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Whether the JSON value which called the function during parsing should be kept (`#!cpp true`) or not (`#!cpp false`). In
the latter case, it is skipped completely, or replaced by `null` if it is the top-level value.
## Notes
For a `parse_event_t::key` event, `parsed` holds the decoded object name, with the same guarantees as the `val`
parameter of [`json_sax::key`](../json_sax/key.md#notes): all escape sequences are decoded and `\uXXXX` escapes are
stored as UTF-8, so different spellings of the same name yield identical strings; the name is validated completely
before the callback is called, so a name with an invalid escape sequence or ill-formed UTF-8 causes a parse error and
no `key` event; no Unicode normalization or case conversion is applied; and a name that occurs several times in the
same object yields one `key` event per occurrence.
## Examples
??? example "Example: skip an object key while parsing"
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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/api/basic_json/parser_callback_t/" rel="canonical"><link href=../parse_event_t/ rel=prev><link href=../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>parser_callback_t - 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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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> parser_callback_t </span> </div> </div> </div> <form class=md-header__option data-md-component=palette> <input class=md-option data-md-color-media="(prefers-color-scheme: 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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/api/basic_json/parser_callback_t/" rel="canonical"><link href=../parse_event_t/ rel=prev><link href=../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>parser_callback_t - 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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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> parser_callback_t </span> </div> </div> </div> <form class=md-header__option data-md-component=palette> <input class=md-option data-md-color-media="(prefers-color-scheme: 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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> <p>With a parser callback function, the result of parsing a <abbr title="JavaScript Object Notation">JSON</abbr> text can be influenced. When passed to <a href=../parse/ ><code>parse</code></a>, it is called on certain events (passed as <a href=../parse_event_t/ ><code>parse_event_t</code></a> via parameter <code>event</code>) with a set recursion depth <code>depth</code> and context <abbr title="JavaScript Object Notation">JSON</abbr> value <code>parsed</code>. The return value of the callback function is a boolean indicating whether the element that emitted the callback shall be kept or not.</p> <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 rLine truncated
</code></pre></div> <p>With a parser callback function, the result of parsing a <abbr title="JavaScript Object Notation">JSON</abbr> text can be influenced. When passed to <a href=../parse/ ><code>parse</code></a>, it is called on certain events (passed as <a href=../parse_event_t/ ><code>parse_event_t</code></a> via parameter <code>event</code>) with a set recursion depth <code>depth</code> and context <abbr title="JavaScript Object Notation">JSON</abbr> value <code>parsed</code>. The return value of the callback function is a boolean indicating whether the element that emitted the callback shall be kept or not.</p> <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 rLine truncated
<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>
@@ -135,4 +135,4 @@
<span class=p>{</span><span class=nt>"IDs"</span><span class=p>:[</span><span class=mi>116</span><span class=p>,</span><span class=mi>943</span><span class=p>]}</span>
<span class=p>{</span><span class=nt>"IDs"</span><span class=p>:[</span><span class=mi>116</span><span class=p>,</span><span class=mi>943</span><span class=p>]}</span>
