSON</abbr> value to <abbr title="Binary Object Notation 8">BON8</abbr></li> <li><a href=../../../api/basic_json/from_bon8/ ><code>from_bon8</code></a> - parse a <abbr title="JavaScript Object Notation">JSON</abbr> value from <abbr title="Binary Object Notation 8">BON8</abbr></li> <li><a href=../../../api/macros/json_strict_binary_utf8/ ><code>JSON_STRICT_BINARY_UTF8</code></a> - reject ill-formed <abbr title="Unicode Transformation Format">UTF</abbr>-8 in strings</li> <li><a href=../../parsing/sax_interface/ ><abbr title="Simple API for XML">SAX</abbr> interface</a> - decode binary formats without building a value</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../bjdata/ class="md-footer__link md-footer__link--prev" aria-label="Previous: BJData"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> BJData </div> </div> </a> <a href=../bson/ class="md-footer__link md-footer__link--next" aria-label="Next: BSON"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> BSON </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 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</code></pre></div> </details> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../ >Binary formats</a> - overview and comparison of the supported binary formats</li> <li><a href=../../binary_values/ >Binary values</a> - how binary values and subtypes map to <abbr title="Binary Object Notation 8">BON8</abbr></li> <li><a href=../../../api/basic_json/to_bon8/ ><code>to_bon8</code></a> - serialize a <abbr title="JavaScript Object Notation">JSON</abbr> value to <abbr title="Binary Object Notation 8">BON8</abbr></li> <li><a href=../../../api/basic_json/from_bon8/ ><code>from_bon8</code></a> - parse a <abbr title="JavaScript Object Notation">JSON</abbr> value from <abbr title="Binary Object Notation 8">BON8</abbr></li> <li><a href=../../../api/macros/json_strict_binary_utf8/ ><code>JSON_STRICT_BINARY_UTF8</code></a> - reject ill-formed <abbr title="Unicode Transformation Format">UTF</abbr>-8 in strings</li> <li><a href=../../parsing/sax_interface/ ><abbr title="Simple API for XML">SAX</abbr> interface</a> - decode binary formats without building a value</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../bjdata/ class="md-footer__link md-footer__link--prev" aria-label="Previous: BJData"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> BJData </div> </div> </a> <a href=../bson/ class="md-footer__link md-footer__link--next" aria-label="Next: BSON"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> BSON </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 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View File
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<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=description content="JSON for Modern C++ is a header-only C++11 library that makes JSON a first-class data type."><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/binary_formats/bson/" rel="canonical"><link href=../bon8/ rel=prev><link href=../cbor/ rel=next><link rel=icon href=../../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>BSON - JSON for Modern C++</title><link rel=stylesheet href=../../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../../assets/stylesheets/palette.ab4e12ef.min.css><link rel=stylesheet href=../../../css/custom.css><link rel=stylesheet href=../../../css/brand.css><link rel=stylesheet href=../../../css/fig-sax.css><link rel=stylesheet href=../../../css/fig-jsonview.css><link rel=stylesheet href=../../../css/fig-exceptions.css><link rel=stylesheet href=../../../css/fig-pointer.css><link rel=stylesheet href=../../../css/fig-charts.css><link rel=stylesheet href=../../../css/fig-integration.css><link rel=stylesheet href=../../../css/fig-macros.css><script>__md_scope=new URL("../../..",location),__md_hash=e=>[...e].reduce(((e,_)=>(e<<5)-e+_.charCodeAt(0)),0),__md_get=(e,_=localStorage,t=__md_scope)=>JSON.parse(_.getItem(t.pathname+"."+e)),__md_set=(e,_,t=localStorage,a=__md_scope)=>{try{t.setItem(a.pathname+"."+e,JSON.stringify(_))}catch(e){}}</script></head> <body dir=ltr data-md-color-scheme=default data-md-color-primary=custom data-md-color-accent=custom> <input class=md-toggle data-md-toggle=drawer type=checkbox id=__drawer autocomplete=off> <input class=md-toggle data-md-toggle=search type=checkbox id=__search autocomplete=off> <label class=md-overlay for=__drawer></label> <div data-md-component=skip> <a href=#bson class=md-skip> Skip to content </a> </div> <div data-md-component=announce> </div> <header class=md-header data-md-component=header> <nav class="md-header__inner md-grid" aria-label=Header> <a href=../../.. title="JSON for Modern C++" class="md-header__button md-logo" aria-label="JSON for Modern C++" data-md-component=logo> <img src=../../../images/wordmark-header.svg alt=logo> </a> <label class="md-header__button md-icon" for=__drawer> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M3 6h18v2H3zm0 5h18v2H3zm0 5h18v2H3z"/></svg> </label> <div class=md-header__title data-md-component=header-title> <div class=md-header__ellipsis> <div class=md-header__topic> <span class=md-ellipsis> JSON for Modern C++ </span> </div> <div class=md-header__topic data-md-component=header-topic> <span class=md-ellipsis> BSON </span> </div> </div> </div> <form class=md-header__option data-md-component=palette> <input class=md-option data-md-color-media="(prefers-color-scheme: light)" data-md-color-scheme=default 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0 24 24"><path d="M9.5 3A6.5 6.5 0 0 1 16 9.5c0 1.61-.59 3.09-1.56 Line truncated
<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=description content="JSON for Modern C++ is a header-only C++11 library that makes JSON a first-class data type."><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/binary_formats/bson/" rel="canonical"><link href=../bon8/ rel=prev><link href=../cbor/ rel=next><link rel=icon href=../../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.8.0"><title>BSON - JSON for Modern C++</title><link rel=stylesheet href=../../../assets/stylesheets/main.57a0d79d.min.css><link rel=stylesheet href=../../../assets/stylesheets/palette.fb2fffe2.min.css><link rel=stylesheet href=../../../css/custom.css><link rel=stylesheet href=../../../css/brand.css><link rel=stylesheet href=../../../css/fig-sax.css><link rel=stylesheet href=../../../css/fig-jsonview.css><link rel=stylesheet href=../../../css/fig-exceptions.css><link rel=stylesheet href=../../../css/fig-pointer.css><link rel=stylesheet href=../../../css/fig-charts.css><link rel=stylesheet href=../../../css/fig-integration.css><link rel=stylesheet href=../../../css/fig-macros.css><script>__md_scope=new URL("../../..",location),__md_hash=e=>[...e].reduce(((e,_)=>(e<<5)-e+_.charCodeAt(0)),0),__md_get=(e,_=localStorage,t=__md_scope)=>JSON.parse(_.getItem(t.pathname+"."+e)),__md_set=(e,_,t=localStorage,a=__md_scope)=>{try{t.setItem(a.pathname+"."+e,JSON.stringify(_))}catch(e){}}</script></head> <body dir=ltr data-md-color-scheme=default data-md-color-primary=custom data-md-color-accent=custom> <input class=md-toggle data-md-toggle=drawer type=checkbox id=__drawer autocomplete=off> <input class=md-toggle data-md-toggle=search type=checkbox id=__search autocomplete=off> <label class=md-overlay for=__drawer></label> <div data-md-component=skip> <a href=#bson class=md-skip> Skip to content </a> </div> <div data-md-component=announce> </div> <header class=md-header data-md-component=header> <nav class="md-header__inner md-grid" aria-label=Header> <a href=../../.. title="JSON for Modern C++" class="md-header__button md-logo" aria-label="JSON for Modern C++" data-md-component=logo> <img src=../../../images/wordmark-header.svg alt=logo> </a> <label class="md-header__button md-icon" for=__drawer> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M3 6h18v2H3zm0 5h18v2H3zm0 5h18v2H3z"/></svg> </label> <div class=md-header__title data-md-component=header-title> <div class=md-header__ellipsis> <div class=md-header__topic> <span class=md-ellipsis> JSON for Modern C++ </span> </div> <div class=md-header__topic data-md-component=header-topic> <span class=md-ellipsis> BSON </span> </div> </div> </div> <form class=md-header__option data-md-component=palette> <input class=md-option data-md-color-media="(prefers-color-scheme: light)" data-md-color-scheme=default data-md-color-primary=custom data-md-color-accent=custom aria-label="Switch to dark mode" type=radio name=__palette id=__palette_0> <label class="md-header__button md-icon" title="Switch to dark mode" for=__palette_1 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 8a4 4 0 0 0-4 4 4 4 0 0 0 4 4 4 4 0 0 0 4-4 4 4 0 0 0-4-4m0 10a6 6 0 0 1-6-6 6 6 0 0 1 6-6 6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> <input class=md-option data-md-color-media="(prefers-color-scheme: dark)" data-md-color-scheme=slate data-md-color-primary=custom data-md-color-accent=custom aria-label="Switch to light mode" type=radio name=__palette id=__palette_1> <label class="md-header__button md-icon" title="Switch to light mode" for=__palette_0 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 18c-.89 0-1.74-.2-2.5-.55C11.56 16.5 13 14.42 13 12s-1.44-4.5-3.5-5.45C10.26 6.2 11.11 6 12 6a6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> </form> <script>var palette=__md_get("__palette");if(palette&&palette.color){if("(prefers-color-scheme)"===palette.color.media){var media=matchMedia("(prefers-color-scheme: light)"),input=document.querySelector(media.matches?"[data-md-color-media='(prefers-color-scheme: light)']":"[data-md-color-media='(prefers-color-scheme: dark)']");palette.color.media=input.getAttribute("data-md-color-media"),palette.color.scheme=input.getAttribute("data-md-color-scheme"),palette.color.primary=input.getAttribute("data-md-color-primary"),palette.color.accent=input.getAttribute("data-md-color-accent")}for(var[key,value]of Object.entries(palette.color))document.body.setAttribute("data-md-color-"+key,value)}</script> <label class="md-header__button md-icon" for=__search> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M9.5 3A6.5 6.5 0 0 1 16 9.5c0 1.61-.59 3.09-1.56 Line truncated
<span class=w> </span><span class=nt>"id"</span><span class=p>:</span><span class=w> </span><span class=mi>300</span><span class=p>,</span>
<span class=w> </span><span class=nt>"name"</span><span class=p>:</span><span class=w> </span><span class=s2>"json"</span><span class=p>,</span>
<span class=w> </span><span class=nt>"pi"</span><span class=p>:</span><span class=w> </span><span class=mf>3.14</span><span class=p>,</span>
@@ -55,4 +55,4 @@
<span class=w> </span><span class=nt>"compact"</span><span class=p>:</span><span class=w> </span><span class=kc>true</span><span class=p>,</span>
<span class=w> </span><span class=nt>"schema"</span><span class=p>:</span><span class=w> </span><span class=mi>0</span>
<span class=p>}</span>
</code></pre></div> </details> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../ >Binary formats</a> - overview and comparison of the supported binary formats</li> <li><a href=../../binary_values/ >Binary values</a> - how binary values and subtypes map to <abbr title="Binary JSON">BSON</abbr></li> <li><a href=../../../api/basic_json/to_bson/ ><code>to_bson</code></a> - serialize a <abbr title="JavaScript Object Notation">JSON</abbr> value to <abbr title="Binary JSON">BSON</abbr></li> <li><a href=../../../api/basic_json/from_bson/ ><code>from_bson</code></a> - parse a <abbr title="JavaScript Object Notation">JSON</abbr> value from <abbr title="Binary JSON">BSON</abbr></li> <li><a href=../../../api/macros/json_strict_binary_utf8/ ><code>JSON_STRICT_BINARY_UTF8</code></a> - reject ill-formed <abbr title="Unicode Transformation Format">UTF</abbr>-8 in strings</li> <li><a href=../../parsing/sax_interface/ ><abbr title="Simple API for XML">SAX</abbr> interface</a> - decode binary formats without building a value</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../bon8/ class="md-footer__link md-footer__link--prev" aria-label="Previous: BON8"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> BON8 </div> </div> </a> <a href=../cbor/ class="md-footer__link md-footer__link--next" aria-label="Next: CBOR"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> CBOR </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 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</code></pre></div> </details> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../ >Binary formats</a> - overview and comparison of the supported binary formats</li> <li><a href=../../binary_values/ >Binary values</a> - how binary values and subtypes map to <abbr title="Binary JSON">BSON</abbr></li> <li><a href=../../../api/basic_json/to_bson/ ><code>to_bson</code></a> - serialize a <abbr title="JavaScript Object Notation">JSON</abbr> value to <abbr title="Binary JSON">BSON</abbr></li> <li><a href=../../../api/basic_json/from_bson/ ><code>from_bson</code></a> - parse a <abbr title="JavaScript Object Notation">JSON</abbr> value from <abbr title="Binary JSON">BSON</abbr></li> <li><a href=../../../api/macros/json_strict_binary_utf8/ ><code>JSON_STRICT_BINARY_UTF8</code></a> - reject ill-formed <abbr title="Unicode Transformation Format">UTF</abbr>-8 in strings</li> <li><a href=../../parsing/sax_interface/ ><abbr title="Simple API for XML">SAX</abbr> interface</a> - decode binary formats without building a value</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../bon8/ class="md-footer__link md-footer__link--prev" aria-label="Previous: BON8"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> BON8 </div> </div> </a> <a href=../cbor/ class="md-footer__link md-footer__link--next" aria-label="Next: CBOR"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> CBOR </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 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+2 -2
View File
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<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=description content="JSON for Modern C++ is a header-only C++11 library that makes JSON a first-class data type."><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/binary_formats/cbor/" rel="canonical"><link href=../bson/ rel=prev><link href=../messagepack/ rel=next><link rel=icon href=../../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>CBOR - JSON for Modern C++</title><link rel=stylesheet href=../../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../../assets/stylesheets/palette.ab4e12ef.min.css><link rel=stylesheet href=../../../css/custom.css><link rel=stylesheet href=../../../css/brand.css><link rel=stylesheet href=../../../css/fig-sax.css><link rel=stylesheet href=../../../css/fig-jsonview.css><link rel=stylesheet href=../../../css/fig-exceptions.css><link rel=stylesheet href=../../../css/fig-pointer.css><link rel=stylesheet href=../../../css/fig-charts.css><link rel=stylesheet href=../../../css/fig-integration.css><link rel=stylesheet href=../../../css/fig-macros.css><script>__md_scope=new URL("../../..",location),__md_hash=e=>[...e].reduce(((e,_)=>(e<<5)-e+_.charCodeAt(0)),0),__md_get=(e,_=localStorage,t=__md_scope)=>JSON.parse(_.getItem(t.pathname+"."+e)),__md_set=(e,_,t=localStorage,a=__md_scope)=>{try{t.setItem(a.pathname+"."+e,JSON.stringify(_))}catch(e){}}</script></head> <body dir=ltr data-md-color-scheme=default data-md-color-primary=custom data-md-color-accent=custom> <input class=md-toggle data-md-toggle=drawer type=checkbox id=__drawer autocomplete=off> <input class=md-toggle data-md-toggle=search type=checkbox id=__search autocomplete=off> <label class=md-overlay for=__drawer></label> <div data-md-component=skip> <a href=#cbor class=md-skip> Skip to content </a> </div> <div data-md-component=announce> </div> <header class=md-header data-md-component=header> <nav class="md-header__inner md-grid" aria-label=Header> <a href=../../.. title="JSON for Modern C++" class="md-header__button md-logo" aria-label="JSON for Modern C++" data-md-component=logo> <img src=../../../images/wordmark-header.svg alt=logo> </a> <label class="md-header__button md-icon" for=__drawer> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M3 6h18v2H3zm0 5h18v2H3zm0 5h18v2H3z"/></svg> </label> <div class=md-header__title data-md-component=header-title> <div class=md-header__ellipsis> <div class=md-header__topic> <span class=md-ellipsis> JSON for Modern C++ </span> </div> <div class=md-header__topic data-md-component=header-topic> <span class=md-ellipsis> CBOR </span> </div> </div> </div> <form class=md-header__option data-md-component=palette> <input class=md-option data-md-color-media="(prefers-color-scheme: light)" data-md-color-scheme=default 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0 24 24"><path d="M9.5 3A6.5 6.5 0 0 1 16 9.5c0 1.61-.59 3.0Line truncated
<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=description content="JSON for Modern C++ is a header-only C++11 library that makes JSON a first-class data type."><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/binary_formats/cbor/" rel="canonical"><link href=../bson/ rel=prev><link href=../messagepack/ rel=next><link rel=icon href=../../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.8.0"><title>CBOR - JSON for Modern C++</title><link rel=stylesheet href=../../../assets/stylesheets/main.57a0d79d.min.css><link rel=stylesheet href=../../../assets/stylesheets/palette.fb2fffe2.min.css><link rel=stylesheet href=../../../css/custom.css><link rel=stylesheet href=../../../css/brand.css><link rel=stylesheet href=../../../css/fig-sax.css><link rel=stylesheet href=../../../css/fig-jsonview.css><link rel=stylesheet 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data-md-component=announce> </div> <header class=md-header data-md-component=header> <nav class="md-header__inner md-grid" aria-label=Header> <a href=../../.. title="JSON for Modern C++" class="md-header__button md-logo" aria-label="JSON for Modern C++" data-md-component=logo> <img src=../../../images/wordmark-header.svg alt=logo> </a> <label class="md-header__button md-icon" for=__drawer> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M3 6h18v2H3zm0 5h18v2H3zm0 5h18v2H3z"/></svg> </label> <div class=md-header__title data-md-component=header-title> <div class=md-header__ellipsis> <div class=md-header__topic> <span class=md-ellipsis> JSON for Modern C++ </span> </div> <div class=md-header__topic data-md-component=header-topic> <span class=md-ellipsis> CBOR </span> </div> </div> </div> <form class=md-header__option data-md-component=palette> <input class=md-option data-md-color-media="(prefers-color-scheme: light)" data-md-color-scheme=default data-md-color-primary=custom data-md-color-accent=custom aria-label="Switch to dark mode" type=radio name=__palette id=__palette_0> <label class="md-header__button md-icon" title="Switch to dark mode" for=__palette_1 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 8a4 4 0 0 0-4 4 4 4 0 0 0 4 4 4 4 0 0 0 4-4 4 4 0 0 0-4-4m0 10a6 6 0 0 1-6-6 6 6 0 0 1 6-6 6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> <input class=md-option data-md-color-media="(prefers-color-scheme: dark)" data-md-color-scheme=slate data-md-color-primary=custom data-md-color-accent=custom aria-label="Switch to light mode" type=radio name=__palette id=__palette_1> <label class="md-header__button md-icon" title="Switch to light mode" for=__palette_0 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 18c-.89 0-1.74-.2-2.5-.55C11.56 16.5 13 14.42 13 12s-1.44-4.5-3.5-5.45C10.26 6.2 11.11 6 12 6a6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> </form> <script>var palette=__md_get("__palette");if(palette&&palette.color){if("(prefers-color-scheme)"===palette.color.media){var media=matchMedia("(prefers-color-scheme: light)"),input=document.querySelector(media.matches?"[data-md-color-media='(prefers-color-scheme: light)']":"[data-md-color-media='(prefers-color-scheme: dark)']");palette.color.media=input.getAttribute("data-md-color-media"),palette.color.scheme=input.getAttribute("data-md-color-scheme"),palette.color.primary=input.getAttribute("data-md-color-primary"),palette.color.accent=input.getAttribute("data-md-color-accent")}for(var[key,value]of Object.entries(palette.color))document.body.setAttribute("data-md-color-"+key,value)}</script> <label class="md-header__button md-icon" for=__search> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M9.5 3A6.5 6.5 0 0 1 16 9.5c0 1.61-.59 3.0Line truncated
<span class=w> </span><span class=nt>"id"</span><span class=p>:</span><span class=w> </span><span class=mi>300</span><span class=p>,</span>
<span class=w> </span><span class=nt>"name"</span><span class=p>:</span><span class=w> </span><span class=s2>"json"</span><span class=p>,</span>
<span class=w> </span><span class=nt>"pi"</span><span class=p>:</span><span class=w> </span><span class=mf>3.14</span><span class=p>,</span>
@@ -55,4 +55,4 @@
<span class=w> </span><span class=nt>"compact"</span><span class=p>:</span><span class=w> </span><span class=kc>true</span><span class=p>,</span>
<span class=w> </span><span class=nt>"schema"</span><span class=p>:</span><span class=w> </span><span class=mi>0</span>
<span class=p>}</span>
</code></pre></div> </details> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../ >Binary formats</a> - overview and comparison of the supported binary formats</li> <li><a href=../../binary_values/ >Binary values</a> - how binary values and subtypes map to <abbr title="Concise Binary Object Representation">CBOR</abbr></li> <li><a href=../../../api/basic_json/to_cbor/ ><code>to_cbor</code></a> - serialize a <abbr title="JavaScript Object Notation">JSON</abbr> value to <abbr title="Concise Binary Object Representation">CBOR</abbr></li> <li><a href=../../../api/basic_json/from_cbor/ ><code>from_cbor</code></a> - parse a <abbr title="JavaScript Object Notation">JSON</abbr> value from <abbr title="Concise Binary Object Representation">CBOR</abbr></li> <li><a href=../../../api/macros/json_strict_binary_utf8/ ><code>JSON_STRICT_BINARY_UTF8</code></a> - reject ill-formed <abbr title="Unicode Transformation Format">UTF</abbr>-8 in strings</li> <li><a href=../../parsing/sax_interface/ ><abbr title="Simple API for XML">SAX</abbr> interface</a> - decode binary formats without building a value</li> <li><a href=../../../api/basic_json/cbor_tag_handler_t/ ><code>cbor_tag_handler_t</code></a> - how <abbr title="Concise Binary Object Representation">CBOR</abbr> tags are handled when parsing</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../bson/ class="md-footer__link md-footer__link--prev" aria-label="Previous: BSON"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> BSON </div> </div> </a> <a href=../messagepack/ class="md-footer__link md-footer__link--next" aria-label="Next: MessagePack"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> MessagePack </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 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</code></pre></div> </details> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../ >Binary formats</a> - overview and comparison of the supported binary formats</li> <li><a href=../../binary_values/ >Binary values</a> - how binary values and subtypes map to <abbr title="Concise Binary Object Representation">CBOR</abbr></li> <li><a href=../../../api/basic_json/to_cbor/ ><code>to_cbor</code></a> - serialize a <abbr title="JavaScript Object Notation">JSON</abbr> value to <abbr title="Concise Binary Object Representation">CBOR</abbr></li> <li><a href=../../../api/basic_json/from_cbor/ ><code>from_cbor</code></a> - parse a <abbr title="JavaScript Object Notation">JSON</abbr> value from <abbr title="Concise Binary Object Representation">CBOR</abbr></li> <li><a href=../../../api/macros/json_strict_binary_utf8/ ><code>JSON_STRICT_BINARY_UTF8</code></a> - reject ill-formed <abbr title="Unicode Transformation Format">UTF</abbr>-8 in strings</li> <li><a href=../../parsing/sax_interface/ ><abbr title="Simple API for XML">SAX</abbr> interface</a> - decode binary formats without building a value</li> <li><a href=../../../api/basic_json/cbor_tag_handler_t/ ><code>cbor_tag_handler_t</code></a> - how <abbr title="Concise Binary Object Representation">CBOR</abbr> tags are handled when parsing</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../bson/ class="md-footer__link md-footer__link--prev" aria-label="Previous: BSON"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> BSON </div> </div> </a> <a href=../messagepack/ class="md-footer__link md-footer__link--next" aria-label="Next: MessagePack"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> MessagePack </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 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+1 -1
View File
@@ -1 +1 @@
<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=description content="JSON for Modern C++ is a header-only C++11 library that makes JSON a first-class data type."><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/binary_formats/" rel="canonical"><link href=../type_recipes/ rel=prev><link href=bjdata/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>Binary Formats - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.ab4e12ef.min.css><link rel=stylesheet href=../../css/custom.css><link rel=stylesheet href=../../css/brand.css><link rel=stylesheet href=../../css/fig-sax.css><link rel=stylesheet href=../../css/fig-jsonview.css><link rel=stylesheet 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@@ -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=description content="JSON for Modern C++ is a header-only C++11 library that makes JSON a first-class data type."><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/binary_formats/messagepack/" rel="canonical"><link href=../cbor/ rel=prev><link href=../ubjson/ rel=next><link rel=icon href=../../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>MessagePack - JSON for Modern C++</title><link rel=stylesheet href=../../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../../assets/stylesheets/palette.ab4e12ef.min.css><link rel=stylesheet href=../../../css/custom.css><link rel=stylesheet href=../../../css/brand.css><link rel=stylesheet href=../../../css/fig-sax.css><link rel=stylesheet href=../../../css/fig-jsonview.css><link 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<span class=w> </span><span class=nt>"id"</span><span class=p>:</span><span class=w> </span><span class=mi>300</span><span class=p>,</span>
<span class=w> </span><span class=nt>"name"</span><span class=p>:</span><span class=w> </span><span class=s2>"json"</span><span class=p>,</span>
<span class=w> </span><span class=nt>"pi"</span><span class=p>:</span><span class=w> </span><span class=mf>3.14</span><span class=p>,</span>
@@ -55,4 +55,4 @@
<span class=w> </span><span class=nt>"compact"</span><span class=p>:</span><span class=w> </span><span class=kc>true</span><span class=p>,</span>
<span class=w> </span><span class=nt>"schema"</span><span class=p>:</span><span class=w> </span><span class=mi>0</span>
<span class=p>}</span>
</code></pre></div> </details> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../ >Binary formats</a> - overview and comparison of the supported binary formats</li> <li><a href=../../binary_values/ >Binary values</a> - how binary values and subtypes map to MessagePack</li> <li><a href=../../../api/basic_json/to_msgpack/ ><code>to_msgpack</code></a> - serialize a <abbr title="JavaScript Object Notation">JSON</abbr> value to MessagePack</li> <li><a href=../../../api/basic_json/from_msgpack/ ><code>from_msgpack</code></a> - parse a <abbr title="JavaScript Object Notation">JSON</abbr> value from MessagePack</li> <li><a href=../../../api/macros/json_strict_binary_utf8/ ><code>JSON_STRICT_BINARY_UTF8</code></a> - reject ill-formed <abbr title="Unicode Transformation Format">UTF</abbr>-8 in strings</li> <li><a href=../../parsing/sax_interface/ ><abbr title="Simple API for XML">SAX</abbr> interface</a> - decode binary formats without building a value</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../cbor/ class="md-footer__link md-footer__link--prev" aria-label="Previous: CBOR"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> CBOR </div> </div> </a> <a href=../ubjson/ class="md-footer__link md-footer__link--next" aria-label="Next: UBJSON"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> UBJSON </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 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</code></pre></div> </details> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../ >Binary formats</a> - overview and comparison of the supported binary formats</li> <li><a href=../../binary_values/ >Binary values</a> - how binary values and subtypes map to MessagePack</li> <li><a href=../../../api/basic_json/to_msgpack/ ><code>to_msgpack</code></a> - serialize a <abbr title="JavaScript Object Notation">JSON</abbr> value to MessagePack</li> <li><a href=../../../api/basic_json/from_msgpack/ ><code>from_msgpack</code></a> - parse a <abbr title="JavaScript Object Notation">JSON</abbr> value from MessagePack</li> <li><a href=../../../api/macros/json_strict_binary_utf8/ ><code>JSON_STRICT_BINARY_UTF8</code></a> - reject ill-formed <abbr title="Unicode Transformation Format">UTF</abbr>-8 in strings</li> <li><a href=../../parsing/sax_interface/ ><abbr title="Simple API for XML">SAX</abbr> interface</a> - decode binary formats without building a value</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../cbor/ class="md-footer__link md-footer__link--prev" aria-label="Previous: CBOR"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> CBOR </div> </div> </a> <a href=../ubjson/ class="md-footer__link md-footer__link--next" aria-label="Next: UBJSON"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> UBJSON </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 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<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=description content="JSON for Modern C++ is a header-only C++11 library that makes JSON a first-class data type."><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/binary_formats/ubjson/" rel="canonical"><link href=../messagepack/ rel=prev><link href=../../binary_values/ rel=next><link rel=icon href=../../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>UBJSON - JSON for Modern C++</title><link rel=stylesheet href=../../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../../assets/stylesheets/palette.ab4e12ef.min.css><link rel=stylesheet href=../../../css/custom.css><link rel=stylesheet href=../../../css/brand.css><link rel=stylesheet href=../../../css/fig-sax.css><link rel=stylesheet href=../../../css/fig-jsonview.css><link 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12s-1.44-4.5-3.5-5.45C10.26 6.2 11.11 6 12 6a6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> </form> <script>var palette=__md_get("__palette");if(palette&&palette.color){if("(prefers-color-scheme)"===palette.color.media){var media=matchMedia("(prefers-color-scheme: light)"),input=document.querySelector(media.matches?"[data-md-color-media='(prefers-color-scheme: light)']":"[data-md-color-media='(prefers-color-scheme: dark)']");palette.color.media=input.getAttribute("data-md-color-media"),palette.color.scheme=input.getAttribute("data-md-color-scheme"),palette.color.primary=input.getAttribute("data-md-color-primary"),palette.color.accent=input.getAttribute("data-md-color-accent")}for(var[key,value]of Object.entries(palette.color))document.body.setAttribute("data-md-color-"+key,value)}</script> <label class="md-header__button md-icon" for=__search> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M9.5 3A6.5 6.5 0 0 1 1Line truncated
<span class=w> </span><span class=nt>"id"</span><span class=p>:</span><span class=w> </span><span class=mi>300</span><span class=p>,</span>
<span class=w> </span><span class=nt>"name"</span><span class=p>:</span><span class=w> </span><span class=s2>"json"</span><span class=p>,</span>
<span class=w> </span><span class=nt>"pi"</span><span class=p>:</span><span class=w> </span><span class=mf>3.14</span><span class=p>,</span>
@@ -99,4 +99,4 @@
<span class=w> </span><span class=nt>"compact"</span><span class=p>:</span><span class=w> </span><span class=kc>true</span><span class=p>,</span>
<span class=w> </span><span class=nt>"schema"</span><span class=p>:</span><span class=w> </span><span class=mi>0</span>
<span class=p>}</span>
</code></pre></div> </details> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../ >Binary formats</a> - overview and comparison of the supported binary formats</li> <li><a href=../../binary_values/ >Binary values</a> - how binary values and subtypes map to <abbr title="Universal Binary JSON">UBJSON</abbr></li> <li><a href=../../../api/basic_json/to_ubjson/ ><code>to_ubjson</code></a> - serialize a <abbr title="JavaScript Object Notation">JSON</abbr> value to <abbr title="Universal Binary JSON">UBJSON</abbr></li> <li><a href=../../../api/basic_json/from_ubjson/ ><code>from_ubjson</code></a> - parse a <abbr title="JavaScript Object Notation">JSON</abbr> value from <abbr title="Universal Binary JSON">UBJSON</abbr></li> <li><a href=../../../api/macros/json_strict_binary_utf8/ ><code>JSON_STRICT_BINARY_UTF8</code></a> - reject ill-formed <abbr title="Unicode Transformation Format">UTF</abbr>-8 in strings</li> <li><a href=../../parsing/sax_interface/ ><abbr title="Simple API for XML">SAX</abbr> interface</a> - decode binary formats without building a value</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../messagepack/ class="md-footer__link md-footer__link--prev" aria-label="Previous: MessagePack"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> MessagePack </div> </div> </a> <a href=../../binary_values/ class="md-footer__link md-footer__link--next" aria-label="Next: Binary Values"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> Binary Values </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 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</code></pre></div> </details> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../ >Binary formats</a> - overview and comparison of the supported binary formats</li> <li><a href=../../binary_values/ >Binary values</a> - how binary values and subtypes map to <abbr title="Universal Binary JSON">UBJSON</abbr></li> <li><a href=../../../api/basic_json/to_ubjson/ ><code>to_ubjson</code></a> - serialize a <abbr title="JavaScript Object Notation">JSON</abbr> value to <abbr title="Universal Binary JSON">UBJSON</abbr></li> <li><a href=../../../api/basic_json/from_ubjson/ ><code>from_ubjson</code></a> - parse a <abbr title="JavaScript Object Notation">JSON</abbr> value from <abbr title="Universal Binary JSON">UBJSON</abbr></li> <li><a href=../../../api/macros/json_strict_binary_utf8/ ><code>JSON_STRICT_BINARY_UTF8</code></a> - reject ill-formed <abbr title="Unicode Transformation Format">UTF</abbr>-8 in strings</li> <li><a href=../../parsing/sax_interface/ ><abbr title="Simple API for XML">SAX</abbr> interface</a> - decode binary formats without building a value</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../messagepack/ class="md-footer__link md-footer__link--prev" aria-label="Previous: MessagePack"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> MessagePack </div> </div> </a> <a href=../../binary_values/ class="md-footer__link md-footer__link--next" aria-label="Next: Binary Values"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> Binary Values </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 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<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=description content="JSON for Modern C++ is a header-only C++11 library that makes JSON a first-class data type."><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/binary_values/" rel="canonical"><link href=../binary_formats/ubjson/ rel=prev><link href=../comments/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>Binary Values - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.ab4e12ef.min.css><link rel=stylesheet href=../../css/custom.css><link rel=stylesheet href=../../css/brand.css><link rel=stylesheet href=../../css/fig-sax.css><link rel=stylesheet href=../../css/fig-jsonview.css><link rel=stylesheet 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<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=description content="JSON for Modern C++ is a header-only C++11 library that makes JSON a first-class data type."><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/binary_values/" rel="canonical"><link href=../binary_formats/ubjson/ rel=prev><link href=../comments/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.8.0"><title>Binary Values - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.57a0d79d.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.fb2fffe2.min.css><link rel=stylesheet href=../../css/custom.css><link rel=stylesheet href=../../css/brand.css><link rel=stylesheet href=../../css/fig-sax.css><link rel=stylesheet href=../../css/fig-jsonview.css><link rel=stylesheet 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class binary_t ["json::binary_t"] {
+void set_subtype(std::uint64_t subtype)
@@ -173,4 +173,4 @@ vector &lt;|-- binary_t</code></pre> <p class=nj-caption>Class diagram of <code>
</code></pre></div> <p>Note that subtype (42) is <strong>not</strong> serialized and that <abbr title="Universal Binary JSON">UBJSON</abbr> has <strong>no binary type</strong>, and deserializing <code>v</code> would yield the following value:</p> <div class=highlight><pre><span></span><code><span class=p>{</span>
<span class=w> </span><span class=nt>"binary"</span><span class=p>:</span><span class=w> </span><span class=p>[</span><span class=mi>202</span><span class=p>,</span><span class=w> </span><span class=mi>254</span><span class=p>,</span><span class=w> </span><span class=mi>186</span><span class=p>,</span><span class=w> </span><span class=mi>190</span><span class=p>]</span>
<span class=p>}</span>
</code></pre></div> </details> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../binary_formats/ >Binary formats</a> - the formats that can store binary values</li> <li><a href=../../api/basic_json/binary_t/ ><code>binary_t</code></a> - type for binary values</li> <li><a href=../../api/byte_container_with_subtype/ ><code>byte_container_with_subtype</code></a> - container for binary values with subtype</li> <li><a href=../../api/basic_json/binary/ ><code>binary</code></a> - create a binary value</li> <li><a href=../../api/basic_json/get_binary/ ><code>get_binary</code></a> - access the stored binary value</li> <li><a href=../../api/basic_json/is_binary/ ><code>is_binary</code></a> - check whether a value is binary</li> <li><a href=../../api/basic_json/cbor_tag_handler_t/ ><code>cbor_tag_handler_t</code></a> - store or ignore <abbr title="Concise Binary Object Representation">CBOR</abbr> tags when parsing</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../binary_formats/ubjson/ class="md-footer__link md-footer__link--prev" aria-label="Previous: UBJSON"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> UBJSON </div> </div> </a> <a href=../comments/ class="md-footer__link md-footer__link--next" aria-label="Next: Comments"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> Comments </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 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</code></pre></div> </details> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../binary_formats/ >Binary formats</a> - the formats that can store binary values</li> <li><a href=../../api/basic_json/binary_t/ ><code>binary_t</code></a> - type for binary values</li> <li><a href=../../api/byte_container_with_subtype/ ><code>byte_container_with_subtype</code></a> - container for binary values with subtype</li> <li><a href=../../api/basic_json/binary/ ><code>binary</code></a> - create a binary value</li> <li><a href=../../api/basic_json/get_binary/ ><code>get_binary</code></a> - access the stored binary value</li> <li><a href=../../api/basic_json/is_binary/ ><code>is_binary</code></a> - check whether a value is binary</li> <li><a href=../../api/basic_json/cbor_tag_handler_t/ ><code>cbor_tag_handler_t</code></a> - store or ignore <abbr title="Concise Binary Object Representation">CBOR</abbr> tags when parsing</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../binary_formats/ubjson/ class="md-footer__link md-footer__link--prev" aria-label="Previous: UBJSON"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> UBJSON </div> </div> </a> <a href=../comments/ class="md-footer__link md-footer__link--next" aria-label="Next: Comments"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> Comments </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 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+2 -2
View File
@@ -1,4 +1,4 @@
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<span class=w> </span><span class=c1>// update in 2006: removed Pluto</span>
<span class=w> </span><span class=nt>"planets"</span><span class=p>:</span><span class=w> </span><span class=p>[</span><span class=s2>"Mercury"</span><span class=p>,</span><span class=w> </span><span class=s2>"Venus"</span><span class=p>,</span><span class=w> </span><span class=s2>"Earth"</span><span class=p>,</span><span class=w> </span><span class=s2>"Mars"</span><span class=p>,</span>
<span class=w> </span><span class=s2>"Jupiter"</span><span class=p>,</span><span class=w> </span><span class=s2>"Uranus"</span><span class=p>,</span><span class=w> </span><span class=s2>"Neptune"</span><span class=w> </span><span class=cm>/*, "Pluto" */</span><span class=p>]</span>
@@ -45,4 +45,4 @@
"Neptune"
]
}
</code></pre></div> </div> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../../api/basic_json/parse/ ><code>parse</code></a> - deserialize from a compatible input</li> <li><a href=../trailing_commas/ >Trailing Commas</a> - accept trailing commas in arrays and objects</li> <li><a href=../parsing/ >Parsing</a> - overview of the parsing functions</li> <li><a href=../parsing/untrusted_input/ >Parsing Untrusted Input</a> - limits and checks for input you do not control</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../binary_values/ class="md-footer__link md-footer__link--prev" aria-label="Previous: Binary Values"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> Binary Values </div> </div> </a> <a href=../conversions/ class="md-footer__link md-footer__link--next" aria-label="Next: Converting values"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> Converting values </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 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</code></pre></div> </div> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../../api/basic_json/parse/ ><code>parse</code></a> - deserialize from a compatible input</li> <li><a href=../trailing_commas/ >Trailing Commas</a> - accept trailing commas in arrays and objects</li> <li><a href=../parsing/ >Parsing</a> - overview of the parsing functions</li> <li><a href=../parsing/untrusted_input/ >Parsing Untrusted Input</a> - limits and checks for input you do not control</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../binary_values/ class="md-footer__link md-footer__link--prev" aria-label="Previous: Binary Values"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> Binary Values </div> </div> </a> <a href=../conversions/ class="md-footer__link md-footer__link--next" aria-label="Next: Converting values"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> Converting values </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 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<span class=k>auto</span><span class=w> </span><span class=n>name</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>j</span><span class=p>[</span><span class=s>"name"</span><span class=p>].</span><span class=n>get</span><span class=o>&lt;</span><span class=n>std</span><span class=o>::</span><span class=n>string</span><span class=o>&gt;</span><span class=p>();</span><span class=w> </span><span class=c1>// "Mary"</span>
<span class=k>auto</span><span class=w> </span><span class=n>age</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>j</span><span class=p>[</span><span class=s>"age"</span><span class=p>].</span><span class=n>get</span><span class=o>&lt;</span><span class=kt>int</span><span class=o>&gt;</span><span class=p>();</span><span class=w> </span><span class=c1>// 42</span>
@@ -120,4 +120,4 @@
</code></pre></div> <div class="admonition info"> <p class=admonition-title>Constructing from a C++20 range view</p> <p>A <code>json</code> array can also be constructed directly from a C++20 range view (<code>std::ranges::view</code>), such as the result of <code>std::views::filter</code> or <code>std::views::transform</code> — no intermediate container is needed:</p> <div class=highlight><pre><span></span><code><span class=n>std</span><span class=o>::</span><span class=n>vector</span><span class=o>&lt;</span><span class=kt>int</span><span class=o>&gt;</span><span class=w> </span><span class=n>nums</span><span class=p>{</span><span class=mi>1</span><span class=p>,</span><span class=w> </span><span class=mi>2</span><span class=p>,</span><span class=w> </span><span class=mi>37</span><span class=p>,</span><span class=w> </span><span class=mi>42</span><span class=p>,</span><span class=w> </span><span class=mi>21</span><span class=p>};</span>
<span class=k>auto</span><span class=w> </span><span class=n>filtered</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>nums</span><span class=w> </span><span class=o>|</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>views</span><span class=o>::</span><span class=n>filter</span><span class=p>([](</span><span class=kt>int</span><span class=w> </span><span class=n>i</span><span class=p>)</span><span class=w> </span><span class=p>{</span><span class=w> </span><span class=k>return</span><span class=w> </span><span class=n>i</span><span class=w> </span><span class=o>&gt;</span><span class=w> </span><span class=mi>10</span><span class=p>;</span><span class=w> </span><span class=p>});</span>
