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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-9.7.7"><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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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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0 24 24"><path d="M9.5 3A6.5 6.5 0 0 1 16 9.5c0 1.61-.59 3.09-1.56 4.23l.27.27h.79l5 5-1.5 1.5-5-5v-.79l-Line truncated
I["JSON input"] --&gt; P["parse()"]
I --&gt; S["sax_parse()"]
I --&gt; A["accept()"]
P --&gt;|"optional parser callback filters values"| D["basic_json value (DOM)"]
S --&gt; H["events delivered to a user SAX handler"]
A --&gt; V["bool: is the input valid JSON?"]</code></pre> <h2 id=input>Input<a class=headerlink href=#input title="Permanent link">&para;</a></h2> <p>The <a href=../../api/basic_json/parse/ ><code>parse</code></a> function reads a <abbr title="JavaScript Object Notation">JSON</abbr> value from an input. The input can be</p> <ul> <li>a string (<code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>string</span></code>, C string, or string literal),</li> <li>a wide string (<code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>u16string</span></code>, <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>u32string</span></code>, or <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>wstring</span></code>, or a wide string literal),</li> <li>a <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>istream</span></code> (e.g., an <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>ifstream</span></code> reading from a file),</li> <li>a <code class=highlight><span class=kt>FILE</span><span class=o>*</span></code> pointer,</li> <li>a pair of iterators over a contiguous range (e.g., a <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>vector</span><span class=o>&lt;</span><span class=n>std</span><span class=o>::</span><span class=kt>uint8_t</span><span class=o>&gt;</span></code>), or</li> <li>a contiguous container.</li> </ul> <div class=highlight><pre><span></span><code><span class=c1>// parse from a string</span>
A --&gt; V["bool: is the input valid JSON?"]</code></pre> <p class=nj-caption>The three entry points for parsing: <code>parse()</code> builds a <code>basic_json</code> value, optionally filtered by a parser callback, <code>sax_parse()</code> delivers events to a <abbr title="Simple API for XML">SAX</abbr> handler, and <code>accept()</code> only checks whether the input is valid.</p> <h2 id=input>Input<a class=headerlink href=#input title="Permanent link">&para;</a></h2> <p>The <a href=../../api/basic_json/parse/ ><code>parse</code></a> function reads a <abbr title="JavaScript Object Notation">JSON</abbr> value from an input. The input can be</p> <ul> <li>a string (<code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>string</span></code>, C string, or string literal),</li> <li>a wide string (<code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>u16string</span></code>, <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>u32string</span></code>, or <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>wstring</span></code>, or a wide string literal),</li> <li>a <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>istream</span></code> (e.g., an <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>ifstream</span></code> reading from a file),</li> <li>a <code class=highlight><span class=kt>FILE</span><span class=o>*</span></code> pointer,</li> <li>a pair of iterators over a contiguous range (e.g., a <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>vector</span><span class=o>&lt;</span><span class=n>std</span><span class=o>::</span><span class=kt>uint8_t</span><span class=o>&gt;</span></code>), or</li> <li>a contiguous container.</li> </ul> <div class=highlight><pre><span></span><code><span class=c1>// parse from a string</span>
<span class=n>json</span><span class=w> </span><span class=n>j</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>parse</span><span class=p>(</span><span class=sa>R</span><span class=s>"</span><span class=dl>(</span><span class=s>{"happy": true, "pi": 3.141}</span><span class=dl>)</span><span class=s>"</span><span class=p>);</span>
<span class=c1>// parse from a file</span>
<span class=n>std</span><span class=o>::</span><span class=n>ifstream</span><span class=w> </span><span class=n>f</span><span class=p>(</span><span class=s>"example.json"</span><span class=p>);</span>
<span class=n>json</span><span class=w> </span><span class=n>data</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>parse</span><span class=p>(</span><span class=n>f</span><span class=p>);</span>