<span class=kc>null</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=../parse/ ><code>parse</code></a> - deserialize from a compatible input</li> <li><a href=../parse_event_t/ ><code>parse_event_t</code></a> - enumeration of parser events</li> <li><a href=../../../features/parsing/parser_callbacks/ >Parser Callbacks</a> - filter or inspect values while parsing</li> </ul> <h2 id=version-history>Version history<a class=headerlink href=#version-history title="Permanent link">&para;</a></h2> <ul> <li>Added in version 1.0.0.</li> <li>Fixed 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> to also remove discarded values from a parent object; before, discarding an array or a value stored under an object key left a discarded member behind, which made the parse result serialize to invalid <abbr title="JavaScript Object Notation">JSON</abbr>.</li> <li>Fixed 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> so that discarding an array or object at its start event also hides its content from the callback, as documented above; before, the callback was still called for the content, and the key of every member of a discarded object was kept in memory until the parse ended.</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_event_t/ class="md-footer__link md-footer__link--prev" aria-label="Previous: parse_event_t"> <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> parse_event_t </div> </div> </a> <a href=../patch/ class="md-footer__link md-footer__link--next" aria-label="Next: patch"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> 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=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../parse/ ><code>parse</code></a> - deserialize from a compatible input</li> <li><a href=../parse_event_t/ ><code>parse_event_t</code></a> - enumeration of parser events</li> <li><a href=../../../features/parsing/parser_callbacks/ >Parser Callbacks</a> - filter or inspect values while parsing</li> </ul> <h2 id=version-history>Version history<a class=headerlink href=#version-history title="Permanent link">&para;</a></h2> <ul> <li>Added in version 1.0.0.</li> <li>Fixed 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> to also remove discarded values from a parent object; before, discarding an array or a value stored under an object key left a discarded member behind, which made the parse result serialize to invalid <abbr title="JavaScript Object Notation">JSON</abbr>.</li> <li>Fixed 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> so that discarding an array or object at its start event also hides its content from the callback, as documented above; before, the callback was still called for the content, and the key of every member of a discarded object was kept in memory until the parse ended.</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=../parse_event_t/ class="md-footer__link md-footer__link--prev" aria-label="Previous: parse_event_t"> <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> parse_event_t </div> </div> </a> <a href=../patch/ class="md-footer__link md-footer__link--next" aria-label="Next: patch"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> 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; 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2025 Fonticons, Inc.--><path d="M173.9 397.4c0 2-2.3 3.6-5.2 3.6-3.3.3-5.6-1.3-5.6-3.6 0-2 2.3-3.6 5.2-3.6 3-.3 5.6 1.3 5.6 3.6m-31.1-4.5c-.7 2 1.3 4.3 4.3 4.9 2.6 1 5.6 0 6.2-2s-1.3-4.3-4.3-5.2c-2.6-.7-5.5.3-6.2 2.3m44.2-1.7c-2.9.7-4.9 2.6-4.6 4.9.3 2 2.9 3.3 5.9 2.6 2.9-.7 4.9-2.6 4.6-4.6-.3-1.9-3-3.2-5.9-2.9M252.8 8C114.1 8 8 113.3 8 252c0 110.9 69.8 205.8 169.5 239.2 12.8 2.3 17.3-5.6 17.3-12.1 0-6.2-.3-40.4-.3-61.4 0 0-70 15-84.7-29.8 0 0-11.4-29.1-27.8-36.6 0 0-22.9-15.7 1.6-15.4 0 0 24.9 2 38.6 25.8 21.9 38.6 58.6 27Line truncated
@@ -40,6 +40,10 @@ Discarding a value (i.e., returning `false`) has different effects depending on
Whether the JSON value which called the function during parsing should be kept (`true`) or not (`false`). In the latter case, it is skipped completely, or replaced by `null` if it is the top-level value.
## Notes
For a `parse_event_t::key` event, `parsed` holds the decoded object name, with the same guarantees as the `val` parameter of [`json_sax::key`](https://json.nlohmann.me/api/json_sax/key/#notes): all escape sequences are decoded and `\uXXXX` escapes are stored as UTF-8, so different spellings of the same name yield identical strings; the name is validated completely before the callback is called, so a name with an invalid escape sequence or ill-formed UTF-8 causes a parse error and no `key` event; no Unicode normalization or case conversion is applied; and a name that occurs several times in the same object yields one `key` event per occurrence.
## Examples
Example: skip an object key while parsing
+21
View File
@@ -19,6 +19,23 @@ Whether parsing should proceed.
It is safe to move the passed object key value.
When parsing JSON text, `val` is the decoded object name, not its spelling in the input:
- All escape sequences (`\"`, `\\`, `\/`, `\b`, `\f`, `\n`, `\r`, `\t`, `\uXXXX`, and surrogate pairs) are
decoded, and `\uXXXX` escapes are stored as UTF-8. Different spellings of the same name, such as `#!json "/"`
and `#!json "\/"`, or `#!json "\u00e9"` and `#!json "\u00E9"`, yield identical strings. An escaped `\u0000` yields
an embedded NUL byte.