<span class=n>json</span><span class=w> </span><span class=nf>j</span><span class=p>(</span><span class=n>filtered</span><span class=p>);</span><span class=w> </span><span class=c1>// [37,42,21]</span>
</code></pre></div> <p>This requires <a href=../../api/macros/json_has_ranges/ ><code>JSON_HAS_RANGES</code></a> to be enabled and is unavailable on <abbr title="Minimalist GNU for Windows">MinGW</abbr> due to incomplete C++20 ranges support there; see <a href=../../api/macros/json_has_range_view_conversion/ ><code>JSON_HAS_RANGE_VIEW_CONVERSION</code></a>.</p> </div> <h2 id=your-own-types>Your own types<a class=headerlink href=#your-own-types title="Permanent link">&para;</a></h2> <p>The conversions above are built in for standard types. To make the same syntax work for <strong>your own</strong> types, provide <code>to_json</code>/<code>from_json</code> functions (or use one of the convenience macros). This is described in detail on the <a href=../arbitrary_types/ >arbitrary types conversions</a> page. Enums can be mapped to strings as described in <a href=../enum_conversion/ >specializing enum conversion</a>.</p> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../../api/basic_json/get/ ><code>get</code></a> - get a copy converted to a given type</li> <li><a href=../../api/basic_json/get_to/ ><code>get_to</code></a> - convert into an existing variable</li> <li><a href=../../api/basic_json/get_ref/ ><code>get_ref</code></a> - / <a href=../../api/basic_json/get_ptr/ ><code>get_ptr</code></a> - access the stored value without copying</li> <li><a href=../arbitrary_types/ >Arbitrary types conversions</a> - support your own types</li> <li><a href=../../api/macros/json_use_implicit_conversions/ ><code>JSON_USE_IMPLICIT_CONVERSIONS</code></a> - toggle implicit conversions</li> <li><a href=../json_view/ >Zero-copy <abbr title="JavaScript Object Notation">JSON</abbr> views</a> - get values out of a document or a view</li> <li><a href=../../api/adl_serializer/ ><code>adl_serializer</code></a> - the default serializer behind the conversions</li> <li><a href=../../api/basic_json/type/ ><code>type</code></a> - query the type of a value</li> <li><a href=../../api/macros/json_disable_tuple_reference_conversion/ ><code>JSON_DISABLE_TUPLE_REFERENCE_CONVERSION</code></a> - toggle conversion to tuples of references</li> <li><a href=../../api/macros/json_has_range_view_conversion/ ><code>JSON_HAS_RANGE_VIEW_CONVERSION</code></a> - toggle conversion from ranges</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../comments/ class="md-footer__link md-footer__link--prev" aria-label="Previous: Comments"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> Comments </div> </div> </a> <a href=../creating_values/ class="md-footer__link md-footer__link--next" aria-label="Next: Creating JSON values"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> Creating JSON values </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &Line truncated
</code></pre></div> <p>This requires <a href=../../api/macros/json_has_ranges/ ><code>JSON_HAS_RANGES</code></a> to be enabled and is unavailable on <abbr title="Minimalist GNU for Windows">MinGW</abbr> due to incomplete C++20 ranges support there; see <a href=../../api/macros/json_has_range_view_conversion/ ><code>JSON_HAS_RANGE_VIEW_CONVERSION</code></a>.</p> </div> <h2 id=your-own-types>Your own types<a class=headerlink href=#your-own-types title="Permanent link">&para;</a></h2> <p>The conversions above are built in for standard types. To make the same syntax work for <strong>your own</strong> types, provide <code>to_json</code>/<code>from_json</code> functions (or use one of the convenience macros). This is described in detail on the <a href=../arbitrary_types/ >arbitrary types conversions</a> page. Enums can be mapped to strings as described in <a href=../enum_conversion/ >specializing enum conversion</a>.</p> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../../api/basic_json/get/ ><code>get</code></a> - get a copy converted to a given type</li> <li><a href=../../api/basic_json/get_to/ ><code>get_to</code></a> - convert into an existing variable</li> <li><a href=../../api/basic_json/get_ref/ ><code>get_ref</code></a> - / <a href=../../api/basic_json/get_ptr/ ><code>get_ptr</code></a> - access the stored value without copying</li> <li><a href=../arbitrary_types/ >Arbitrary types conversions</a> - support your own types</li> <li><a href=../../api/macros/json_use_implicit_conversions/ ><code>JSON_USE_IMPLICIT_CONVERSIONS</code></a> - toggle implicit conversions</li> <li><a href=../json_view/ >Zero-copy <abbr title="JavaScript Object Notation">JSON</abbr> views</a> - get values out of a document or a view</li> <li><a href=../../api/adl_serializer/ ><code>adl_serializer</code></a> - the default serializer behind the conversions</li> <li><a href=../../api/basic_json/type/ ><code>type</code></a> - query the type of a value</li> <li><a href=../../api/macros/json_disable_tuple_reference_conversion/ ><code>JSON_DISABLE_TUPLE_REFERENCE_CONVERSION</code></a> - toggle conversion to tuples of references</li> <li><a href=../../api/macros/json_has_range_view_conversion/ ><code>JSON_HAS_RANGE_VIEW_CONVERSION</code></a> - toggle conversion from ranges</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../comments/ class="md-footer__link md-footer__link--prev" aria-label="Previous: Comments"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> Comments </div> </div> </a> <a href=../creating_values/ class="md-footer__link md-footer__link--next" aria-label="Next: Creating JSON values"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> Creating JSON values </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &Line truncated
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<span class=n>json</span><span class=w> </span><span class=n>j_float</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=mf>3.141</span><span class=p>;</span>
<span class=n>json</span><span class=w> </span><span class=n>j_string</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=s>"Hello"</span><span class=p>;</span>
<span class=n>json</span><span class=w> </span><span class=n>j_boolean</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=nb>true</span><span class=p>;</span>
@@ -42,4 +42,4 @@
<span class=w> </span><span class=nt>"answer"</span><span class=p>:</span><span class=w> </span><span class=mi>42</span><span class=p>,</span>
<span class=w> </span><span class=nt>"hello"</span><span class=p>:</span><span class=w> </span><span class=s2>"world"</span>
<span class=p>}</span>
</code></pre></div> </details> <p>Note this <strong>parses</strong> the string, so <code class=highlight><span class=s>"42"</span><span class=n>_json</span></code> is the number <code class=highlight><span class=mi>42</span></code>, whereas <code class=highlight><span class=n>json</span><span class=p>(</span><span class=s>"42"</span><span class=p>)</span></code> is the <abbr title="JavaScript Object Notation">JSON</abbr> string <code class=highlight><span class=s2>"42"</span></code>.</p> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../../api/basic_json/basic_json/ ><code>basic_json</code> constructors</a> - all ways to construct a value</li> <li><a href=../../api/basic_json/array/ ><code>array</code></a> - / <a href=../../api/basic_json/object/ ><code>object</code></a> - force array or object type</li> <li><a href=../../api/operator_literal_json/ ><code>operator""_json</code></a> - the <code>_json</code> literal</li> <li><a href=../conversions/ >Converting values</a> - which C++ types can be used</li> <li><a href=../parsing/ >Parsing</a> - create a value from <abbr title="JavaScript Object Notation">JSON</abbr> text</li> <li><a href=../json_view/ >Zero-copy <abbr title="JavaScript Object Notation">JSON</abbr> views</a> - create a document and a view from <abbr title="JavaScript Object Notation">JSON</abbr> text instead of a value</li> <li><a href=../../api/basic_json/ ><code>basic_json</code></a> - the class template and its member functions</li> <li><a href=../types/ >Types</a> - the <abbr title="JavaScript Object Notation">JSON</abbr> value types</li> <li><a href=../../api/macros/json_brace_init_copy_semantics/ ><code>JSON_BRACE_INIT_COPY_SEMANTICS</code></a> - change how a single braced value is interpreted</li> <li><a href=../../api/basic_json/operator%3D/ ><code>operator=</code></a> - assign a new value</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../conversions/ class="md-footer__link md-footer__link--prev" aria-label="Previous: Converting values"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> Converting values </div> </div> </a> <a href=../element_access/ class="md-footer__link md-footer__link--next" aria-label="Next: Element Access"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> Element Access </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 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</code></pre></div> </details> <p>Note this <strong>parses</strong> the string, so <code class=highlight><span class=s>"42"</span><span class=n>_json</span></code> is the number <code class=highlight><span class=mi>42</span></code>, whereas <code class=highlight><span class=n>json</span><span class=p>(</span><span class=s>"42"</span><span class=p>)</span></code> is the <abbr title="JavaScript Object Notation">JSON</abbr> string <code class=highlight><span class=s2>"42"</span></code>.</p> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../../api/basic_json/basic_json/ ><code>basic_json</code> constructors</a> - all ways to construct a value</li> <li><a href=../../api/basic_json/array/ ><code>array</code></a> - / <a href=../../api/basic_json/object/ ><code>object</code></a> - force array or object type</li> <li><a href=../../api/operator_literal_json/ ><code>operator""_json</code></a> - the <code>_json</code> literal</li> <li><a href=../conversions/ >Converting values</a> - which C++ types can be used</li> <li><a href=../parsing/ >Parsing</a> - create a value from <abbr title="JavaScript Object Notation">JSON</abbr> text</li> <li><a href=../json_view/ >Zero-copy <abbr title="JavaScript Object Notation">JSON</abbr> views</a> - create a document and a view from <abbr title="JavaScript Object Notation">JSON</abbr> text instead of a value</li> <li><a href=../../api/basic_json/ ><code>basic_json</code></a> - the class template and its member functions</li> <li><a href=../types/ >Types</a> - the <abbr title="JavaScript Object Notation">JSON</abbr> value types</li> <li><a href=../../api/macros/json_brace_init_copy_semantics/ ><code>JSON_BRACE_INIT_COPY_SEMANTICS</code></a> - change how a single braced value is interpreted</li> <li><a href=../../api/basic_json/operator%3D/ ><code>operator=</code></a> - assign a new value</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../conversions/ class="md-footer__link md-footer__link--prev" aria-label="Previous: Converting values"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> Converting values </div> </div> </a> <a href=../element_access/ class="md-footer__link md-footer__link--next" aria-label="Next: Element Access"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> Element Access </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 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@@ -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=description content="JSON for Modern C++ is a header-only C++11 library that makes JSON a first-class data type."><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/element_access/checked_access/" rel="canonical"><link href=../unchecked_access/ rel=prev><link href=../default_value/ rel=next><link rel=icon href=../../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>Checked access: at - JSON for Modern C++</title><link rel=stylesheet href=../../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../../assets/stylesheets/palette.ab4e12ef.min.css><link rel=stylesheet href=../../../css/custom.css><link rel=stylesheet href=../../../css/brand.css><link rel=stylesheet href=../../../css/fig-sax.css><link rel=stylesheet 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<span class=w> </span><span class=nt>"name"</span><span class=p>:</span><span class=w> </span><span class=s2>"Mary Smith"</span><span class=p>,</span>
<span class=w> </span><span class=nt>"age"</span><span class=p>:</span><span class=w> </span><span class=mi>42</span><span class=p>,</span>
<span class=w> </span><span class=nt>"hobbies"</span><span class=p>:</span><span class=w> </span><span class=p>[</span><span class=s2>"hiking"</span><span class=p>,</span><span class=w> </span><span class=s2>"reading"</span><span class=p>]</span>
@@ -12,4 +12,4 @@
</code></pre></div> </details> <p>When accessing an invalid index (i.e., an index greater than or equal to the array size) or the passed object key is non-existing, an exception is thrown.</p> <details class=example> <summary>Example: access via invalid index or missing key</summary> <div class=highlight><pre><span></span><code><span class=n>j</span><span class=p>.</span><span class=n>at</span><span class=p>(</span><span class=s>"hobbies"</span><span class=p>).</span><span class=n>at</span><span class=p>(</span><span class=mi>3</span><span class=p>)</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=s>"cooking"</span><span class=p>;</span>
</code></pre></div> <p>This code produces the following exception:</p> <div class=highlight><pre><span></span><code>[json.exception.out_of_range.401] array index 3 is out of range
</code></pre></div> <p>When <a href=../../../home/exceptions/#extended-diagnostic-messages>extended diagnostic messages</a> are enabled by defining <a href=../../../api/macros/json_diagnostics/ ><code>JSON_DIAGNOSTICS</code></a>, the exception further gives information where the key or index is missing or out of range.</p> <div class=highlight><pre><span></span><code>[json.exception.out_of_range.401] (/hobbies) array index 3 is out of range
</code></pre></div> </details> <h2 id=notes>Notes<a class=headerlink href=#notes title="Permanent link">&para;</a></h2> <div class="admonition failure"> <p class=admonition-title>Exceptions</p> <ul> <li><a href=../../../api/basic_json/at/ ><code>at</code></a> can only be used with objects (with a string argument) or with arrays (with a numeric argument). For other types, a <a href=../../../home/exceptions/#jsonexceptiontype_error304><code>basic_json::type_error</code></a> is thrown.</li> <li><a href=../../../home/exceptions/#out-of-range><code>basic_json::out_of_range</code> exception</a> exceptions are thrown if the provided key is not found in an object or the provided index is invalid.</li> </ul> </div> <h2 id=summary>Summary<a class=headerlink href=#summary title="Permanent link">&para;</a></h2> <table> <thead> <tr> <th>scenario</th> <th>non-const value</th> <th>const value</th> </tr> </thead> <tbody> <tr> <td>access to existing object key</td> <td>reference to existing value is returned</td> <td>const reference to existing value is returned</td> </tr> <tr> <td>access to valid array index</td> <td>reference to existing value is returned</td> <td>const reference to existing value is returned</td> </tr> <tr> <td>access to non-existing object key</td> <td><code>basic_json::out_of_range</code> exception is thrown</td> <td><code>basic_json::out_of_range</code> exception is thrown</td> </tr> <tr> <td>access to invalid array index</td> <td><code>basic_json::out_of_range</code> exception is thrown</td> <td><code>basic_json::out_of_range</code> exception is thrown</td> </tr> </tbody> </table> <p class=nj-table-caption>Result of <a href=../../../api/basic_json/at/ ><code>at</code></a> in each scenario, for a non-const and a const value.</p> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../ >Element Access</a> - overview of all ways to access elements</li> <li><a href=../unchecked_access/ >Unchecked access: operator[]</a> - access without range checking</li> <li><a href=../default_value/ >Access with default value: value</a> - fall back to a default instead of throwing</li> <li><a href=../../../api/basic_json/at/ ><code>at</code></a> - checked access by reference</li> <li><a href=../../json_pointer/ ><abbr title="JavaScript Object Notation">JSON</abbr> Pointer</a> - access nested values with <code>at</code> and a pointer</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../unchecked_access/ class="md-footer__link md-footer__link--prev" aria-label="Previous: Unchecked access: operator[]"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> Unchecked access: operator[] </div> </div> </a> <a href=../default_value/ class="md-footer__link md-footer__link--next" aria-label="Next: Access with default value: value"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> Access with default value: value </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11z"/></svg> </div> </a> </nav> <div clasLine truncated
</code></pre></div> </details> <h2 id=notes>Notes<a class=headerlink href=#notes title="Permanent link">&para;</a></h2> <div class="admonition failure"> <p class=admonition-title>Exceptions</p> <ul> <li><a href=../../../api/basic_json/at/ ><code>at</code></a> can only be used with objects (with a string argument) or with arrays (with a numeric argument). For other types, a <a href=../../../home/exceptions/#jsonexceptiontype_error304><code>basic_json::type_error</code></a> is thrown.</li> <li><a href=../../../home/exceptions/#out-of-range><code>basic_json::out_of_range</code> exception</a> exceptions are thrown if the provided key is not found in an object or the provided index is invalid.</li> </ul> </div> <h2 id=summary>Summary<a class=headerlink href=#summary title="Permanent link">&para;</a></h2> <table> <thead> <tr> <th>scenario</th> <th>non-const value</th> <th>const value</th> </tr> </thead> <tbody> <tr> <td>access to existing object key</td> <td>reference to existing value is returned</td> <td>const reference to existing value is returned</td> </tr> <tr> <td>access to valid array index</td> <td>reference to existing value is returned</td> <td>const reference to existing value is returned</td> </tr> <tr> <td>access to non-existing object key</td> <td><code>basic_json::out_of_range</code> exception is thrown</td> <td><code>basic_json::out_of_range</code> exception is thrown</td> </tr> <tr> <td>access to invalid array index</td> <td><code>basic_json::out_of_range</code> exception is thrown</td> <td><code>basic_json::out_of_range</code> exception is thrown</td> </tr> </tbody> </table> <p class=nj-table-caption>Result of <a href=../../../api/basic_json/at/ ><code>at</code></a> in each scenario, for a non-const and a const value.</p> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../ >Element Access</a> - overview of all ways to access elements</li> <li><a href=../unchecked_access/ >Unchecked access: operator[]</a> - access without range checking</li> <li><a href=../default_value/ >Access with default value: value</a> - fall back to a default instead of throwing</li> <li><a href=../../../api/basic_json/at/ ><code>at</code></a> - checked access by reference</li> <li><a href=../../json_pointer/ ><abbr title="JavaScript Object Notation">JSON</abbr> Pointer</a> - access nested values with <code>at</code> and a pointer</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../unchecked_access/ class="md-footer__link md-footer__link--prev" aria-label="Previous: Unchecked access: operator[]"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> Unchecked access: operator[] </div> </div> </a> <a href=../default_value/ class="md-footer__link md-footer__link--next" aria-label="Next: Access with default value: value"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> Access with default value: value </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11z"/></svg> </div> </a> </nav> <div clasLine truncated
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class=md-option data-md-color-media="(prefers-color-scheme: light)" data-md-color-scheme=default data-md-color-primary=custom data-md-color-accent=custom aria-label="Switch to dark mode" type=radio name=__palette id=__palette_0> <label class="md-header__button md-icon" title="Switch to dark mode" for=__palette_1 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 8a4 4 0 0 0-4 4 4 4 0 0 0 4 4 4 4 0 0 0 4-4 4 4 0 0 0-4-4m0 10a6 6 0 0 1-6-6 6 6 0 0 1 6-6 6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> <input class=md-option data-md-color-media="(prefers-color-scheme: dark)" data-md-color-scheme=slate data-md-color-primary=custom data-md-color-accent=custom aria-label="Switch to light mode" type=radio name=__palette id=__palette_1> <label class="md-header__button md-icon" title="Switch to light mode" for=__palette_0 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 18c-.89 0-1.74-.2-2.5-.55C11.56 16.5 13 14.42 13 12s-1.44-4.5-3.5-5.45C10.26 6.2 11.11 6 12 6a6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> </form> <script>var palette=__md_get("__palette");if(palette&&palette.color){if("(prefers-color-scheme)"===palette.color.media){var media=matchMedia("(prefers-color-scheme: light)"),input=document.querySelector(media.matches?"[data-md-color-media='(prefers-color-scheme: light)']":"[data-md-color-media='(prefers-color-scheme: dark)']");palette.color.media=input.getAttribute("data-md-color-media"),palette.color.scheme=input.getAttribute("data-md-color-scheme"),palette.color.primary=input.getAttribute("data-md-color-primary"),palette.color.accent=input.getAttribute("data-md-color-accent")}for(var[key,value]of Object.entries(palette.color))document.body.setAttribute("data-md-color-"+key,value)}</script> <label class="md-header__button md-icon" for=__search> <svg xmlnsLine truncated
<span class=w> </span><span class=nt>"logOutput"</span><span class=p>:</span><span class=w> </span><span class=s2>"result.log"</span><span class=p>,</span>
<span class=w> </span><span class=nt>"append"</span><span class=p>:</span><span class=w> </span><span class=kc>true</span>
<span class=p>}</span>
@@ -48,4 +48,4 @@
<span class=err>de</span><span class=kc>fault</span><span class=w> </span><span class=err>value</span><span class=w> </span><span class=err>(i</span><span class=kc>nt</span><span class=err>)</span><span class=p>:</span><span class=w> </span><span class=mi>-1</span>
<span class=err>de</span><span class=kc>fault</span><span class=w> </span><span class=err>value</span><span class=w> </span><span class=err>(ui</span><span class=kc>nt</span><span class=mi>64</span><span class=err>_</span><span class=kc>t</span><span class=err>)</span><span class=p>:</span><span class=w> </span><span class=mi>18446744073709551615</span>
<span class=err>explici</span><span class=kc>t</span><span class=w> </span><span class=err>re</span><span class=kc>turn</span><span class=w> </span><span class=err>value</span><span class=w> </span><span class=kc>t</span><span class=err>ype</span><span class=p>:</span><span class=w> </span><span class=mi>18446744073709551615</span>
</code></pre></div> </div> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../../../api/basic_json/value/ ><code>value</code></a> - for access with default value</li> <li><a href=../checked_access/ >Checked access: at</a> - access with bounds checking</li> <li><a href=../../json_pointer/ ><abbr title="JavaScript Object Notation">JSON</abbr> Pointer</a> - addressing values by path</li> <li><a href=../../../api/basic_json/contains/ ><code>contains</code></a> - to check whether a key or <abbr title="JavaScript Object Notation">JSON</abbr> Pointer exists</li> <li><a href=../../../api/json_pointer/operator_slasheq/ ><code>json_pointer::operator/=</code></a> - to build a pointer token by token</li> <li><a href=../unchecked_access/ >Unchecked access: operator[]</a> - access without range checking</li> <li><a href=../ >Element Access</a> - overview of all ways to access elements</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../checked_access/ class="md-footer__link md-footer__link--prev" aria-label="Previous: Checked access: at"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> Checked access: at </div> </div> </a> <a href=../../iterators/ class="md-footer__link md-footer__link--next" aria-label="Next: Iterators"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> Iterators </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 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</code></pre></div> </div> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../../../api/basic_json/value/ ><code>value</code></a> - for access with default value</li> <li><a href=../checked_access/ >Checked access: at</a> - access with bounds checking</li> <li><a href=../../json_pointer/ ><abbr title="JavaScript Object Notation">JSON</abbr> Pointer</a> - addressing values by path</li> <li><a href=../../../api/basic_json/contains/ ><code>contains</code></a> - to check whether a key or <abbr title="JavaScript Object Notation">JSON</abbr> Pointer exists</li> <li><a href=../../../api/json_pointer/operator_slasheq/ ><code>json_pointer::operator/=</code></a> - to build a pointer token by token</li> <li><a href=../unchecked_access/ >Unchecked access: operator[]</a> - access without range checking</li> <li><a href=../ >Element Access</a> - overview of all ways to access elements</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../checked_access/ class="md-footer__link md-footer__link--prev" aria-label="Previous: Checked access: at"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> Checked access: at </div> </div> </a> <a href=../../files/ class="md-footer__link md-footer__link--next" aria-label="Next: Reading and writing files"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> Reading and writing files </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 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+2 -2
View File
@@ -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=description content="JSON for Modern C++ is a header-only C++11 library that makes JSON a first-class data type."><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/element_access/" rel="canonical"><link href=../creating_values/ rel=prev><link href=unchecked_access/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>Element Access - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.ab4e12ef.min.css><link rel=stylesheet href=../../css/custom.css><link rel=stylesheet href=../../css/brand.css><link rel=stylesheet href=../../css/fig-sax.css><link rel=stylesheet href=../../css/fig-jsonview.css><link rel=stylesheet 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A["getting a value out"] --&gt; P{{"nested path such as /server/port?"}}
P --&gt;|"yes"| JP["JSON Pointer: at(ptr), value(ptr, default), contains(ptr)"]
P --&gt;|"no, one key or index"| T{{"only testing whether it exists?"}}
@@ -20,4 +20,4 @@
<span class=p>}</span>
</code></pre></div> <p><code>contains</code> also returns <code>true</code> for a key whose value is <code>null</code>; use <code>j.contains("k") &amp;&amp; !j["k"].is_null()</code> if a <code>null</code> should count as missing. To read a value with a fallback instead, see <a href=default_value/ ><code>value</code></a>.</p> <div class="admonition warning"> <p class=admonition-title><code>operator[]</code> inserts missing keys</p> <p>Like <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>map</span><span class=o>::</span><span class=k>operator</span><span class=p>[]</span></code>, the non-const <a href=unchecked_access/ ><code>operator[]</code></a> adds a <code>null</code> value if the key is missing. Testing a key with it therefore <em>changes the object</em>:</p> <div class=highlight><pre><span></span><code><span class=n>json</span><span class=w> </span><span class=n>j</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=p>{{</span><span class=s>"foo"</span><span class=p>,</span><span class=w> </span><span class=mi>1</span><span class=p>}};</span>
<span class=kt>bool</span><span class=w> </span><span class=n>absent</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>j</span><span class=p>[</span><span class=s>"bar"</span><span class=p>].</span><span class=n>is_null</span><span class=p>();</span><span class=w> </span><span class=c1>// true, but j is now {"bar": null, "foo": 1}</span>
</code></pre></div> <p>Use <code>contains</code>/<code>find</code> to test, and <a href=checked_access/ ><code>at</code></a> to access a value that must exist (it throws an <a href=../../home/exceptions/#out-of-range><code>out_of_range</code></a> exception otherwise).</p> </div> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../../api/basic_json/operator%5B%5D/ ><code>operator[]</code></a> - unchecked access</li> <li><a href=../../api/basic_json/at/ ><code>at</code></a> - checked access</li> <li><a href=../../api/basic_json/value/ ><code>value</code></a> - access with default value</li> <li><a href=../../api/basic_json/contains/ ><code>contains</code></a> - and <a href=../../api/basic_json/find/ ><code>find</code></a> - test for existence</li> <li><a href=../../api/basic_json/front/ ><code>front</code></a> - and <a href=../../api/basic_json/back/ ><code>back</code></a> - first and last element</li> <li><a href=../../api/basic_json/items/ ><code>items</code></a> - iterate over keys and values</li> <li><a href=checked_access/ >Checked access: at</a> - access with range checking</li> <li><a href=unchecked_access/ >Unchecked access: operator[]</a> - access without range checking</li> <li><a href=default_value/ >Access with default value: value</a> - fall back to a default instead of throwing</li> <li><a href=../iterators/ >Iterators</a> - visit all elements of arrays and objects</li> <li><a href=../json_pointer/ ><abbr title="JavaScript Object Notation">JSON</abbr> Pointer</a> - address nested values with a single string</li> <li><a href=../modifying_values/ >Modifying values</a> - add, merge and remove elements</li> <li><a href=../../api/basic_json/ ><code>basic_json</code></a> - all member functions</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../creating_values/ class="md-footer__link md-footer__link--prev" aria-label="Previous: Creating JSON values"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> Creating JSON values </div> </div> </a> <a href=unchecked_access/ class="md-footer__link md-footer__link--next" aria-label="Next: Unchecked access: operator[]"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> Unchecked access: operator[] </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 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</code></pre></div> <p>Use <code>contains</code>/<code>find</code> to test, and <a href=checked_access/ ><code>at</code></a> to access a value that must exist (it throws an <a href=../../home/exceptions/#out-of-range><code>out_of_range</code></a> exception otherwise).</p> </div> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../../api/basic_json/operator%5B%5D/ ><code>operator[]</code></a> - unchecked access</li> <li><a href=../../api/basic_json/at/ ><code>at</code></a> - checked access</li> <li><a href=../../api/basic_json/value/ ><code>value</code></a> - access with default value</li> <li><a href=../../api/basic_json/contains/ ><code>contains</code></a> - and <a href=../../api/basic_json/find/ ><code>find</code></a> - test for existence</li> <li><a href=../../api/basic_json/front/ ><code>front</code></a> - and <a href=../../api/basic_json/back/ ><code>back</code></a> - first and last element</li> <li><a href=../../api/basic_json/items/ ><code>items</code></a> - iterate over keys and values</li> <li><a href=checked_access/ >Checked access: at</a> - access with range checking</li> <li><a href=unchecked_access/ >Unchecked access: operator[]</a> - access without range checking</li> <li><a href=default_value/ >Access with default value: value</a> - fall back to a default instead of throwing</li> <li><a href=../iterators/ >Iterators</a> - visit all elements of arrays and objects</li> <li><a href=../json_pointer/ ><abbr title="JavaScript Object Notation">JSON</abbr> Pointer</a> - address nested values with a single string</li> <li><a href=../modifying_values/ >Modifying values</a> - add, merge and remove elements</li> <li><a href=../../api/basic_json/ ><code>basic_json</code></a> - all member functions</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../creating_values/ class="md-footer__link md-footer__link--prev" aria-label="Previous: Creating JSON values"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> Creating JSON values </div> </div> </a> <a href=unchecked_access/ class="md-footer__link md-footer__link--next" aria-label="Next: Unchecked access: operator[]"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> Unchecked access: operator[] </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 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<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=description content="JSON for Modern C++ is a header-only C++11 library that makes JSON a first-class data type."><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/element_access/unchecked_access/" rel="canonical"><link href=../ rel=prev><link href=../checked_access/ rel=next><link rel=icon href=../../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.8.0"><title>Unchecked access: operator[] - JSON for Modern C++</title><link rel=stylesheet href=../../../assets/stylesheets/main.57a0d79d.min.css><link rel=stylesheet href=../../../assets/stylesheets/palette.fb2fffe2.min.css><link rel=stylesheet href=../../../css/custom.css><link rel=stylesheet href=../../../css/brand.css><link rel=stylesheet href=../../../css/fig-sax.css><link rel=stylesheet 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<span class=w> </span><span class=nt>"name"</span><span class=p>:</span><span class=w> </span><span class=s2>"Mary Smith"</span><span class=p>,</span>
<span class=w> </span><span class=nt>"age"</span><span class=p>:</span><span class=w> </span><span class=mi>42</span><span class=p>,</span>
<span class=w> </span><span class=nt>"hobbies"</span><span class=p>:</span><span class=w> </span><span class=p>[</span><span class=s2>"hiking"</span><span class=p>,</span><span class=w> </span><span class=s2>"reading"</span><span class=p>]</span>
@@ -24,4 +24,4 @@
<span class=k>for</span><span class=w> </span><span class=p>(</span><span class=kt>int</span><span class=w> </span><span class=n>i</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=mi>0</span><span class=p>;</span><span class=w> </span><span class=n>i</span><span class=w> </span><span class=o>&lt;</span><span class=w> </span><span class=mi>1000</span><span class=p>;</span><span class=w> </span><span class=o>++</span><span class=n>i</span><span class=p>)</span><span class=w> </span><span class=p>{</span>
<span class=w> </span><span class=n>j</span><span class=p>.</span><span class=n>push_back</span><span class=p>(</span><span class=n>i</span><span class=p>);</span>
<span class=p>}</span>
</code></pre></div> <h2 id=summary>Summary<a class=headerlink href=#summary title="Permanent link">&para;</a></h2> <table> <thead> <tr> <th>scenario</th> <th>non-const value</th> <th>const value</th> </tr> </thead> <tbody> <tr> <td>access to existing object key</td> <td>reference to existing value is returned</td> <td>const reference to existing value is returned</td> </tr> <tr> <td>access to valid array index</td> <td>reference to existing value is returned</td> <td>const reference to existing value is returned</td> </tr> <tr> <td>access to non-existing object key</td> <td>reference to newly inserted <code class=highlight><span class=kc>null</span></code> value is returned</td> <td><strong>undefined behavior</strong>; <a href=../../assertions/ >runtime assertion</a> in debug mode</td> </tr> <tr> <td>access to invalid array index</td> <td>reference to newly inserted <code class=highlight><span class=kc>null</span></code> value is returned; any index between previous maximal index and passed index are filled with <code class=highlight><span class=kc>null</span></code></td> <td><strong>undefined behavior</strong>; <a href=../../assertions/ >runtime assertion</a> in debug mode</td> </tr> </tbody> </table> <p class=nj-table-caption>Result of <a href=../../../api/basic_json/operator%5B%5D/ ><code>operator[]</code></a> in each scenario, for a non-const and a const value.</p> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../ >Element Access</a> - overview of all ways to access elements</li> <li><a href=../checked_access/ >Checked access: at</a> - access with range checking</li> <li><a href=../default_value/ >Access with default value: value</a> - fall back to a default instead of throwing</li> <li><a href=../../../api/basic_json/operator%5B%5D/ ><code>operator[]</code></a> - unchecked access by reference</li> <li><a href=../../assertions/ >Runtime assertions</a> - assertions that guard <code>operator[]</code> misuse</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../ class="md-footer__link md-footer__link--prev" aria-label="Previous: Element Access"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> Element Access </div> </div> </a> <a href=../checked_access/ class="md-footer__link md-footer__link--next" aria-label="Next: Checked access: at"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> Checked access: at </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 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</code></pre></div> <h2 id=summary>Summary<a class=headerlink href=#summary title="Permanent link">&para;</a></h2> <table> <thead> <tr> <th>scenario</th> <th>non-const value</th> <th>const value</th> </tr> </thead> <tbody> <tr> <td>access to existing object key</td> <td>reference to existing value is returned</td> <td>const reference to existing value is returned</td> </tr> <tr> <td>access to valid array index</td> <td>reference to existing value is returned</td> <td>const reference to existing value is returned</td> </tr> <tr> <td>access to non-existing object key</td> <td>reference to newly inserted <code class=highlight><span class=kc>null</span></code> value is returned</td> <td><strong>undefined behavior</strong>; <a href=../../assertions/ >runtime assertion</a> in debug mode</td> </tr> <tr> <td>access to invalid array index</td> <td>reference to newly inserted <code class=highlight><span class=kc>null</span></code> value is returned; any index between previous maximal index and passed index are filled with <code class=highlight><span class=kc>null</span></code></td> <td><strong>undefined behavior</strong>; <a href=../../assertions/ >runtime assertion</a> in debug mode</td> </tr> </tbody> </table> <p class=nj-table-caption>Result of <a href=../../../api/basic_json/operator%5B%5D/ ><code>operator[]</code></a> in each scenario, for a non-const and a const value.</p> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../ >Element Access</a> - overview of all ways to access elements</li> <li><a href=../checked_access/ >Checked access: at</a> - access with range checking</li> <li><a href=../default_value/ >Access with default value: value</a> - fall back to a default instead of throwing</li> <li><a href=../../../api/basic_json/operator%5B%5D/ ><code>operator[]</code></a> - unchecked access by reference</li> <li><a href=../../assertions/ >Runtime assertions</a> - assertions that guard <code>operator[]</code> misuse</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../ class="md-footer__link md-footer__link--prev" aria-label="Previous: Element Access"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> Element Access </div> </div> </a> <a href=../checked_access/ class="md-footer__link md-footer__link--next" aria-label="Next: Checked access: at"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> Checked access: at </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 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+2 -2
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@@ -1,4 +1,4 @@
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<span class=k>enum</span><span class=w> </span><span class=nc>TaskState</span><span class=w> </span><span class=p>{</span>
<span class=w> </span><span class=n>TS_STOPPED</span><span class=p>,</span>
<span class=w> </span><span class=n>TS_RUNNING</span><span class=p>,</span>
@@ -146,4 +146,4 @@
<span class=p>}</span>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code><span class=err>deserializa</span><span class=kc>t</span><span class=err>io</span><span class=kc>n</span><span class=w> </span><span class=kc>fa</span><span class=err>iled</span><span class=p>:</span><span class=w> </span><span class=p>[</span><span class=err>jso</span><span class=kc>n</span><span class=err>.excep</span><span class=kc>t</span><span class=err>io</span><span class=kc>n</span><span class=err>.ou</span><span class=kc>t</span><span class=err>_o</span><span class=kc>f</span><span class=err>_ra</span><span class=kc>n</span><span class=err>ge.</span><span class=mi>410</span><span class=p>]</span><span class=w> </span><span class=err>e</span><span class=kc>nu</span><span class=err>m</span><span class=w> </span><span class=err>value</span><span class=w> </span><span class=err>ou</span><span class=kc>t</span><span class=w> </span><span class=err>o</span><span class=kc>f</span><span class=w> </span><span class=err>ra</span><span class=kc>n</span><span class=err>ge</span><span class=w> </span><span class=kc>f</span><span class=err>or</span><span class=w> </span><span class=err>Color</span>
<span class=err>deserializa</span><span class=kc>t</span><span class=err>io</span><span class=kc>n</span><span class=w> </span><span class=kc>fa</span><span class=err>iled</span><span class=p>:</span><span class=w> </span><span class=p>[</span><span class=err>jso</span><span class=kc>n</span><span class=err>.excep</span><span class=kc>t</span><span class=err>io</span><span class=kc>n</span><span class=err>.ou</span><span class=kc>t</span><span class=err>_o</span><span class=kc>f</span><span class=err>_ra</span><span class=kc>n</span><span class=err>ge.</span><span class=mi>410</span><span class=p>]</span><span class=w> </span><span class=err>e</span><span class=kc>nu</span><span class=err>m</span><span class=w> </span><span class=err>value</span><span class=w> </span><span class=err>ou</span><span class=kc>t</span><span class=w> </span><span class=err>o</span><span class=kc>f</span><span class=w> </span><span class=err>ra</span><span class=kc>n</span><span class=err>ge</span><span class=w> </span><span class=kc>f</span><span class=err>or</span><span class=w> </span><span class=err>Color</span><span class=p>:</span><span class=w> </span><span class=s2>"what"</span>
</code></pre></div> </details> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../arbitrary_types/ >Arbitrary Type Conversions</a> - convert your own types</li> <li><a href=../conversions/ >Converting values</a> - the general overview of <code>get</code>/<code>get_to</code></li> <li><a href=../../api/macros/nlohmann_json_serialize_enum/ ><code>NLOHMANN_JSON_SERIALIZE_ENUM</code></a> - define the enum-to-<abbr title="JavaScript Object Notation">JSON</abbr> mapping</li> <li><a href=../../api/macros/nlohmann_json_serialize_enum_strict/ ><code>NLOHMANN_JSON_SERIALIZE_ENUM_STRICT</code></a> - throw on unmapped values</li> <li><a href=../../api/macros/json_disable_enum_serialization/ ><code>JSON_DISABLE_ENUM_SERIALIZATION</code></a> - disable the default enum conversion</li> <li><a href=../../api/macros/json_use_objects_for_enum_keyed_maps/ ><code>JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS</code></a> - serialize maps with enum keys as objects</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../serialization/ class="md-footer__link md-footer__link--prev" aria-label="Previous: Serialization"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> Serialization </div> </div> </a> <a href=../macros/ class="md-footer__link md-footer__link--next" aria-label="Next: Supported Macros"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> Supported Macros </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 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</code></pre></div> </details> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../arbitrary_types/ >Arbitrary Type Conversions</a> - convert your own types</li> <li><a href=../conversions/ >Converting values</a> - the general overview of <code>get</code>/<code>get_to</code></li> <li><a href=../../api/macros/nlohmann_json_serialize_enum/ ><code>NLOHMANN_JSON_SERIALIZE_ENUM</code></a> - define the enum-to-<abbr title="JavaScript Object Notation">JSON</abbr> mapping</li> <li><a href=../../api/macros/nlohmann_json_serialize_enum_strict/ ><code>NLOHMANN_JSON_SERIALIZE_ENUM_STRICT</code></a> - throw on unmapped values</li> <li><a href=../../api/macros/json_disable_enum_serialization/ ><code>JSON_DISABLE_ENUM_SERIALIZATION</code></a> - disable the default enum conversion</li> <li><a href=../../api/macros/json_use_objects_for_enum_keyed_maps/ ><code>JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS</code></a> - serialize maps with enum keys as objects</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../serialization/ class="md-footer__link md-footer__link--prev" aria-label="Previous: Serialization"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> Serialization </div> </div> </a> <a href=../macros/ class="md-footer__link md-footer__link--next" aria-label="Next: Supported Macros"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> Supported Macros </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 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# Reading and writing files
The library has no functions that take a file name. It reads from and writes to streams, so files are handled with
`#!cpp std::ifstream` and `#!cpp std::ofstream` (or a `#!cpp FILE*` when reading). This page collects the recipes for
the common cases and the mistakes that show up again and again.