</code></pre></div> <p>The input must be encoded in <abbr title="Unicode Transformation Format">UTF</abbr>-8; other encodings are not supported. The exception is wide-character input, which is read as <abbr title="Unicode Transformation Format">UTF</abbr>-16 or <abbr title="Unicode Transformation Format">UTF</abbr>-32 -- depending on the character type -- and converted to <abbr title="Unicode Transformation Format">UTF</abbr>-8; see the <a href=../../api/basic_json/parse/ ><code>parse</code></a> page for an example. A single input may contain only one <abbr title="JavaScript Object Notation">JSON</abbr> value. Inputs consisting of multiple values separated by newlines are handled by the <a href=json_lines/ ><abbr title="JavaScript Object Notation">JSON</abbr> Lines</a> format; input that arrives in chunks or contains several values is covered in <a href=large_and_partial_input/ >Large, streamed, and partial input</a>.</p> <p>By default, the library rejects comments and trailing commas. Both can be enabled with parameters of the <code>parse</code> function — see <a href=../comments/ >comments</a> and <a href=../trailing_commas/ >trailing commas</a>.</p> <p><a href=../../api/basic_json_document/parse/ ><code>basic_json_document::parse</code></a> accepts the same inputs with the same options, but keeps the result in a document rather than in a <code>basic_json</code> value; see <a href=#documents-and-views>Documents and views</a> below.</p> <h2 id=strictness-and-trailing-data>Strictness and trailing data<a class=headerlink href=#strictness-and-trailing-data title="Permanent link">&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;</spLine truncated
</code></pre></div> <p>The input must be encoded in <abbr title="Unicode Transformation Format">UTF</abbr>-8; other encodings are not supported. The exception is wide-character input, which is read as <abbr title="Unicode Transformation Format">UTF</abbr>-16 or <abbr title="Unicode Transformation Format">UTF</abbr>-32 — depending on the character type — and converted to <abbr title="Unicode Transformation Format">UTF</abbr>-8; see the <a href=../../api/basic_json/parse/ ><code>parse</code></a> page for an example. A single input may contain only one <abbr title="JavaScript Object Notation">JSON</abbr> value. Inputs consisting of multiple values separated by newlines are handled by the <a href=json_lines/ ><abbr title="JavaScript Object Notation">JSON</abbr> Lines</a> format; input that arrives in chunks or contains several values is covered in <a href=large_and_partial_input/ >Large, streamed, and partial input</a>.</p> <p>By default, the library rejects comments and trailing commas. Both can be enabled with parameters of the <code>parse</code> function — see <a href=../comments/ >comments</a> and <a href=../trailing_commas/ >trailing commas</a>.</p> <p><a href=../../api/basic_json_document/parse/ ><code>basic_json_document::parse</code></a> accepts the same inputs with the same options, but keeps the result in a document rather than in a <code>basic_json</code> value; see <a href=#documents-and-views>Documents and views</a> below.</p> <h2 id=strictness-and-trailing-data>Strictness and trailing data<a class=headerlink href=#strictness-and-trailing-data title="Permanent link">&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
<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=../json_view/ >Zero-copy <abbr title="JavaScript Object Notation">JSON</abbr> views</a> - parse into a document and read it through a view</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 25, 2026 11:07:13 UTC">October 25, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &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 1Line truncated
</code></pre></div> <p>See <a href=../json_view/ >Zero-copy <abbr title="JavaScript Object Notation">JSON</abbr> views</a> for the concept and the trade-offs.</p> <h2 id=exceptions>Exceptions<a class=headerlink href=#exceptions title="Permanent link">&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
+11 -2
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@@ -12,6 +12,8 @@ flowchart LR
A --> V["bool: is the input valid JSON?"]
```
The three entry points for parsing: `parse()` builds a `basic_json` value, optionally filtered by a parser callback, `sax_parse()` delivers events to a SAX handler, and `accept()` only checks whether the input is valid.
## Input
The [`parse`](https://json.nlohmann.me/api/basic_json/parse/index.md) function reads a JSON value from an input. The input can be
@@ -32,7 +34,7 @@ std::ifstream f("example.json");
json data = json::parse(f);
```
The input must be encoded in UTF-8; other encodings are not supported. The exception is wide-character input, which is read as UTF-16 or UTF-32 -- depending on the character type -- and converted to UTF-8; see the [`parse`](https://json.nlohmann.me/api/basic_json/parse/index.md) page for an example. A single input may contain only one JSON value. Inputs consisting of multiple values separated by newlines are handled by the [JSON Lines](https://json.nlohmann.me/features/parsing/json_lines/index.md) format; input that arrives in chunks or contains several values is covered in [Large, streamed, and partial input](https://json.nlohmann.me/features/parsing/large_and_partial_input/index.md).