- The name is scanned and validated completely before `key` is called. If it contains an invalid escape sequence, an
unpaired surrogate, a control character, or ill-formed UTF-8, the parse fails with a
[`parse_error`](../../home/exceptions.md#parse-errors), and `key` is not called for it.
- No Unicode normalization or case conversion is applied: names that differ in any byte are different strings.
- `key` is called before the following `:` is read, so a parse error may still follow for the same member.
- A name that occurs several times in the same object is passed once per occurrence.
For the binary formats, object keys contain no escape sequences. For CBOR, MessagePack, UBJSON, and BSON, `val` holds
the bytes stored in the input, and [`sax_parse`](../basic_json/sax_parse.md) passes them unchanged, without checking them
for valid UTF-8. BON8 encodes characters itself, so an ill-formed name is a parse error and `key` is not called for it.
## Examples
??? example
@@ -213,3 +230,7 @@ It is safe to move the passed object key value.
## Version history
- Added in version 3.2.0.
- The behavior described in the notes above (decoded names, validation before `key` is called, no normalization, `key`
before the `:` check, one call per duplicate, bytes of binary formats passed unchanged) is the same in all versions
from 3.2.0 on, and for the `key` event of a [parser callback](../basic_json/parser_callback_t.md) from 3.0.0 on. BSON
is only supported from version 3.4.0, and BON8 only from version 3.13.0.
+2 -2
View File
@@ -1,5 +1,5 @@
<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=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/api/json_sax/key/" rel="canonical"><link href=../end_object/ rel=prev><link href=../null/ rel=next><link rel=icon href=../../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>key - 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=#nlohmannjson_saxkey 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> key </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
</code></pre></div> <p>An object key was read.</p> <h2 id=parameters>Parameters<a class=headerlink href=#parameters title="Permanent link">&para;</a></h2> <dl> <dt><code>val</code> (in)</dt> <dd>object key</dd> </dl> <h2 id=return-value>Return value<a class=headerlink href=#return-value title="Permanent link">&para;</a></h2> <p>Whether parsing should proceed.</p> <h2 id=notes>Notes<a class=headerlink href=#notes title="Permanent link">&para;</a></h2> <p>It is safe to move the passed object key value.</p> <h2 id=examples>Examples<a class=headerlink href=#examples title="Permanent link">&para;</a></h2> <details class=example> <summary>Example</summary> <p>The example below shows how the <abbr title="Simple API for XML">SAX</abbr> interface is used.</p> <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>An object key was read.</p> <h2 id=parameters>Parameters<a class=headerlink href=#parameters title="Permanent link">&para;</a></h2> <dl> <dt><code>val</code> (in)</dt> <dd>object key</dd> </dl> <h2 id=return-value>Return value<a class=headerlink href=#return-value title="Permanent link">&para;</a></h2> <p>Whether parsing should proceed.</p> <h2 id=notes>Notes<a class=headerlink href=#notes title="Permanent link">&para;</a></h2> <p>It is safe to move the passed object key value.</p> <p>When parsing <abbr title="JavaScript Object Notation">JSON</abbr> text, <code>val</code> is the decoded object name, not its spelling in the input:</p> <ul> <li>All escape sequences (<code>\"</code>, <code>\\</code>, <code>\/</code>, <code>\b</code>, <code>\f</code>, <code>\n</code>, <code>\r</code>, <code>\t</code>, <code>\uXXXX</code>, and surrogate pairs) are decoded, and <code>\uXXXX</code> escapes are stored as <abbr title="Unicode Transformation Format">UTF</abbr>-8. Different spellings of the same name, such as <code class=highlight><span class=s2>"/"</span></code> and <code class=highlight><span class=s2>"\/"</span></code>, or <code class=highlight><span class=s2>"\u00e9"</span></code> and <code class=highlight><span class=s2>"\u00E9"</span></code>, yield identical strings. An escaped <code>\u0000</code> yields an embedded <abbr title="Null character (U+0000, byte value 0)">NUL</abbr> byte.