## Reading a JSON file
Pass the stream to [`parse`](../api/basic_json/parse.md) directly, and check that the file was opened:
```cpp
#include <fstream>
#include <stdexcept>
#include <string>
#include <nlohmann/json.hpp>
using json = nlohmann::json;
json read_json_file(const std::string& path)
{
std::ifstream f(path);
if (!f.is_open())
{
throw std::runtime_error("cannot open " + path);
}
return json::parse(f);
}
```
- **Check that the file is open.** A stream that failed to open looks like an empty input, so `parse` reports
[`parse_error.101`](../home/exceptions.md#jsonexceptionparse_error101) ("unexpected end of input") at line 1,
column 1 instead of saying that the file is missing. A relative path is resolved against the working directory of the
running program, which is often not the directory of the source file (see the
[FAQ](../home/faq.md#parsing-a-file-fails-with-unexpected-end-of-input)).
- **Don't read the file into a string first.** Parsing from a `#!cpp std::ifstream` is only about 10 to 20 % slower
than reading the whole file into a string with one `read()` and parsing that, and it needs no memory for the copy (see
[input sources and performance](parsing/index.md#input-sources-and-performance)). The common way to read a file with
`#!cpp std::istreambuf_iterator<char>` is so slow that it takes about twice as long as parsing from the stream. Read
the file into a string only if you need the text itself, for example for an error message. This advice is for
`#!cpp json::parse`. [`json_document`](json_view.md) reads a stream into a buffer that it owns anyway, so there is no
copy to save: reading the file into a `#!cpp std::string` yourself and passing that as an lvalue, which the document
borrows, was about 6 to 12 % faster than parsing from the stream, with the same single buffer.
- `parse` requires the file to hold exactly one JSON value. Files with one value per line are read with
[JSON Lines](parsing/json_lines.md); files too large for memory are covered in
[Large, streamed, and partial input](parsing/large_and_partial_input.md).
## Writing a JSON file
[`dump`](../api/basic_json/dump.md) the value into a string first, then write the string:
```cpp
void write_json_file(const std::string& path, const json& j)
{
const std::string text = j.dump(4) + '\n'; // may throw: nothing is written yet
std::ofstream f(path, std::ios::binary);
if (!f.is_open())
{
throw std::runtime_error("cannot open " + path);
}
f << text;
f.close();
if (f.fail())
{
throw std::runtime_error("cannot write " + path);
}
}
```
- **Serialize before opening the file.** Opening a `#!cpp std::ofstream` truncates the file. `dump` and
[`operator<<`](../api/operator_ltlt.md) throw [`type_error.316`](../home/exceptions.md#jsonexceptiontype_error316)
when a string is not valid UTF-8 (see [handling invalid UTF-8](serialization.md#handling-invalid-utf-8)), and
`#!cpp f << std::setw(4) << j` may already have written part of the value when it throws. Serializing into a string
first leaves the old file untouched in that case.
- **Check the stream after closing it.** A full disk or a missing permission sets the stream's error state; it does not
throw unless you enabled exceptions on the stream with `exceptions()`.
- `std::ios::binary` writes the text as it is. Without it, Windows writes `\r\n` for every newline of a pretty-printed
value.
## Replacing a file without corrupting it
The library never writes a file itself, so this section is not about the library. It is general C++ advice for a
problem users of the library keep running into.
A program that crashes or is killed while it writes leaves a truncated file behind. To make sure readers see either the
old or the new contents, write to a temporary file in the same directory and rename it over the original
(C++17, `#!cpp <filesystem>`):
```cpp
#include <filesystem>
void replace_json_file(const std::filesystem::path& path, const json& j)
{
std::filesystem::path tmp = path;
tmp += ".tmp"; // use a unique name if several processes may write the file
write_json_file(tmp.string(), j);
std::filesystem::rename(tmp, path); // replaces path
}
```
The temporary file must be in the same directory (more precisely, on the same file system) as the target: a rename
across file systems does not fall back to a copy, it fails and throws `#!cpp std::filesystem::filesystem_error`. On
POSIX systems, the rename atomically replaces the file. Standard C++ has no way to flush a file to the disk (`fsync`),
so this protects against a crashed program, but not necessarily against a power loss.
## Binary formats
The [binary formats](binary_formats/index.md) work the same way, with the streams opened in binary mode:
```cpp
// read
std::ifstream in("data.cbor", std::ios::binary);
json j = json::from_cbor(in);
// write
const std::vector<std::uint8_t> bytes = json::to_cbor(j);
std::ofstream out("data.cbor", std::ios::binary);
out.write(reinterpret_cast<const char*>(bytes.data()), static_cast<std::streamsize>(bytes.size()));
```
Without `std::ios::binary`, Windows turns the bytes `0x0D 0x0A` into `0x0A` and may stop at a byte `0x1A` while reading,
and turns `0x0A` into `0x0D 0x0A` while writing, which corrupts the data.
## See also
- [Parsing](parsing/index.md) - the inputs `parse` accepts
- [Serialization](serialization.md) - the options of `dump`
- [`parse`](../api/basic_json/parse.md), [`dump`](../api/basic_json/dump.md)
- [`operator>>`](../api/operator_gtgt.md), [`operator<<`](../api/operator_ltlt.md)
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<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;stdexcept&gt;</span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;string&gt;</span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;nlohmann/json.hpp&gt;</span>
<span class=k>using</span><span class=w> </span><span class=n>json</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>nlohmann</span><span class=o>::</span><span class=n>json</span><span class=p>;</span>
<span class=n>json</span><span class=w> </span><span class=nf>read_json_file</span><span class=p>(</span><span class=k>const</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>string</span><span class=o>&amp;</span><span class=w> </span><span class=n>path</span><span class=p>)</span>
<span class=p>{</span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>ifstream</span><span class=w> </span><span class=n>f</span><span class=p>(</span><span class=n>path</span><span class=p>);</span>
<span class=w> </span><span class=k>if</span><span class=w> </span><span class=p>(</span><span class=o>!</span><span class=n>f</span><span class=p>.</span><span class=n>is_open</span><span class=p>())</span>
<span class=w> </span><span class=p>{</span>
<span class=w> </span><span class=k>throw</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>runtime_error</span><span class=p>(</span><span class=s>"cannot open "</span><span class=w> </span><span class=o>+</span><span class=w> </span><span class=n>path</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>json</span><span class=o>::</span><span class=n>parse</span><span class=p>(</span><span class=n>f</span><span class=p>);</span>
<span class=p>}</span>
</code></pre></div> <ul> <li><strong>Check that the file is open.</strong> A stream that failed to open looks like an empty input, so <code>parse</code> reports <a href=../../home/exceptions/#jsonexceptionparse_error101><code>parse_error.101</code></a> ("unexpected end of input") at line 1, column 1 instead of saying that the file is missing. A relative path is resolved against the working directory of the running program, which is often not the directory of the source file (see the <a href=../../home/faq/#parsing-a-file-fails-with-unexpected-end-of-input><abbr title="Frequently Asked Questions">FAQ</abbr></a>).</li> <li><strong>Don't read the file into a string first.</strong> Parsing from a <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>ifstream</span></code> is only about 10 to 20 % slower than reading the whole file into a string with one <code>read()</code> and parsing that, and it needs no memory for the copy (see <a href=../parsing/#input-sources-and-performance>input sources and performance</a>). The common way to read a file with <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>istreambuf_iterator</span><span class=o>&lt;</span><span class=kt>char</span><span class=o>&gt;</span></code> is so slow that it takes about twice as long as parsing from the stream. Read the file into a string only if you need the text itself, for example for an error message. This advice is for <code class=highlight><span class=n>json</span><span class=o>::</span><span class=n>parse</span></code>. <a href=../json_view/ ><code>json_document</code></a> reads a stream into a buffer that it owns anyway, so there is no copy to save: reading the file into a <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>string</span></code> yourself and passing that as an lvalue, which the document borrows, was about 6 to 12 % faster than parsing from the stream, with the same single buffer.</li> <li><code>parse</code> requires the file to hold exactly one <abbr title="JavaScript Object Notation">JSON</abbr> value. Files with one value per line are read with <a href=../parsing/json_lines/ ><abbr title="JavaScript Object Notation">JSON</abbr> Lines</a>; files too large for memory are covered in <a href=../parsing/large_and_partial_input/ >Large, streamed, and partial input</a>.</li> </ul> <h2 id=writing-a-json-file>Writing a <abbr title="JavaScript Object Notation">JSON</abbr> file<a class=headerlink href=#writing-a-json-file title="Permanent link">&para;</a></h2> <p><a href=../../api/basic_json/dump/ ><code>dump</code></a> the value into a string first, then write the string:</p> <div class=highlight><pre><span></span><code><span class=kt>void</span><span class=w> </span><span class=nf>write_json_file</span><span class=p>(</span><span class=k>const</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>string</span><span class=o>&amp;</span><span class=w> </span><span class=n>path</span><span class=p>,</span><span class=w> </span><span class=k>const</span><span class=w> </span><span class=n>json</span><span class=o>&amp;</span><span class=w> </span><span class=n>j</span><span class=p>)</span>
<span class=p>{</span>
<span class=w> </span><span class=k>const</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>string</span><span class=w> </span><span class=n>text</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>j</span><span class=p>.</span><span class=n>dump</span><span class=p>(</span><span class=mi>4</span><span class=p>)</span><span class=w> </span><span class=o>+</span><span class=w> </span><span class=sc>'\n'</span><span class=p>;</span><span class=w> </span><span class=c1>// may throw: nothing is written yet</span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>ofstream</span><span class=w> </span><span class=n>f</span><span class=p>(</span><span class=n>path</span><span class=p>,</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>ios</span><span class=o>::</span><span class=n>binary</span><span class=p>);</span>
<span class=w> </span><span class=k>if</span><span class=w> </span><span class=p>(</span><span class=o>!</span><span class=n>f</span><span class=p>.</span><span class=n>is_open</span><span class=p>())</span>
<span class=w> </span><span class=p>{</span>
<span class=w> </span><span class=k>throw</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>runtime_error</span><span class=p>(</span><span class=s>"cannot open "</span><span class=w> </span><span class=o>+</span><span class=w> </span><span class=n>path</span><span class=p>);</span>
<span class=w> </span><span class=p>}</span>
<span class=w> </span><span class=n>f</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>text</span><span class=p>;</span>
<span class=w> </span><span class=n>f</span><span class=p>.</span><span class=n>close</span><span class=p>();</span>
<span class=w> </span><span class=k>if</span><span class=w> </span><span class=p>(</span><span class=n>f</span><span class=p>.</span><span class=n>fail</span><span class=p>())</span>
<span class=w> </span><span class=p>{</span>
<span class=w> </span><span class=k>throw</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>runtime_error</span><span class=p>(</span><span class=s>"cannot write "</span><span class=w> </span><span class=o>+</span><span class=w> </span><span class=n>path</span><span class=p>);</span>
<span class=w> </span><span class=p>}</span>
<span class=p>}</span>
</code></pre></div> <ul> <li><strong>Serialize before opening the file.</strong> Opening a <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>ofstream</span></code> truncates the file. <code>dump</code> and <a href=../../api/operator_ltlt/ ><code>operator&lt;&lt;</code></a> throw <a href=../../home/exceptions/#jsonexceptiontype_error316><code>type_error.316</code></a> when a string is not valid <abbr title="Unicode Transformation Format">UTF</abbr>-8 (see <a href=../serialization/#handling-invalid-utf-8>handling invalid <abbr title="Unicode Transformation Format">UTF</abbr>-8</a>), and <code class=highlight><span class=n>f</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>setw</span><span class=p>(</span><span class=mi>4</span><span class=p>)</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>j</span></code> may already have written part of the value when it throws. Serializing into a string first leaves the old file untouched in that case.</li> <li><strong>Check the stream after closing it.</strong> A full disk or a missing permission sets the stream's error state; it does not throw unless you enabled exceptions on the stream with <code>exceptions()</code>.</li> <li><code>std::ios::binary</code> writes the text as it is. Without it, Windows writes <code>\r\n</code> for every newline of a pretty-printed value.</li> </ul> <h2 id=replacing-a-file-without-corrupting-it>Replacing a file without corrupting it<a class=headerlink href=#replacing-a-file-without-corrupting-it title="Permanent link">&para;</a></h2> <p>The library never writes a file itself, so this section is not about the library. It is general C++ advice for a problem users of the library keep running into.</p> <p>A program that crashes or is killed while it writes leaves a truncated file behind. To make sure readers see either the old or the new contents, write to a temporary file in the same directory and rename it over the original (C++17, <code class=highlight><span class=o>&lt;</span><span class=n>filesystem</span><span class=o>&gt;</span></code>):</p> <div class=highlight><pre><span></span><code><span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;filesystem&gt;</span>
<span class=kt>void</span><span class=w> </span><span class=nf>replace_json_file</span><span class=p>(</span><span class=k>const</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>filesystem</span><span class=o>::</span><span class=n>path</span><span class=o>&amp;</span><span class=w> </span><span class=n>path</span><span class=p>,</span><span class=w> </span><span class=k>const</span><span class=w> </span><span class=n>json</span><span class=o>&amp;</span><span class=w> </span><span class=n>j</span><span class=p>)</span>
<span class=p>{</span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>filesystem</span><span class=o>::</span><span class=n>path</span><span class=w> </span><span class=n>tmp</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>path</span><span class=p>;</span>
<span class=w> </span><span class=n>tmp</span><span class=w> </span><span class=o>+=</span><span class=w> </span><span class=s>".tmp"</span><span class=p>;</span><span class=w> </span><span class=c1>// use a unique name if several processes may write the file</span>
<span class=w> </span><span class=n>write_json_file</span><span class=p>(</span><span class=n>tmp</span><span class=p>.</span><span class=n>string</span><span class=p>(),</span><span class=w> </span><span class=n>j</span><span class=p>);</span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>filesystem</span><span class=o>::</span><span class=n>rename</span><span class=p>(</span><span class=n>tmp</span><span class=p>,</span><span class=w> </span><span class=n>path</span><span class=p>);</span><span class=w> </span><span class=c1>// replaces path</span>
<span class=p>}</span>
</code></pre></div> <p>The temporary file must be in the same directory (more precisely, on the same file system) as the target: a rename across file systems does not fall back to a copy, it fails and throws <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>filesystem</span><span class=o>::</span><span class=n>filesystem_error</span></code>. On POSIX systems, the rename atomically replaces the file. Standard C++ has no way to flush a file to the disk (<code>fsync</code>), so this protects against a crashed program, but not necessarily against a power loss.</p> <h2 id=binary-formats>Binary formats<a class=headerlink href=#binary-formats title="Permanent link">&para;</a></h2> <p>The <a href=../binary_formats/ >binary formats</a> work the same way, with the streams opened in binary mode:</p> <div class=highlight><pre><span></span><code><span class=c1>// read</span>
<span class=n>std</span><span class=o>::</span><span class=n>ifstream</span><span class=w> </span><span class=n>in</span><span class=p>(</span><span class=s>"data.cbor"</span><span class=p>,</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>ios</span><span class=o>::</span><span class=n>binary</span><span class=p>);</span>
<span class=n>json</span><span class=w> </span><span class=n>j</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>from_cbor</span><span class=p>(</span><span class=n>in</span><span class=p>);</span>
<span class=c1>// write</span>
<span class=k>const</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>vector</span><span class=o>&lt;</span><span class=n>std</span><span class=o>::</span><span class=kt>uint8_t</span><span class=o>&gt;</span><span class=w> </span><span class=n>bytes</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>to_cbor</span><span class=p>(</span><span class=n>j</span><span class=p>);</span>
<span class=n>std</span><span class=o>::</span><span class=n>ofstream</span><span class=w> </span><span class=n>out</span><span class=p>(</span><span class=s>"data.cbor"</span><span class=p>,</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>ios</span><span class=o>::</span><span class=n>binary</span><span class=p>);</span>
<span class=n>out</span><span class=p>.</span><span class=n>write</span><span class=p>(</span><span class=k>reinterpret_cast</span><span class=o>&lt;</span><span class=k>const</span><span class=w> </span><span class=kt>char</span><span class=o>*&gt;</span><span class=p>(</span><span class=n>bytes</span><span class=p>.</span><span class=n>data</span><span class=p>()),</span><span class=w> </span><span class=k>static_cast</span><span class=o>&lt;</span><span class=n>std</span><span class=o>::</span><span class=n>streamsize</span><span class=o>&gt;</span><span class=p>(</span><span class=n>bytes</span><span class=p>.</span><span class=n>size</span><span class=p>()));</span>
</code></pre></div> <p>Without <code>std::ios::binary</code>, Windows turns the bytes <code>0x0D 0x0A</code> into <code>0x0A</code> and may stop at a byte <code>0x1A</code> while reading, and turns <code>0x0A</code> into <code>0x0D 0x0A</code> while writing, which corrupts the data.</p> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../parsing/ >Parsing</a> - the inputs <code>parse</code> accepts</li> <li><a href=../serialization/ >Serialization</a> - the options of <code>dump</code></li> <li><a href=../../api/basic_json/parse/ ><code>parse</code></a>, <a href=../../api/basic_json/dump/ ><code>dump</code></a></li> <li><a href=../../api/operator_gtgt/ ><code>operator&gt;&gt;</code></a>, <a href=../../api/operator_ltlt/ ><code>operator&lt;&lt;</code></a></li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="November 4, 2026 16:46:20 UTC">November 4, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../element_access/default_value/ class="md-footer__link md-footer__link--prev" aria-label="Previous: Access with default value: value"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> Access with default value: value </div> </div> </a> <a href=../iterators/ class="md-footer__link md-footer__link--next" aria-label="Next: Iterators"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> Iterators </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 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# Reading and writing files
The library has no functions that take a file name. It reads from and writes to streams, so files are handled with `std::ifstream` and `std::ofstream` (or a `FILE*` when reading). This page collects the recipes for the common cases and the mistakes that show up again and again.
## Reading a JSON file
Pass the stream to [`parse`](https://json.nlohmann.me/api/basic_json/parse/index.md) directly, and check that the file was opened:
```
#include <fstream>
#include <stdexcept>
#include <string>
#include <nlohmann/json.hpp>
using json = nlohmann::json;
json read_json_file(const std::string& path)
{
std::ifstream f(path);
if (!f.is_open())
{
throw std::runtime_error("cannot open " + path);
}
return json::parse(f);
}
```
- **Check that the file is open.** A stream that failed to open looks like an empty input, so `parse` reports [`parse_error.101`](https://json.nlohmann.me/home/exceptions/#jsonexceptionparse_error101) ("unexpected end of input") at line 1, column 1 instead of saying that the file is missing. A relative path is resolved against the working directory of the running program, which is often not the directory of the source file (see the [FAQ](https://json.nlohmann.me/home/faq/#parsing-a-file-fails-with-unexpected-end-of-input)).
- **Don't read the file into a string first.** Parsing from a `std::ifstream` is only about 10 to 20 % slower than reading the whole file into a string with one `read()` and parsing that, and it needs no memory for the copy (see [input sources and performance](https://json.nlohmann.me/features/parsing/#input-sources-and-performance)). The common way to read a file with `std::istreambuf_iterator<char>` is so slow that it takes about twice as long as parsing from the stream. Read the file into a string only if you need the text itself, for example for an error message. This advice is for `json::parse`. [`json_document`](https://json.nlohmann.me/features/json_view/index.md) reads a stream into a buffer that it owns anyway, so there is no copy to save: reading the file into a `std::string` yourself and passing that as an lvalue, which the document borrows, was about 6 to 12 % faster than parsing from the stream, with the same single buffer.
- `parse` requires the file to hold exactly one JSON value. Files with one value per line are read with [JSON Lines](https://json.nlohmann.me/features/parsing/json_lines/index.md); files too large for memory are covered in [Large, streamed, and partial input](https://json.nlohmann.me/features/parsing/large_and_partial_input/index.md).
## Writing a JSON file
[`dump`](https://json.nlohmann.me/api/basic_json/dump/index.md) the value into a string first, then write the string:
```
void write_json_file(const std::string& path, const json& j)
{
const std::string text = j.dump(4) + '\n'; // may throw: nothing is written yet
std::ofstream f(path, std::ios::binary);
if (!f.is_open())
{
throw std::runtime_error("cannot open " + path);
}
f << text;
f.close();
if (f.fail())
{
throw std::runtime_error("cannot write " + path);
}
}
```
- **Serialize before opening the file.** Opening a `std::ofstream` truncates the file. `dump` and [`operator<<`](https://json.nlohmann.me/api/operator_ltlt/index.md) throw [`type_error.316`](https://json.nlohmann.me/home/exceptions/#jsonexceptiontype_error316) when a string is not valid UTF-8 (see [handling invalid UTF-8](https://json.nlohmann.me/features/serialization/#handling-invalid-utf-8)), and `f << std::setw(4) << j` may already have written part of the value when it throws. Serializing into a string first leaves the old file untouched in that case.
- **Check the stream after closing it.** A full disk or a missing permission sets the stream's error state; it does not throw unless you enabled exceptions on the stream with `exceptions()`.
- `std::ios::binary` writes the text as it is. Without it, Windows writes `\r\n` for every newline of a pretty-printed value.
## Replacing a file without corrupting it
The library never writes a file itself, so this section is not about the library. It is general C++ advice for a problem users of the library keep running into.
A program that crashes or is killed while it writes leaves a truncated file behind. To make sure readers see either the old or the new contents, write to a temporary file in the same directory and rename it over the original (C++17, `<filesystem>`):
```
#include <filesystem>
void replace_json_file(const std::filesystem::path& path, const json& j)
{
std::filesystem::path tmp = path;
tmp += ".tmp"; // use a unique name if several processes may write the file
write_json_file(tmp.string(), j);
std::filesystem::rename(tmp, path); // replaces path
}
```
The temporary file must be in the same directory (more precisely, on the same file system) as the target: a rename across file systems does not fall back to a copy, it fails and throws `std::filesystem::filesystem_error`. On POSIX systems, the rename atomically replaces the file. Standard C++ has no way to flush a file to the disk (`fsync`), so this protects against a crashed program, but not necessarily against a power loss.
## Binary formats
The [binary formats](https://json.nlohmann.me/features/binary_formats/index.md) work the same way, with the streams opened in binary mode:
```
// read
std::ifstream in("data.cbor", std::ios::binary);
json j = json::from_cbor(in);
// write
const std::vector<std::uint8_t> bytes = json::to_cbor(j);
std::ofstream out("data.cbor", std::ios::binary);
out.write(reinterpret_cast<const char*>(bytes.data()), static_cast<std::streamsize>(bytes.size()));
```
Without `std::ios::binary`, Windows turns the bytes `0x0D 0x0A` into `0x0A` and may stop at a byte `0x1A` while reading, and turns `0x0A` into `0x0D 0x0A` while writing, which corrupts the data.
## See also
- [Parsing](https://json.nlohmann.me/features/parsing/index.md) - the inputs `parse` accepts
- [Serialization](https://json.nlohmann.me/features/serialization/index.md) - the options of `dump`
- [`parse`](https://json.nlohmann.me/api/basic_json/parse/index.md), [`dump`](https://json.nlohmann.me/api/basic_json/dump/index.md)
- [`operator>>`](https://json.nlohmann.me/api/operator_gtgt/index.md), [`operator<<`](https://json.nlohmann.me/api/operator_ltlt/index.md)
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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 0Line truncated
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@@ -31,6 +31,7 @@ This section describes the features of the library in detail. If you are new to
- [Serialization](https://json.nlohmann.me/features/serialization/index.md) — turn a value back into JSON text with [`dump`](https://json.nlohmann.me/api/basic_json/dump/index.md), including pretty-printing and handling of non-ASCII and invalid UTF-8.
- [Binary formats](https://json.nlohmann.me/features/binary_formats/index.md) — encode values more compactly as [BJData](https://json.nlohmann.me/features/binary_formats/bjdata/index.md), [BON8](https://json.nlohmann.me/features/binary_formats/bon8/index.md), [BSON](https://json.nlohmann.me/features/binary_formats/bson/index.md), [CBOR](https://json.nlohmann.me/features/binary_formats/cbor/index.md), [MessagePack](https://json.nlohmann.me/features/binary_formats/messagepack/index.md), or [UBJSON](https://json.nlohmann.me/features/binary_formats/ubjson/index.md).
- [Binary values](https://json.nlohmann.me/features/binary_values/index.md) — store and exchange raw byte sequences.
- [Reading and writing files](https://json.nlohmann.me/features/files/index.md) — read and write files safely, including replacing a file without corrupting it.
## How values are stored and configured
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<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=description content="JSON for Modern C++ is a header-only C++11 library that makes JSON a first-class data type."><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/iterators/" rel="canonical"><link href=../files/ rel=prev><link href=../merge_patch/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.8.0"><title>Iterators - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.57a0d79d.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.fb2fffe2.min.css><link rel=stylesheet href=../../css/custom.css><link rel=stylesheet href=../../css/brand.css><link rel=stylesheet href=../../css/fig-sax.css><link rel=stylesheet href=../../css/fig-jsonview.css><link rel=stylesheet 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<span class=n>json</span><span class=w> </span><span class=n>j</span><span class=p>;</span>
<span class=n>j</span><span class=p>[</span><span class=s>"one"</span><span class=p>]</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=mi>1</span><span class=p>;</span>
<span class=n>j</span><span class=p>[</span><span class=s>"two"</span><span class=p>]</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=mi>2</span><span class=p>;</span>
@@ -53,4 +53,4 @@
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=o>*</span><span class=n>it</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>endl</span><span class=p>;</span>
<span class=p>}</span>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code><span class=s2>"Hello, world"</span>
</code></pre></div> </details> <h2 id=iterator-invalidation>Iterator invalidation<a class=headerlink href=#iterator-invalidation title="Permanent link">&para;</a></h2> <table> <thead> <tr> <th>Operations</th> <th>invalidated iterators</th> </tr> </thead> <tbody> <tr> <td><code>clear</code></td> <td>all</td> </tr> </tbody> </table> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../../api/basic_json/begin/ ><code>begin</code></a> - / <a href=../../api/basic_json/end/ ><code>end</code></a> - iterators to the first and one past the last element</li> <li><a href=../../api/basic_json/rbegin/ ><code>rbegin</code></a> - / <a href=../../api/basic_json/rend/ ><code>rend</code></a> - reverse iterators</li> <li><a href=../../api/basic_json/cbegin/ ><code>cbegin</code></a> - / <a href=../../api/basic_json/cend/ ><code>cend</code></a> - const iterators</li> <li><a href=../../api/basic_json/items/ ><code>items</code></a> - iterate over keys and values</li> <li><a href=../object_order/ >Object Order</a> - iteration order of object keys</li> <li><a href=../element_access/ >Element Access</a> - access single elements</li> <li><a href=../modifying_values/ >Modifying values</a> - add and remove elements (including iterator invalidation rules)</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../element_access/default_value/ class="md-footer__link md-footer__link--prev" aria-label="Previous: Access with default value: value"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> Access with default value: value </div> </div> </a> <a href=../merge_patch/ class="md-footer__link md-footer__link--next" aria-label="Next: JSON Merge Patch"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> JSON Merge Patch </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 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</code></pre></div> </details> <h2 id=iterator-invalidation>Iterator invalidation<a class=headerlink href=#iterator-invalidation title="Permanent link">&para;</a></h2> <table> <thead> <tr> <th>Operations</th> <th>invalidated iterators</th> </tr> </thead> <tbody> <tr> <td><code>clear</code></td> <td>all</td> </tr> </tbody> </table> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../../api/basic_json/begin/ ><code>begin</code></a> - / <a href=../../api/basic_json/end/ ><code>end</code></a> - iterators to the first and one past the last element</li> <li><a href=../../api/basic_json/rbegin/ ><code>rbegin</code></a> - / <a href=../../api/basic_json/rend/ ><code>rend</code></a> - reverse iterators</li> <li><a href=../../api/basic_json/cbegin/ ><code>cbegin</code></a> - / <a href=../../api/basic_json/cend/ ><code>cend</code></a> - const iterators</li> <li><a href=../../api/basic_json/items/ ><code>items</code></a> - iterate over keys and values</li> <li><a href=../object_order/ >Object Order</a> - iteration order of object keys</li> <li><a href=../element_access/ >Element Access</a> - access single elements</li> <li><a href=../modifying_values/ >Modifying values</a> - add and remove elements (including iterator invalidation rules)</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../files/ class="md-footer__link md-footer__link--prev" aria-label="Previous: Reading and writing files"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> Reading and writing files </div> </div> </a> <a href=../merge_patch/ class="md-footer__link md-footer__link--next" aria-label="Next: JSON Merge Patch"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> JSON Merge Patch </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 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@@ -29,7 +29,8 @@ other three continue on its result.
[`patch`](../api/basic_json/patch.md) leaves the original value unchanged and returns the patched result as a copy.
If the document is large and the original value is no longer needed,
[`patch_inplace`](../api/basic_json/patch_inplace.md) applies the same operations in place instead.
[`patch_inplace`](../api/basic_json/patch_inplace.md) applies the same operations in place instead. Unlike
`patch`, it is not atomic: if an operation fails, the operations before it stay applied.
!!! note "Duplicate member names"
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@@ -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=description content="JSON for Modern C++ is a header-only C++11 library that makes JSON a first-class data type."><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/json_patch/" rel="canonical"><link href=../merge_patch/ rel=prev><link href=../json_pointer/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>JSON Patch and Diff - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.ab4e12ef.min.css><link rel=stylesheet href=../../css/custom.css><link rel=stylesheet href=../../css/brand.css><link rel=stylesheet href=../../css/fig-sax.css><link rel=stylesheet href=../../css/fig-jsonview.css><link rel=stylesheet 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12s-1.44-4.5-3.5-5.45C10.26 6.2 11.11 6 12 6a6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> </form> <script>var palette=__md_get("__palette");if(palette&&palette.color){if("(prefers-color-scheme)"===palette.color.media){var media=matchMedia("(prefers-color-scheme: light)"),input=document.querySelector(media.matches?"[data-md-color-media='(prefers-color-scheme: light)']":"[data-md-color-media='(prefers-color-scheme: dark)']");palette.color.media=input.getAttribute("data-md-color-media"),palette.color.scheme=input.getAttribute("data-md-color-scheme"),palette.color.primary=input.getAttribute("data-md-color-primary"),palette.color.accent=input.getAttribute("data-md-color-accent")}for(var[key,value]of Object.entries(palette.color))document.body.setAttribute("data-md-color-"+key,value)}</script> <label class="md-header__button md-icon" for=__search> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 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<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;iomanip&gt;</span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;nlohmann/json.hpp&gt;</span>
@@ -108,4 +108,4 @@
<span class=w> </span><span class=s2>"world"</span>
<span class=w> </span><span class=p>]</span>
<span class=p>}</span>
</code></pre></div> </details> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../json_pointer/ ><abbr title="JavaScript Object Notation">JSON</abbr> Pointer</a> - the addressing scheme used for patch paths</li> <li><a href=../merge_patch/ ><abbr title="JavaScript Object Notation">JSON</abbr> Merge Patch</a> - a simpler, less expressive alternative patch format</li> <li><a href=../../api/basic_json/patch/ ><code>patch</code></a> - apply a <abbr title="JavaScript Object Notation">JSON</abbr> Patch, returning the result as a copy</li> <li><a href=../../api/basic_json/patch_inplace/ ><code>patch_inplace</code></a> - apply a <abbr title="JavaScript Object Notation">JSON</abbr> Patch without copying</li> <li><a href=../../api/basic_json/diff/ ><code>diff</code></a> - compute a <abbr title="JavaScript Object Notation">JSON</abbr> Patch from two values</li> <li><a href=../../api/json_pointer/ ><code>json_pointer</code></a> - type to represent <abbr title="JavaScript Object Notation">JSON</abbr> Pointers</li> <li><a href=../../api/basic_json/merge_diff/ ><code>merge_diff</code></a> - compute a <abbr title="JavaScript Object Notation">JSON</abbr> Merge Patch from two values</li> <li><a href=../modifying_values/ >Modifying values</a> - simple in-place modifications</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 16:16:08 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../merge_patch/ class="md-footer__link md-footer__link--prev" aria-label="Previous: JSON Merge Patch"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> JSON Merge Patch </div> </div> </a> <a href=../json_pointer/ class="md-footer__link md-footer__link--next" aria-label="Next: JSON Pointer"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> JSON Pointer </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 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</code></pre></div> </details> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../json_pointer/ ><abbr title="JavaScript Object Notation">JSON</abbr> Pointer</a> - the addressing scheme used for patch paths</li> <li><a href=../merge_patch/ ><abbr title="JavaScript Object Notation">JSON</abbr> Merge Patch</a> - a simpler, less expressive alternative patch format</li> <li><a href=../../api/basic_json/patch/ ><code>patch</code></a> - apply a <abbr title="JavaScript Object Notation">JSON</abbr> Patch, returning the result as a copy</li> <li><a href=../../api/basic_json/patch_inplace/ ><code>patch_inplace</code></a> - apply a <abbr title="JavaScript Object Notation">JSON</abbr> Patch without copying</li> <li><a href=../../api/basic_json/diff/ ><code>diff</code></a> - compute a <abbr title="JavaScript Object Notation">JSON</abbr> Patch from two values</li> <li><a href=../../api/json_pointer/ ><code>json_pointer</code></a> - type to represent <abbr title="JavaScript Object Notation">JSON</abbr> Pointers</li> <li><a href=../../api/basic_json/merge_diff/ ><code>merge_diff</code></a> - compute a <abbr title="JavaScript Object Notation">JSON</abbr> Merge Patch from two values</li> <li><a href=../modifying_values/ >Modifying values</a> - simple in-place modifications</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="November 4, 2026 16:46:20 UTC">November 4, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../merge_patch/ class="md-footer__link md-footer__link--prev" aria-label="Previous: JSON Merge Patch"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> JSON Merge Patch </div> </div> </a> <a href=../json_pointer/ class="md-footer__link md-footer__link--next" aria-label="Next: JSON Pointer"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> JSON Pointer </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 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+1 -1
View File
@@ -19,7 +19,7 @@ Stepper through the six operations: *Next* applies the next operation, *Back* un
Applying a patch without copying
[`patch`](https://json.nlohmann.me/api/basic_json/patch/index.md) leaves the original value unchanged and returns the patched result as a copy. If the document is large and the original value is no longer needed, [`patch_inplace`](https://json.nlohmann.me/api/basic_json/patch_inplace/index.md) applies the same operations in place instead.