The input must be encoded in UTF-8; other encodings are not supported. The exception is wide-character input, which is read as UTF-16 or UTF-32 — depending on the character type — and converted to UTF-8; see the [`parse`](https://json.nlohmann.me/api/basic_json/parse/index.md) page for an example. A single input may contain only one JSON value. Inputs consisting of multiple values separated by newlines are handled by the [JSON Lines](https://json.nlohmann.me/features/parsing/json_lines/index.md) format; input that arrives in chunks or contains several values is covered in [Large, streamed, and partial input](https://json.nlohmann.me/features/parsing/large_and_partial_input/index.md).
By default, the library rejects comments and trailing commas. Both can be enabled with parameters of the `parse` function — see [comments](https://json.nlohmann.me/features/comments/index.md) and [trailing commas](https://json.nlohmann.me/features/trailing_commas/index.md).
@@ -103,4 +105,11 @@ When the input is not valid JSON, the `parse` function throws an exception by de
- [Zero-copy JSON views](https://json.nlohmann.me/features/json_view/index.md) - parse into a read-only document and read it through views
- [parsing and exceptions](https://json.nlohmann.me/features/parsing/parse_exceptions/index.md) - control error handling
- [parsing untrusted input](https://json.nlohmann.me/features/parsing/untrusted_input/index.md) - what to consider when parsing input from untrusted sources
- [Zero-copy JSON views](https://json.nlohmann.me/features/json_view/index.md) - parse into a document and read it through a view
- [`operator>>`](https://json.nlohmann.me/api/operator_gtgt/index.md) - deserialize from stream
- [`json_sax`](https://json.nlohmann.me/api/json_sax/index.md) - documentation of the SAX interface
- [`parse_error`](https://json.nlohmann.me/api/basic_json/parse_error/index.md) - exception thrown on a parse error
- [Comments](https://json.nlohmann.me/features/comments/index.md) - ignore `//` and `/* */` comments while parsing
- [Trailing Commas](https://json.nlohmann.me/features/trailing_commas/index.md) - accept trailing commas in arrays and objects
- [Serialization](https://json.nlohmann.me/features/serialization/index.md) - pretty-printing, UTF-8 handling, and number output
- [Working Without Exceptions](https://json.nlohmann.me/features/no_exceptions/index.md) - error handling when exceptions are disabled
- [Exceptions](https://json.nlohmann.me/home/exceptions/index.md) - list of exceptions and error codes
+8 -1
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@@ -87,5 +87,12 @@ JSON Lines input with more than one value is treated as invalid JSON by the [`pa
}
```
A value that is a number must be followed by whitespace -- see the [notes](../../api/operator_gtgt.md#notes) of
A value that is a number must be followed by whitespace — see the [notes](../../api/operator_gtgt.md#notes) of
`operator>>` for details.