</li> <li>The name is scanned and validated completely before <code>key</code> is called. If it contains an invalid escape sequence, an unpaired surrogate, a control character, or ill-formed <abbr title="Unicode Transformation Format">UTF</abbr>-8, the parse fails with a <a href=../../../home/exceptions/#parse-errors><code>parse_error</code></a>, and <code>key</code> is not called for it.</li> <li>No Unicode normalization or case conversion is applied: names that differ in any byte are different strings.</li> <li><code>key</code> is called before the following <code>:</code> is read, so a parse error may still follow for the same member.</li> <li>A name that occurs several times in the same object is passed once per occurrence.</li> </ul> <p>For the binary formats, object keys contain no escape sequences. For <abbr title="Concise Binary Object Representation">CBOR</abbr>, MessagePack, <abbr title="Universal Binary JSON">UBJSON</abbr>, and <abbr title="Binary JSON">BSON</abbr>, <code>val</code> holds the bytes stored in the input, and <a href=../../basic_json/sax_parse/ ><code>sax_parse</code></a> passes them unchanged, without checking them for valid <abbr title="Unicode Transformation Format">UTF</abbr>-8. <abbr title="Binary Object Notation 8">BON8</abbr> encodes characters itself, so an ill-formed name is a parse error and <code>key</code> is not called for it.</p> <h2 id=examples>Examples<a class=headerlink href=#examples title="Permanent link">&para;</a></h2> <details class=example> <summary>Example</summary> <p>The example below shows how the <abbr title="Simple API for XML">SAX</abbr> interface is used.</p> <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;iomanip&gt;</span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;sstream&gt;</span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;nlohmann/json.hpp&gt;</span>
@@ -167,4 +167,4 @@
<span class=w> </span><span class=err>ex=</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>.parse_error.</span><span class=mi>101</span><span class=p>]</span><span class=w> </span><span class=err>parse</span><span class=w> </span><span class=err>error</span><span class=w> </span><span class=err>a</span><span class=kc>t</span><span class=w> </span><span class=err>li</span><span class=kc>ne</span><span class=w> </span><span class=mi>17</span><span class=p>,</span><span class=w> </span><span class=err>colum</span><span class=kc>n</span><span class=w> </span><span class=mi>6</span><span class=p>:</span><span class=w> </span><span class=err>sy</span><span class=kc>nta</span><span class=err>x</span><span class=w> </span><span class=err>error</span><span class=w> </span><span class=err>while</span><span class=w> </span><span class=err>parsi</span><span class=kc>n</span><span class=err>g</span><span class=w> </span><span class=err>value</span><span class=w> </span><span class=mi>-</span><span class=w> </span><span class=err>u</span><span class=kc>ne</span><span class=err>xpec</span><span class=kc>te</span><span class=err>d</span><span class=w> </span><span class=err>'</span><span class=p>]</span><span class=err>';</span><span class=w> </span><span class=err>expec</span><span class=kc>te</span><span class=err>d</span><span class=w> </span><span class=err>e</span><span class=kc>n</span><span class=err>d</span><span class=w> </span><span class=err>o</span><span class=kc>f</span><span class=w> </span><span class=err>i</span><span class=kc>n</span><span class=err>pu</span><span class=kc>t</span><span class=err>)</span>
<span class=err>resul</span><span class=kc>t</span><span class=p>:</span><span class=w> </span><span class=kc>false</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=../start_object/ ><code>start_object</code></a> - the beginning of an object was read</li> <li><a href=../end_object/ ><code>end_object</code></a> - the end of an object was read</li> <li><a href=../../basic_json/sax_parse/ ><code>sax_parse</code></a> - <abbr title="Simple API for XML">SAX</abbr> parser</li> <li><a href=../../../features/parsing/sax_interface/ ><abbr title="Simple API for XML">SAX</abbr> Interface</a> - guide to event-based parsing</li> </ul> <h2 id=version-history>Version history<a class=headerlink href=#version-history title="Permanent link">&para;</a></h2> <ul> <li>Added in version 3.2.0.