[`patch`](https://json.nlohmann.me/api/basic_json/patch/index.md) leaves the original value unchanged and returns the patched result as a copy. If the document is large and the original value is no longer needed, [`patch_inplace`](https://json.nlohmann.me/api/basic_json/patch_inplace/index.md) applies the same operations in place instead. Unlike `patch`, it is not atomic: if an operation fails, the operations before it stay applied.
Duplicate member names
+2 -2
View File
@@ -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=description content="JSON for Modern C++ is a header-only C++11 library that makes JSON a first-class data type."><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/json_pointer/" rel="canonical"><link href=../json_patch/ rel=prev><link href=../modifying_values/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>JSON Pointer - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.ab4e12ef.min.css><link rel=stylesheet href=../../css/custom.css><link rel=stylesheet href=../../css/brand.css><link rel=stylesheet href=../../css/fig-sax.css><link rel=stylesheet href=../../css/fig-jsonview.css><link rel=stylesheet 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<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=description content="JSON for Modern C++ is a header-only C++11 library that makes JSON a first-class data type."><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/json_pointer/" rel="canonical"><link href=../json_patch/ rel=prev><link href=../modifying_values/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.8.0"><title>JSON Pointer - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.57a0d79d.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.fb2fffe2.min.css><link rel=stylesheet href=../../css/custom.css><link rel=stylesheet href=../../css/brand.css><link rel=stylesheet href=../../css/fig-sax.css><link rel=stylesheet href=../../css/fig-jsonview.css><link rel=stylesheet href=../../css/fig-exceptions.css><link rel=stylesheet href=../../css/fig-pointer.css><link rel=stylesheet href=../../css/fig-charts.css><link rel=stylesheet href=../../css/fig-integration.css><link rel=stylesheet href=../../css/fig-macros.css><script>__md_scope=new URL("../..",location),__md_hash=e=>[...e].reduce(((e,_)=>(e<<5)-e+_.charCodeAt(0)),0),__md_get=(e,_=localStorage,t=__md_scope)=>JSON.parse(_.getItem(t.pathname+"."+e)),__md_set=(e,_,t=localStorage,a=__md_scope)=>{try{t.setItem(a.pathname+"."+e,JSON.stringify(_))}catch(e){}}</script></head> <body dir=ltr data-md-color-scheme=default data-md-color-primary=custom data-md-color-accent=custom> <input class=md-toggle data-md-toggle=drawer type=checkbox id=__drawer autocomplete=off> <input class=md-toggle data-md-toggle=search type=checkbox id=__search autocomplete=off> <label class=md-overlay for=__drawer></label> <div data-md-component=skip> <a href=#json-pointer class=md-skip> Skip to content </a> </div> <div data-md-component=announce> </div> <header class=md-header data-md-component=header> <nav class="md-header__inner md-grid" aria-label=Header> <a href=../.. title="JSON for Modern C++" class="md-header__button md-logo" aria-label="JSON for Modern C++" data-md-component=logo> <img src=../../images/wordmark-header.svg alt=logo> </a> <label class="md-header__button md-icon" for=__drawer> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M3 6h18v2H3zm0 5h18v2H3zm0 5h18v2H3z"/></svg> </label> <div class=md-header__title data-md-component=header-title> <div class=md-header__ellipsis> <div class=md-header__topic> <span class=md-ellipsis> JSON for Modern C++ </span> </div> <div class=md-header__topic data-md-component=header-topic> <span class=md-ellipsis> JSON Pointer </span> </div> </div> </div> <form class=md-header__option data-md-component=palette> <input class=md-option data-md-color-media="(prefers-color-scheme: light)" data-md-color-scheme=default data-md-color-primary=custom data-md-color-accent=custom aria-label="Switch to dark mode" type=radio name=__palette id=__palette_0> <label class="md-header__button md-icon" title="Switch to dark mode" for=__palette_1 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 8a4 4 0 0 0-4 4 4 4 0 0 0 4 4 4 4 0 0 0 4-4 4 4 0 0 0-4-4m0 10a6 6 0 0 1-6-6 6 6 0 0 1 6-6 6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> <input class=md-option data-md-color-media="(prefers-color-scheme: dark)" data-md-color-scheme=slate data-md-color-primary=custom data-md-color-accent=custom aria-label="Switch to light mode" type=radio name=__palette id=__palette_1> <label class="md-header__button md-icon" title="Switch to light mode" for=__palette_0 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 18c-.89 0-1.74-.2-2.5-.55C11.56 16.5 13 14.42 13 12s-1.44-4.5-3.5-5.45C10.26 6.2 11.11 6 12 6a6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> </form> <script>var palette=__md_get("__palette");if(palette&&palette.color){if("(prefers-color-scheme)"===palette.color.media){var media=matchMedia("(prefers-color-scheme: light)"),input=document.querySelector(media.matches?"[data-md-color-media='(prefers-color-scheme: light)']":"[data-md-color-media='(prefers-color-scheme: dark)']");palette.color.media=input.getAttribute("data-md-color-media"),palette.color.scheme=input.getAttribute("data-md-color-scheme"),palette.color.primary=input.getAttribute("data-md-color-primary"),palette.color.accent=input.getAttribute("data-md-color-accent")}for(var[key,value]of Object.entries(palette.color))document.body.setAttribute("data-md-color-"+key,value)}</script> <label class="md-header__button md-icon" for=__search> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M9.5 3A6.5 6.5 0 0 1 16 9.5c0 1.61-.59 3.09-1.56 4.23l.27.2Line truncated
<span class=w> </span><span class=nt>"array"</span><span class=p>:</span><span class=w> </span><span class=p>[</span><span class=s2>"A"</span><span class=p>,</span><span class=w> </span><span class=s2>"B"</span><span class=p>,</span><span class=w> </span><span class=s2>"C"</span><span class=p>],</span>
<span class=w> </span><span class=nt>"nested"</span><span class=p>:</span><span class=w> </span><span class=p>{</span>
<span class=w> </span><span class=nt>"one"</span><span class=p>:</span><span class=w> </span><span class=mi>1</span><span class=p>,</span>
@@ -45,4 +45,4 @@
<span class=w> </span><span class=nt>"/nested/three/1"</span><span class=p>:</span><span class=w> </span><span class=kc>false</span>
<span class=p>}</span>
</code></pre></div> <p>The reverse function, <a href=../../api/basic_json/unflatten/ ><code>unflatten</code></a> recreates the original value.</p> <div class=highlight><pre><span></span><code><span class=k>auto</span><span class=w> </span><span class=n>j_original</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>j_flat</span><span class=p>.</span><span class=n>unflatten</span><span class=p>();</span>
</code></pre></div> <p>Each leaf of the document becomes one key of the flat object, and <code>unflatten()</code> reverses the mapping:</p> <div class=nj-fig-wrap> <p><svg class="nj-fig nj-pointer-fl" viewbox="0 0 760 442" role=img aria-labelledby=fig-flat-title xmlns=http://www.w3.org/2000/svg> <title id=fig-flat-title>flatten() turns the nested example document into a flat object, and unflatten() turns it back. Left, the document as a tree: array with the elements A, B, C, and nested with one = 1, two = 2, and three with the elements true and false. Right, the flat object with one key per leaf: /array/0 = A, /array/1 = B, /array/2 = C, /nested/one = 1, /nested/two = 2, /nested/three/0 = true, /nested/three/1 = false. Each leaf is joined to its key by a line.</title> <defs><marker id=fig-flat-arrow viewbox="0 0 10 10" refx=9 refy=5 markerwidth=7 markerheight=7 orient=auto><path class=fg-head d="M0 1 L10 5 L0 9 z"/></marker></defs> <text class=fg-h x=20 y=26>nested value</text> <text class=fg-h x=470 y=26>flat object: key = J&#8203;SON Pointer</text> <line class=fg-arrow x1=250 y1=52 x2=450 y2=52 marker-end=url(#fig-flat-arrow) /> <text class=nj-pointer-fl-op x=350 y=44>flatten()</text> <line class=fg-arrow x1=450 y1=420 x2=250 y2=420 marker-end=url(#fig-flat-arrow) /> <text class=nj-pointer-fl-op x=350 y=412>unflatten()</text> <path class=nj-pointer-fl-edge d="M25 99 V115 H42"/> <path class=nj-pointer-fl-edge d="M51 127 V143 H68"/> <path class=nj-pointer-fl-edge d="M51 127 V171 H68"/> <path class=nj-pointer-fl-edge d="M51 127 V199 H68"/> <path class=nj-pointer-fl-edge d="M25 99 V227 H42"/> <path class=nj-pointer-fl-edge d="M51 239 V255 H68"/> <path class=nj-pointer-fl-edge d="M51 239 V283 H68"/> <path class=nj-pointer-fl-edge d="M51 239 V311 H68"/> <path class=nj-pointer-fl-edge d="M77 323 V339 H94"/> <path class=nj-pointer-fl-edge d="M77 323 V367 H94"/> <path class="nj-pointer-fl-link nj-pointer-fl--s" d="M139.6 143 C259.6 143 340 171 460 171"/> <circle cx=139.6 cy=143 r=3.5 class="nj-pointer-fl-dot nj-pointer-fl--s"/> <text class=fg-code x=470 y=176 xml:space=preserve><tspan class=nj-pointer-fl-key>"/array/0"</tspan>: <tspan class="nj-pointer-fl-v nj-pointer-fl--s">"A"</tspan></text> <path class="nj-pointer-fl-link nj-pointer-fl--s" d="M139.6 171 C259.6 171 340 199 460 199"/> <circle cx=139.6 cy=171 r=3.5 class="nj-pointer-fl-dot nj-pointer-fl--s"/> <text class=fg-code x=470 y=204 xml:space=preserve><tspan class=nj-pointer-fl-key>"/array/1"</tspan>: <tspan class="nj-pointer-fl-v nj-pointer-fl--s">"B"</tspan></text> <path class="nj-pointer-fl-link nj-pointer-fl--s" d="M139.6 199 C259.6 199 340 227 460 227"/> <circle cx=139.6 cy=199 r=3.5 class="nj-pointer-fl-dot nj-pointer-fl--s"/> <text class=fg-code x=470 y=232 xml:space=preserve><tspan class=nj-pointer-fl-key>"/array/2"</tspan>: <tspan class="nj-pointer-fl-v nj-pointer-fl--s">"C"</tspan></text> <path class="nj-pointer-fl-link nj-pointer-fl--n" d="M158.8 255 C278.8 255 340 255 460 255"/> <circle cx=158.8 cy=255 r=3.5 class="nj-pointer-fl-dot nj-pointer-fl--n"/> <text class=fg-code x=470 y=260 xml:space=preserve><tspan class=nj-pointer-fl-key>"/nested/one"</tspan>: <tspan class="nj-pointer-fl-v nj-pointer-fl--n">1</tspan></text> <path class="nj-pointer-fl-link nj-pointer-fl--n" d="M158.8 283 C278.8 283 340 283 460 283"/> <circle cx=158.8 cy=283 r=3.5 class="nj-pointer-fl-dot nj-pointer-fl--n"/> <text class=fg-code x=470 y=288 xml:space=preserve><tspan class=nj-pointer-fl-key>"/nested/two"</tspan>: <tspan class="nj-pointer-fl-v nj-pointer-fl--n">2</tspan></text> <path class="nj-pointer-fl-link nj-pointer-fl--b" d="M175.2 339 C295.2 339 340 311 460 311"/> <circle cx=175.2 cy=339 r=3.5 class="nj-pointer-fl-dot nj-pointer-fl--b"/> <text class=fg-code x=470 y=316 xml:space=preserve><tspan class=nj-pointer-fl-key>"/nested/three/0"</tspan>: <tspan class="nj-pointer-fl-v nj-pointer-fl--b">true</tspan></text> <path class="nj-pointer-fl-link nj-pointer-fl--b" d="M184.8 367 C304.8 367 340 339 460 339"/> <circle cx=184.8 cy=367 r=3.5 class="nj-pointer-fl-dot nj-pointer-fl--b"/> <text class=fg-code x=470 y=344 xml:space=preserve><tspan class=nj-pointer-fl-key>"/nested/three/1"</tspan>: <tspan class="nj-pointer-fl-v nj-pointer-fl--b">false</tspan></text> <text class=fg-h x=20 y=92>root</text> <text class="fg-code nj-pointer-fl-inner" x=46 y=120>"array"</text> <text class=fg-code x=72 y=148 xml:space=preserve><tspan class=nj-pointer-fl-k>0</tspan>: <tspan class="nj-pointer-fl-v nj-pointer-fl--s">"A"</tspan></text> <text class=fg-code x=72 y=176 xml:space=preserve><tspan class=nj-pointer-fl-k>1</tspan>: <tspan class="nj-pointer-fl-v nj-pointer-fl--s">"B"</tspan></text> <text class=fg-code x=72 y=204 xml:space=preserve><tspan class=nj-pointer-fl-k>2</tspan>: <tspan class="nj-pointer-fl-v nj-pointer-fl--s">"C"</tspan></text> <text class="fg-code nj-pointer-fl-inner" x=46 y=232>"nested"</text> <text class=fg-code x=72 y=260 xml:space=preserve><tspan class=nj-pointer-fl-k>Line truncated
</code></pre></div> <p>Each leaf of the document becomes one key of the flat object, and <code>unflatten()</code> reverses the mapping:</p> <div class=nj-fig-wrap> <p><svg class="nj-fig nj-pointer-fl" viewbox="0 0 760 442" role=img aria-labelledby=fig-flat-title xmlns=http://www.w3.org/2000/svg> <title id=fig-flat-title>flatten() turns the nested example document into a flat object, and unflatten() turns it back. Left, the document as a tree: array with the elements A, B, C, and nested with one = 1, two = 2, and three with the elements true and false. Right, the flat object with one key per leaf: /array/0 = A, /array/1 = B, /array/2 = C, /nested/one = 1, /nested/two = 2, /nested/three/0 = true, /nested/three/1 = false. Each leaf is joined to its key by a line.</title> <defs><marker id=fig-flat-arrow viewbox="0 0 10 10" refx=9 refy=5 markerwidth=7 markerheight=7 orient=auto><path class=fg-head d="M0 1 L10 5 L0 9 z"/></marker></defs> <text class=fg-h x=20 y=26>nested value</text> <text class=fg-h x=470 y=26>flat object: key = J&#8203;SON Pointer</text> <line class=fg-arrow x1=250 y1=52 x2=450 y2=52 marker-end=url(#fig-flat-arrow) /> <text class=nj-pointer-fl-op x=350 y=44>flatten()</text> <line class=fg-arrow x1=450 y1=420 x2=250 y2=420 marker-end=url(#fig-flat-arrow) /> <text class=nj-pointer-fl-op x=350 y=412>unflatten()</text> <path class=nj-pointer-fl-edge d="M25 99 V115 H42"/> <path class=nj-pointer-fl-edge d="M51 127 V143 H68"/> <path class=nj-pointer-fl-edge d="M51 127 V171 H68"/> <path class=nj-pointer-fl-edge d="M51 127 V199 H68"/> <path class=nj-pointer-fl-edge d="M25 99 V227 H42"/> <path class=nj-pointer-fl-edge d="M51 239 V255 H68"/> <path class=nj-pointer-fl-edge d="M51 239 V283 H68"/> <path class=nj-pointer-fl-edge d="M51 239 V311 H68"/> <path class=nj-pointer-fl-edge d="M77 323 V339 H94"/> <path class=nj-pointer-fl-edge d="M77 323 V367 H94"/> <path class="nj-pointer-fl-link nj-pointer-fl--s" d="M139.6 143 C259.6 143 340 171 460 171"/> <circle cx=139.6 cy=143 r=3.5 class="nj-pointer-fl-dot nj-pointer-fl--s"/> <text class=fg-code x=470 y=176 xml:space=preserve><tspan class=nj-pointer-fl-key>"/array/0"</tspan>: <tspan class="nj-pointer-fl-v nj-pointer-fl--s">"A"</tspan></text> <path class="nj-pointer-fl-link nj-pointer-fl--s" d="M139.6 171 C259.6 171 340 199 460 199"/> <circle cx=139.6 cy=171 r=3.5 class="nj-pointer-fl-dot nj-pointer-fl--s"/> <text class=fg-code x=470 y=204 xml:space=preserve><tspan class=nj-pointer-fl-key>"/array/1"</tspan>: <tspan class="nj-pointer-fl-v nj-pointer-fl--s">"B"</tspan></text> <path class="nj-pointer-fl-link nj-pointer-fl--s" d="M139.6 199 C259.6 199 340 227 460 227"/> <circle cx=139.6 cy=199 r=3.5 class="nj-pointer-fl-dot nj-pointer-fl--s"/> <text class=fg-code x=470 y=232 xml:space=preserve><tspan class=nj-pointer-fl-key>"/array/2"</tspan>: <tspan class="nj-pointer-fl-v nj-pointer-fl--s">"C"</tspan></text> <path class="nj-pointer-fl-link nj-pointer-fl--n" d="M158.8 255 C278.8 255 340 255 460 255"/> <circle cx=158.8 cy=255 r=3.5 class="nj-pointer-fl-dot nj-pointer-fl--n"/> <text class=fg-code x=470 y=260 xml:space=preserve><tspan class=nj-pointer-fl-key>"/nested/one"</tspan>: <tspan class="nj-pointer-fl-v nj-pointer-fl--n">1</tspan></text> <path class="nj-pointer-fl-link nj-pointer-fl--n" d="M158.8 283 C278.8 283 340 283 460 283"/> <circle cx=158.8 cy=283 r=3.5 class="nj-pointer-fl-dot nj-pointer-fl--n"/> <text class=fg-code x=470 y=288 xml:space=preserve><tspan class=nj-pointer-fl-key>"/nested/two"</tspan>: <tspan class="nj-pointer-fl-v nj-pointer-fl--n">2</tspan></text> <path class="nj-pointer-fl-link nj-pointer-fl--b" d="M175.2 339 C295.2 339 340 311 460 311"/> <circle cx=175.2 cy=339 r=3.5 class="nj-pointer-fl-dot nj-pointer-fl--b"/> <text class=fg-code x=470 y=316 xml:space=preserve><tspan class=nj-pointer-fl-key>"/nested/three/0"</tspan>: <tspan class="nj-pointer-fl-v nj-pointer-fl--b">true</tspan></text> <path class="nj-pointer-fl-link nj-pointer-fl--b" d="M184.8 367 C304.8 367 340 339 460 339"/> <circle cx=184.8 cy=367 r=3.5 class="nj-pointer-fl-dot nj-pointer-fl--b"/> <text class=fg-code x=470 y=344 xml:space=preserve><tspan class=nj-pointer-fl-key>"/nested/three/1"</tspan>: <tspan class="nj-pointer-fl-v nj-pointer-fl--b">false</tspan></text> <text class=fg-h x=20 y=92>root</text> <text class="fg-code nj-pointer-fl-inner" x=46 y=120>"array"</text> <text class=fg-code x=72 y=148 xml:space=preserve><tspan class=nj-pointer-fl-k>0</tspan>: <tspan class="nj-pointer-fl-v nj-pointer-fl--s">"A"</tspan></text> <text class=fg-code x=72 y=176 xml:space=preserve><tspan class=nj-pointer-fl-k>1</tspan>: <tspan class="nj-pointer-fl-v nj-pointer-fl--s">"B"</tspan></text> <text class=fg-code x=72 y=204 xml:space=preserve><tspan class=nj-pointer-fl-k>2</tspan>: <tspan class="nj-pointer-fl-v nj-pointer-fl--s">"C"</tspan></text> <text class="fg-code nj-pointer-fl-inner" x=46 y=232>"nested"</text> <text class=fg-code x=72 y=260 xml:space=preserve><tspan class=nj-pointer-fl-k>Line truncated
+3 -1
View File
@@ -458,7 +458,9 @@ document: `#!cpp auto v = json_document::parse(text).root();` does not compile.
wide. The limit is 16 bytes below 2^32, because the parser reads in 16-byte steps, and a position one step past the
end must still fit.
- **A stream is always read to its end.** There is no partial/streaming read of an `#!cpp std::istream`: the stream
is read to its end into a buffer the document owns, and parsing starts after that.
is read to its end into a buffer the document owns, and parsing starts after that. For a file, reading it into a
`#!cpp std::string` yourself and parsing that (an lvalue is borrowed) is therefore not more expensive in memory, and
was about 6 to 12 % faster than parsing from an `#!cpp std::ifstream`.
- **Fewer source positions after edits.** With
[`JSON_DIAGNOSTIC_POSITIONS`](../api/macros/json_diagnostic_positions.md) enabled,
[`materialize()`](../api/basic_json_view/materialize.md) sets the same positions as `#!cpp basic_json::parse()`.
+3 -3
View File
@@ -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=description content="JSON for Modern C++ is a header-only C++11 library that makes JSON a first-class data type."><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/json_view/" rel="canonical"><link href=../types/template_parameters/ rel=prev><link href=../../integration/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>Zero-copy JSON views - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.ab4e12ef.min.css><link rel=stylesheet href=../../css/custom.css><link rel=stylesheet href=../../css/brand.css><link rel=stylesheet href=../../css/fig-sax.css><link rel=stylesheet href=../../css/fig-jsonview.css><link 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<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=description content="JSON for Modern C++ is a header-only C++11 library that makes JSON a first-class data type."><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/json_view/" rel="canonical"><link href=../types/template_parameters/ rel=prev><link href=../../integration/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.8.0"><title>Zero-copy JSON views - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.57a0d79d.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.fb2fffe2.min.css><link rel=stylesheet href=../../css/custom.css><link rel=stylesheet href=../../css/brand.css><link rel=stylesheet href=../../css/fig-sax.css><link rel=stylesheet href=../../css/fig-jsonview.css><link 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<div data-md-component=announce> </div> <header class=md-header data-md-component=header> <nav class="md-header__inner md-grid" aria-label=Header> <a href=../.. title="JSON for Modern C++" class="md-header__button md-logo" aria-label="JSON for Modern C++" data-md-component=logo> <img src=../../images/wordmark-header.svg alt=logo> </a> <label class="md-header__button md-icon" for=__drawer> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M3 6h18v2H3zm0 5h18v2H3zm0 5h18v2H3z"/></svg> </label> <div class=md-header__title data-md-component=header-title> <div class=md-header__ellipsis> <div class=md-header__topic> <span class=md-ellipsis> JSON for Modern C++ </span> </div> <div class=md-header__topic data-md-component=header-topic> <span class=md-ellipsis> Zero-copy JSON views </span> </div> </div> </div> <form class=md-header__option data-md-component=palette> <input class=md-option data-md-color-media="(prefers-color-scheme: light)" data-md-color-scheme=default 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<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;nlohmann/json_view.hpp&gt;</span>
<span class=k>using</span><span class=w> </span><span class=n>nlohmann</span><span class=o>::</span><span class=n>json_document</span><span class=p>;</span>
@@ -53,7 +53,7 @@
<span class=kc>true</span>
<span class=kc>true</span>
<span class=kc>true</span>
</code></pre></div> </details> <h2 id=what-is-the-same-as-parse>What is the same as <code>parse()</code><a class=headerlink href=#what-is-the-same-as-parse title="Permanent link">&para;</a></h2> <ul> <li><strong>Accept/reject.</strong> <a href=../../api/basic_json_document/accept/ ><code>accept()</code></a> and <a href=../../api/basic_json_document/parse/ ><code>parse()</code></a> accept and reject exactly the same inputs as <a href=../../api/basic_json/accept/ ><code>basic_json::accept()</code></a>/<a href=../../api/basic_json/parse/ ><code>basic_json::parse()</code></a>, with the same <code>ignore_comments</code> and <code>ignore_trailing_commas</code> options.</li> <li><strong>Errors.</strong> A failing parse throws the same exception — the same id, message, and position — because on failure the library's own parser is run on the same bytes to produce the diagnostic. Valid input never pays for this, and the replay builds no values: it costs about a lexing pass up to the error. <code class=highlight><span class=n>allow_exceptions</span><span class=w> </span><span class=o>==</span><span class=w> </span><span class=nb>false</span></code> skips the replay and gives a <a href=../../api/basic_json_document/is_discarded/ >discarded</a> document instead of throwing, just as it gives a discarded value for <code class=highlight><span class=n>basic_json</span><span class=o>::</span><span class=n>parse</span><span class=p>()</span></code>.</li> <li><strong>Number classification.</strong> An integer literal that does not fit into the 64-bit integer type becomes a floating-point value, exactly as it does for <code class=highlight><span class=n>basic_json</span><span class=o>::</span><span class=n>parse</span><span class=p>()</span></code>.</li> <li> <p><strong>Macros.</strong> <a href=../../api/macros/json_strict_nul_handling/ ><code>JSON_STRICT_NUL_HANDLING</code></a> and <a href=../../api/macros/json_noexception/ ><code>JSON_NOEXCEPTION</code></a>/<a href=../../api/macros/json_throw_user/ ><code>JSON_THROW_USER</code></a> behave the same way they do for <code>&lt;nlohmann/json.hpp&gt;</code>. These are the macros that change what the parser accepts or how it reports errors. Two more macros add context to the exceptions of views, as they do for those of <code class=highlight><span class=n>basic_json</span></code>:</p> <ul> <li>With <a href=../../api/macros/json_diagnostics/ ><code>JSON_DIAGNOSTICS</code></a>, an exception names the <abbr title="JavaScript Object Notation">JSON</abbr> Pointer path of the value within the document.</li> <li>With <a href=../../api/macros/json_diagnostic_positions/ ><code>JSON_DIAGNOSTIC_POSITIONS</code></a>, an exception names the bytes of the value in the source text.</li> </ul> <p>A view has neither parent pointers nor stored positions, so both are found from the document's index when the exception is thrown. This takes time linear in the size of the document. One exception has neither: <code>key()</code> of an iterator over an array or a primitive value (<a href=../../home/exceptions/#jsonexceptioninvalid_iterator207><code>invalid_iterator.207</code></a>), because a view's iterator does not know the value it iterates over.</p> <p>The other macros only affect the values <code>materialize()</code> produces, through <code>BasicJsonType</code>.</p> </li> </ul> <h2 id=what-is-different>What is different<a class=headerlink href=#what-is-different title="Permanent link">&para;</a></h2> <ul> <li><strong>Only 64-bit integers.</strong> <code>basic_json_document&lt;BasicJsonType&gt;</code> requires <code>BasicJsonType::number_integer_t</code> and <code>number_unsigned_t</code> to both be 64 bits wide; this is a compile-time <code class=highlight><span class=k>static_assert</span></code>.</li> <li><strong>A 4 <abbr title="Gibibyte (2^30 bytes)">GiB</abbr> input limit.</strong> An input of 4294967280 bytes (4 <abbr title="Gibibyte (2^30 bytes)">GiB</abbr> minus 16 bytes) or more throws <a href=../../home/exceptions/#jsonexceptionout_of_range416><code>out_of_range.416</code></a>, a limit <code class=highlight><span class=n>basic_json</span><span class=o>::</span><span class=n>parse</span><span class=p>()</span></code> does not have. It keeps an index entry at 16 bytes: offsets and counts are 32 bits wide. The limit is 16 bytes below 2^32, because the parser reads in 16-byte steps, and a position one step past the end must still fit.</li> <li><strong>A stream is always read to its end.</strong> There is no partial/streaming read of an <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>istream</span></code>: the stream is read to its end into a buffer the document owns, and parsing starts after that.</li> <li><strong>Fewer source positions after edits.</strong> With <a href=../../api/macros/json_diagnostic_positions/ ><code>JSON_DIAGNOSTIC_POSITIONS</code></a> enabled, <a href=../../api/basic_json_view/materialize/ ><code>materialize()</code></a> sets the same poLine truncated
</code></pre></div> </details> <h2 id=what-is-the-same-as-parse>What is the same as <code>parse()</code><a class=headerlink href=#what-is-the-same-as-parse title="Permanent link">&para;</a></h2> <ul> <li><strong>Accept/reject.</strong> <a href=../../api/basic_json_document/accept/ ><code>accept()</code></a> and <a href=../../api/basic_json_document/parse/ ><code>parse()</code></a> accept and reject exactly the same inputs as <a href=../../api/basic_json/accept/ ><code>basic_json::accept()</code></a>/<a href=../../api/basic_json/parse/ ><code>basic_json::parse()</code></a>, with the same <code>ignore_comments</code> and <code>ignore_trailing_commas</code> options.</li> <li><strong>Errors.</strong> A failing parse throws the same exception — the same id, message, and position — because on failure the library's own parser is run on the same bytes to produce the diagnostic. Valid input never pays for this, and the replay builds no values: it costs about a lexing pass up to the error. <code class=highlight><span class=n>allow_exceptions</span><span class=w> </span><span class=o>==</span><span class=w> </span><span class=nb>false</span></code> skips the replay and gives a <a href=../../api/basic_json_document/is_discarded/ >discarded</a> document instead of throwing, just as it gives a discarded value for <code class=highlight><span class=n>basic_json</span><span class=o>::</span><span class=n>parse</span><span class=p>()</span></code>.</li> <li><strong>Number classification.</strong> An integer literal that does not fit into the 64-bit integer type becomes a floating-point value, exactly as it does for <code class=highlight><span class=n>basic_json</span><span class=o>::</span><span class=n>parse</span><span class=p>()</span></code>.</li> <li> <p><strong>Macros.</strong> <a href=../../api/macros/json_strict_nul_handling/ ><code>JSON_STRICT_NUL_HANDLING</code></a> and <a href=../../api/macros/json_noexception/ ><code>JSON_NOEXCEPTION</code></a>/<a href=../../api/macros/json_throw_user/ ><code>JSON_THROW_USER</code></a> behave the same way they do for <code>&lt;nlohmann/json.hpp&gt;</code>. These are the macros that change what the parser accepts or how it reports errors. Two more macros add context to the exceptions of views, as they do for those of <code class=highlight><span class=n>basic_json</span></code>:</p> <ul> <li>With <a href=../../api/macros/json_diagnostics/ ><code>JSON_DIAGNOSTICS</code></a>, an exception names the <abbr title="JavaScript Object Notation">JSON</abbr> Pointer path of the value within the document.</li> <li>With <a href=../../api/macros/json_diagnostic_positions/ ><code>JSON_DIAGNOSTIC_POSITIONS</code></a>, an exception names the bytes of the value in the source text.</li> </ul> <p>A view has neither parent pointers nor stored positions, so both are found from the document's index when the exception is thrown. This takes time linear in the size of the document. One exception has neither: <code>key()</code> of an iterator over an array or a primitive value (<a href=../../home/exceptions/#jsonexceptioninvalid_iterator207><code>invalid_iterator.207</code></a>), because a view's iterator does not know the value it iterates over.</p> <p>The other macros only affect the values <code>materialize()</code> produces, through <code>BasicJsonType</code>.</p> </li> </ul> <h2 id=what-is-different>What is different<a class=headerlink href=#what-is-different title="Permanent link">&para;</a></h2> <ul> <li><strong>Only 64-bit integers.</strong> <code>basic_json_document&lt;BasicJsonType&gt;</code> requires <code>BasicJsonType::number_integer_t</code> and <code>number_unsigned_t</code> to both be 64 bits wide; this is a compile-time <code class=highlight><span class=k>static_assert</span></code>.</li> <li><strong>A 4 <abbr title="Gibibyte (2^30 bytes)">GiB</abbr> input limit.</strong> An input of 4294967280 bytes (4 <abbr title="Gibibyte (2^30 bytes)">GiB</abbr> minus 16 bytes) or more throws <a href=../../home/exceptions/#jsonexceptionout_of_range416><code>out_of_range.416</code></a>, a limit <code class=highlight><span class=n>basic_json</span><span class=o>::</span><span class=n>parse</span><span class=p>()</span></code> does not have. It keeps an index entry at 16 bytes: offsets and counts are 32 bits wide. The limit is 16 bytes below 2^32, because the parser reads in 16-byte steps, and a position one step past the end must still fit.</li> <li><strong>A stream is always read to its end.</strong> There is no partial/streaming read of an <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>istream</span></code>: the stream is read to its end into a buffer the document owns, and parsing starts after that. For a file, reading it into a <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>string</span></code> yourself and parsing that (an lvalue is borrowed) is therefore not more expensive in memory, and was about 6 to 12 % faster than parsLine truncated
<span class=n>json_view</span><span class=w> </span><span class=n>last</span><span class=p>;</span>
<span class=k>for</span><span class=w> </span><span class=p>(</span><span class=k>const</span><span class=w> </span><span class=k>auto</span><span class=w> </span><span class=n>item</span><span class=w> </span><span class=o>:</span><span class=w> </span><span class=n>view</span><span class=p>.</span><span class=n>items</span><span class=p>())</span>
<span class=p>{</span>
@@ -171,4 +171,4 @@
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code><span class=mi>316</span>
<span class=kc>true</span>
<span class=kc>true</span>
</code></pre></div> </details> <div class="admonition warning"> <p class=admonition-title>Experimental</p> <p>The image format is versioned but not yet stable, and may change in an incompatible way before it is declared stable. It is little-endian only (on big-endian targets, <code>save()</code> and <code>load()</code> do not exist), and tied to the library build that wrote it — use it to cache a document or to hand one to another process running the <em>same</em> build, not as a long-term storage format; keep the original <abbr title="JavaScript Object Notation">JSON</abbr> text if a saved document needs to be readable by a future library version.</p> </div> <p>The idea of a document you can read without parsing comes from zero-copy formats such as <a href="https://github.com/google/flatbuffers">FlatBuffers</a> and <a href="https://github.com/yandex/yaff">YaFF</a>; the check <a href=../../api/basic_json_document/load/ ><code>load()</code></a> runs follows the idea of FlatBuffers' Verifier. No code is taken from either.</p> <h2 id=choosing-between-json-ordered_json-the-sax-interface-and-json_view>Choosing between <code>json</code>, <code>ordered_json</code>, the <abbr title="Simple API for XML">SAX</abbr> interface, and <code>json_view</code><a class=headerlink href=#choosing-between-json-ordered_json-the-sax-interface-and-json_view title="Permanent link">&para;</a></h2> <table> <thead> <tr> <th></th> <th><a href=../../api/json/ ><code>json</code></a> / <a href=../../api/ordered_json/ ><code>ordered_json</code></a></th> <th><a href=../parsing/sax_interface/ ><abbr title="Simple API for XML">SAX</abbr> interface</a></th> <th><a href=../../api/json_document/ ><code>json_document</code></a> / <a href=../../api/json_view/ ><code>json_view</code></a></th> <th><a href=../../api/json_editable_document/ ><code>json_editable_document</code></a> / <a href=../../api/json_editable_view/ ><code>json_editable_view</code></a></th> </tr> </thead> <tbody> <tr> <td><strong>Ownership</strong></td> <td>owns every value</td> <td>owns nothing; you decide what to keep, in your handler</td> <td>borrows or owns the <em>text</em>; the index is always owned by the document</td> <td>same as <code>json_document</code>; edits go to storage the document owns</td> </tr> <tr> <td><strong>Mutability</strong></td> <td>freely mutable</td> <td>not applicable (a one-shot event stream)</td> <td>read-only</td> <td><a href=../../api/basic_json_document/set/ ><code>set</code></a>/<a href=../../api/basic_json_document/push_back/ ><code>push_back</code></a>/<a href=../../api/basic_json_document/insert/ ><code>insert</code></a>/<a href=../../api/basic_json_document/erase/ ><code>erase</code></a> edit in place; the source text is never rewritten</td> </tr> <tr> <td><strong>What you get</strong></td> <td>a full tree you can read, write, and keep as long as you like</td> <td>a sequence of callbacks; whatever your handler builds from them</td> <td>a flat index plus, on demand, <a href=../../api/basic_json_view/materialize/ ><code>materialize()</code></a>d <code>json</code>/<code>ordered_json</code> values for the parts you actually use</td> <td>the same, plus <a href=../../api/basic_json_view/dump/ ><code>dump()</code></a> of an edited document that keeps the member order and, with <a href=../../api/basic_json_view/number_format/ ><code>number_format::source</code></a>, the spelling of every untouched number</td> </tr> <tr> <td><strong>Typical use</strong></td> <td>general-purpose <abbr title="JavaScript Object Notation">JSON</abbr> handling: config, request/response bodies you build or modify, anything you hold onto</td> <td>validating or projecting a text into your own data structure without ever holding the whole thing as <abbr title="JavaScript Object Notation">JSON</abbr></td> <td>large or high-volume input where you only need part of it, or need it repeatedly, and can keep the source text (or a copy) alive for as long as the document lives</td> <td>a document you read, patch a few fields of, and write back — a configuration file, for instance — where the rest of it should come back exactly as it was</td> </tr> <tr> <td><strong>Caching/reload</strong></td> <td>not applicable — re-parse, or roll your own serialization</td> <td>not applicable</td> <td><a href=../../api/basic_json_document/save/ ><code>save()</code></a>/<a href=../../api/basic_json_document/load/ ><code>load()</code></a>: cache the parsed index as an image and reload it without parsing</td> <td>same, saving the document's current — possibly edited — state</td> </tr> </tbody> </table> <p class=nj-table-caption>Each column is one way to work with <abbr title="JavaScript Object Notation">JSON</abbr> in this library, and each row compares one property of them.</p> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../../api/basic_json_document/ ><code>basic_json_document</code></a> - the document that owns the parsLine truncated
</code></pre></div> </details> <div class="admonition warning"> <p class=admonition-title>Experimental</p> <p>The image format is versioned but not yet stable, and may change in an incompatible way before it is declared stable. It is little-endian only (on big-endian targets, <code>save()</code> and <code>load()</code> do not exist), and tied to the library build that wrote it — use it to cache a document or to hand one to another process running the <em>same</em> build, not as a long-term storage format; keep the original <abbr title="JavaScript Object Notation">JSON</abbr> text if a saved document needs to be readable by a future library version.</p> </div> <p>The idea of a document you can read without parsing comes from zero-copy formats such as <a href="https://github.com/google/flatbuffers">FlatBuffers</a> and <a href="https://github.com/yandex/yaff">YaFF</a>; the check <a href=../../api/basic_json_document/load/ ><code>load()</code></a> runs follows the idea of FlatBuffers' Verifier. No code is taken from either.</p> <h2 id=choosing-between-json-ordered_json-the-sax-interface-and-json_view>Choosing between <code>json</code>, <code>ordered_json</code>, the <abbr title="Simple API for XML">SAX</abbr> interface, and <code>json_view</code><a class=headerlink href=#choosing-between-json-ordered_json-the-sax-interface-and-json_view title="Permanent link">&para;</a></h2> <table> <thead> <tr> <th></th> <th><a href=../../api/json/ ><code>json</code></a> / <a href=../../api/ordered_json/ ><code>ordered_json</code></a></th> <th><a href=../parsing/sax_interface/ ><abbr title="Simple API for XML">SAX</abbr> interface</a></th> <th><a href=../../api/json_document/ ><code>json_document</code></a> / <a href=../../api/json_view/ ><code>json_view</code></a></th> <th><a href=../../api/json_editable_document/ ><code>json_editable_document</code></a> / <a href=../../api/json_editable_view/ ><code>json_editable_view</code></a></th> </tr> </thead> <tbody> <tr> <td><strong>Ownership</strong></td> <td>owns every value</td> <td>owns nothing; you decide what to keep, in your handler</td> <td>borrows or owns the <em>text</em>; the index is always owned by the document</td> <td>same as <code>json_document</code>; edits go to storage the document owns</td> </tr> <tr> <td><strong>Mutability</strong></td> <td>freely mutable</td> <td>not applicable (a one-shot event stream)</td> <td>read-only</td> <td><a href=../../api/basic_json_document/set/ ><code>set</code></a>/<a href=../../api/basic_json_document/push_back/ ><code>push_back</code></a>/<a href=../../api/basic_json_document/insert/ ><code>insert</code></a>/<a href=../../api/basic_json_document/erase/ ><code>erase</code></a> edit in place; the source text is never rewritten</td> </tr> <tr> <td><strong>What you get</strong></td> <td>a full tree you can read, write, and keep as long as you like</td> <td>a sequence of callbacks; whatever your handler builds from them</td> <td>a flat index plus, on demand, <a href=../../api/basic_json_view/materialize/ ><code>materialize()</code></a>d <code>json</code>/<code>ordered_json</code> values for the parts you actually use</td> <td>the same, plus <a href=../../api/basic_json_view/dump/ ><code>dump()</code></a> of an edited document that keeps the member order and, with <a href=../../api/basic_json_view/number_format/ ><code>number_format::source</code></a>, the spelling of every untouched number</td> </tr> <tr> <td><strong>Typical use</strong></td> <td>general-purpose <abbr title="JavaScript Object Notation">JSON</abbr> handling: config, request/response bodies you build or modify, anything you hold onto</td> <td>validating or projecting a text into your own data structure without ever holding the whole thing as <abbr title="JavaScript Object Notation">JSON</abbr></td> <td>large or high-volume input where you only need part of it, or need it repeatedly, and can keep the source text (or a copy) alive for as long as the document lives</td> <td>a document you read, patch a few fields of, and write back — a configuration file, for instance — where the rest of it should come back exactly as it was</td> </tr> <tr> <td><strong>Caching/reload</strong></td> <td>not applicable — re-parse, or roll your own serialization</td> <td>not applicable</td> <td><a href=../../api/basic_json_document/save/ ><code>save()</code></a>/<a href=../../api/basic_json_document/load/ ><code>load()</code></a>: cache the parsed index as an image and reload it without parsing</td> <td>same, saving the document's current — possibly edited — state</td> </tr> </tbody> </table> <p class=nj-table-caption>Each column is one way to work with <abbr title="JavaScript Object Notation">JSON</abbr> in this library, and each row compares one property of them.</p> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../../api/basic_json_document/ ><code>basic_json_document</code></a> - the document that owns the parsLine truncated
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- **Only 64-bit integers.** `basic_json_document<BasicJsonType>` requires `BasicJsonType::number_integer_t` and `number_unsigned_t` to both be 64 bits wide; this is a compile-time `static_assert`.