## See also
- [Parsing](index.md) - overview of the parsing functions
- [Large, Streamed, and Partial Input](large_and_partial_input.md) - chunked, concatenated, and very large input
- [`parse`](../../api/basic_json/parse.md) - deserialize from a compatible input
- [`operator>>`](../../api/operator_gtgt.md) - deserialize from stream
+2 -2
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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=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/parsing/json_lines/" rel="canonical"><link href=../ rel=prev><link href=../large_and_partial_input/ rel=next><link rel=icon href=../../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>JSON Lines - JSON for Modern C++</title><link rel=stylesheet href=../../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../../assets/stylesheets/palette.ab4e12ef.min.css><link rel="stylesheet" href="../../../assets/external/fonts.googleapis.com/css.61a430c9.css"><style>:root{--md-text-font:"Roboto";--md-code-font:"JetBrains Mono"}</style><link rel=stylesheet href=../../../css/custom.css><script>__md_scope=new 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<span class=p>{</span><span class=nt>"name"</span><span class=p>:</span><span class=w> </span><span class=s2>"Alexa"</span><span class=p>,</span><span class=w> </span><span class=nt>"wins"</span><span class=p>:</span><span class=w> </span><span class=p>[[</span><span class=s2>"two pair"</span><span class=p>,</span><span class=w> </span><span class=s2>"4♠"</span><span class=p>],</span><span class=w> </span><span class=p>[</span><span class=s2>"two pair"</span><span class=p>,</span><span class=w> </span><span class=s2>"9♠"</span><span class=p>]]}</span>
<span class=p>{</span><span class=nt>"name"</span><span class=p>:</span><span class=w> </span><span class=s2>"May"</span><span class=p>,</span><span class=w> </span><span class=nt>"wins"</span><span class=p>:</span><span class=w> </span><span class=p>[]}</span>
<span class=p>{</span><span class=nt>"name"</span><span class=p>:</span><span class=w> </span><span class=s2>"Deloise"</span><span class=p>,</span><span class=w> </span><span class=nt>"wins"</span><span class=p>:</span><span class=w> </span><span class=p>[[</span><span class=s2>"three of a kind"</span><span class=p>,</span><span class=w> </span><span class=s2>"5♣"</span><span class=p>]]}</span>
@@ -39,4 +39,4 @@
<span class=w> </span><span class=n>input</span><span class=w> </span><span class=o>&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> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 2, 2026 09:32:15 UTC">October 2, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &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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+8 -1
View File
@@ -79,4 +79,11 @@ while (input >> std::ws && input.peek() != std::char_traits<char>::eof())
}
```
A value that is a number must be followed by whitespace -- see the [notes](https://json.nlohmann.me/api/operator_gtgt/#notes) of `operator>>` for details.
A value that is a number must be followed by whitespace — see the [notes](https://json.nlohmann.me/api/operator_gtgt/#notes) of `operator>>` for details.
## See also
- [Parsing](https://json.nlohmann.me/features/parsing/index.md) - overview of the parsing functions
- [Large, Streamed, and Partial Input](https://json.nlohmann.me/features/parsing/large_and_partial_input/index.md) - chunked, concatenated, and very large input
- [`parse`](https://json.nlohmann.me/api/basic_json/parse/index.md) - deserialize from a compatible input
- [`operator>>`](https://json.nlohmann.me/api/operator_gtgt/index.md) - deserialize from stream
@@ -17,6 +17,8 @@ flowchart TD
H -->|"yes"| I["stop early with SAX, or read through a view"]
```
<p class="nj-caption" markdown>Questions about the input and the approach this page describes for each; an input can match more than one question.</p>
## Several values in one stream
For newline-delimited input, read line by line and parse each line, as described in [JSON Lines](json_lines.md). For
@@ -182,6 +184,64 @@ exception is one past the buffer size in the first case:
The example feeds a document in five chunks. The first four are reported as incomplete, and a syntax error is reported as
such, not as missing data.
The chunks of the example, and the bytes each attempt parses again:
<div class="nj-fig-wrap" markdown>
<svg class="nj-fig nj-fig--sax-buf" viewBox="0 0 700 442" role="img" aria-labelledby="fig-sax-buf-title fig-sax-buf-desc" xmlns="http://www.w3.org/2000/svg">
<title id="fig-sax-buf-title">Buffering chunks until the value is complete: five chunks of {&quot;name&quot;: &quot;Mary&quot;, &quot;list&quot;: [1, 2, 3]} appended to a buffer.</title>