</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=../end_object/ class="md-footer__link md-footer__link--prev" aria-label="Previous: end_object"> <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> end_object </div> </div> </a> <a href=../null/ class="md-footer__link md-footer__link--next" aria-label="Next: null"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> null </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=../start_object/ ><code>start_object</code></a> - the beginning of an object was read</li> <li><a href=../end_object/ ><code>end_object</code></a> - the end of an object was read</li> <li><a href=../../basic_json/sax_parse/ ><code>sax_parse</code></a> - <abbr title="Simple API for XML">SAX</abbr> parser</li> <li><a href=../../../features/parsing/sax_interface/ ><abbr title="Simple API for XML">SAX</abbr> Interface</a> - guide to event-based parsing</li> </ul> <h2 id=version-history>Version history<a class=headerlink href=#version-history title="Permanent link">&para;</a></h2> <ul> <li>Added in version 3.2.0.</li> <li>The behavior described in the notes above (decoded names, validation before <code>key</code> is called, no normalization, <code>key</code> before the <code>:</code> check, one call per duplicate, bytes of binary formats passed unchanged) is the same in all versions from 3.2.0 on, and for the <code>key</code> event of a <a href=../../basic_json/parser_callback_t/ >parser callback</a> from 3.0.0 on. <abbr title="Binary JSON">BSON</abbr> is only supported from version 3.4.0, and <abbr title="Binary Object Notation 8">BON8</abbr> only from 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 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=../end_object/ class="md-footer__link md-footer__link--prev" aria-label="Previous: end_object"> <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> end_object </div> </div> </a> <a href=../null/ class="md-footer__link md-footer__link--next" aria-label="Next: null"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> null </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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+11
View File
@@ -18,6 +18,16 @@ Whether parsing should proceed.
It is safe to move the passed object key value.
When parsing JSON text, `val` is the decoded object name, not its spelling in the input:
- All escape sequences (`\"`, `\\`, `\/`, `\b`, `\f`, `\n`, `\r`, `\t`, `\uXXXX`, and surrogate pairs) are decoded, and `\uXXXX` escapes are stored as UTF-8. Different spellings of the same name, such as `"/"` and `"\/"`, or `"\u00e9"` and `"\u00E9"`, yield identical strings. An escaped `\u0000` yields an embedded NUL byte.
- The name is scanned and validated completely before `key` is called. If it contains an invalid escape sequence, an unpaired surrogate, a control character, or ill-formed UTF-8, the parse fails with a [`parse_error`](https://json.nlohmann.me/home/exceptions/#parse-errors), and `key` is not called for it.
- No Unicode normalization or case conversion is applied: names that differ in any byte are different strings.
- `key` is called before the following `:` is read, so a parse error may still follow for the same member.
- A name that occurs several times in the same object is passed once per occurrence.
For the binary formats, object keys contain no escape sequences. For CBOR, MessagePack, UBJSON, and BSON, `val` holds the bytes stored in the input, and [`sax_parse`](https://json.nlohmann.me/api/basic_json/sax_parse/index.md) passes them unchanged, without checking them for valid UTF-8. BON8 encodes characters itself, so an ill-formed name is a parse error and `key` is not called for it.
## Examples
Example
@@ -210,3 +220,4 @@ result: false
## Version history
- Added in version 3.2.0.
- The behavior described in the notes above (decoded names, validation before `key` is called, no normalization, `key` before the `:` check, one call per duplicate, bytes of binary formats passed unchanged) is the same in all versions from 3.2.0 on, and for the `key` event of a [parser callback](https://json.nlohmann.me/api/basic_json/parser_callback_t/index.md) from 3.0.0 on. BSON is only supported from version 3.4.0, and BON8 only from version 3.13.0 unreleased.