- **A 4 GiB input limit.** An input of 4294967280 bytes (4 GiB minus 16 bytes) or more throws [`out_of_range.416`](https://json.nlohmann.me/home/exceptions/#jsonexceptionout_of_range416), a limit `basic_json::parse()` does not have. It keeps an index entry at 16 bytes: offsets and counts are 32 bits wide. The limit is 16 bytes below 2^32, because the parser reads in 16-byte steps, and a position one step past the end must still fit.
- **A stream is always read to its end.** There is no partial/streaming read of an `std::istream`: the stream is read to its end into a buffer the document owns, and parsing starts after that.
- **A stream is always read to its end.** There is no partial/streaming read of an `std::istream`: the stream is read to its end into a buffer the document owns, and parsing starts after that. For a file, reading it into a `std::string` yourself and parsing that (an lvalue is borrowed) is therefore not more expensive in memory, and was about 6 to 12 % faster than parsing from an `std::ifstream`.
- **Fewer source positions after edits.** With [`JSON_DIAGNOSTIC_POSITIONS`](https://json.nlohmann.me/api/macros/json_diagnostic_positions/index.md) enabled, [`materialize()`](https://json.nlohmann.me/api/basic_json_view/materialize/index.md) sets the same positions as `basic_json::parse()`. In an [editable document](https://json.nlohmann.me/api/json_editable_document/index.md), a value written by an edit has none, and neither has an array or object whose elements changed or whose last element has none; a `basic_json` value keeps its positions when it is changed. See [`materialize()`](https://json.nlohmann.me/api/basic_json_view/materialize/#notes).
- **Objects iterate in document order.** [`begin()`](https://json.nlohmann.me/api/basic_json_view/begin/index.md)/ [`end()`](https://json.nlohmann.me/api/basic_json_view/end/index.md) and [`items()`](https://json.nlohmann.me/api/basic_json_view/items/index.md) visit an object's members in the order they appear in the source text. `basic_json`'s default `object_t` is a `std::map`, which sorts by key, so iterating a [`materialize()`](https://json.nlohmann.me/api/basic_json_view/materialize/index.md)d value can print members in a different order than iterating the view they came from.
- **Chained access is safe.** [`operator[]`](https://json.nlohmann.me/api/basic_json_view/operator%5B%5D/index.md) with a missing key, an index out of range, or an unresolvable JSON pointer returns a [discarded](https://json.nlohmann.me/api/basic_json_view/is_discarded/index.md) view, and `operator[]` on a discarded view returns a discarded view without throwing: `v["a"]["b"][0]` can be tested once at the end. Type errors on values that exist (a key on an array, an index on an object) still throw, and [`at`](https://json.nlohmann.me/api/basic_json_view/at/index.md) throws for every missing value.
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<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=description content="JSON for Modern C++ is a header-only C++11 library that makes JSON a first-class data type."><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/merge_patch/" rel="canonical"><link href=../iterators/ rel=prev><link href=../json_patch/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>JSON Merge Patch - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.ab4e12ef.min.css><link rel=stylesheet href=../../css/custom.css><link rel=stylesheet href=../../css/brand.css><link rel=stylesheet href=../../css/fig-sax.css><link rel=stylesheet href=../../css/fig-jsonview.css><link rel=stylesheet 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<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=description content="JSON for Modern C++ is a header-only C++11 library that makes JSON a first-class data type."><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/merge_patch/" rel="canonical"><link href=../iterators/ rel=prev><link href=../json_patch/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.8.0"><title>JSON Merge Patch - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.57a0d79d.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.fb2fffe2.min.css><link rel=stylesheet href=../../css/custom.css><link rel=stylesheet href=../../css/brand.css><link rel=stylesheet href=../../css/fig-sax.css><link rel=stylesheet href=../../css/fig-jsonview.css><link rel=stylesheet 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data-md-component=announce> </div> <header class=md-header data-md-component=header> <nav class="md-header__inner md-grid" aria-label=Header> <a href=../.. title="JSON for Modern C++" class="md-header__button md-logo" aria-label="JSON for Modern C++" data-md-component=logo> <img src=../../images/wordmark-header.svg alt=logo> </a> <label class="md-header__button md-icon" for=__drawer> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M3 6h18v2H3zm0 5h18v2H3zm0 5h18v2H3z"/></svg> </label> <div class=md-header__title data-md-component=header-title> <div class=md-header__ellipsis> <div class=md-header__topic> <span class=md-ellipsis> JSON for Modern C++ </span> </div> <div class=md-header__topic data-md-component=header-topic> <span class=md-ellipsis> JSON Merge Patch </span> </div> </div> </div> <form class=md-header__option data-md-component=palette> <input class=md-option data-md-color-media="(prefers-color-scheme: light)" data-md-color-scheme=default 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12s-1.44-4.5-3.5-5.45C10.26 6.2 11.11 6 12 6a6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> </form> <script>var palette=__md_get("__palette");if(palette&&palette.color){if("(prefers-color-scheme)"===palette.color.media){var media=matchMedia("(prefers-color-scheme: light)"),input=document.querySelector(media.matches?"[data-md-color-media='(prefers-color-scheme: light)']":"[data-md-color-media='(prefers-color-scheme: dark)']");palette.color.media=input.getAttribute("data-md-color-media"),palette.color.scheme=input.getAttribute("data-md-color-scheme"),palette.color.primary=input.getAttribute("data-md-color-primary"),palette.color.accent=input.getAttribute("data-md-color-accent")}for(var[key,value]of Object.entries(palette.color))document.body.setAttribute("data-md-color-"+key,value)}</script> <label class="md-header__button md-icon" for=__search> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M9.5 3A6.5 6.5 0 0 1 16 9.5c0 1.61-.59 3.09-1.56 4.23l.Line truncated
<span class=w> </span><span class=nt>"title"</span><span class=p>:</span><span class=w> </span><span class=s2>"Goodbye!"</span><span class=p>,</span>
<span class=w> </span><span class=nt>"author"</span><span class=p>:</span><span class=w> </span><span class=p>{</span>
<span class=w> </span><span class=nt>"givenName"</span><span class=p>:</span><span class=w> </span><span class=s2>"John"</span><span class=p>,</span>
@@ -81,4 +81,4 @@
<span class=w> </span><span class=p>],</span>
<span class=w> </span><span class=nt>"title"</span><span class=p>:</span><span class=w> </span><span class=s2>"Hello!"</span>
<span class=p>}</span>
</code></pre></div> </details> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../json_patch/ ><abbr title="JavaScript Object Notation">JSON</abbr> Patch and Diff</a> - a more expressive alternative that describes a sequence of operations</li> <li><a href=../json_pointer/ ><abbr title="JavaScript Object Notation">JSON</abbr> Pointer</a> - the addressing scheme used by <abbr title="JavaScript Object Notation">JSON</abbr> Patch</li> <li><a href=../../api/basic_json/merge_patch/ ><code>merge_patch</code></a> - apply a <abbr title="JavaScript Object Notation">JSON</abbr> Merge Patch to a value</li> <li><a href=../../api/basic_json/merge_diff/ ><code>merge_diff</code></a> - create a <abbr title="JavaScript Object Notation">JSON</abbr> Merge Patch from two values</li> <li>Function <a href=../../api/basic_json/update/ ><code>update</code></a> - merge objects, optionally recursively</li> <li><a href=../modifying_values/ >Modifying values</a> - simple in-place modifications</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../iterators/ class="md-footer__link md-footer__link--prev" aria-label="Previous: Iterators"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> Iterators </div> </div> </a> <a href=../json_patch/ class="md-footer__link md-footer__link--next" aria-label="Next: JSON Patch and Diff"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> JSON Patch and Diff </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 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</code></pre></div> </details> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../json_patch/ ><abbr title="JavaScript Object Notation">JSON</abbr> Patch and Diff</a> - a more expressive alternative that describes a sequence of operations</li> <li><a href=../json_pointer/ ><abbr title="JavaScript Object Notation">JSON</abbr> Pointer</a> - the addressing scheme used by <abbr title="JavaScript Object Notation">JSON</abbr> Patch</li> <li><a href=../../api/basic_json/merge_patch/ ><code>merge_patch</code></a> - apply a <abbr title="JavaScript Object Notation">JSON</abbr> Merge Patch to a value</li> <li><a href=../../api/basic_json/merge_diff/ ><code>merge_diff</code></a> - create a <abbr title="JavaScript Object Notation">JSON</abbr> Merge Patch from two values</li> <li>Function <a href=../../api/basic_json/update/ ><code>update</code></a> - merge objects, optionally recursively</li> <li><a href=../modifying_values/ >Modifying values</a> - simple in-place modifications</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../iterators/ class="md-footer__link md-footer__link--prev" aria-label="Previous: Iterators"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> Iterators </div> </div> </a> <a href=../json_patch/ class="md-footer__link md-footer__link--next" aria-label="Next: JSON Patch and Diff"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> JSON Patch and Diff </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 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+2 -2
View File
@@ -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=description content="JSON for Modern C++ is a header-only C++11 library that makes JSON a first-class data type."><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/modifying_values/" rel="canonical"><link href=../json_pointer/ rel=prev><link href=../modules/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>Modifying values - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.ab4e12ef.min.css><link rel=stylesheet href=../../css/custom.css><link rel=stylesheet href=../../css/brand.css><link rel=stylesheet href=../../css/fig-sax.css><link rel=stylesheet href=../../css/fig-jsonview.css><link rel=stylesheet 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<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=description content="JSON for Modern C++ is a header-only C++11 library that makes JSON a first-class data type."><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/modifying_values/" rel="canonical"><link href=../json_pointer/ rel=prev><link href=../modules/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.8.0"><title>Modifying values - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.57a0d79d.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.fb2fffe2.min.css><link rel=stylesheet href=../../css/custom.css><link rel=stylesheet href=../../css/brand.css><link rel=stylesheet href=../../css/fig-sax.css><link rel=stylesheet href=../../css/fig-jsonview.css><link rel=stylesheet 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<span class=n>j</span><span class=p>.</span><span class=n>push_back</span><span class=p>(</span><span class=mi>1</span><span class=p>);</span><span class=w> </span><span class=c1>// [1]</span>
<span class=n>j</span><span class=p>.</span><span class=n>push_back</span><span class=p>(</span><span class=mi>2</span><span class=p>);</span><span class=w> </span><span class=c1>// [1,2]</span>
<span class=n>j</span><span class=p>.</span><span class=n>emplace_back</span><span class=p>(</span><span class=mi>3</span><span class=p>);</span><span class=w> </span><span class=c1>// [1,2,3]</span>
@@ -60,4 +60,4 @@
<span class=n>json</span><span class=w> </span><span class=n>a</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=p>{</span><span class=mi>1</span><span class=p>,</span><span class=w> </span><span class=mi>2</span><span class=p>,</span><span class=w> </span><span class=mi>3</span><span class=p>,</span><span class=w> </span><span class=mi>4</span><span class=p>};</span>
<span class=n>a</span><span class=p>.</span><span class=n>erase</span><span class=p>(</span><span class=mi>1</span><span class=p>);</span><span class=w> </span><span class=c1>// [1,3,4] (erase by index)</span>
</code></pre></div> <h2 id=editing-a-document>Editing a document<a class=headerlink href=#editing-a-document title="Permanent link">&para;</a></h2> <p>The operations above work on a <code>basic_json</code> value. A <abbr title="JavaScript Object Notation">JSON</abbr> text that was parsed into a <a href=../json_view/#editing-a-document>document</a> can also be changed in place, without building a <code>basic_json</code> tree for the parts that are not touched: <a href=../../api/basic_json_document/set/ ><code>set</code></a> replaces a value, adds or replaces an object member, or replaces an array element (also through a <abbr title="JavaScript Object Notation">JSON</abbr> pointer), <a href=../../api/basic_json_document/push_back/ ><code>push_back</code></a> appends to an array, <a href=../../api/basic_json_document/insert/ ><code>insert</code></a> inserts into an array (unlike <code>basic_json::insert</code>, it has no object overload; members are added with <code>set</code>), and <a href=../../api/basic_json_document/erase/ ><code>erase</code></a> removes a member or an element. These functions are available on the editable document types, such as <a href=../../api/json_editable_document/ ><code>json_editable_document</code></a>.</p> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../../api/basic_json/push_back/ ><code>push_back</code></a> - / <a href=../../api/basic_json/emplace_back/ ><code>emplace_back</code></a> - append to an array</li> <li><a href=../../api/basic_json/emplace/ ><code>emplace</code></a> - insert into an object if the key is absent</li> <li><a href=../../api/basic_json/update/ ><code>update</code></a> - merge objects (shallow, or recursive with <code>merge_objects</code>)</li> <li><a href=../../api/basic_json/merge_patch/ ><code>merge_patch</code></a> - apply an <abbr title="Request for Comments">RFC</abbr> 7396 merge patch</li> <li><a href=../../api/basic_json/erase/ ><code>erase</code></a> - / <a href=../../api/basic_json/clear/ ><code>clear</code></a> - remove elements</li> <li><a href=../json_patch/ ><abbr title="JavaScript Object Notation">JSON</abbr> Patch and Diff</a> - and <a href=../merge_patch/ ><abbr title="JavaScript Object Notation">JSON</abbr> Merge Patch</a> - structured modifications</li> <li><a href=../json_view/#editing-a-document>Editing a document</a> - change a parsed document in place</li> <li><a href=../../api/basic_json/insert/ ><code>insert</code></a> - insert elements at an iterator position or merge a range</li> <li><a href=../../api/basic_json/operator%2B%3D/ ><code>operator+=</code></a> - shorthand for <code>push_back</code></li> <li><a href=../element_access/ >Element Access</a> - read elements</li> <li><a href=../iterators/ >Iterators</a> - iterate over elements</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../json_pointer/ class="md-footer__link md-footer__link--prev" aria-label="Previous: JSON Pointer"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> JSON Pointer </div> </div> </a> <a href=../modules/ class="md-footer__link md-footer__link--next" aria-label="Next: Modules"> <div class=md-footeLine truncated
</code></pre></div> <h2 id=editing-a-document>Editing a document<a class=headerlink href=#editing-a-document title="Permanent link">&para;</a></h2> <p>The operations above work on a <code>basic_json</code> value. A <abbr title="JavaScript Object Notation">JSON</abbr> text that was parsed into a <a href=../json_view/#editing-a-document>document</a> can also be changed in place, without building a <code>basic_json</code> tree for the parts that are not touched: <a href=../../api/basic_json_document/set/ ><code>set</code></a> replaces a value, adds or replaces an object member, or replaces an array element (also through a <abbr title="JavaScript Object Notation">JSON</abbr> pointer), <a href=../../api/basic_json_document/push_back/ ><code>push_back</code></a> appends to an array, <a href=../../api/basic_json_document/insert/ ><code>insert</code></a> inserts into an array (unlike <code>basic_json::insert</code>, it has no object overload; members are added with <code>set</code>), and <a href=../../api/basic_json_document/erase/ ><code>erase</code></a> removes a member or an element. These functions are available on the editable document types, such as <a href=../../api/json_editable_document/ ><code>json_editable_document</code></a>.</p> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../../api/basic_json/push_back/ ><code>push_back</code></a> - / <a href=../../api/basic_json/emplace_back/ ><code>emplace_back</code></a> - append to an array</li> <li><a href=../../api/basic_json/emplace/ ><code>emplace</code></a> - insert into an object if the key is absent</li> <li><a href=../../api/basic_json/update/ ><code>update</code></a> - merge objects (shallow, or recursive with <code>merge_objects</code>)</li> <li><a href=../../api/basic_json/merge_patch/ ><code>merge_patch</code></a> - apply an <abbr title="Request for Comments">RFC</abbr> 7396 merge patch</li> <li><a href=../../api/basic_json/erase/ ><code>erase</code></a> - / <a href=../../api/basic_json/clear/ ><code>clear</code></a> - remove elements</li> <li><a href=../json_patch/ ><abbr title="JavaScript Object Notation">JSON</abbr> Patch and Diff</a> - and <a href=../merge_patch/ ><abbr title="JavaScript Object Notation">JSON</abbr> Merge Patch</a> - structured modifications</li> <li><a href=../json_view/#editing-a-document>Editing a document</a> - change a parsed document in place</li> <li><a href=../../api/basic_json/insert/ ><code>insert</code></a> - insert elements at an iterator position or merge a range</li> <li><a href=../../api/basic_json/operator%2B%3D/ ><code>operator+=</code></a> - shorthand for <code>push_back</code></li> <li><a href=../element_access/ >Element Access</a> - read elements</li> <li><a href=../iterators/ >Iterators</a> - iterate over elements</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../json_pointer/ class="md-footer__link md-footer__link--prev" aria-label="Previous: JSON Pointer"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> JSON Pointer </div> </div> </a> <a href=../modules/ class="md-footer__link md-footer__link--next" aria-label="Next: Modules"> <div class=md-footeLine truncated
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@@ -1,4 +1,4 @@
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<span class=k>import</span><span class=w> </span><span class=n>nlohmann</span><span class=p>.</span><span class=n>json</span><span class=p>;</span>
<span class=k>using</span><span class=w> </span><span class=n>nlohmann</span><span class=o>::</span><span class=n>json</span><span class=p>;</span>
@@ -7,4 +7,4 @@
<span class=n>std</span><span class=o>::</span><span class=n>ifstream</span><span class=w> </span><span class=n>f</span><span class=p>(</span><span class=s>"example.json"</span><span class=p>);</span>
<span class=n>json</span><span class=w> </span><span class=n>data</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>parse</span><span class=p>(</span><span class=n>f</span><span class=p>);</span>
</code></pre></div></p> <h2 id=modules-do-not-export-macros>Modules do not export macros<a class=headerlink href=#modules-do-not-export-macros title="Permanent link">&para;</a></h2> <p>It should be noted that as modules do not export macros, the <code>nlohmann.json</code> module will not export any macros.</p> <h2 id=exported-symbols>Exported symbols<a class=headerlink href=#exported-symbols title="Permanent link">&para;</a></h2> <p>Only the following symbols are exported from <code>nlohmann.json</code>:</p> <ul> <li><code>nlohmann::adl_serializer</code></li> <li><code>nlohmann::basic_json</code></li> <li><code>nlohmann::basic_json_document</code></li> <li><code>nlohmann::basic_json_view</code></li> <li><code>nlohmann::json</code></li> <li><code>nlohmann::json_document</code></li> <li><code>nlohmann::json_pointer</code></li> <li><code>nlohmann::json_view</code></li> <li><code>nlohmann::ordered_json</code></li> <li><code>nlohmann::ordered_json_document</code></li> <li><code>nlohmann::ordered_json_view</code></li> <li><code>nlohmann::ordered_map</code></li> <li><code>nlohmann::to_string</code></li> <li><code>nlohmann::literals::json_literals::operator""_json</code></li> <li><code>nlohmann::literals::json_literals::operator""_json_pointer</code></li> </ul> <p>The module always exports the two user-defined string literals, even if <a href=../../api/macros/json_no_automatic_udls/ ><code>JSON_NO_AUTOMATIC_UDLS</code></a> is defined when building it.</p> <p>Additionally, the following <code>nlohmann::detail</code> symbols are exported, solely to work around an <abbr title="Microsoft Visual C++">MSVC</abbr> compilation issue (<a href="https://github.com/nlohmann/json/issues/3970">#3970</a>). They are implementation details, not part of the public <abbr title="Application Programming Interface">API</abbr>, and should not be used directly:</p> <ul> <li><code>nlohmann::detail::json_sax_dom_callback_parser</code></li> <li><code>nlohmann::detail::unknown_size</code></li> </ul> <h2 id=known-issues>Known issues<a class=headerlink href=#known-issues title="Permanent link">&para;</a></h2> <p>C++20 modules support is exercised in <abbr title="Continuous Integration">CI</abbr> against current <abbr title="GNU Compiler Collection">GCC</abbr> and Clang on Ubuntu, and the default <abbr title="Microsoft Visual C++">MSVC</abbr> toolset on Windows Server 2022 — there is no documented minimum compiler version, unlike feature-test-macro-gated features such as <a href=../../api/macros/json_has_ranges/ ><code>JSON_HAS_RANGES</code></a>.</p> <div class="admonition info"> <p class=admonition-title>Known compiler issues</p> <ul> <li><strong><abbr title="GNU Compiler Collection">GCC</abbr></strong> may emit "redefinition" errors when <code>#include &lt;nlohmann/json.hpp&gt;</code> appears in a module preamble together with other imports. This is an upstream <abbr title="GNU Compiler Collection">GCC</abbr> bug, not yet resolved as of <abbr title="GNU Compiler Collection">GCC</abbr> 16. Workarounds: include <code>nlohmann/json.hpp</code> before other <code>#include</code>s, use <code>import nlohmann.json;</code> instead, or upgrade <abbr title="GNU Compiler Collection">GCC</abbr>. (<a href="https://github.com/nlohmann/json/issues/5103">issue #5103</a>)</li> <li><strong><abbr title="Microsoft Visual C++">MSVC</abbr></strong> could fail with <code>C2039: 'json_sax_dom_callback_parser' is not a member of ... detail</code>; fixed by exporting the required internal symbols from <code>json.cppm</code> (see <a href=#exported-symbols>Exported symbols</a> above). (<a href="https://github.com/nlohmann/json/issues/3970">issue #3970</a>)</li> </ul> <p>If you hit a different module-related build failure, search <a href="https://github.com/nlohmann/json/issues?q=is%3Aissue+modules">existing issues</a> before filing a new one.</p> </div> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../macros/ >Supported Macros</a> - macros are not exported by modules</li> <li><a href=../../api/macros/ >Macros</a> - reference of all macros</li> <li><a href=../namespace/ ><code>nlohmann</code> Namespace</a> - the library namespace</li> <li><a href=../../integration/cmake/ >CMake</a> - enable the module build in CMake</li> <li><a href=../../integration/compile_times/ >Compile times</a> - reduce compile times</li> <li><a href=../../api/basic_json/ ><code>basic_json</code></a> - the main class and all its member functions</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.Line truncated
</code></pre></div></p> <h2 id=modules-do-not-export-macros>Modules do not export macros<a class=headerlink href=#modules-do-not-export-macros title="Permanent link">&para;</a></h2> <p>It should be noted that as modules do not export macros, the <code>nlohmann.json</code> module will not export any macros.</p> <h2 id=exported-symbols>Exported symbols<a class=headerlink href=#exported-symbols title="Permanent link">&para;</a></h2> <p>Only the following symbols are exported from <code>nlohmann.json</code>:</p> <ul> <li><code>nlohmann::adl_serializer</code></li> <li><code>nlohmann::basic_json</code></li> <li><code>nlohmann::basic_json_document</code></li> <li><code>nlohmann::basic_json_view</code></li> <li><code>nlohmann::json</code></li> <li><code>nlohmann::json_document</code></li> <li><code>nlohmann::json_pointer</code></li> <li><code>nlohmann::json_view</code></li> <li><code>nlohmann::ordered_json</code></li> <li><code>nlohmann::ordered_json_document</code></li> <li><code>nlohmann::ordered_json_view</code></li> <li><code>nlohmann::ordered_map</code></li> <li><code>nlohmann::to_string</code></li> <li><code>nlohmann::literals::json_literals::operator""_json</code></li> <li><code>nlohmann::literals::json_literals::operator""_json_pointer</code></li> </ul> <p>The module always exports the two user-defined string literals, even if <a href=../../api/macros/json_no_automatic_udls/ ><code>JSON_NO_AUTOMATIC_UDLS</code></a> is defined when building it.</p> <p>Additionally, the following <code>nlohmann::detail</code> symbols are exported, solely to work around an <abbr title="Microsoft Visual C++">MSVC</abbr> compilation issue (<a href="https://github.com/nlohmann/json/issues/3970">#3970</a>). They are implementation details, not part of the public <abbr title="Application Programming Interface">API</abbr>, and should not be used directly:</p> <ul> <li><code>nlohmann::detail::json_sax_dom_callback_parser</code></li> <li><code>nlohmann::detail::unknown_size</code></li> </ul> <h2 id=known-issues>Known issues<a class=headerlink href=#known-issues title="Permanent link">&para;</a></h2> <p>C++20 modules support is exercised in <abbr title="Continuous Integration">CI</abbr> against current <abbr title="GNU Compiler Collection">GCC</abbr> and Clang on Ubuntu, and the default <abbr title="Microsoft Visual C++">MSVC</abbr> toolset on Windows Server 2022 — there is no documented minimum compiler version, unlike feature-test-macro-gated features such as <a href=../../api/macros/json_has_ranges/ ><code>JSON_HAS_RANGES</code></a>.</p> <div class="admonition info"> <p class=admonition-title>Known compiler issues</p> <ul> <li><strong><abbr title="GNU Compiler Collection">GCC</abbr></strong> may emit "redefinition" errors when <code>#include &lt;nlohmann/json.hpp&gt;</code> appears in a module preamble together with other imports. This is an upstream <abbr title="GNU Compiler Collection">GCC</abbr> bug, not yet resolved as of <abbr title="GNU Compiler Collection">GCC</abbr> 16. Workarounds: include <code>nlohmann/json.hpp</code> before other <code>#include</code>s, use <code>import nlohmann.json;</code> instead, or upgrade <abbr title="GNU Compiler Collection">GCC</abbr>. (<a href="https://github.com/nlohmann/json/issues/5103">issue #5103</a>)</li> <li><strong><abbr title="Microsoft Visual C++">MSVC</abbr></strong> could fail with <code>C2039: 'json_sax_dom_callback_parser' is not a member of ... detail</code>; fixed by exporting the required internal symbols from <code>json.cppm</code> (see <a href=#exported-symbols>Exported symbols</a> above). (<a href="https://github.com/nlohmann/json/issues/3970">issue #3970</a>)</li> </ul> <p>If you hit a different module-related build failure, search <a href="https://github.com/nlohmann/json/issues?q=is%3Aissue+modules">existing issues</a> before filing a new one.</p> </div> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../macros/ >Supported Macros</a> - macros are not exported by modules</li> <li><a href=../../api/macros/ >Macros</a> - reference of all macros</li> <li><a href=../namespace/ ><code>nlohmann</code> Namespace</a> - the library namespace</li> <li><a href=../../integration/cmake/ >CMake</a> - enable the module build in CMake</li> <li><a href=../../integration/compile_times/ >Compile times</a> - reduce compile times</li> <li><a href=../../api/basic_json/ ><code>basic_json</code></a> - the main class and all its member functions</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.Line truncated
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@@ -1,4 +1,4 @@
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<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=description content="JSON for Modern C++ is a header-only C++11 library that makes JSON a first-class data type."><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/namespace/" rel="canonical"><link href=../modules/ rel=prev><link href=../no_exceptions/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.8.0"><title>nlohmann Namespace - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.57a0d79d.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.fb2fffe2.min.css><link rel=stylesheet href=../../css/custom.css><link rel=stylesheet href=../../css/brand.css><link rel=stylesheet href=../../css/fig-sax.css><link rel=stylesheet href=../../css/fig-jsonview.css><link rel=stylesheet 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</code></pre></div> <p>The name is assembled from these segments; pick a version and macros to see the resulting name:</p> <figure class=nj-integration-ns data-nj-integration-ns aria-label="How the inline namespace name is assembled from its segments"> <div class=nj-integration-ns__strip aria-live=polite> <span class="nj-integration-ns__seg nj-integration-ns__seg--root"><code>nlohmann</code><small>root namespace</small></span> <span class="nj-integration-ns__seg nj-integration-ns__seg--sep"><code>::</code><small>&nbsp;</small></span> <span class="nj-integration-ns__seg nj-integration-ns__seg--abi"><code>json_abi</code><small>always</small></span> <span class="nj-integration-ns__seg nj-integration-ns__seg--tag"><code>_diag</code><small>JSON_DIAGNOSTICS=1</small></span> <span class="nj-integration-ns__seg nj-integration-ns__seg--ver"><code>_v3_11_2</code><small>version 3.11.2</small></span> </div> <figcaption>Version 3.11.2 with <code>JSON_DIAGNOSTICS</code> defined to <code>1</code>. ABI tags are appended in the order of the list above, the version suffix comes last.</figcaption> </figure> <p class=nj-caption>The name of the inline namespace, assembled from its segments. Select a version and macros to see which <abbr title="Application Binary Interface">ABI</abbr> tags appear; a tag the chosen version does not have is disabled.</p> <h2 id=purpose>Purpose<a class=headerlink href=#purpose title="Permanent link">&para;</a></h2> <p>Several incompatibilities have been observed. Amongst the most common ones is linking code compiled with different definitions of <a href=../../api/macros/json_diagnostics/ ><code>JSON_DIAGNOSTICS</code></a>. This is illustrated in the diagram below.</p> <pre class=mermaid><code>graph
json["&lt;strong&gt;nlohmann_json (v3.10.5)&lt;/strong&gt;&lt;br&gt;JSON_DIAGNOSTICS=0"]
json_diag["&lt;strong&gt;nlohmann_json (v3.10.5)&lt;/strong&gt;&lt;br&gt;JSON_DIAGNOSTICS=1"]
@@ -9,4 +9,4 @@
app --&gt; json_diag
app --&gt; library</code></pre> <p class=nj-caption>The application uses the library with <code>JSON_DIAGNOSTICS=1</code> and links <code>some library</code>, which uses the same library version with <code>JSON_DIAGNOSTICS=0</code>. Arrows point from a component to what it uses.</p> <p>In releases prior to 3.11.0, mixing any version of the <abbr title="JavaScript Object Notation">JSON</abbr> library with different <code>JSON_DIAGNOSTICS</code> settings would result in a crashing application. If <code>some_library</code> never passes instances of <abbr title="JavaScript Object Notation">JSON</abbr> library types to the application, this scenario became safe in version 3.11.0 and above due to the inline namespace yielding distinct symbol names.</p> <h2 id=limitations>Limitations<a class=headerlink href=#limitations title="Permanent link">&para;</a></h2> <p>Neither the compiler nor the linker will issue as much as a warning when translation units – intended to be linked together and that include different versions and/or configurations of the <abbr title="JavaScript Object Notation">JSON</abbr> library – exchange and use library types.</p> <p>There is an exception when forward declarations are used (i.e., when including <code>json_fwd.hpp</code>) in which case the linker may complain about undefined references.</p> <h2 id=disabling-the-version-component>Disabling the version component<a class=headerlink href=#disabling-the-version-component title="Permanent link">&para;</a></h2> <p>Different versions are not necessarily <abbr title="Application Binary Interface">ABI</abbr>-incompatible, but the project does not actively track changes in the <abbr title="Application Binary Interface">ABI</abbr> and recommends that all parts of a codebase exchanging library types be built with the same version. Users can, <strong>at their own risk</strong>, disable the version component of the inline namespace, allowing different versions – but not configurations – to be used in cases where the linker would otherwise output undefined reference errors.</p> <p>To do so, define <a href=../../api/macros/nlohmann_json_namespace_no_version/ ><code>NLOHMANN_JSON_NAMESPACE_NO_VERSION</code></a> to <code>1</code>.</p> <p>This applies to version 3.11.2 and above only; versions 3.11.0 and 3.11.1 can apply the technique described in the next section to emulate the effect of the <code>NLOHMANN_JSON_NAMESPACE_NO_VERSION</code> macro.</p> <div class="admonition danger"> <p class=admonition-title>Use at your own risk</p> <p>Disabling the namespace version component and mixing <abbr title="Application Binary Interface">ABI</abbr>-incompatible versions will result in crashes or incorrect behavior. You have been warned!</p> </div> <h2 id=disabling-the-inline-namespace-completely>Disabling the inline namespace completely<a class=headerlink href=#disabling-the-inline-namespace-completely title="Permanent link">&para;</a></h2> <p>When interoperability with code using a pre-3.11.0 version of the library is required, users can, <strong>at their own risk</strong> restore the old namespace layout by redefining <a href=../../api/macros/nlohmann_json_namespace_begin/ ><code>NLOHMANN_JSON_NAMESPACE_BEGIN, NLOHMANN_JSON_NAMESPACE_END</code></a> as follows:</p> <div class=highlight><pre><span></span><code><span class=cp>#define NLOHMANN_JSON_NAMESPACE_BEGIN namespace nlohmann {</span>
<span class=cp>#define NLOHMANN_JSON_NAMESPACE_END }</span>
</code></pre></div> <div class="admonition danger"> <p class=admonition-title>Use at your own risk</p> <p>Overriding the namespace and mixing <abbr title="Application Binary Interface">ABI</abbr>-incompatible versions will result in crashes or incorrect behavior. You have been warned!</p> </div> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../../api/macros/nlohmann_json_namespace/ ><code>NLOHMANN_JSON_NAMESPACE</code></a> - the library namespace as macro</li> <li><a href=../../api/macros/nlohmann_json_namespace_begin/ ><code>NLOHMANN_JSON_NAMESPACE_BEGIN</code>, <code>NLOHMANN_JSON_NAMESPACE_END</code></a> - open and close the library namespace</li> <li><a href=../../api/macros/nlohmann_json_namespace_no_version/ ><code>NLOHMANN_JSON_NAMESPACE_NO_VERSION</code></a> - disable the version component</li> <li><a href=../macros/ >Supported Macros</a> - all supported macros</li> <li><a href=../../integration/migration_guide/ >Migration Guide</a> - do not hard-code the library namespace</li> <li><a href=../../api/basic_json/ ><code>basic_json</code></a> - the main class and all its member functions</li> </ul> <h2 id=version-history>Version history<a class=headerlink href=#version-history title="Permanent link">&para;</a></h2> <ul> <li>Introduced inline namespace (<code>json_v3_11_0[_abi-tag]*</code>) in version 3.11.0.</li> <li>Changed structure of inline namespace in version 3.11.2.</li> <li>Added <abbr title="Application Binary Interface">ABI</abbr> tag <code>_dp</code> (<a href=../../api/macros/json_diagnostic_positions/ ><code>JSON_DIAGNOSTIC_POSITIONS</code></a>) in version 3.12.0.</li> <li>Added <abbr title="Application Binary Interface">ABI</abbr> tags <code>_bics</code> (<a href=../../api/macros/json_brace_init_copy_semantics/ ><code>JSON_BRACE_INIT_COPY_SEMANTICS</code></a>), <code>_psp</code> (<a href=../../api/macros/json_precise_stream_position/ ><code>JSON_PRECISE_STREAM_POSITION</code></a>), <code>_snul</code> (<a href=../../api/macros/json_strict_nul_handling/ ><code>JSON_STRICT_NUL_HANDLING</code></a>), <code>_sbu8</code> (<a href=../../api/macros/json_strict_binary_utf8/ ><code>JSON_STRICT_BINARY_UTF8</code></a>), <code>_ekmo</code> (<a href=../../api/macros/json_use_objects_for_enum_keyed_maps/ ><code>JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS</code></a>), <code>_dsbs</code> (<a href=../../api/macros/json_disable_stream_bulk_scan/ ><code>JSON_DISABLE_STREAM_BULK_SCAN</code></a>), <code>_dufio</code> (<a href=../../api/macros/json_disable_unlocked_file_io/ ><code>JSON_DISABLE_UNLOCKED_FILE_IO</code></a>), and <code>_ns</code> (<a href=../../api/macros/json_no_simd/ ><code>JSON_NO_SIMD</code></a>) in version 3.13.0 <span class=unreleased-version title="Not part of a release yet; the latest release is 3.12.0.">unreleased</span>.</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../modules/ class="md-footer__link md-footer__link--prev" aria-label="Previous: Modules"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> Modules </div> </div> </a> <a href=../no_exceptions/ class="md-footer__link md-footer__link--next" aria-label="Next: Working Without ELine truncated