<desc id="fig-sax-buf-desc">After chunks 1 to 4 the buffer has 4, 12, 19, and 30 bytes, and parsing it fails at byte 5, 13, 20, and 31: one past the end, so the input is incomplete and the program waits for more. After chunk 5 the buffer of 35 bytes parses as one complete value. For the input [1, ] of 5 bytes the error is at byte 5, before the end, so it is a real syntax error. Every attempt parses the buffer from byte 1, so 4 + 12 + 19 + 30 + 35 = 100 bytes are parsed for a 35-byte value, and the work grows quadratically.</desc>
<text class="fg-h" x="16" y="24">chunk</text>
<text class="fg-h" x="96" y="24">buffer, parsed from byte 1 each time</text>
<text class="fg-h" x="420" y="24">result</text>
<text class="fg-note fg-note--l" x="16" y="67">chunk 1</text>
<rect class="nj-sax-new" x="96" y="50" width="36.6" height="24" rx="4"/>
<text class="nj-sax-buf" x="96" y="67" xml:space="preserve">{&quot;na</text>
<path class="nj-sax-caret nj-sax-s-wait" d="M133.8 76 l-5 8 h10 z"/>
<text class="nj-sax-status nj-sax-s-wait" x="420" y="61">error at byte 5 = size + 1</text>
<text class="fg-note fg-note--l" x="420" y="77">incomplete: wait for more</text>
<text class="fg-note fg-note--l" x="16" y="117">chunk 2</text>
<rect class="nj-sax-old" x="93" y="100" width="36.6" height="24" rx="4"/>
<rect class="nj-sax-new" x="129.6" y="100" width="70.2" height="24" rx="4"/>
<text class="nj-sax-buf" x="96" y="117" xml:space="preserve">{&quot;name&quot;: &quot;Ma</text>
<path class="nj-sax-caret nj-sax-s-wait" d="M201 126 l-5 8 h10 z"/>
<text class="nj-sax-status nj-sax-s-wait" x="420" y="111">error at byte 13 = size + 1</text>
<text class="fg-note fg-note--l" x="420" y="127">incomplete: wait for more</text>
<text class="fg-note fg-note--l" x="16" y="167">chunk 3</text>
<rect class="nj-sax-old" x="93" y="150" width="103.8" height="24" rx="4"/>
<rect class="nj-sax-new" x="196.8" y="150" width="61.8" height="24" rx="4"/>
<text class="nj-sax-buf" x="96" y="167" xml:space="preserve">{&quot;name&quot;: &quot;Mary&quot;, &quot;l</text>
<path class="nj-sax-caret nj-sax-s-wait" d="M259.8 176 l-5 8 h10 z"/>
<text class="nj-sax-status nj-sax-s-wait" x="420" y="161">error at byte 20 = size + 1</text>
<text class="fg-note fg-note--l" x="420" y="177">incomplete: wait for more</text>
<text class="fg-note fg-note--l" x="16" y="217">chunk 4</text>
<rect class="nj-sax-old" x="93" y="200" width="162.6" height="24" rx="4"/>
<rect class="nj-sax-new" x="255.6" y="200" width="95.4" height="24" rx="4"/>
<text class="nj-sax-buf" x="96" y="217" xml:space="preserve">{&quot;name&quot;: &quot;Mary&quot;, &quot;list&quot;: [1, 2</text>
<path class="nj-sax-caret nj-sax-s-wait" d="M352.2 226 l-5 8 h10 z"/>
<text class="nj-sax-status nj-sax-s-wait" x="420" y="211">error at byte 31 = size + 1</text>
<text class="fg-note fg-note--l" x="420" y="227">incomplete: wait for more</text>
<text class="fg-note fg-note--l" x="16" y="267">chunk 5</text>
<rect class="nj-sax-old" x="93" y="250" width="255" height="24" rx="4"/>
<rect class="nj-sax-new" x="348" y="250" width="45" height="24" rx="4"/>
<text class="nj-sax-buf" x="96" y="267" xml:space="preserve">{&quot;name&quot;: &quot;Mary&quot;, &quot;list&quot;: [1, 2, 3]}</text>
<text class="nj-sax-status nj-sax-s-ok" x="420" y="261">parsed, size 35</text>
<text class="fg-note fg-note--l" x="420" y="277">complete: one value</text>
<path class="nj-sax-sep" d="M16 290 H684"/>
<text class="fg-note fg-note--l" x="16" y="325">other input</text>
<rect class="nj-sax-new" x="96" y="308" width="45" height="24" rx="4"/>
<text class="nj-sax-buf" x="96" y="325" xml:space="preserve">[1, ]</text>
<path class="nj-sax-caret nj-sax-s-err" d="M133.8 334 l-5 8 h10 z"/>
<text class="nj-sax-status nj-sax-s-err" x="420" y="319">error at byte 5, size 5</text>
<text class="fg-note fg-note--l" x="420" y="335">before the end: a real error</text>
<text class="fg-note fg-note--l" x="16" y="366">Bytes parsed: 4 + 12 + 19 + 30 + 35 = 100 for a value of 35 bytes.</text>
<text class="fg-note fg-note--l" x="16" y="386">Every attempt starts again at byte 1, so the total work grows</text>
<text class="fg-note fg-note--l" x="16" y="406">quadratically with the size of the value.</text>
</svg>
</div>
<p class="nj-caption" markdown>Each row is one parse attempt on the growing buffer: an error at the byte after the end of the buffer means the value is incomplete, an error before the end is a real syntax error. The last line adds up the bytes parsed over all attempts.</p>
!!! warning "Limits of buffering"
- The buffer must contain exactly one value. Data that belongs to the next value is a syntax error. Use framing if
@@ -195,6 +255,34 @@ such, not as missing data.