</code></pre></div> <div class="admonition danger"> <p class=admonition-title>Use at your own risk</p> <p>Overriding the namespace and mixing <abbr title="Application Binary Interface">ABI</abbr>-incompatible versions will result in crashes or incorrect behavior. You have been warned!</p> </div> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../../api/macros/nlohmann_json_namespace/ ><code>NLOHMANN_JSON_NAMESPACE</code></a> - the library namespace as macro</li> <li><a href=../../api/macros/nlohmann_json_namespace_begin/ ><code>NLOHMANN_JSON_NAMESPACE_BEGIN</code>, <code>NLOHMANN_JSON_NAMESPACE_END</code></a> - open and close the library namespace</li> <li><a href=../../api/macros/nlohmann_json_namespace_no_version/ ><code>NLOHMANN_JSON_NAMESPACE_NO_VERSION</code></a> - disable the version component</li> <li><a href=../macros/ >Supported Macros</a> - all supported macros</li> <li><a href=../../integration/migration_guide/ >Migration Guide</a> - do not hard-code the library namespace</li> <li><a href=../../api/basic_json/ ><code>basic_json</code></a> - the main class and all its member functions</li> </ul> <h2 id=version-history>Version history<a class=headerlink href=#version-history title="Permanent link">&para;</a></h2> <ul> <li>Introduced inline namespace (<code>json_v3_11_0[_abi-tag]*</code>) in version 3.11.0.</li> <li>Changed structure of inline namespace in version 3.11.2.</li> <li>Added <abbr title="Application Binary Interface">ABI</abbr> tag <code>_dp</code> (<a href=../../api/macros/json_diagnostic_positions/ ><code>JSON_DIAGNOSTIC_POSITIONS</code></a>) in version 3.12.0.</li> <li>Added <abbr title="Application Binary Interface">ABI</abbr> tags <code>_bics</code> (<a href=../../api/macros/json_brace_init_copy_semantics/ ><code>JSON_BRACE_INIT_COPY_SEMANTICS</code></a>), <code>_psp</code> (<a href=../../api/macros/json_precise_stream_position/ ><code>JSON_PRECISE_STREAM_POSITION</code></a>), <code>_snul</code> (<a href=../../api/macros/json_strict_nul_handling/ ><code>JSON_STRICT_NUL_HANDLING</code></a>), <code>_sbu8</code> (<a href=../../api/macros/json_strict_binary_utf8/ ><code>JSON_STRICT_BINARY_UTF8</code></a>), <code>_ekmo</code> (<a href=../../api/macros/json_use_objects_for_enum_keyed_maps/ ><code>JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS</code></a>), <code>_dsbs</code> (<a href=../../api/macros/json_disable_stream_bulk_scan/ ><code>JSON_DISABLE_STREAM_BULK_SCAN</code></a>), <code>_dufio</code> (<a href=../../api/macros/json_disable_unlocked_file_io/ ><code>JSON_DISABLE_UNLOCKED_FILE_IO</code></a>), and <code>_ns</code> (<a href=../../api/macros/json_no_simd/ ><code>JSON_NO_SIMD</code></a>) in version 3.13.0 <span class=unreleased-version title="Not part of a release yet; the latest release is 3.12.0.">unreleased</span>.</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../modules/ class="md-footer__link md-footer__link--prev" aria-label="Previous: Modules"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> Modules </div> </div> </a> <a href=../no_exceptions/ class="md-footer__link md-footer__link--next" aria-label="Next: Working Without ELine truncated
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<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;iostream&gt;</span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;vector&gt;</span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;nlohmann/json.hpp&gt;</span>
@@ -129,4 +129,4 @@ get_ptr&lt;string*&gt;: name = 1, age = 0
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code>person: Mary, 42
not a person: {"age":"forty-two","name":"Mary"}
not a person: {"name":"Mary"}
</code></pre></div> </details> <h2 id=overriding-what-happens-instead-of-an-exception>Overriding what happens instead of an exception<a class=headerlink href=#overriding-what-happens-instead-of-an-exception title="Permanent link">&para;</a></h2> <p>With exceptions off, you can replace the <code>abort()</code> by your own code, for example to log the message first. See <a href=../../home/exceptions/#switch-off-exceptions>Switch off exceptions</a> and <a href=../../api/macros/json_throw_user/ ><code>JSON_THROW_USER</code></a>. The handler must not return normally.</p> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../../home/exceptions/ >Exceptions</a> - all exceptions and their ids</li> <li><a href=../parsing/parse_exceptions/ >Parsing and exceptions</a> - the parsing alternatives in detail</li> <li><a href=../parsing/untrusted_input/ >Parsing untrusted input</a> - harden an application that parses data it does not control</li> <li><a href=../../api/macros/json_noexception/ ><code>JSON_NOEXCEPTION</code></a> - disable exceptions in the library</li> <li><a href=../../api/basic_json/exception/ ><code>exception</code></a> - base class of all exceptions thrown by the library</li> <li><a href=../../api/macros/json_throw_user/ ><code>JSON_THROW_USER</code></a> - replace the throw statements of the library</li> <li><a href=../../api/basic_json/parse/ ><code>parse</code></a> - deserialize from a compatible input</li> <li><a href=../../api/basic_json/accept/ ><code>accept</code></a> - check whether the input is valid <abbr title="JavaScript Object Notation">JSON</abbr></li> <li><a href=../macros/ >Supported Macros</a> - all supported macros</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../namespace/ class="md-footer__link md-footer__link--prev" aria-label="Previous: nlohmann Namespace"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> nlohmann Namespace </div> </div> </a> <a href=../object_order/ class="md-footer__link md-footer__link--next" aria-label="Next: Object Order"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> Object Order </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2025 Fonticons, Inc.--><path d="M173.9 397.4c0 2-2.3 3.6-5.2 3.6-3.3.3-5.6-1.3-5.6-3.6 0-2 2.3-3.6 5.2-3.6 3-.3 5.6 1.3 5.6 3.6m-31.1-4.5c-.7 2 1.3 4.3 4.3 4.9 2.6 1 5.6 0 6.2-2s-1.3-4.3-4.3-5.2Line truncated
</code></pre></div> </details> <h2 id=overriding-what-happens-instead-of-an-exception>Overriding what happens instead of an exception<a class=headerlink href=#overriding-what-happens-instead-of-an-exception title="Permanent link">&para;</a></h2> <p>With exceptions off, you can replace the <code>abort()</code> by your own code, for example to log the message first. See <a href=../../home/exceptions/#switch-off-exceptions>Switch off exceptions</a> and <a href=../../api/macros/json_throw_user/ ><code>JSON_THROW_USER</code></a>. The handler must not return normally.</p> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../../home/exceptions/ >Exceptions</a> - all exceptions and their ids</li> <li><a href=../parsing/parse_exceptions/ >Parsing and exceptions</a> - the parsing alternatives in detail</li> <li><a href=../parsing/untrusted_input/ >Parsing untrusted input</a> - harden an application that parses data it does not control</li> <li><a href=../../api/macros/json_noexception/ ><code>JSON_NOEXCEPTION</code></a> - disable exceptions in the library</li> <li><a href=../../api/basic_json/exception/ ><code>exception</code></a> - base class of all exceptions thrown by the library</li> <li><a href=../../api/macros/json_throw_user/ ><code>JSON_THROW_USER</code></a> - replace the throw statements of the library</li> <li><a href=../../api/basic_json/parse/ ><code>parse</code></a> - deserialize from a compatible input</li> <li><a href=../../api/basic_json/accept/ ><code>accept</code></a> - check whether the input is valid <abbr title="JavaScript Object Notation">JSON</abbr></li> <li><a href=../macros/ >Supported Macros</a> - all supported macros</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../namespace/ class="md-footer__link md-footer__link--prev" aria-label="Previous: nlohmann Namespace"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> nlohmann Namespace </div> </div> </a> <a href=../object_order/ class="md-footer__link md-footer__link--next" aria-label="Next: Object Order"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> Object Order </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2025 Fonticons, Inc.--><path d="M173.9 397.4c0 2-2.3 3.6-5.2 3.6-3.3.3-5.6-1.3-5.6-3.6 0-2 2.3-3.6 5.2-3.6 3-.3 5.6 1.3 5.6 3.6m-31.1-4.5c-.7 2 1.3 4.3 4.3 4.9 2.6 1 5.6 0 6.2-2s-1.3-4.3-4.3-5.2Line truncated
+2 -2
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@@ -1,4 +1,4 @@
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<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;nlohmann/json.hpp&gt;</span>
<span class=k>using</span><span class=w> </span><span class=n>nlohmann</span><span class=o>::</span><span class=n>json</span><span class=p>;</span>
@@ -57,4 +57,4 @@
<span class=w> </span><span class=nt>"three"</span><span class=p>:</span><span class=w> </span><span class=mi>3</span><span class=p>,</span>
<span class=w> </span><span class=nt>"two"</span><span class=p>:</span><span class=w> </span><span class=mi>2</span>
<span class=p>}</span>
</code></pre></div> </details> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../../api/ordered_json/ ><code>ordered_json</code></a> - <abbr title="JavaScript Object Notation">JSON</abbr> type that preserves insertion order</li> <li><a href=../../api/ordered_map/ ><code>ordered_map</code></a> - the container behind <code>ordered_json</code></li> <li><a href=../../api/basic_json/object_t/ ><code>object_t</code></a> - type used to store <abbr title="JavaScript Object Notation">JSON</abbr> objects</li> <li><a href=../iterators/ >Iterators</a> - iteration order of object keys</li> <li><a href=../element_access/ >Element Access</a> - access elements by key</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../no_exceptions/ class="md-footer__link md-footer__link--prev" aria-label="Previous: Working Without Exceptions"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> Working Without Exceptions </div> </div> </a> <a href=../performance/ class="md-footer__link md-footer__link--next" aria-label="Next: Performance"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> Performance </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 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</code></pre></div> </details> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../../api/ordered_json/ ><code>ordered_json</code></a> - <abbr title="JavaScript Object Notation">JSON</abbr> type that preserves insertion order</li> <li><a href=../../api/ordered_map/ ><code>ordered_map</code></a> - the container behind <code>ordered_json</code></li> <li><a href=../../api/basic_json/object_t/ ><code>object_t</code></a> - type used to store <abbr title="JavaScript Object Notation">JSON</abbr> objects</li> <li><a href=../iterators/ >Iterators</a> - iteration order of object keys</li> <li><a href=../element_access/ >Element Access</a> - access elements by key</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../no_exceptions/ class="md-footer__link md-footer__link--prev" aria-label="Previous: Working Without Exceptions"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> Working Without Exceptions </div> </div> </a> <a href=../performance/ class="md-footer__link md-footer__link--next" aria-label="Next: Performance"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> Performance </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 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+5
View File
@@ -35,6 +35,8 @@ std::ifstream f("example.json");
json data = json::parse(f);
```
See [Reading and writing files](../files.md) for checking that the file was opened and for writing files safely.
The input must be encoded in UTF-8; other encodings are not supported. The exception is wide-character input, which
is read as UTF-16 or UTF-32 — depending on the character type — and converted to UTF-8; see the
[`parse`](../../api/basic_json/parse.md) page for an example. A single input may contain only one JSON value.
@@ -84,6 +86,9 @@ ahead: after parsing, it is positioned exactly as if every character had been re
[`operator>>`](../../api/operator_gtgt.md) can still read several values from one stream (see also
[JSON Lines](json_lines.md)).
This does not hold for [`json_document`](../json_view.md): it reads a stream into a buffer it owns, so the stream saves
no memory, and reading the file into a `#!cpp std::string` yourself and borrowing it is about 6 to 12 % faster.
Two macros switch these optimizations off without changing any result:
[`JSON_DISABLE_STREAM_BULK_SCAN`](../../api/macros/json_disable_stream_bulk_scan.md) for stream buffers that do not
follow the usual `std::streambuf` contract, and
+3 -3
View File
@@ -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=description content="JSON for Modern C++ is a header-only C++11 library that makes JSON a first-class data type."><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/parsing/" rel="canonical"><link href=../performance/ rel=prev><link href=json_lines/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>Parsing - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.ab4e12ef.min.css><link rel=stylesheet href=../../css/custom.css><link rel=stylesheet href=../../css/brand.css><link rel=stylesheet href=../../css/fig-sax.css><link rel=stylesheet href=../../css/fig-jsonview.css><link rel=stylesheet 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<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=description content="JSON for Modern C++ is a header-only C++11 library that makes JSON a first-class data type."><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/parsing/" rel="canonical"><link href=../performance/ rel=prev><link href=json_lines/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.8.0"><title>Parsing - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.57a0d79d.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.fb2fffe2.min.css><link rel=stylesheet href=../../css/custom.css><link rel=stylesheet href=../../css/brand.css><link rel=stylesheet href=../../css/fig-sax.css><link rel=stylesheet href=../../css/fig-jsonview.css><link rel=stylesheet 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data-md-component=announce> </div> <header class=md-header data-md-component=header> <nav class="md-header__inner md-grid" aria-label=Header> <a href=../.. title="JSON for Modern C++" class="md-header__button md-logo" aria-label="JSON for Modern C++" data-md-component=logo> <img src=../../images/wordmark-header.svg alt=logo> </a> <label class="md-header__button md-icon" for=__drawer> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M3 6h18v2H3zm0 5h18v2H3zm0 5h18v2H3z"/></svg> </label> <div class=md-header__title data-md-component=header-title> <div class=md-header__ellipsis> <div class=md-header__topic> <span class=md-ellipsis> JSON for Modern C++ </span> </div> <div class=md-header__topic data-md-component=header-topic> <span class=md-ellipsis> Parsing </span> </div> </div> </div> <form class=md-header__option data-md-component=palette> <input class=md-option data-md-color-media="(prefers-color-scheme: light)" data-md-color-scheme=default 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12s-1.44-4.5-3.5-5.45C10.26 6.2 11.11 6 12 6a6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> </form> <script>var palette=__md_get("__palette");if(palette&&palette.color){if("(prefers-color-scheme)"===palette.color.media){var media=matchMedia("(prefers-color-scheme: light)"),input=document.querySelector(media.matches?"[data-md-color-media='(prefers-color-scheme: light)']":"[data-md-color-media='(prefers-color-scheme: dark)']");palette.color.media=input.getAttribute("data-md-color-media"),palette.color.scheme=input.getAttribute("data-md-color-scheme"),palette.color.primary=input.getAttribute("data-md-color-primary"),palette.color.accent=input.getAttribute("data-md-color-accent")}for(var[key,value]of Object.entries(palette.color))document.body.setAttribute("data-md-color-"+key,value)}</script> <label class="md-header__button md-icon" for=__search> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 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I["JSON input"] --&gt; P["parse()"]
I --&gt; S["sax_parse()"]
I --&gt; A["accept()"]
@@ -10,8 +10,8 @@
<span class=c1>// parse from a file</span>
<span class=n>std</span><span class=o>::</span><span class=n>ifstream</span><span class=w> </span><span class=n>f</span><span class=p>(</span><span class=s>"example.json"</span><span class=p>);</span>
<span class=n>json</span><span class=w> </span><span class=n>data</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>parse</span><span class=p>(</span><span class=n>f</span><span class=p>);</span>
</code></pre></div> <p>The input must be encoded in <abbr title="Unicode Transformation Format">UTF</abbr>-8; other encodings are not supported. The exception is wide-character input, which is read as <abbr title="Unicode Transformation Format">UTF</abbr>-16 or <abbr title="Unicode Transformation Format">UTF</abbr>-32 — depending on the character type — and converted to <abbr title="Unicode Transformation Format">UTF</abbr>-8; see the <a href=../../api/basic_json/parse/ ><code>parse</code></a> page for an example. A single input may contain only one <abbr title="JavaScript Object Notation">JSON</abbr> value. Inputs consisting of multiple values separated by newlines are handled by the <a href=json_lines/ ><abbr title="JavaScript Object Notation">JSON</abbr> Lines</a> format; input that arrives in chunks or contains several values is covered in <a href=large_and_partial_input/ >Large, streamed, and partial input</a>.</p> <p>By default, the library rejects comments and trailing commas. Both can be enabled with parameters of the <code>parse</code> function — see <a href=../comments/ >comments</a> and <a href=../trailing_commas/ >trailing commas</a>.</p> <p><a href=../../api/basic_json_document/parse/ ><code>basic_json_document::parse</code></a> accepts the same inputs with the same options, but keeps the result in a document rather than in a <code>basic_json</code> value; see <a href=#documents-and-views>Documents and views</a> below.</p> <h2 id=strictness-and-trailing-data>Strictness and trailing data<a class=headerlink href=#strictness-and-trailing-data title="Permanent link">&para;</a></h2> <p><a href=../../api/basic_json/parse/ ><code>parse</code></a> reads a single <abbr title="JavaScript Object Notation">JSON</abbr> value and requires the whole input to be consumed: any non-whitespace data after the value is reported as a parse error. Use it when you want to guarantee that an input is exactly one complete <abbr title="JavaScript Object Notation">JSON</abbr> document.</p> <p><a href=../../api/operator_gtgt/ ><code>operator&gt;&gt;</code></a> follows relaxed <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>istream</span></code> semantics instead: it parses one <abbr title="JavaScript Object Notation">JSON</abbr> value and leaves the stream positioned right after it, without requiring the rest of the stream to be consumed. This is what makes it possible to read several concatenated values from the same stream, but it also means that "a valid document followed by trailing bytes" is accepted rather than rejected. If you are validating conformance, or need to reject any input that is not exactly one <abbr title="JavaScript Object Notation">JSON</abbr> document, prefer <code>parse</code>.</p> <p>When using <code>operator&gt;&gt;</code> to read several concatenated values this way, a value that is a number must be followed by whitespace, because <code>operator&gt;&gt;</code> consumes the character that terminates a number, unless <a href=../../api/macros/json_precise_stream_position/ ><code>JSON_PRECISE_STREAM_POSITION</code></a> is defined to <code>1</code> — see the <a href=../../api/operator_gtgt/#notes><code>operator&gt;&gt;</code> notes</a> for details and examples.</p> <h2 id=input-sources-and-performance>Input sources and performance<a class=headerlink href=#input-sources-and-performance title="Permanent link">&para;</a></h2> <p>All inputs produce the same values and the same exceptions, but they are read in different ways, which makes some of them faster than others:</p> <table> <thead> <tr> <th>input</th> <th>how it is read</th> </tr> </thead> <tbody> <tr> <td>strings, string views, string literals, contiguous containers, pointer or iterator ranges over contiguous memory</td> <td>the whole input is scanned in bulk: runs of string characters, numbers, and indentation are recognized several bytes at a time</td> </tr> <tr> <td><code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>istream</span></code> (<code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>ifstream</span></code>, <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>istringstream</span></code>, ...)</td> <td>the characters the stream buffer holds are scanned in bulk like a string; the stream buffer is asked for more when they are used up</td> </tr> <tr> <td><code class=highlight><span class=kt>FILE</span><span class=o>*</span></code></td> <td>one character at a time, with the file locked once for the whole parse</td> </tr> <tr> <td>other iterator ranges (e.g., <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>list</span><span class=o>&lt;</span><span class=kt>char</span><span class=o>&gt;</span></code>, <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>istreambuf_iterator</span><span class=o>&lt;</span><span class=kt>char</span><span class=o>&gt;</Line truncated
</code></pre></div> <p>See <a href=../files/ >Reading and writing files</a> for checking that the file was opened and for writing files safely.</p> <p>The input must be encoded in <abbr title="Unicode Transformation Format">UTF</abbr>-8; other encodings are not supported. The exception is wide-character input, which is read as <abbr title="Unicode Transformation Format">UTF</abbr>-16 or <abbr title="Unicode Transformation Format">UTF</abbr>-32 — depending on the character type — and converted to <abbr title="Unicode Transformation Format">UTF</abbr>-8; see the <a href=../../api/basic_json/parse/ ><code>parse</code></a> page for an example. A single input may contain only one <abbr title="JavaScript Object Notation">JSON</abbr> value. Inputs consisting of multiple values separated by newlines are handled by the <a href=json_lines/ ><abbr title="JavaScript Object Notation">JSON</abbr> Lines</a> format; input that arrives in chunks or contains several values is covered in <a href=large_and_partial_input/ >Large, streamed, and partial input</a>.</p> <p>By default, the library rejects comments and trailing commas. Both can be enabled with parameters of the <code>parse</code> function — see <a href=../comments/ >comments</a> and <a href=../trailing_commas/ >trailing commas</a>.</p> <p><a href=../../api/basic_json_document/parse/ ><code>basic_json_document::parse</code></a> accepts the same inputs with the same options, but keeps the result in a document rather than in a <code>basic_json</code> value; see <a href=#documents-and-views>Documents and views</a> below.</p> <h2 id=strictness-and-trailing-data>Strictness and trailing data<a class=headerlink href=#strictness-and-trailing-data title="Permanent link">&para;</a></h2> <p><a href=../../api/basic_json/parse/ ><code>parse</code></a> reads a single <abbr title="JavaScript Object Notation">JSON</abbr> value and requires the whole input to be consumed: any non-whitespace data after the value is reported as a parse error. Use it when you want to guarantee that an input is exactly one complete <abbr title="JavaScript Object Notation">JSON</abbr> document.</p> <p><a href=../../api/operator_gtgt/ ><code>operator&gt;&gt;</code></a> follows relaxed <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>istream</span></code> semantics instead: it parses one <abbr title="JavaScript Object Notation">JSON</abbr> value and leaves the stream positioned right after it, without requiring the rest of the stream to be consumed. This is what makes it possible to read several concatenated values from the same stream, but it also means that "a valid document followed by trailing bytes" is accepted rather than rejected. If you are validating conformance, or need to reject any input that is not exactly one <abbr title="JavaScript Object Notation">JSON</abbr> document, prefer <code>parse</code>.</p> <p>When using <code>operator&gt;&gt;</code> to read several concatenated values this way, a value that is a number must be followed by whitespace, because <code>operator&gt;&gt;</code> consumes the character that terminates a number, unless <a href=../../api/macros/json_precise_stream_position/ ><code>JSON_PRECISE_STREAM_POSITION</code></a> is defined to <code>1</code> — see the <a href=../../api/operator_gtgt/#notes><code>operator&gt;&gt;</code> notes</a> for details and examples.</p> <h2 id=input-sources-and-performance>Input sources and performance<a class=headerlink href=#input-sources-and-performance title="Permanent link">&para;</a></h2> <p>All inputs produce the same values and the same exceptions, but they are read in different ways, which makes some of them faster than others:</p> <table> <thead> <tr> <th>input</th> <th>how it is read</th> </tr> </thead> <tbody> <tr> <td>strings, string views, string literals, contiguous containers, pointer or iterator ranges over contiguous memory</td> <td>the whole input is scanned in bulk: runs of string characters, numbers, and indentation are recognized several bytes at a time</td> </tr> <tr> <td><code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>istream</span></code> (<code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>ifstream</span></code>, <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>istringstream</span></code>, ...)</td> <td>the characters the stream buffer holds are scanned in bulk like a string; the stream buffer is asked for more when they are used up</td> </tr> <tr> <td><code class=highlight><span class=kt>FILE</span><span class=o>*</span></code></td> <td>one character at a time, with the file locked once for the whole parse</td> </tr> <tr> <td>other iterator ranges (e.g., <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>list</span><span class=o>&lt;</span><span class=kt>char</span><span class=o>&gt;</span></code>, <code class=highlight><span class=n>std</span><span Line truncated
<span class=n>json_document</span><span class=w> </span><span class=n>doc</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>json_document</span><span class=o>::</span><span class=n>parse</span><span class=p>(</span><span class=sa>R</span><span class=s>"</span><span class=dl>(</span><span class=s>{"happy": true, "pi": 3.141}</span><span class=dl>)</span><span class=s>"</span><span class=p>);</span>
<span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>doc</span><span class=p>.</span><span class=n>root</span><span class=p>()[</span><span class=s>"happy"</span><span class=p>].</span><span class=n>get</span><span class=o>&lt;</span><span class=kt>bool</span><span class=o>&gt;</span><span class=p>()</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=sc>'\n'</span><span class=p>;</span>
</code></pre></div> <p>See <a href=../json_view/ >Zero-copy <abbr title="JavaScript Object Notation">JSON</abbr> views</a> for the concept and the trade-offs.</p> <h2 id=exceptions>Exceptions<a class=headerlink href=#exceptions title="Permanent link">&para;</a></h2> <p>When the input is not valid <abbr title="JavaScript Object Notation">JSON</abbr>, the <code>parse</code> function throws an exception by default. If exceptions are undesired or unavailable, the parser can instead return a discarded value, or <a href=../../api/basic_json/accept/ ><code>accept</code></a> can be used to only check whether an input is valid <abbr title="JavaScript Object Notation">JSON</abbr>. See <a href=parse_exceptions/ >parsing and exceptions</a> for the available options.</p> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../../api/basic_json/parse/ ><code>parse</code></a> - deserialize from a compatible input</li> <li><a href=../../api/basic_json_document/parse/ ><code>basic_json_document::parse</code></a> - deserialize into a document instead of a value</li> <li><a href=../../api/basic_json/accept/ ><code>accept</code></a> - check if the input is valid <abbr title="JavaScript Object Notation">JSON</abbr></li> <li><a href=../../api/basic_json/sax_parse/ ><code>sax_parse</code></a> - generate <abbr title="Simple API for XML">SAX</abbr> events</li> <li><a href=json_lines/ ><abbr title="JavaScript Object Notation">JSON</abbr> Lines</a> - parse newline-delimited <abbr title="JavaScript Object Notation">JSON</abbr></li> <li><a href=large_and_partial_input/ >Large, streamed, and partial input</a> - chunked, concatenated, and very large input</li> <li><a href=parser_callbacks/ >parser callbacks</a> - influence the parsing by a callback function</li> <li><a href=sax_interface/ ><abbr title="Simple API for XML">SAX</abbr> interface</a> - implement a custom <abbr title="Simple API for XML">SAX</abbr> handler</li> <li><a href=../json_view/ >Zero-copy <abbr title="JavaScript Object Notation">JSON</abbr> views</a> - parse into a read-only document and read it through views</li> <li><a href=parse_exceptions/ >parsing and exceptions</a> - control error handling</li> <li><a href=untrusted_input/ >parsing untrusted input</a> - what to consider when parsing input from untrusted sources</li> <li><a href=../../api/operator_gtgt/ ><code>operator&gt;&gt;</code></a> - deserialize from stream</li> <li><a href=../../api/json_sax/ ><code>json_sax</code></a> - documentation of the <abbr title="Simple API for XML">SAX</abbr> interface</li> <li><a href=../../api/basic_json/parse_error/ ><code>parse_error</code></a> - exception thrown on a parse error</li> <li><a href=../comments/ >Comments</a> - ignore <code>//</code> and <code>/* */</code> comments while parsing</li> <li><a href=../trailing_commas/ >Trailing Commas</a> - accept trailing commas in arrays and objects</li> <li><a href=../serialization/ >Serialization</a> - pretty-printing, <abbr title="Unicode Transformation Format">UTF</abbr>-8 handling, and number output</li> <li><a href=../no_exceptions/ >Working Without Exceptions</a> - error handling when exceptions are disabled</li> <li><a href=../../home/exceptions/ >Exceptions</a> - list of exceptions and error codes</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../performance/ class="md-footer__link md-footer__link--preLine truncated
</code></pre></div> <p>See <a href=../json_view/ >Zero-copy <abbr title="JavaScript Object Notation">JSON</abbr> views</a> for the concept and the trade-offs.</p> <h2 id=exceptions>Exceptions<a class=headerlink href=#exceptions title="Permanent link">&para;</a></h2> <p>When the input is not valid <abbr title="JavaScript Object Notation">JSON</abbr>, the <code>parse</code> function throws an exception by default. If exceptions are undesired or unavailable, the parser can instead return a discarded value, or <a href=../../api/basic_json/accept/ ><code>accept</code></a> can be used to only check whether an input is valid <abbr title="JavaScript Object Notation">JSON</abbr>. See <a href=parse_exceptions/ >parsing and exceptions</a> for the available options.</p> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../../api/basic_json/parse/ ><code>parse</code></a> - deserialize from a compatible input</li> <li><a href=../../api/basic_json_document/parse/ ><code>basic_json_document::parse</code></a> - deserialize into a document instead of a value</li> <li><a href=../../api/basic_json/accept/ ><code>accept</code></a> - check if the input is valid <abbr title="JavaScript Object Notation">JSON</abbr></li> <li><a href=../../api/basic_json/sax_parse/ ><code>sax_parse</code></a> - generate <abbr title="Simple API for XML">SAX</abbr> events</li> <li><a href=json_lines/ ><abbr title="JavaScript Object Notation">JSON</abbr> Lines</a> - parse newline-delimited <abbr title="JavaScript Object Notation">JSON</abbr></li> <li><a href=large_and_partial_input/ >Large, streamed, and partial input</a> - chunked, concatenated, and very large input</li> <li><a href=parser_callbacks/ >parser callbacks</a> - influence the parsing by a callback function</li> <li><a href=sax_interface/ ><abbr title="Simple API for XML">SAX</abbr> interface</a> - implement a custom <abbr title="Simple API for XML">SAX</abbr> handler</li> <li><a href=../json_view/ >Zero-copy <abbr title="JavaScript Object Notation">JSON</abbr> views</a> - parse into a read-only document and read it through views</li> <li><a href=parse_exceptions/ >parsing and exceptions</a> - control error handling</li> <li><a href=untrusted_input/ >parsing untrusted input</a> - what to consider when parsing input from untrusted sources</li> <li><a href=../../api/operator_gtgt/ ><code>operator&gt;&gt;</code></a> - deserialize from stream</li> <li><a href=../../api/json_sax/ ><code>json_sax</code></a> - documentation of the <abbr title="Simple API for XML">SAX</abbr> interface</li> <li><a href=../../api/basic_json/parse_error/ ><code>parse_error</code></a> - exception thrown on a parse error</li> <li><a href=../comments/ >Comments</a> - ignore <code>//</code> and <code>/* */</code> comments while parsing</li> <li><a href=../trailing_commas/ >Trailing Commas</a> - accept trailing commas in arrays and objects</li> <li><a href=../serialization/ >Serialization</a> - pretty-printing, <abbr title="Unicode Transformation Format">UTF</abbr>-8 handling, and number output</li> <li><a href=../no_exceptions/ >Working Without Exceptions</a> - error handling when exceptions are disabled</li> <li><a href=../../home/exceptions/ >Exceptions</a> - list of exceptions and error codes</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="November 4, 2026 16:46:20 UTC">November 4, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../performance/ class="md-footer__link md-footer__link--preLine truncated
+4
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@@ -34,6 +34,8 @@ std::ifstream f("example.json");
json data = json::parse(f);
```
See [Reading and writing files](https://json.nlohmann.me/features/files/index.md) for checking that the file was opened and for writing files safely.
The input must be encoded in UTF-8; other encodings are not supported. The exception is wide-character input, which is read as UTF-16 or UTF-32 — depending on the character type — and converted to UTF-8; see the [`parse`](https://json.nlohmann.me/api/basic_json/parse/index.md) page for an example. A single input may contain only one JSON value. Inputs consisting of multiple values separated by newlines are handled by the [JSON Lines](https://json.nlohmann.me/features/parsing/json_lines/index.md) format; input that arrives in chunks or contains several values is covered in [Large, streamed, and partial input](https://json.nlohmann.me/features/parsing/large_and_partial_input/index.md).
By default, the library rejects comments and trailing commas. Both can be enabled with parameters of the `parse` function — see [comments](https://json.nlohmann.me/features/comments/index.md) and [trailing commas](https://json.nlohmann.me/features/trailing_commas/index.md).
@@ -61,6 +63,8 @@ All inputs produce the same values and the same exceptions, but they are read in
Parsing a file from a `std::ifstream` is therefore only about 10 to 20 % slower than reading the whole file into a `std::string` first and parsing that, and it needs no memory for a copy of the file. The stream is never read ahead: after parsing, it is positioned exactly as if every character had been read one at a time, so [`operator>>`](https://json.nlohmann.me/api/operator_gtgt/index.md) can still read several values from one stream (see also [JSON Lines](https://json.nlohmann.me/features/parsing/json_lines/index.md)).
This does not hold for [`json_document`](https://json.nlohmann.me/features/json_view/index.md): it reads a stream into a buffer it owns, so the stream saves no memory, and reading the file into a `std::string` yourself and borrowing it is about 6 to 12 % faster.
Two macros switch these optimizations off without changing any result: [`JSON_DISABLE_STREAM_BULK_SCAN`](https://json.nlohmann.me/api/macros/json_disable_stream_bulk_scan/index.md) for stream buffers that do not follow the usual `std::streambuf` contract, and [`JSON_DISABLE_UNLOCKED_FILE_IO`](https://json.nlohmann.me/api/macros/json_disable_unlocked_file_io/index.md) for programs in which another thread must use the same `FILE` object while it is parsed.
Where the target has them, strings are scanned with vector instructions: SSE2 on x86-64 (and SSSE3 for UTF-8 text, when the CPU has it) and NEON on AArch64. [`JSON_NO_SIMD`](https://json.nlohmann.me/api/macros/json_no_simd/index.md) selects portable code instead, again without changing any result.
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@@ -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=description content="JSON for Modern C++ is a header-only C++11 library that makes JSON a first-class data type."><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/parsing/json_lines/" rel="canonical"><link href=../ rel=prev><link href=../large_and_partial_input/ rel=next><link rel=icon href=../../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>JSON Lines - JSON for Modern C++</title><link rel=stylesheet href=../../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../../assets/stylesheets/palette.ab4e12ef.min.css><link rel=stylesheet href=../../../css/custom.css><link rel=stylesheet href=../../../css/brand.css><link rel=stylesheet href=../../../css/fig-sax.css><link rel=stylesheet href=../../../css/fig-jsonview.css><link 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</div> <div data-md-component=announce> </div> <header class=md-header data-md-component=header> <nav class="md-header__inner md-grid" aria-label=Header> <a href=../../.. title="JSON for Modern C++" class="md-header__button md-logo" aria-label="JSON for Modern C++" data-md-component=logo> <img src=../../../images/wordmark-header.svg alt=logo> </a> <label class="md-header__button md-icon" for=__drawer> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M3 6h18v2H3zm0 5h18v2H3zm0 5h18v2H3z"/></svg> </label> <div class=md-header__title data-md-component=header-title> <div class=md-header__ellipsis> <div class=md-header__topic> <span class=md-ellipsis> JSON for Modern C++ </span> </div> <div class=md-header__topic data-md-component=header-topic> <span class=md-ellipsis> JSON Lines </span> </div> </div> </div> <form class=md-header__option data-md-component=palette> <input class=md-option data-md-color-media="(prefers-color-scheme: light)" data-md-color-scheme=default 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<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=description content="JSON for Modern C++ is a header-only C++11 library that makes JSON a first-class data type."><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/parsing/json_lines/" rel="canonical"><link href=../ rel=prev><link href=../large_and_partial_input/ rel=next><link rel=icon href=../../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.8.0"><title>JSON Lines - JSON for Modern C++</title><link rel=stylesheet href=../../../assets/stylesheets/main.57a0d79d.min.css><link rel=stylesheet href=../../../assets/stylesheets/palette.fb2fffe2.min.css><link rel=stylesheet href=../../../css/custom.css><link rel=stylesheet href=../../../css/brand.css><link rel=stylesheet href=../../../css/fig-sax.css><link rel=stylesheet href=../../../css/fig-jsonview.css><link 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<span class=p>{</span><span class=nt>"name"</span><span class=p>:</span><span class=w> </span><span class=s2>"Alexa"</span><span class=p>,</span><span class=w> </span><span class=nt>"wins"</span><span class=p>:</span><span class=w> </span><span class=p>[[</span><span class=s2>"two pair"</span><span class=p>,</span><span class=w> </span><span class=s2>"4♠"</span><span class=p>],</span><span class=w> </span><span class=p>[</span><span class=s2>"two pair"</span><span class=p>,</span><span class=w> </span><span class=s2>"9♠"</span><span class=p>]]}</span>
<span class=p>{</span><span class=nt>"name"</span><span class=p>:</span><span class=w> </span><span class=s2>"May"</span><span class=p>,</span><span class=w> </span><span class=nt>"wins"</span><span class=p>:</span><span class=w> </span><span class=p>[]}</span>
<span class=p>{</span><span class=nt>"name"</span><span class=p>:</span><span class=w> </span><span class=s2>"Deloise"</span><span class=p>,</span><span class=w> </span><span class=nt>"wins"</span><span class=p>:</span><span class=w> </span><span class=p>[[</span><span class=s2>"three of a kind"</span><span class=p>,</span><span class=w> </span><span class=s2>"5♣"</span><span class=p>]]}</span>
@@ -39,4 +39,4 @@
<span class=w> </span><span class=n>input</span><span class=w> </span><span class=o>&gt;&gt;</span><span class=w> </span><span class=n>j</span><span class=p>;</span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>j</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>endl</span><span class=p>;</span>
<span class=p>}</span>
</code></pre></div> <p>A value that is a number must be followed by whitespace — see the <a href=../../../api/operator_gtgt/#notes>notes</a> of <code>operator&gt;&gt;</code> for details.</p> </div> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../ >Parsing</a> - overview of the parsing functions</li> <li><a href=../large_and_partial_input/ >Large, Streamed, and Partial Input</a> - chunked, concatenated, and very large input</li> <li><a href=../../../api/basic_json/parse/ ><code>parse</code></a> - deserialize from a compatible input</li> <li><a href=../../../api/operator_gtgt/ ><code>operator&gt;&gt;</code></a> - deserialize from stream</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../ class="md-footer__link md-footer__link--prev" aria-label="Previous: Parsing"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> Parsing </div> </div> </a> <a href=../large_and_partial_input/ class="md-footer__link md-footer__link--next" aria-label="Next: Large, Streamed, and Partial Input"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> Large, Streamed, and Partial Input </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 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</code></pre></div> <p>A value that is a number must be followed by whitespace — see the <a href=../../../api/operator_gtgt/#notes>notes</a> of <code>operator&gt;&gt;</code> for details.</p> </div> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../ >Parsing</a> - overview of the parsing functions</li> <li><a href=../large_and_partial_input/ >Large, Streamed, and Partial Input</a> - chunked, concatenated, and very large input</li> <li><a href=../../../api/basic_json/parse/ ><code>parse</code></a> - deserialize from a compatible input</li> <li><a href=../../../api/operator_gtgt/ ><code>operator&gt;&gt;</code></a> - deserialize from stream</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../ class="md-footer__link md-footer__link--prev" aria-label="Previous: Parsing"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> Parsing </div> </div> </a> <a href=../large_and_partial_input/ class="md-footer__link md-footer__link--next" aria-label="Next: Large, Streamed, and Partial Input"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> Large, Streamed, and Partial Input </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 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@@ -1,4 +1,4 @@
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class="md-header__button md-icon" for=__searchLine truncated
A["my input is..."] --&gt; B{"several JSON values in one stream?"}
B --&gt;|"yes"| C["read them one at a time"]
A --&gt; D{"arriving in chunks (socket, pipe)?"}
@@ -222,4 +222,4 @@ result: []
<span class=p>}</span>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code>parse completed: false
id: 42
</code></pre></div> </details> <p>Note that <code>sax_parse</code> returns <code class=highlight><span class=nb>false</span></code> both when a handler stopped the parse and when the input is invalid, so record the reason in the handler (here: <code>id</code>) if you need to tell the two apart.</p> <h3 id=read-parts-of-a-large-document-with-a-view>Read parts of a large document with a view<a class=headerlink href=#read-parts-of-a-large-document-with-a-view title="Permanent link">&para;</a></h3> <p>If the whole text is available and you only need to read some of it, a <a href=../../json_view/ ><code>json_document</code></a> from <code>&lt;nlohmann/json_view.hpp&gt;</code> (added in version 3.13.0 <span class=unreleased-version title="Not part of a release yet; the latest release is 3.12.0.">unreleased</span>) avoids the <code>basic_json</code> tree altogether: it keeps the text, builds a flat index of the values, and gives read-only <a href=../../../api/basic_json_view/ >views</a> of the parts you look at. Only what you <a href=../../../api/basic_json_view/materialize/ ><code>materialize()</code></a> becomes a <code>basic_json</code> value. See <a href=../../json_view/ >Zero-copy <abbr title="JavaScript Object Notation">JSON</abbr> views</a> for details and for the lifetime rules.</p> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../ >Parsing</a> - overview of the parsing functions</li> <li><a href=../json_lines/ ><abbr title="JavaScript Object Notation">JSON</abbr> Lines</a> - newline-delimited <abbr title="JavaScript Object Notation">JSON</abbr></li> <li><a href=../parser_callbacks/ >Parser callbacks</a> - discard values while parsing</li> <li><a href=../sax_interface/ ><abbr title="Simple API for XML">SAX</abbr> interface</a> - event-based parsing</li> <li><a href=../../json_view/ >Zero-copy <abbr title="JavaScript Object Notation">JSON</abbr> views</a> - read parts of a large document without building a tree</li> <li><a href=../untrusted_input/ >Untrusted input</a> - limits for input you do not control</li> <li><a href=../../../api/operator_gtgt/ ><code>operator&gt;&gt;</code></a> - deserialize from stream</li> <li><a href=../../../api/basic_json/parse/ ><code>parse</code></a> - deserialize from a compatible input</li> <li><a href=../../../api/basic_json/sax_parse/ ><code>sax_parse</code></a> - generate <abbr title="Simple API for XML">SAX</abbr> events</li> <li><a href=../../../api/macros/json_precise_stream_position/ ><code>JSON_PRECISE_STREAM_POSITION</code></a> - leave the stream positioned right after a parsed number</li> <li><a href=../../../api/macros/json_disable_stream_bulk_scan/ ><code>JSON_DISABLE_STREAM_BULK_SCAN</code></a> - read a <code>std::istream</code> one character at a time</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../json_lines/ class="md-footer__link md-footer__link--prev" aria-label="Previous: JSON Lines"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> JSON Lines </div> </div> </a> <a href=../parse_exceptions/ class="md-footer__link md-footer__link--next" aria-label="Next: Parsing and Exceptions"> <div class=md-foLine truncated
</code></pre></div> </details> <p>Note that <code>sax_parse</code> returns <code class=highlight><span class=nb>false</span></code> both when a handler stopped the parse and when the input is invalid, so record the reason in the handler (here: <code>id</code>) if you need to tell the two apart.</p> <h3 id=read-parts-of-a-large-document-with-a-view>Read parts of a large document with a view<a class=headerlink href=#read-parts-of-a-large-document-with-a-view title="Permanent link">&para;</a></h3> <p>If the whole text is available and you only need to read some of it, a <a href=../../json_view/ ><code>json_document</code></a> from <code>&lt;nlohmann/json_view.hpp&gt;</code> (added in version 3.13.0 <span class=unreleased-version title="Not part of a release yet; the latest release is 3.12.0.">unreleased</span>) avoids the <code>basic_json</code> tree altogether: it keeps the text, builds a flat index of the values, and gives read-only <a href=../../../api/basic_json_view/ >views</a> of the parts you look at. Only what you <a href=../../../api/basic_json_view/materialize/ ><code>materialize()</code></a> becomes a <code>basic_json</code> value. See <a href=../../json_view/ >Zero-copy <abbr title="JavaScript Object Notation">JSON</abbr> views</a> for details and for the lifetime rules.</p> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../ >Parsing</a> - overview of the parsing functions</li> <li><a href=../json_lines/ ><abbr title="JavaScript Object Notation">JSON</abbr> Lines</a> - newline-delimited <abbr title="JavaScript Object Notation">JSON</abbr></li> <li><a href=../parser_callbacks/ >Parser callbacks</a> - discard values while parsing</li> <li><a href=../sax_interface/ ><abbr title="Simple API for XML">SAX</abbr> interface</a> - event-based parsing</li> <li><a href=../../json_view/ >Zero-copy <abbr title="JavaScript Object Notation">JSON</abbr> views</a> - read parts of a large document without building a tree</li> <li><a href=../untrusted_input/ >Untrusted input</a> - limits for input you do not control</li> <li><a href=../../../api/operator_gtgt/ ><code>operator&gt;&gt;</code></a> - deserialize from stream</li> <li><a href=../../../api/basic_json/parse/ ><code>parse</code></a> - deserialize from a compatible input</li> <li><a href=../../../api/basic_json/sax_parse/ ><code>sax_parse</code></a> - generate <abbr title="Simple API for XML">SAX</abbr> events</li> <li><a href=../../../api/macros/json_precise_stream_position/ ><code>JSON_PRECISE_STREAM_POSITION</code></a> - leave the stream positioned right after a parsed number</li> <li><a href=../../../api/macros/json_disable_stream_bulk_scan/ ><code>JSON_DISABLE_STREAM_BULK_SCAN</code></a> - read a <code>std::istream</code> one character at a time</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../json_lines/ class="md-footer__link md-footer__link--prev" aria-label="Previous: JSON Lines"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> JSON Lines </div> </div> </a> <a href=../parse_exceptions/ class="md-footer__link md-footer__link--next" aria-label="Next: Parsing and Exceptions"> <div class=md-foLine truncated
+2 -2
View File
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md-icon" for=__search> <svg xmlns=http://www.w3.oLine truncated
<span class=k>try</span>
<span class=p>{</span>
<span class=w> </span><span class=n>j</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>parse</span><span class=p>(</span><span class=n>my_input</span><span class=p>);</span>
@@ -157,4 +157,4 @@ byte 20 = line 3, column 8
<span class=p>}</span>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code>[json.exception.type_error.302] (bytes 29-40) type must be number, but is string
the value starts at line 3, column 10
</code></pre></div> </details> <p>Positions are only available for values created by <a href=../../../api/basic_json/parse/ ><code>parse</code></a>, they are not updated when a value is changed, and the macro makes every value 16 bytes larger on 64-bit platforms, so use it where the cost is acceptable, for example for configuration files. Note that <code>start_pos()</code> is a 0-based index into the input, whereas <code>byte</code> of a <code>parse_error</code> starts at 1.</p> <p>For binary formats such as <abbr title="Concise Binary Object Representation">CBOR</abbr>, there are no lines; <code>byte</code> is the position in the byte sequence. To find out how many bytes a successfully parsed value used, see <a href=../large_and_partial_input/#several-values-in-one-stream>Several values in one stream</a>.</p> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../../../api/basic_json/parse/ ><code>parse</code></a> - deserialize from a compatible input</li> <li><a href=../../../api/basic_json/accept/ ><code>accept</code></a> - check if the input is valid <abbr title="JavaScript Object Notation">JSON</abbr></li> <li><a href=../../../api/basic_json/sax_parse/ ><code>sax_parse</code></a> - generate <abbr title="Simple API for XML">SAX</abbr> events</li> <li><a href=../../../api/basic_json/parse_error/ ><code>parse_error</code></a> - exception thrown on a parse error</li> <li><a href=../../../api/macros/json_diagnostic_positions/ ><code>JSON_DIAGNOSTIC_POSITIONS</code></a> - record start and end positions of parsed values</li> <li><a href=../../no_exceptions/ >Working Without Exceptions</a> - error handling when exceptions are disabled</li> <li><a href=../../../home/exceptions/ >Exceptions</a> - list of exceptions and error codes</li> <li><a href=../untrusted_input/ >Parsing Untrusted Input</a> - limits and checks for input you do not control</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../large_and_partial_input/ class="md-footer__link md-footer__link--prev" aria-label="Previous: Large, Streamed, and Partial Input"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> Large, Streamed, and Partial Input </div> </div> </a> <a href=../parser_callbacks/ class="md-footer__link md-footer__link--next" aria-label="Next: Parser Callbacks"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> Parser Callbacks </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/frLine truncated
</code></pre></div> </details> <p>Positions are only available for values created by <a href=../../../api/basic_json/parse/ ><code>parse</code></a>, they are not updated when a value is changed, and the macro makes every value 16 bytes larger on 64-bit platforms, so use it where the cost is acceptable, for example for configuration files. Note that <code>start_pos()</code> is a 0-based index into the input, whereas <code>byte</code> of a <code>parse_error</code> starts at 1.</p> <p>For binary formats such as <abbr title="Concise Binary Object Representation">CBOR</abbr>, there are no lines; <code>byte</code> is the position in the byte sequence. To find out how many bytes a successfully parsed value used, see <a href=../large_and_partial_input/#several-values-in-one-stream>Several values in one stream</a>.</p> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../../../api/basic_json/parse/ ><code>parse</code></a> - deserialize from a compatible input</li> <li><a href=../../../api/basic_json/accept/ ><code>accept</code></a> - check if the input is valid <abbr title="JavaScript Object Notation">JSON</abbr></li> <li><a href=../../../api/basic_json/sax_parse/ ><code>sax_parse</code></a> - generate <abbr title="Simple API for XML">SAX</abbr> events</li> <li><a href=../../../api/basic_json/parse_error/ ><code>parse_error</code></a> - exception thrown on a parse error</li> <li><a href=../../../api/macros/json_diagnostic_positions/ ><code>JSON_DIAGNOSTIC_POSITIONS</code></a> - record start and end positions of parsed values</li> <li><a href=../../no_exceptions/ >Working Without Exceptions</a> - error handling when exceptions are disabled</li> <li><a href=../../../home/exceptions/ >Exceptions</a> - list of exceptions and error codes</li> <li><a href=../untrusted_input/ >Parsing Untrusted Input</a> - limits and checks for input you do not control</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../large_and_partial_input/ class="md-footer__link md-footer__link--prev" aria-label="Previous: Large, Streamed, and Partial Input"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> Large, Streamed, and Partial Input </div> </div> </a> <a href=../parser_callbacks/ class="md-footer__link md-footer__link--next" aria-label="Next: Parser Callbacks"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> Parser Callbacks </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/frLine truncated
+2 -2
View File
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<span class=k>using</span><span class=w> </span><span class=n>parser_callback_t</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>function</span><span class=o>&lt;</span><span class=kt>bool</span><span class=p>(</span><span class=kt>int</span><span class=w> </span><span class=n>depth</span><span class=p>,</span><span class=w> </span><span class=n>parse_event_t</span><span class=w> </span><span class=n>event</span><span class=p>,</span><span class=w> </span><span class=n>BasicJsonType</span><span class=o>&amp;</span><span class=w> </span><span class=n>parsed</span><span class=p>)</span><span class=o>&gt;</span><span class=p>;</span>
</code></pre></div> <h2 id=callback-event-types>Callback event types<a class=headerlink href=#callback-event-types title="Permanent link">&para;</a></h2> <p>We distinguish six scenarios (determined by the event type) in which the callback function can be called. The following table describes the values of the parameters <code>depth</code>, <code>event</code>, and <code>parsed</code>.</p> <table> <thead> <tr> <th>parameter <code>event</code></th> <th>description</th> <th>parameter <code>depth</code></th> <th>parameter <code>parsed</code></th> </tr> </thead> <tbody> <tr> <td><code>parse_event_t::object_start</code></td> <td>the parser read <code>{</code> and started to process a <abbr title="JavaScript Object Notation">JSON</abbr> object</td> <td>depth of the parent of the <abbr title="JavaScript Object Notation">JSON</abbr> object</td> <td>a <abbr title="JavaScript Object Notation">JSON</abbr> value with type discarded</td> </tr> <tr> <td><code>parse_event_t::key</code></td> <td>the parser read a key of a value in an object</td> <td>depth of the currently parsed <abbr title="JavaScript Object Notation">JSON</abbr> object</td> <td>a <abbr title="JavaScript Object Notation">JSON</abbr> string containing the key</td> </tr> <tr> <td><code>parse_event_t::object_end</code></td> <td>the parser read <code>}</code> and finished processing a <abbr title="JavaScript Object Notation">JSON</abbr> object</td> <td>depth of the parent of the <abbr title="JavaScript Object Notation">JSON</abbr> object</td> <td>the parsed <abbr title="JavaScript Object Notation">JSON</abbr> object</td> </tr> <tr> <td><code>parse_event_t::array_start</code></td> <td>the parser read <code>[</code> and started to process a <abbr title="JavaScript Object Notation">JSON</abbr> array</td> <td>depth of the parent of the <abbr title="JavaScript Object Notation">JSON</abbr> array</td> <td>a <abbr title="JavaScript Object Notation">JSON</abbr> value with type discarded</td> </tr> <tr> <td><code>parse_event_t::array_end</code></td> <td>the parser read <code>]</code> and finished processing a <abbr title="JavaScript Object Notation">JSON</abbr> array</td> <td>depth of the parent of the <abbr title="JavaScript Object Notation">JSON</abbr> array</td> <td>the parsed <abbr title="JavaScript Object Notation">JSON</abbr> array</td> </tr> <tr> <td><code>parse_event_t::value</code></td> <td>the parser finished reading a <abbr title="JavaScript Object Notation">JSON</abbr> value</td> <td>depth of the value</td> <td>the parsed <abbr title="JavaScript Object Notation">JSON</abbr> value</td> </tr> </tbody> </table> <div class=nj-fig-wrap> <p><svg class="nj-fig nj-fig--cb" viewbox="0 0 480 262" role=img aria-labelledby=fig-cb-title xmlns=http://www.w3.org/2000/svg> <title id=fig-cb-title>Example of the parse events a callback sees for the J&#8203;SON text {&quot;name&quot;: &quot;Berlin&quot;, &quot;location&quot;: [52.519444, 13.406667]}: object_start after the opening brace, key after each key, array_start after the opening bracket, value after each value, array_end after the closing bracket, and object_end after the closing brace. Every array and object also produces a value event when it is complete.</title> <text class=fg-h x=16 y=28>J&#8203;SON to parse</text> <text class=fg-h x=320 y=28>callback events</text> <text class=fg-code x=16 y=74 xml:space=preserve>{</text> <circle class=cb-os cx=35.6 cy=69 r=5.5 /> <text class=fg-code x=35.2 y=102 xml:space=preserve>&quot;name&quot;</text> <circle class=cb-k cx=102.8 cy=97 r=5.5 /> <text class=fg-code x=112.8 y=102 xml:space=preserve>: &quot;Berlin&quot;</text> <circle class=cb-v cx=218.8 cy=97 r=5.5 /> <text class=fg-code x=228.8 y=102 xml:space=preserve>,</text> <text class=fg-code x=35.2 y=130 xml:space=preserve>&quot;location&quot;</text> <circle class=cb-k cx=141.2 cy=125 r=5.5 /> <text class=fg-code x=151.2 y=130 xml:space=preserve>: [</text> <circle class=cb-as cx=190 cy=125 r=5.5 /> <text class=fg-code x=54.4 y=158 xml:space=preserve>52.519444</text> <circle class=cb-v cx=150.8 cy=153 r=5.5 /> <text class=fg-code x=160.8 y=158 xml:space=preserve>,</text> <text class=fg-code x=54.4 y=186 xml:space=preserve>13.406667</text> <circle class=cb-v cx=150.8 cy=181 r=5.5 /> <text class=fg-code x=35.2 y=214 xml:space=preserve>]</text> <circle class=cb-ae cx=54.8 cy=209 r=5.5 /> <circle class=cb-v cx=74.8 cy=209 r=5.5 /> <text class=fg-code x=16 y=242 xml:space=preserve>}</text> <circle class=cb-oe cx=35.6 cy=237 r=5.5 /> <circle class=cb-v cx=55.6 cy=237 r=5.5 /> <circle class=cb-os cx=327 cy=69 r=6 /> <text class="fg-code fg-leg" x=344 y=74>object_start</text> <circle class=cb-k cx=327 cy=99 r=6 /> <text class="fg-code fg-leg" x=344 y=104>key</text> <circle class=cb-oe cx=327 cy=129 r=6 /> <text class="fg-code fg-leg" x=344 y=134>object_end</text> <circle class=cb-as cx=327 cy=159 r=6 /> <text class="fg-code fg-leg" x=344 y=164>array_start</text> <circle class=cb-ae cx=327 cy=189 r=6 /> <text class="fg-code fg-leg" xLine truncated
@@ -174,4 +174,4 @@
<span class=p>};</span>
<span class=n>json</span><span class=o>::</span><span class=n>parse</span><span class=p>(</span><span class=n>input</span><span class=p>,</span><span class=w> </span><span class=n>callback</span><span class=p>);</span>
</code></pre></div> </details> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../ >Parsing</a> - overview of the parsing functions</li> <li><a href=../sax_interface/ ><abbr title="Simple API for XML">SAX</abbr> Interface</a> - guide to event-based parsing</li> <li><a href=../../../api/basic_json/parse/ ><code>parse</code></a> - deserialize from a compatible input</li> <li><a href=../../../api/basic_json/parser_callback_t/ ><code>parser_callback_t</code></a> - callback function type for the parser</li> <li><a href=../../../api/basic_json/parse_event_t/ ><code>parse_event_t</code></a> - enumeration of parser events</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../parse_exceptions/ class="md-footer__link md-footer__link--prev" aria-label="Previous: Parsing and Exceptions"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> Parsing and Exceptions </div> </div> </a> <a href=../sax_interface/ class="md-footer__link md-footer__link--next" aria-label="Next: SAX Interface"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> SAX Interface </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 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</code></pre></div> </details> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../ >Parsing</a> - overview of the parsing functions</li> <li><a href=../sax_interface/ ><abbr title="Simple API for XML">SAX</abbr> Interface</a> - guide to event-based parsing</li> <li><a href=../../../api/basic_json/parse/ ><code>parse</code></a> - deserialize from a compatible input</li> <li><a href=../../../api/basic_json/parser_callback_t/ ><code>parser_callback_t</code></a> - callback function type for the parser</li> <li><a href=../../../api/basic_json/parse_event_t/ ><code>parse_event_t</code></a> - enumeration of parser events</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../parse_exceptions/ class="md-footer__link md-footer__link--prev" aria-label="Previous: Parsing and Exceptions"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> Parsing and Exceptions </div> </div> </a> <a href=../sax_interface/ class="md-footer__link md-footer__link--next" aria-label="Next: SAX Interface"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> SAX Interface </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 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@@ -1,4 +1,4 @@
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class sax_t ["json::sax_t"] {
&lt;&lt;interface&gt;&gt;
@@ -55,4 +55,4 @@ class sax_t ["json::sax_t"] {
true
],
"b": "x"
}</code></pre><p class=nj-sax__note>After <code>end_object()</code> the tree is complete and no container is open. The number of elements is passed as -1: the parser does not know it in advance.</p></div></div> </div> <p class=nj-caption>The three lanes show the input consumed so far, the events called so far, and the state of the selected handler (<em>build a tree</em> or <em>keep only /b</em>). Check <em>return false at this event</em> to stop the parse at the current event and see which events are no longer called.</p> <p>To implement your own <abbr title="Simple API for XML">SAX</abbr> handler, proceed as follows:</p> <ol> <li>Implement the <abbr title="Simple API for XML">SAX</abbr> interface in a class. You can use class <code>nlohmann::json_sax&lt;json&gt;</code> as base class, but you can also use any class where the functions described above are implemented and public.</li> <li>Create an object of your <abbr title="Simple API for XML">SAX</abbr> interface class, e.g. <code>my_sax</code>.</li> <li>Call <code class=highlight><span class=kt>bool</span><span class=w> </span><span class=nf>json::sax_parse</span><span class=p>(</span><span class=n>input</span><span class=p>,</span><span class=w> </span><span class=o>&amp;</span><span class=n>my_sax</span><span class=p>);</span></code> where the first parameter can be any input like a string or an input stream and the second parameter is a pointer to your <abbr title="Simple API for XML">SAX</abbr> interface.</li> </ol> <p>Note the <code>sax_parse</code> function only returns a <code class=highlight><span class=kt>bool</span></code> indicating the result of the last executed <abbr title="Simple API for XML">SAX</abbr> event. It does not return <code>json</code> value - it is up to you to decide what to do with the <abbr title="Simple API for XML">SAX</abbr> events. Furthermore, no exceptions are thrown in case of a parse error - it is up to you what to do with the exception object passed to your <code>parse_error</code> implementation. Internally, the <abbr title="Simple API for XML">SAX</abbr> interface is used for the <abbr title="Document Object Model">DOM</abbr> parser (class <code>json_sax_dom_parser</code>) as well as the acceptor (<code>json_sax_acceptor</code>), see file <code>json_sax.hpp</code>.</p> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../../../api/json_sax/ ><code>json_sax</code></a> - documentation of the <abbr title="Simple API for XML">SAX</abbr> interface</li> <li><a href=../../../api/basic_json/sax_parse/ ><code>sax_parse</code></a> - <abbr title="Simple API for XML">SAX</abbr> parser</li> <li><a href=../ >Parsing</a> - overview of the parsing functions</li> <li><a href=../parse_exceptions/ >Parsing and Exceptions</a> - handling parse errors and locating them in the input</li> <li><a href=../../../api/basic_json/parse/ ><code>parse</code></a> - deserialize from a compatible input</li> <li><a href=../../binary_formats/ >Binary Formats</a> - <abbr title="Concise Binary Object Representation">CBOR</abbr>, MessagePack, <abbr title="Binary JSON">BSON</abbr>, and other formats</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 29, 2026 06:37:33 UTC">October 29, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../parser_callbacks/ class="md-footer__link md-footer__link--prev" aria-label="Previous: Parser Callbacks"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbLine truncated
}</code></pre><p class=nj-sax__note>After <code>end_object()</code> the tree is complete and no container is open. The number of elements is passed as -1: the parser does not know it in advance.</p></div></div> </div> <p class=nj-caption>The three lanes show the input consumed so far, the events called so far, and the state of the selected handler (<em>build a tree</em> or <em>keep only /b</em>). Check <em>return false at this event</em> to stop the parse at the current event and see which events are no longer called.</p> <p>To implement your own <abbr title="Simple API for XML">SAX</abbr> handler, proceed as follows:</p> <ol> <li>Implement the <abbr title="Simple API for XML">SAX</abbr> interface in a class. You can use class <code>nlohmann::json_sax&lt;json&gt;</code> as base class, but you can also use any class where the functions described above are implemented and public.</li> <li>Create an object of your <abbr title="Simple API for XML">SAX</abbr> interface class, e.g. <code>my_sax</code>.</li> <li>Call <code class=highlight><span class=kt>bool</span><span class=w> </span><span class=nf>json::sax_parse</span><span class=p>(</span><span class=n>input</span><span class=p>,</span><span class=w> </span><span class=o>&amp;</span><span class=n>my_sax</span><span class=p>);</span></code> where the first parameter can be any input like a string or an input stream and the second parameter is a pointer to your <abbr title="Simple API for XML">SAX</abbr> interface.</li> </ol> <p>Note the <code>sax_parse</code> function only returns a <code class=highlight><span class=kt>bool</span></code> indicating the result of the last executed <abbr title="Simple API for XML">SAX</abbr> event. It does not return <code>json</code> value - it is up to you to decide what to do with the <abbr title="Simple API for XML">SAX</abbr> events. Furthermore, no exceptions are thrown in case of a parse error - it is up to you what to do with the exception object passed to your <code>parse_error</code> implementation. Internally, the <abbr title="Simple API for XML">SAX</abbr> interface is used for the <abbr title="Document Object Model">DOM</abbr> parser (class <code>json_sax_dom_parser</code>) as well as the acceptor (<code>json_sax_acceptor</code>), see file <code>json_sax.hpp</code>.</p> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../../../api/json_sax/ ><code>json_sax</code></a> - documentation of the <abbr title="Simple API for XML">SAX</abbr> interface</li> <li><a href=../../../api/basic_json/sax_parse/ ><code>sax_parse</code></a> - <abbr title="Simple API for XML">SAX</abbr> parser</li> <li><a href=../ >Parsing</a> - overview of the parsing functions</li> <li><a href=../parse_exceptions/ >Parsing and Exceptions</a> - handling parse errors and locating them in the input</li> <li><a href=../../../api/basic_json/parse/ ><code>parse</code></a> - deserialize from a compatible input</li> <li><a href=../../binary_formats/ >Binary Formats</a> - <abbr title="Concise Binary Object Representation">CBOR</abbr>, MessagePack, <abbr title="Binary JSON">BSON</abbr>, and other formats</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 29, 2026 06:37:33 UTC">October 29, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../parser_callbacks/ class="md-footer__link md-footer__link--prev" aria-label="Previous: Parser Callbacks"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbLine truncated
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@@ -39,7 +39,9 @@ iterator pair over non-contiguous but random-access storage (e.g. `#!cpp std::de
optimization but not the second, since the byte-scanning fast path requires contiguous storage.
Practically: a file can be passed directly to `parse`, which also avoids the memory for a copy of the file. Reading it
into a string first is no longer necessary for speed:
into a string first is no longer necessary for speed (this is for `#!cpp json::parse`; a
[`json_document`](json_view.md) reads a stream into an owned buffer anyway, so for it reading the file into a string
and borrowing that is the faster way):
```cpp
// fast: the stream buffer's get area is scanned in bulk
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<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=description content="JSON for Modern C++ is a header-only C++11 library that makes JSON a first-class data type."><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/performance/" rel="canonical"><link href=../object_order/ rel=prev><link href=../parsing/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.8.0"><title>Performance - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.57a0d79d.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.fb2fffe2.min.css><link rel=stylesheet href=../../css/custom.css><link rel=stylesheet href=../../css/brand.css><link rel=stylesheet href=../../css/fig-sax.css><link rel=stylesheet href=../../css/fig-jsonview.css><link rel=stylesheet 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<span class=n>std</span><span class=o>::</span><span class=n>ifstream</span><span class=w> </span><span class=n>f</span><span class=p>(</span><span class=s>"example.json"</span><span class=p>);</span>
<span class=n>json</span><span class=w> </span><span class=n>j</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>parse</span><span class=p>(</span><span class=n>f</span><span class=p>);</span>
</code></pre></div> <p>Two macros switch the stream optimizations off without changing any result: <a href=../../api/macros/json_disable_stream_bulk_scan/ ><code>JSON_DISABLE_STREAM_BULK_SCAN</code></a> for stream buffers that do not follow the usual <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>streambuf</span></code> contract, and <a href=../../api/macros/json_disable_unlocked_file_io/ ><code>JSON_DISABLE_UNLOCKED_FILE_IO</code></a> for programs in which another thread must use the same <code class=highlight><span class=kt>FILE</span></code> object while it is parsed.</p> <p>For contiguous input with many non-<abbr title="American Standard Code for Information Interchange">ASCII</abbr> characters, <a href=../../api/macros/json_use_simdutf/ ><code>JSON_USE_SIMDUTF</code></a> can additionally speed up <abbr title="Unicode Transformation Format">UTF</abbr>-8 validation by using the <a href="https://github.com/simdutf/simdutf">simdutf</a> library instead of the built-in scalar validator; streaming inputs (files, <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>istream</span></code>, wide strings, user-defined adapters) always use the scalar path regardless of this macro.</p> <h2 id=large-documents>Large documents<a class=headerlink href=#large-documents title="Permanent link">&para;</a></h2> <p>Parsing always produces <abbr title="Simple API for XML">SAX</abbr> events internally; <a href=../../api/basic_json/parse/ ><code>parse</code></a> simply feeds them to a consumer that builds a complete <code>basic_json</code> value tree (a <abbr title="Document Object Model">DOM</abbr>) in memory. For documents too large to comfortably hold as a <abbr title="Document Object Model">DOM</abbr>, three alternatives avoid building it:</p> <ul> <li>Implement the <a href=../parsing/sax_interface/ ><abbr title="Simple API for XML">SAX</abbr> interface</a> directly and pass it to <a href=../../api/basic_json/sax_parse/ ><code>sax_parse</code></a>; only the parts of the input you choose to keep ever become <code>basic_json</code> values.</li> <li>Pass a <a href=../parsing/parser_callbacks/ >parser callback</a> to <code>parse</code>. This still builds a <abbr title="Document Object Model">DOM</abbr>, but the callback can discard finished elements as soon as they are handled, so memory usage stays bounded by one element (plus the unparsed remainder of the input) instead of the whole document — see the <a href=../parsing/parser_callbacks/#recipe-streaming-a-large-homogeneous-array>recipe for streaming a large homogeneous array</a>.</li> <li>Parse into a <a href=../json_view/ ><code>json_document</code></a> (<code class=highlight><span class=o>&lt;</span><span class=n>nlohmann</span><span class=o>/</span><span class=n>json_view</span><span class=p>.</span><span class=n>hpp</span><span class=o>&gt;</span></code>) instead of a <code>basic_json</code>. It keeps the input text and builds a flat index of 16 bytes per value; strings and floating-point numbers are not copied, but read from the text when needed. Read-only <a href=../../api/basic_json_view/ >views</a> give the familiar element access, and only the parts you <a href=../../api/basic_json_view/materialize/ ><code>materialize()</code></a> become <code>basic_json</code> values. A document that borrows the text instead of owning a copy needs the text to outlive it; see <a href=../json_view/#choosing-between-json-ordered_json-the-sax-interface-and-json_view>choosing between <code>json</code>, the <abbr title="Simple API for XML">SAX</abbr> interface, and <code>json_view</code></a>.</li> </ul> <p>If the data is naturally record-oriented, consider <a href=../parsing/json_lines/ ><abbr title="JavaScript Object Notation">JSON</abbr> Lines</a> instead of one large <abbr title="JavaScript Object Notation">JSON</abbr> document: reading and parsing it line by line with <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>getline</span></code> means only one line's value is ever in memory at a time, and a malformed line does not invalidate lines already processed.</p> <h2 id=binary-formats>Binary formats<a class=headerlink href=#binary-formats title="Permanent link">&para;</a></h2> <p><abbr title="JavaScript Object Notation">JSON</abbr> text is not a compact format. If the data is only exchanged between programs (not read by humans), the <a href=../binary_formats/ >binary formats</a> — <abbr title="Binary JData">BJData</abbr>, <abbr title="Binary Object Notation 8">BON8</abbr>, <abbr title="Binary JSON">BSON</abbr>, <abbr title="Concise Binary Object Representation">CBOR</abbr>, MessagePack, and <abbr title="Universal Binary JSON">UBJSON</abbr> — encode the same values more compactly, which reduces both the bytes transferred and, for most of them, the work needed to parse them back. The <a href=../binary_formats/#sizes>size comparison</a> on that page, measured against minified <abbr Line truncated
@@ -14,4 +14,4 @@
<span class=cp>#include</span><span class=w> </span><span class=cpf>"my_type.hpp"</span>
<span class=n>nlohmann</span><span class=o>::</span><span class=n>json</span><span class=w> </span><span class=nf>my_type::config</span><span class=p>()</span><span class=w> </span><span class=k>const</span><span class=w> </span><span class=p>{</span><span class=w> </span><span class=cm>/* ... */</span><span class=w> </span><span class=p>}</span>
</code></pre></div> <p>One caveat: <abbr title="Application Binary Interface">ABI</abbr>-affecting macros such as <code>JSON_DIAGNOSTICS</code> and <code>JSON_DIAGNOSTIC_POSITIONS</code> are encoded into the library's <a href=../namespace/#limitations>inline namespace name</a>. Every translation unit — whether it includes <code>json_fwd.hpp</code> or the full header — must define them the same way, or linking fails with undefined references instead of a compile error.</p> <p>If I/O support is not needed at all, <a href=../../api/macros/json_no_io/ ><code>JSON_NO_IO</code></a> excludes <code>&lt;cstdio&gt;</code>, <code>&lt;ios&gt;</code>, <code>&lt;iosfwd&gt;</code>, <code>&lt;istream&gt;</code>, and <code>&lt;ostream&gt;</code> outright and drops the <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>istream</span></code>/<code class=highlight><span class=kt>FILE</span><span class=o>*</span></code> <code>parse</code> overloads and <a href=../../api/operator_ltlt/ ><code>operator&lt;&lt;</code></a> that depend on them (<code>dump()</code> itself is unaffected, since it only returns a string); it exists for environments where those headers are unavailable (such as Intel <abbr title="Software Guard Extensions">SGX</abbr>), and as a side effect those headers are then never processed by the compiler at all.</p> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../../home/design_goals/ >Design goals</a> - why this library does not optimize for speed first</li> <li><a href=../../home/architecture/ >Architecture</a> - how input adapters, the lexer, and the serializer fit together</li> <li><a href=../parsing/ >Parsing</a> - the available parsing functions and inputs</li> <li><a href=../parsing/sax_interface/ ><abbr title="Simple API for XML">SAX</abbr> interface</a> - parse without building a <abbr title="Document Object Model">DOM</abbr></li> <li><a href=../json_view/ >Zero-copy <abbr title="JavaScript Object Notation">JSON</abbr> views</a> - parse into a flat index of the text and read it without building a <abbr title="Document Object Model">DOM</abbr></li> <li><a href=../binary_formats/ >Binary formats</a> - compact alternatives to <abbr title="JavaScript Object Notation">JSON</abbr> text</li> <li><a href=../object_order/ >Object Order</a> - <code>json</code> vs. <code>ordered_json</code> and other <code>ObjectType</code> choices</li> <li><a href=../types/template_parameters/ >Template Parameter Requirements</a> - custom container and allocator types</li> <li><a href=../macros/ >Supported macros</a> - overview of all configuration macros, including the diagnostics ones above</li> <li><a href=../../api/macros/json_no_simd/ ><code>JSON_NO_SIMD</code></a> - disable <abbr title="Single Instruction, Multiple Data">SIMD</abbr> acceleration</li> <li><a href=../../api/macros/json_use_simdutf/ ><code>JSON_USE_SIMDUTF</code></a> - use simdutf for <abbr title="Unicode Transformation Format">UTF</abbr>-8 validation</li> <li><a href=../../api/macros/json_disable_stream_bulk_scan/ ><code>JSON_DISABLE_STREAM_BULK_SCAN</code></a> - disable the stream bulk scan</li> <li><a href=../../api/basic_json/parse/ ><code>parse</code></a> - deserialize from a compatible input</li> <li><a href=../../api/basic_json/sax_parse/ ><code>sax_parse</code></a> - parse with a <abbr title="Simple API for XML">SAX</abbr> interface</li> <li><a href=../../api/json_view/ ><code>json_view</code></a> - a non-owning view of a value in a document</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><pathLine truncated
</code></pre></div> <p>One caveat: <abbr title="Application Binary Interface">ABI</abbr>-affecting macros such as <code>JSON_DIAGNOSTICS</code> and <code>JSON_DIAGNOSTIC_POSITIONS</code> are encoded into the library's <a href=../namespace/#limitations>inline namespace name</a>. Every translation unit — whether it includes <code>json_fwd.hpp</code> or the full header — must define them the same way, or linking fails with undefined references instead of a compile error.</p> <p>If I/O support is not needed at all, <a href=../../api/macros/json_no_io/ ><code>JSON_NO_IO</code></a> excludes <code>&lt;cstdio&gt;</code>, <code>&lt;ios&gt;</code>, <code>&lt;iosfwd&gt;</code>, <code>&lt;istream&gt;</code>, and <code>&lt;ostream&gt;</code> outright and drops the <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>istream</span></code>/<code class=highlight><span class=kt>FILE</span><span class=o>*</span></code> <code>parse</code> overloads and <a href=../../api/operator_ltlt/ ><code>operator&lt;&lt;</code></a> that depend on them (<code>dump()</code> itself is unaffected, since it only returns a string); it exists for environments where those headers are unavailable (such as Intel <abbr title="Software Guard Extensions">SGX</abbr>), and as a side effect those headers are then never processed by the compiler at all.</p> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../../home/design_goals/ >Design goals</a> - why this library does not optimize for speed first</li> <li><a href=../../home/architecture/ >Architecture</a> - how input adapters, the lexer, and the serializer fit together</li> <li><a href=../parsing/ >Parsing</a> - the available parsing functions and inputs</li> <li><a href=../parsing/sax_interface/ ><abbr title="Simple API for XML">SAX</abbr> interface</a> - parse without building a <abbr title="Document Object Model">DOM</abbr></li> <li><a href=../json_view/ >Zero-copy <abbr title="JavaScript Object Notation">JSON</abbr> views</a> - parse into a flat index of the text and read it without building a <abbr title="Document Object Model">DOM</abbr></li> <li><a href=../binary_formats/ >Binary formats</a> - compact alternatives to <abbr title="JavaScript Object Notation">JSON</abbr> text</li> <li><a href=../object_order/ >Object Order</a> - <code>json</code> vs. <code>ordered_json</code> and other <code>ObjectType</code> choices</li> <li><a href=../types/template_parameters/ >Template Parameter Requirements</a> - custom container and allocator types</li> <li><a href=../macros/ >Supported macros</a> - overview of all configuration macros, including the diagnostics ones above</li> <li><a href=../../api/macros/json_no_simd/ ><code>JSON_NO_SIMD</code></a> - disable <abbr title="Single Instruction, Multiple Data">SIMD</abbr> acceleration</li> <li><a href=../../api/macros/json_use_simdutf/ ><code>JSON_USE_SIMDUTF</code></a> - use simdutf for <abbr title="Unicode Transformation Format">UTF</abbr>-8 validation</li> <li><a href=../../api/macros/json_disable_stream_bulk_scan/ ><code>JSON_DISABLE_STREAM_BULK_SCAN</code></a> - disable the stream bulk scan</li> <li><a href=../../api/basic_json/parse/ ><code>parse</code></a> - deserialize from a compatible input</li> <li><a href=../../api/basic_json/sax_parse/ ><code>sax_parse</code></a> - parse with a <abbr title="Simple API for XML">SAX</abbr> interface</li> <li><a href=../../api/json_view/ ><code>json_view</code></a> - a non-owning view of a value in a document</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="November 4, 2026 16:46:20 UTC">November 4, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><pathLine truncated
+1 -1
View File
@@ -17,7 +17,7 @@ A `std::istream` (including `std::ifstream`) gets the same bulk scanning: `input
Inputs that are still read one character at a time: iterator ranges that are not contiguous (e.g. `std::list<char>` or `std::istreambuf_iterator<char>`), wide strings, and user-defined adapters. An iterator pair over non-contiguous but random-access storage (e.g. `std::deque<char>::iterator`) gets the first optimization but not the second, since the byte-scanning fast path requires contiguous storage.