The input does not have to become one huge `json` value. The two tools below process it piece by piece.
How the memory use develops while a large array of records is parsed (illustrative, not a measurement):
<div class="nj-fig-wrap" markdown>
<svg class="nj-fig nj-fig--sax-mem" viewBox="0 0 660 312" role="img" aria-labelledby="fig-sax-mem-title fig-sax-mem-desc" xmlns="http://www.w3.org/2000/svg">
<title id="fig-sax-mem-title">Illustrative memory use over time while a large array of records is parsed, not a measurement.</title>
<desc id="fig-sax-mem-desc">Four curves. D&#8203;OM, parse(): rises steadily until the whole tree is in memory. json_document: starts at about the size of the text, which it keeps, and grows a little while its index is built. Parser callback that discards every record: stays flat at the size of one record. S&#8203;AX: stays flat and lowest, because no value is built and only what the handler keeps is in memory.</desc>
<path class="nj-sax-axis" d="M56 36 V272 H448"/>
<text class="fg-h" x="16" y="24">memory</text>
<text class="fg-note fg-note--l" x="56" y="294">time: records parsed</text>
<text class="fg-note fg-note--r nj-sax-illus" x="648" y="24">illustrative, not a measurement</text>
<path class="nj-sax-curve nj-sax-c-dom" d="M56 268 L440 52"/>
<path class="nj-sax-curve nj-sax-c-doc" d="M56 168 L440 146"/>
<path class="nj-sax-curve nj-sax-c-cb" d="M56 250 L88 238 L88 250 L120 238 L120 250 L152 238 L152 250 L184 238 L184 250 L216 238 L216 250 L248 238 L248 250 L280 238 L280 250 L312 238 L312 250 L344 238 L344 250 L376 238 L376 250 L408 238 L408 250 L440 238 L440 250"/>
<path class="nj-sax-curve nj-sax-c-sax" d="M56 264 H440"/>
<text class="nj-sax-lab nj-sax-c-dom" x="456" y="48">D&#8203;OM: parse()</text>
<text class="fg-note fg-note--l" x="456" y="65">the whole tree</text>
<text class="nj-sax-lab nj-sax-c-doc" x="456" y="142">json_document</text>
<text class="fg-note fg-note--l" x="456" y="159">the text plus its index</text>
<text class="nj-sax-lab nj-sax-c-cb" x="456" y="227">callback, discard</text>
<text class="fg-note fg-note--l" x="456" y="244">one record at a time</text>
<text class="nj-sax-lab nj-sax-c-sax" x="456" y="269">S&#8203;AX</text>
<text class="fg-note fg-note--l" x="456" y="286">only what the handler keeps</text>
</svg>
</div>
<p class="nj-caption" markdown>Memory use over the number of records parsed: `parse()` keeps the whole tree, `json_document` the text plus its index, a discarding callback one record at a time, and SAX only what the handler keeps.</p>
### Handle and discard elements with a callback
A [parser callback](parser_callbacks.md) is called for every finished value. If it returns `#!cpp false`, the value is
@@ -368,3 +456,8 @@ and gives read-only [views](../../api/basic_json_view/index.md) of the parts you
- [SAX interface](sax_interface.md) - event-based parsing
- [Zero-copy JSON views](../json_view.md) - read parts of a large document without building a tree
- [Untrusted input](untrusted_input.md) - limits for input you do not control
- [`operator>>`](../../api/operator_gtgt.md) - deserialize from stream
- [`parse`](../../api/basic_json/parse.md) - deserialize from a compatible input
- [`sax_parse`](../../api/basic_json/sax_parse.md) - generate SAX events
- [`JSON_PRECISE_STREAM_POSITION`](../../api/macros/json_precise_stream_position.md) - leave the stream positioned right after a parsed number
- [`JSON_DISABLE_STREAM_BULK_SCAN`](../../api/macros/json_disable_stream_bulk_scan.md) - read a `std::istream` one character at a time
@@ -1,4 +1,4 @@
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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)?"}
@@ -6,7 +6,7 @@