Practically: a file can be passed directly to `parse`, which also avoids the memory for a copy of the file. Reading it into a string first is no longer necessary for speed:
Practically: a file can be passed directly to `parse`, which also avoids the memory for a copy of the file. Reading it into a string first is no longer necessary for speed (this is for `json::parse`; a [`json_document`](https://json.nlohmann.me/features/json_view/index.md) reads a stream into an owned buffer anyway, so for it reading the file into a string and borrowing that is the faster way):
```
// fast: the stream buffer's get area is scanned in bulk
+3
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@@ -17,6 +17,9 @@ To write a value directly to a stream (for example, a file or `#!cpp std::cout`)
std::cout << j << std::endl;
```
To write a file without leaving a truncated file behind when serialization fails, see
[Reading and writing files](files.md#writing-a-json-file).
!!! note "String, not raw value"
`dump` always returns a **JSON text**. Serializing a JSON string therefore includes the surrounding quotes and
+3 -3
View File
@@ -1,8 +1,8 @@
<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=description content="JSON for Modern C++ is a header-only C++11 library that makes JSON a first-class data type."><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/serialization/" rel="canonical"><link href=../assertions/ rel=prev><link href=../enum_conversion/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>Serialization - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.ab4e12ef.min.css><link rel=stylesheet href=../../css/custom.css><link rel=stylesheet href=../../css/brand.css><link rel=stylesheet href=../../css/fig-sax.css><link rel=stylesheet href=../../css/fig-jsonview.css><link rel=stylesheet 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data-md-component=announce> </div> <header class=md-header data-md-component=header> <nav class="md-header__inner md-grid" aria-label=Header> <a href=../.. title="JSON for Modern C++" class="md-header__button md-logo" aria-label="JSON for Modern C++" data-md-component=logo> <img src=../../images/wordmark-header.svg alt=logo> </a> <label class="md-header__button md-icon" for=__drawer> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M3 6h18v2H3zm0 5h18v2H3zm0 5h18v2H3z"/></svg> </label> <div class=md-header__title data-md-component=header-title> <div class=md-header__ellipsis> <div class=md-header__topic> <span class=md-ellipsis> JSON for Modern C++ </span> </div> <div class=md-header__topic data-md-component=header-topic> <span class=md-ellipsis> Serialization </span> </div> </div> </div> <form class=md-header__option data-md-component=palette> <input class=md-option data-md-color-media="(prefers-color-scheme: light)" data-md-color-scheme=default 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<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=description content="JSON for Modern C++ is a header-only C++11 library that makes JSON a first-class data type."><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/serialization/" rel="canonical"><link href=../assertions/ rel=prev><link href=../enum_conversion/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.8.0"><title>Serialization - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.57a0d79d.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.fb2fffe2.min.css><link rel=stylesheet href=../../css/custom.css><link rel=stylesheet href=../../css/brand.css><link rel=stylesheet href=../../css/fig-sax.css><link rel=stylesheet href=../../css/fig-jsonview.css><link rel=stylesheet 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data-md-component=announce> </div> <header class=md-header data-md-component=header> <nav class="md-header__inner md-grid" aria-label=Header> <a href=../.. title="JSON for Modern C++" class="md-header__button md-logo" aria-label="JSON for Modern C++" data-md-component=logo> <img src=../../images/wordmark-header.svg alt=logo> </a> <label class="md-header__button md-icon" for=__drawer> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M3 6h18v2H3zm0 5h18v2H3zm0 5h18v2H3z"/></svg> </label> <div class=md-header__title data-md-component=header-title> <div class=md-header__ellipsis> <div class=md-header__topic> <span class=md-ellipsis> JSON for Modern C++ </span> </div> <div class=md-header__topic data-md-component=header-topic> <span class=md-ellipsis> Serialization </span> </div> </div> </div> <form class=md-header__option data-md-component=palette> <input class=md-option data-md-color-media="(prefers-color-scheme: light)" data-md-color-scheme=default 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<span class=n>std</span><span class=o>::</span><span class=n>string</span><span class=w> </span><span class=n>s</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>j</span><span class=p>.</span><span class=n>dump</span><span class=p>();</span><span class=w> </span><span class=c1>// {"happy":true,"pi":3.141}</span>
</code></pre></div> <p>To write a value directly to a stream (for example, a file or <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>cout</span></code>), the <a href=../../api/operator_ltlt/ ><code>operator&lt;&lt;</code></a> is provided:</p> <div class=highlight><pre><span></span><code><span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>j</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>endl</span><span class=p>;</span>
</code></pre></div> <div class="admonition note"> <p class=admonition-title>String, not raw value</p> <p><code>dump</code> always returns a <strong><abbr title="JavaScript Object Notation">JSON</abbr> text</strong>. Serializing a <abbr title="JavaScript Object Notation">JSON</abbr> string therefore includes the surrounding quotes and escapes special characters. To obtain the <em>contained</em> string value without quotes, use <a href=../conversions/ ><code>get&lt;std::string&gt;()</code></a> instead of <code>dump</code>. See the <a href=../conversions/ >converting values</a> page.</p> </div> <h2 id=pretty-printing>Pretty-printing<a class=headerlink href=#pretty-printing title="Permanent link">&para;</a></h2> <p>By default, <code>dump</code> produces the most compact representation without any superfluous whitespace. Passing a non-negative <code>indent</code> argument pretty-prints the output with the given number of spaces per level:</p> <details class=example> <summary>Example: pretty-print <abbr title="JavaScript Object Notation">JSON</abbr> values with <code>dump()</code></summary> <div class=highlight><pre><span></span><code><span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;iostream&gt;</span>
</code></pre></div> <p>To write a file without leaving a truncated file behind when serialization fails, see <a href=../files/#writing-a-json-file>Reading and writing files</a>.</p> <div class="admonition note"> <p class=admonition-title>String, not raw value</p> <p><code>dump</code> always returns a <strong><abbr title="JavaScript Object Notation">JSON</abbr> text</strong>. Serializing a <abbr title="JavaScript Object Notation">JSON</abbr> string therefore includes the surrounding quotes and escapes special characters. To obtain the <em>contained</em> string value without quotes, use <a href=../conversions/ ><code>get&lt;std::string&gt;()</code></a> instead of <code>dump</code>. See the <a href=../conversions/ >converting values</a> page.</p> </div> <h2 id=pretty-printing>Pretty-printing<a class=headerlink href=#pretty-printing title="Permanent link">&para;</a></h2> <p>By default, <code>dump</code> produces the most compact representation without any superfluous whitespace. Passing a non-negative <code>indent</code> argument pretty-prints the output with the given number of spaces per level:</p> <details class=example> <summary>Example: pretty-print <abbr title="JavaScript Object Notation">JSON</abbr> values with <code>dump()</code></summary> <div class=highlight><pre><span></span><code><span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;iostream&gt;</span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;nlohmann/json.hpp&gt;</span>
<span class=k>using</span><span class=w> </span><span class=n>nlohmann</span><span class=o>::</span><span class=n>json</span><span class=p>;</span>
@@ -140,4 +140,4 @@
</code></pre></div> </details> <div class="admonition tip"> <p class=admonition-title>Avoiding invalid <abbr title="Unicode Transformation Format">UTF</abbr>-8</p> <p>The best fix is to ensure that all strings are <abbr title="Unicode Transformation Format">UTF</abbr>-8 encoded before storing them. See the <a href=../../home/faq/#parse-errors-reading-non-ascii-characters><abbr title="Frequently Asked Questions">FAQ</abbr> on non-<abbr title="American Standard Code for Information Interchange">ASCII</abbr> characters</a> for how to convert wide or Latin-1 strings.</p> </div> <h2 id=numbers-nan-and-binary-values>Numbers, <abbr title="Not a Number">NaN</abbr>, and binary values<a class=headerlink href=#numbers-nan-and-binary-values title="Permanent link">&para;</a></h2> <ul> <li><strong>Numbers</strong> are serialized with enough precision to round-trip; see <a href=../types/number_handling/#number-serialization>number serialization</a>.</li> <li><strong><abbr title="Not a Number">NaN</abbr> and infinity</strong> cannot be represented in <abbr title="JavaScript Object Notation">JSON</abbr> and are serialized as <code class=highlight><span class=kc>null</span></code>; see <a href=../types/number_handling/#nan-handling><abbr title="Not a Number">NaN</abbr> handling</a>. The <a href=../binary_formats/ >binary formats</a> can preserve them.</li> <li><strong>Binary values</strong> have no <abbr title="JavaScript Object Notation">JSON</abbr> representation and are serialized as a helper object for debugging only; see <a href=../binary_values/#serialization>binary values</a>.</li> </ul> <h2 id=using-stdformat-stdprint-and-fmt>Using <code>std::format</code>, <code>std::print</code>, and <code>fmt</code><a class=headerlink href=#using-stdformat-stdprint-and-fmt title="Permanent link">&para;</a></h2> <p>Since version 3.12.0, <abbr title="JavaScript Object Notation">JSON</abbr> values can be formatted directly with C++20's <a href="https://en.cppreference.com/w/cpp/utility/format/format"><code>std::format</code></a> whenever the standard library provides the <code>&lt;format&gt;</code> header (controlled by <a href=../../api/macros/json_has_std_format/ ><code>JSON_HAS_STD_FORMAT</code></a>). This is enabled by the <a href=../../api/basic_json/std_formatter/ ><code>std::formatter&lt;basic_json&gt;</code></a> specialization, which also makes <abbr title="JavaScript Object Notation">JSON</abbr> values work with <code>std::format_to</code> and with C++23's <code>std::print</code>/<code>std::println</code>:</p> <div class=highlight><pre><span></span><code><span class=n>std</span><span class=o>::</span><span class=n>print</span><span class=p>(</span><span class=s>"{}"</span><span class=p>,</span><span class=w> </span><span class=n>j</span><span class=p>);</span><span class=w> </span><span class=c1>// compact, like j.dump()</span>
<span class=n>std</span><span class=o>::</span><span class=n>print</span><span class=p>(</span><span class=s>"{:2}"</span><span class=p>,</span><span class=w> </span><span class=n>j</span><span class=p>);</span><span class=w> </span><span class=c1>// pretty-printed with indent 2 (like j.dump(2))</span>
<span class=n>std</span><span class=o>::</span><span class=n>println</span><span class=p>(</span><span class=s>"{:#}"</span><span class=p>,</span><span class=w> </span><span class=n>j</span><span class=p>);</span><span class=w> </span><span class=c1>// pretty-printed with the default indent</span>
</code></pre></div> <p>The format spec mirrors the <code>dump</code> parameters: <code class=highlight><span class=s>"{:#}"</span></code> pretty-prints, a width such as <code class=highlight><span class=s>"{:2}"</span></code> sets the indent, and a fill-and-align prefix such as <code class=highlight><span class=s>"{:.&gt;#}"</span></code> sets the indent character.</p> <p>For the <a href="https://github.com/fmtlib/fmt">{fmt}</a> library, the library ships a <a href=../../api/basic_json/format_as/ ><code>format_as</code></a> helper. Note its behavior depends on the <code>fmt</code> version; see the <a href=../../home/faq/#using-json-values-with-stdformat-or-fmt><abbr title="Frequently Asked Questions">FAQ</abbr> entry</a> for the details and a recipe for a full <code>fmt::formatter</code> specialization.</p> <h2 id=serializing-to-other-formats>Serializing to other formats<a class=headerlink href=#serializing-to-other-formats title="Permanent link">&para;</a></h2> <p>Besides <abbr title="JavaScript Object Notation">JSON</abbr> text, a value can also be serialized to the more compact <a href=../binary_formats/ >binary formats</a> (<abbr title="Binary JData">BJData</abbr>, <abbr title="Binary Object Notation 8">BON8</abbr>, <abbr title="Binary JSON">BSON</abbr>, <abbr title="Concise Binary Object Representation">CBOR</abbr>, MessagePack, <abbr title="Universal Binary JSON">UBJSON</abbr>).</p> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../../api/basic_json/dump/ ><code>dump</code></a> - serialize to a <abbr title="JavaScript Object Notation">JSON</abbr>-formatted string</li> <li><a href=../../api/operator_ltlt/ ><code>operator&lt;&lt;</code></a> - serialize to a stream</li> <li><a href=../../api/basic_json/to_string/ ><code>to_string</code></a> - user-defined-conversion helper</li> <li><a href=../../api/basic_json/std_formatter/ ><code>std::formatter&lt;basic_json&gt;</code></a> - use <abbr title="JavaScript Object Notation">JSON</abbr> values with <code>std::format</code> and <code>std::print</code></li> <li><a href=../../api/basic_json/format_as/ ><code>format_as</code></a> - use <abbr title="JavaScript Object Notation">JSON</abbr> values with the {fmt} library</li> <li><a href=../parsing/ >Parsing</a> - the reverse operation</li> <li><a href=../../api/basic_json/error_handler_t/ ><code>error_handler_t</code></a> - how to treat invalid <abbr title="Unicode Transformation Format">UTF</abbr>-8 when dumping</li> <li><a href=../../api/basic_json_view/dump/ ><code>basic_json_view::dump</code></a> - serialize a view directly</li> <li><a href=../../api/macros/json_no_io/ ><code>JSON_NO_IO</code></a> - remove stream and file I/O</li> <li><a href=../../api/macros/json_no_simd/ ><code>JSON_NO_SIMD</code></a> - disable <abbr title="Single Instruction, Multiple Data">SIMD</abbr> acceleration</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../assertions/ class="md-footer__link md-footer__link--prev" aria-label="Previous: Runtime Assertions"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> Runtime Assertions </div> </div> </a> <a href=../enum_conversion/ classLine truncated
</code></pre></div> <p>The format spec mirrors the <code>dump</code> parameters: <code class=highlight><span class=s>"{:#}"</span></code> pretty-prints, a width such as <code class=highlight><span class=s>"{:2}"</span></code> sets the indent, and a fill-and-align prefix such as <code class=highlight><span class=s>"{:.&gt;#}"</span></code> sets the indent character.</p> <p>For the <a href="https://github.com/fmtlib/fmt">{fmt}</a> library, the library ships a <a href=../../api/basic_json/format_as/ ><code>format_as</code></a> helper. Note its behavior depends on the <code>fmt</code> version; see the <a href=../../home/faq/#using-json-values-with-stdformat-or-fmt><abbr title="Frequently Asked Questions">FAQ</abbr> entry</a> for the details and a recipe for a full <code>fmt::formatter</code> specialization.</p> <h2 id=serializing-to-other-formats>Serializing to other formats<a class=headerlink href=#serializing-to-other-formats title="Permanent link">&para;</a></h2> <p>Besides <abbr title="JavaScript Object Notation">JSON</abbr> text, a value can also be serialized to the more compact <a href=../binary_formats/ >binary formats</a> (<abbr title="Binary JData">BJData</abbr>, <abbr title="Binary Object Notation 8">BON8</abbr>, <abbr title="Binary JSON">BSON</abbr>, <abbr title="Concise Binary Object Representation">CBOR</abbr>, MessagePack, <abbr title="Universal Binary JSON">UBJSON</abbr>).</p> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../../api/basic_json/dump/ ><code>dump</code></a> - serialize to a <abbr title="JavaScript Object Notation">JSON</abbr>-formatted string</li> <li><a href=../../api/operator_ltlt/ ><code>operator&lt;&lt;</code></a> - serialize to a stream</li> <li><a href=../../api/basic_json/to_string/ ><code>to_string</code></a> - user-defined-conversion helper</li> <li><a href=../../api/basic_json/std_formatter/ ><code>std::formatter&lt;basic_json&gt;</code></a> - use <abbr title="JavaScript Object Notation">JSON</abbr> values with <code>std::format</code> and <code>std::print</code></li> <li><a href=../../api/basic_json/format_as/ ><code>format_as</code></a> - use <abbr title="JavaScript Object Notation">JSON</abbr> values with the {fmt} library</li> <li><a href=../parsing/ >Parsing</a> - the reverse operation</li> <li><a href=../../api/basic_json/error_handler_t/ ><code>error_handler_t</code></a> - how to treat invalid <abbr title="Unicode Transformation Format">UTF</abbr>-8 when dumping</li> <li><a href=../../api/basic_json_view/dump/ ><code>basic_json_view::dump</code></a> - serialize a view directly</li> <li><a href=../../api/macros/json_no_io/ ><code>JSON_NO_IO</code></a> - remove stream and file I/O</li> <li><a href=../../api/macros/json_no_simd/ ><code>JSON_NO_SIMD</code></a> - disable <abbr title="Single Instruction, Multiple Data">SIMD</abbr> acceleration</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="November 4, 2026 16:46:20 UTC">November 4, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../assertions/ class="md-footer__link md-footer__link--prev" aria-label="Previous: Runtime Assertions"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> Runtime Assertions </div> </div> </a> <a href=../enum_conversion/ classLine truncated
+2
View File
@@ -14,6 +14,8 @@ To write a value directly to a stream (for example, a file or `std::cout`), the
std::cout << j << std::endl;
```
To write a file without leaving a truncated file behind when serialization fails, see [Reading and writing files](https://json.nlohmann.me/features/files/#writing-a-json-file).
String, not raw value
`dump` always returns a **JSON text**. Serializing a JSON string therefore includes the surrounding quotes and escapes special characters. To obtain the *contained* string value without quotes, use [`get<std::string>()`](https://json.nlohmann.me/features/conversions/index.md) instead of `dump`. See the [converting values](https://json.nlohmann.me/features/conversions/index.md) page.
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@@ -1,4 +1,4 @@
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<span class=w> </span><span class=nt>"planets"</span><span class=p>:</span><span class=w> </span><span class=p>[</span>
<span class=w> </span><span class=s2>"Mercury"</span><span class=p>,</span>
<span class=w> </span><span class=s2>"Venus"</span><span class=p>,</span>
@@ -58,4 +58,4 @@
"Neptune"
]
}
</code></pre></div> </div> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../../api/basic_json/parse/ ><code>parse</code></a> - deserialize from a compatible input</li> <li><a href=../comments/ >Comments</a> - ignore <code>//</code> and <code>/* */</code> comments while parsing</li> <li><a href=../parsing/ >Parsing</a> - overview of the parsing functions</li> <li><a href=../parsing/untrusted_input/ >Parsing Untrusted Input</a> - limits and checks for input you do not control</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../macros/ class="md-footer__link md-footer__link--prev" aria-label="Previous: Supported Macros"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> Supported Macros </div> </div> </a> <a href=../types/ class="md-footer__link md-footer__link--next" aria-label="Next: Types"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> Types </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 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</code></pre></div> </div> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../../api/basic_json/parse/ ><code>parse</code></a> - deserialize from a compatible input</li> <li><a href=../comments/ >Comments</a> - ignore <code>//</code> and <code>/* */</code> comments while parsing</li> <li><a href=../parsing/ >Parsing</a> - overview of the parsing functions</li> <li><a href=../parsing/untrusted_input/ >Parsing Untrusted Input</a> - limits and checks for input you do not control</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../macros/ class="md-footer__link md-footer__link--prev" aria-label="Previous: Supported Macros"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> Supported Macros </div> </div> </a> <a href=../types/ class="md-footer__link md-footer__link--next" aria-label="Next: Types"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> Types </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 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+2 -2
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<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=description content="JSON for Modern C++ is a header-only C++11 library that makes JSON a first-class data type."><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/type_recipes/" rel="canonical"><link href=../arbitrary_types/ rel=prev><link href=../binary_formats/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.8.0"><title>Recipes for Common Types - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.57a0d79d.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.fb2fffe2.min.css><link rel=stylesheet href=../../css/custom.css><link rel=stylesheet href=../../css/brand.css><link rel=stylesheet href=../../css/fig-sax.css><link rel=stylesheet href=../../css/fig-jsonview.css><link rel=stylesheet 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<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;memory&gt;</span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;vector&gt;</span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;nlohmann/json.hpp&gt;</span>
@@ -363,4 +363,4 @@ as milliseconds: 1500
error: unexpected unit "ms"
time point: {"unit":"ms","value":1700000000123}
round trip: true
</code></pre></div> </details> <p>A <code>system_clock::time_point</code> with millisecond precision is written as the duration since the Unix epoch, that is, as an object with <code>unit</code> and <code>value</code>. The time point serializer is limited to <code>system_clock</code>, because the epoch of other clocks (for example <code>steady_clock</code>) has no meaning outside the running program.</p> <p>If you prefer a text representation such as <abbr title="International Organization for Standardization">ISO</abbr> 8601 (<code>2023-11-14T22:13:20Z</code>), write the string with <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>format</span><span class=p>(</span><span class=s>"{:%FT%TZ}"</span><span class=p>,</span><span class=w> </span><span class=n>t</span><span class=p>)</span></code> and read it with <code>std::chrono::parse</code> (both C++20), or with a date library, in the same two functions.</p> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../arbitrary_types/ >Arbitrary type conversions</a> - the conversion mechanism and macros</li> <li><a href=../arbitrary_types/#why-isnt-my-to_json-or-from_json-found>Why isn't my <code>to_json</code> found?</a> - troubleshoot conversion functions that are not picked up</li> <li><a href=../conversions/ >Converting values</a> - <code>get</code>, implicit conversions, and <code>std::optional</code></li> <li><a href=../../api/macros/nlohmann_define_derived_type/ ><code>NLOHMANN_DEFINE_DERIVED_TYPE</code></a> - conversion functions for derived classes</li> <li><a href=../../api/adl_serializer/ ><code>adl_serializer</code></a> - customize conversions for third-party types</li> <li><a href=../../api/basic_json/get/ ><code>get</code></a> - convert a <abbr title="JavaScript Object Notation">JSON</abbr> value to a C++ type</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../arbitrary_types/ class="md-footer__link md-footer__link--prev" aria-label="Previous: Arbitrary Type Conversions"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> Arbitrary Type Conversions </div> </div> </a> <a href=../binary_formats/ class="md-footer__link md-footer__link--next" aria-label="Next: Binary Formats"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> Binary Formats </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) CopyrighLine truncated
</code></pre></div> </details> <p>A <code>system_clock::time_point</code> with millisecond precision is written as the duration since the Unix epoch, that is, as an object with <code>unit</code> and <code>value</code>. The time point serializer is limited to <code>system_clock</code>, because the epoch of other clocks (for example <code>steady_clock</code>) has no meaning outside the running program.</p> <p>If you prefer a text representation such as <abbr title="International Organization for Standardization">ISO</abbr> 8601 (<code>2023-11-14T22:13:20Z</code>), write the string with <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>format</span><span class=p>(</span><span class=s>"{:%FT%TZ}"</span><span class=p>,</span><span class=w> </span><span class=n>t</span><span class=p>)</span></code> and read it with <code>std::chrono::parse</code> (both C++20), or with a date library, in the same two functions.</p> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../arbitrary_types/ >Arbitrary type conversions</a> - the conversion mechanism and macros</li> <li><a href=../arbitrary_types/#why-isnt-my-to_json-or-from_json-found>Why isn't my <code>to_json</code> found?</a> - troubleshoot conversion functions that are not picked up</li> <li><a href=../conversions/ >Converting values</a> - <code>get</code>, implicit conversions, and <code>std::optional</code></li> <li><a href=../../api/macros/nlohmann_define_derived_type/ ><code>NLOHMANN_DEFINE_DERIVED_TYPE</code></a> - conversion functions for derived classes</li> <li><a href=../../api/adl_serializer/ ><code>adl_serializer</code></a> - customize conversions for third-party types</li> <li><a href=../../api/basic_json/get/ ><code>get</code></a> - convert a <abbr title="JavaScript Object Notation">JSON</abbr> value to a C++ type</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../arbitrary_types/ class="md-footer__link md-footer__link--prev" aria-label="Previous: Arbitrary Type Conversions"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> Arbitrary Type Conversions </div> </div> </a> <a href=../binary_formats/ class="md-footer__link md-footer__link--next" aria-label="Next: Binary Formats"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> Binary Formats </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) CopyrighLine truncated
+2 -2
View File
@@ -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=description content="JSON for Modern C++ is a header-only C++11 library that makes JSON a first-class data type."><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/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=../../css/custom.css><link rel=stylesheet href=../../css/brand.css><link rel=stylesheet href=../../css/fig-sax.css><link rel=stylesheet href=../../css/fig-jsonview.css><link rel=stylesheet 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class value_t {
&lt;&lt;enumeration&gt;&gt;
@@ -81,4 +81,4 @@ basic_json .. value_t</code></pre> <p class=nj-caption>Storage of a <code>basic_
<span class=w> </span><span class=n>basic_json</span><span class=p>,</span><span class=w> </span><span class=c1>// value_type</span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>allocator</span><span class=o>&lt;</span><span class=n>basic_json</span><span class=o>&gt;</span><span class=w> </span><span class=c1>// allocator_type</span>
<span class=o>&gt;</span>
</code></pre></div> <h3 id=limits_1>Limits<a class=headerlink href=#limits_1 title="Permanent link">&para;</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">&para;</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">&para;</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">&para;</a></h3> <p>With the default values for <em>StringType</em> (<code>std::string</code>), the default type 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">&para;</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">&para;</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">&para;</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">&para;</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">&para;</a></h3> <p>With the default values for <em>BooleanType</em> (<code class=highlight><span class=kt>bool</span></code>), the default type 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">&para;</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">&para;</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 represLine truncated
</code></pre></div> <h3 id=limits_1>Limits<a class=headerlink href=#limits_1 title="Permanent link">&para;</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">&para;</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">&para;</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">&para;</a></h3> <p>With the default values for <em>StringType</em> (<code>std::string</code>), the default type 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">&para;</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">&para;</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">&para;</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">&para;</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">&para;</a></h3> <p>With the default values for <em>BooleanType</em> (<code class=highlight><span class=kt>bool</span></code>), the default type 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">&para;</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">&para;</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 represLine truncated
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@@ -1,4 +1,4 @@
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A["number literal"] --&gt; B{{"has a fraction (.) or exponent (e/E)?"}}
B --&gt;|"yes"| F["number_float_t"]
B --&gt;|"no"| C{{"has a leading minus sign?"}}
@@ -60,4 +60,4 @@ val=nan
<span class=w> </span><span class=c1>// ...</span>
<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">&para;</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> <p class=nj-table-caption>Return value of each function for a value that stores an unsigned integer, a signed integer, or a floating-point number, and for a string for comparison.</p> <h3 id=template-number-types>Template number types<a class=headerlink href=#template-number-types title="Permanent link">&para;</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></Line truncated
</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> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../ >Types</a> - how all value types are stored</li> <li><a href=../../conversions/ >Converting values</a> - convert numbers to C++ types</li> <li><a href=../../../api/basic_json/number_integer_t/ ><code>number_integer_t</code></a> - the signed integer type</li> <li><a href=../../../api/basic_json/number_unsigned_t/ ><code>number_unsigned_t</code></a> - the unsigned integer type</li> <li><a href=../../../api/basic_json/number_float_t/ ><code>number_float_t</code></a> - the floating-point type</li> <li><a href=../../../api/basic_json/is_number/ ><code>is_number</code></a> - check for a number of any kind</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../ class="md-footer__link md-footer__link--prev" aria-label="Previous: Types"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> Types </div> </div> </a> <a href=../template_parameters/ class="md-footer__link md-footer__link--next" aria-label="Next: Template Parameter Requirements"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> Template Parameter Requirements </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 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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> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../ >Types</a> - how all value types are stored</li> <li><a href=../../conversions/ >Converting values</a> - convert numbers to C++ types</li> <li><a href=../../../api/basic_json/number_integer_t/ ><code>number_integer_t</code></a> - the signed integer type</li> <li><a href=../../../api/basic_json/number_unsigned_t/ ><code>number_unsigned_t</code></a> - the unsigned integer type</li> <li><a href=../../../api/basic_json/number_float_t/ ><code>number_float_t</code></a> - the floating-point type</li> <li><a href=../../../api/basic_json/is_number/ ><code>is_number</code></a> - check for a number of any kind</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 27, 2026 06:41:42 UTC">October 27, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../ class="md-footer__link md-footer__link--prev" aria-label="Previous: Types"> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> Types </div> </div> </a> <a href=../template_parameters/ class="md-footer__link md-footer__link--next" aria-label="Next: Template Parameter Requirements"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> Template Parameter Requirements </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 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+1
View File
@@ -758,6 +758,7 @@ using array_t = ArrayType<basic_json, AllocatorType<basic_json>>;
| `#!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()` |
| [`basic_json_view::dump`](../../api/basic_json_view/dump.md) | `reserve(n)` and `resize(n)` (one argument); `capacity()` is **not** required |
### Compatible types
@@ -1,4 +1,4 @@
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class="md-header__button md-icon" for=__search> <svg xmlnsLine truncated
<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>&lt;</span><span class=n>std</span><span class=o>::</span><span class=n>pair</span><span class=o>&lt;</span><span class=k>const</span><span class=w> </span><span class=n>StringType</span><span class=p>,</span>
@@ -374,7 +374,7 @@
<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">&para;</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">&para;</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">&para;</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">&para;</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>strtold</span></code> (only used to parse a <code class=highlight><span class=kt>long</span><span class=w> </span><span class=kt>double</span></code> that is not <abbr title="Institute of Electrical and Electronics Engineers">IEEE</abbr> 754 binary64, see <a href=#numberfloattype><code>NumberFloatType</code></a>). <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 Line truncated
</code></pre></div> </details> <h3 id=compatible-containers_1>Compatible containers<a class=headerlink href=#compatible-containers_1 title="Permanent link">&para;</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">&para;</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">&para;</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">&para;</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>strtold</span></code> (only used to parse a <code class=highlight><span class=kt>long</span><span class=w> </span><span class=kt>double</span></code> that is not <abbr title="Institute of Electrical and Electronics Engineers">IEEE</abbr> 754 binary64, see <a href=#numberfloattype><code>NumberFloatType</code></a>). <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 Line truncated
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;cstddef&gt;</span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;ostream&gt;</span>
@@ -678,4 +678,4 @@
<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">&para;</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>&lt;</span><span class=n>std</span><span class=o>::</span><span class=kt>uint8_t</span><span class=o>&gt;</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>&lt;</span><span class=kt>char</span><span class=o>&gt;</span></code>, <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>vector</span><span class=o>&lt;</span><span class=n>std</span><span class=o>::</span><span class=kt>byte</span><span class=o>&gt;</span></code></td> <td><code>dump()</code> writes the bytes as 0..255 whichever is used</td> </tr> <tr> <td><code>boost::container::vector&lt;std::uint8_t&gt;</code>, <code>boost::container::small_vector&lt;std::uint8_t, N&gt;</code></td> <td></td> </tr> <tr> <td><code>absl::InlinedVector&lt;std::uint8_t, N&gt;</code></td> <td>usable here, unlike as an <code>ArrayType</code>, because the value type is complete</td> </tr> <tr> <td><code>eastl::vector&lt;std::uint8_t&gt;</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&lt;std::uint8_t&gt;</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">&para;</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>&lt;</span><span class=n>std</span><span class=o>::</span><span class=kt>uint8_t</span><span class=o>&gt;</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">&para;</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">&para;</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">&para;</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">&para;</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> </trLine truncated
</code></pre></div> </details> <h3 id=compatible-containers_2>Compatible containers<a class=headerlink href=#compatible-containers_2 title="Permanent link">&para;</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>&lt;</span><span class=n>std</span><span class=o>::</span><span class=kt>uint8_t</span><span class=o>&gt;</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>&lt;</span><span class=kt>char</span><span class=o>&gt;</span></code>, <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>vector</span><span class=o>&lt;</span><span class=n>std</span><span class=o>::</span><span class=kt>byte</span><span class=o>&gt;</span></code></td> <td><code>dump()</code> writes the bytes as 0..255 whichever is used</td> </tr> <tr> <td><code>boost::container::vector&lt;std::uint8_t&gt;</code>, <code>boost::container::small_vector&lt;std::uint8_t, N&gt;</code></td> <td></td> </tr> <tr> <td><code>absl::InlinedVector&lt;std::uint8_t, N&gt;</code></td> <td>usable here, unlike as an <code>ArrayType</code>, because the value type is complete</td> </tr> <tr> <td><code>eastl::vector&lt;std::uint8_t&gt;</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&lt;std::uint8_t&gt;</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">&para;</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>&lt;</span><span class=n>std</span><span class=o>::</span><span class=kt>uint8_t</span><span class=o>&gt;</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">&para;</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">&para;</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">&para;</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">&para;</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> </trLine truncated
@@ -639,6 +639,7 @@ true
| `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()` |
| [`basic_json_view::dump`](https://json.nlohmann.me/api/basic_json_view/dump/index.md) | `reserve(n)` and `resize(n)` (one argument); `capacity()` is **not** required |
### Compatible types