A --&gt; F{"too large to keep in memory?"}
F --&gt;|"yes"| G["discard parts with a callback, or use SAX"]
A --&gt; H{"only a small part is needed?"}
H --&gt;|"yes"| I["stop early with SAX, or read through a view"]</code></pre> <h2 id=several-values-in-one-stream>Several values in one stream<a class=headerlink href=#several-values-in-one-stream title="Permanent link">&para;</a></h2> <p>For newline-delimited input, read line by line and parse each line, as described in <a href=../json_lines/ ><abbr title="JavaScript Object Notation">JSON</abbr> Lines</a>. For values that are simply concatenated, <a href=../../../api/operator_gtgt/ ><code>operator&gt;&gt;</code></a> reads one value and leaves the stream positioned after it. Check for the end of the stream before each read, because a read at the end throws:</p> <details class=example> <summary>Example: read concatenated values</summary> <div class=highlight><pre><span></span><code><span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;iostream&gt;</span>
H --&gt;|"yes"| I["stop early with SAX, or read through a view"]</code></pre> <p class=nj-caption>Questions about the input and the approach this page describes for each; an input can match more than one question.</p> <h2 id=several-values-in-one-stream>Several values in one stream<a class=headerlink href=#several-values-in-one-stream title="Permanent link">&para;</a></h2> <p>For newline-delimited input, read line by line and parse each line, as described in <a href=../json_lines/ ><abbr title="JavaScript Object Notation">JSON</abbr> Lines</a>. For values that are simply concatenated, <a href=../../../api/operator_gtgt/ ><code>operator&gt;&gt;</code></a> reads one value and leaves the stream positioned after it. Check for the end of the stream before each read, because a read at the end throws:</p> <details class=example> <summary>Example: read concatenated values</summary> <div class=highlight><pre><span></span><code><span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;iostream&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>
@@ -108,7 +108,7 @@ chunk ist": [1, 2 -&gt; incomplete
chunk , 3]} -&gt; complete
{"list":[1,2,3],"name":"Mary"}
[json.exception.parse_error.101] parse error at line 1, column 5: syntax error while parsing value - unexpected ']'; expected '[', '{', or a literal
</code></pre></div> </details> <p>The example feeds a document in five chunks. The first four are reported as incomplete, and a syntax error is reported as such, not as missing data.</p> <div class="admonition warning"> <p class=admonition-title>Limits of buffering</p> <ul> <li>The buffer must contain exactly one value. Data that belongs to the next value is a syntax error. Use framing if values follow each other without a separator.</li> <li>A number at the end of the buffer is a complete value, even if more digits are still to come (<code>12</code> may be the start of <code>123</code>). Buffering cannot detect this; use framing for top-level numbers.</li> <li>Every attempt parses the buffer from the beginning, so the total work grows quadratically with the size of the value. Limit the size of the buffer for input you do not trust (see <a href=../untrusted_input/ >untrusted input</a>).</li> </ul> </div> <h2 id=input-too-large-for-memory>Input too large for memory<a class=headerlink href=#input-too-large-for-memory title="Permanent link">&para;</a></h2> <p>The input does not have to become one huge <code>json</code> value. The two tools below process it piece by piece.</p> <h3 id=handle-and-discard-elements-with-a-callback>Handle and discard elements with a callback<a class=headerlink href=#handle-and-discard-elements-with-a-callback title="Permanent link">&para;</a></h3> <p>A <a href=../parser_callbacks/ >parser callback</a> is called for every finished value. If it returns <code class=highlight><span class=nb>false</span></code>, the value is discarded and its memory is released at once. For a file that consists of one huge array (or one huge object) of records, handle each record at depth 1 and discard it:</p> <details class=example> <summary>Example: handle and discard each record of a large array</summary> <div class=highlight><pre><span></span><code><span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;iostream&gt;</span>
</code></pre></div> </details> <p>The example feeds a document in five chunks. The first four are reported as incomplete, and a syntax error is reported as such, not as missing data.</p> <p>The chunks of the example, and the bytes each attempt parses again:</p> <div class=nj-fig-wrap> <p><svg class="nj-fig nj-fig--sax-buf" viewbox="0 0 700 442" role=img aria-labelledby="fig-sax-buf-title fig-sax-buf-desc" xmlns=http://www.w3.org/2000/svg> <title id=fig-sax-buf-title>Buffering chunks until the value is complete: five chunks of {&quot;name&quot;: &quot;Mary&quot;, &quot;list&quot;: [1, 2, 3]} appended to a buffer.</title> <desc id=fig-sax-buf-desc>After chunks 1 to 4 the buffer has 4, 12, 19, and 30 bytes, and parsing it fails at byte 5, 13, 20, and 31: one past the end, so the input is incomplete and the program waits for more. After chunk 5 the buffer of 35 bytes parses as one complete value. For the input [1, ] of 5 bytes the error is at byte 5, before the end, so it is a real syntax error. Every attempt parses the buffer from byte 1, so 4 + 12 + 19 + 30 + 35 = 100 bytes are parsed for a 35-byte value, and the work grows quadratically.</desc> <text class=fg-h x=16 y=24>chunk</text> <text class=fg-h x=96 y=24>buffer, parsed from byte 1 each time</text> <text class=fg-h x=420 y=24>result</text> <text class="fg-note fg-note--l" x=16 y=67>chunk 1</text> <rect class=nj-sax-new x=96 y=50 width=36.6 height=24 rx=4 /> <text class=nj-sax-buf x=96 y=67 xml:space=preserve>{&quot;na</text> <path class="nj-sax-caret nj-sax-s-wait" d="M133.8 76 l-5 8 h10 z"/> <text class="nj-sax-status nj-sax-s-wait" x=420 y=61>error at byte 5 = size + 1</text> <text class="fg-note fg-note--l" x=420 y=77>incomplete: wait for more</text> <text class="fg-note fg-note--l" x=16 y=117>chunk 2</text> <rect class=nj-sax-old x=93 y=100 width=36.6 height=24 rx=4 /> <rect class=nj-sax-new x=129.6 y=100 width=70.2 height=24 rx=4 /> <text class=nj-sax-buf x=96 y=117 xml:space=preserve>{&quot;name&quot;: &quot;Ma</text> <path class="nj-sax-caret nj-sax-s-wait" d="M201 126 l-5 8 h10 z"/> <text class="nj-sax-status nj-sax-s-wait" x=420 y=111>error at byte 13 = size + 1</text> <text class="fg-note fg-note--l" x=420 y=127>incomplete: wait for more</text> <text class="fg-note fg-note--l" x=16 y=167>chunk 3</text> <rect class=nj-sax-old x=93 y=150 width=103.8 height=24 rx=4 /> <rect class=nj-sax-new x=196.8 y=150 width=61.8 height=24 rx=4 /> <text class=nj-sax-buf x=96 y=167 xml:space=preserve>{&quot;name&quot;: &quot;Mary&quot;, &quot;l</text> <path class="nj-sax-caret nj-sax-s-wait" d="M259.8 176 l-5 8 h10 z"/> <text class="nj-sax-status nj-sax-s-wait" x=420 y=161>error at byte 20 = size + 1</text> <text class="fg-note fg-note--l" x=420 y=177>incomplete: wait for more</text> <text class="fg-note fg-note--l" x=16 y=217>chunk 4</text> <rect class=nj-sax-old x=93 y=200 width=162.6 height=24 rx=4 /> <rect class=nj-sax-new x=255.6 y=200 width=95.4 height=24 rx=4 /> <text class=nj-sax-buf x=96 y=217 xml:space=preserve>{&quot;name&quot;: &quot;Mary&quot;, &quot;list&quot;: [1, 2</text> <path class="nj-sax-caret nj-sax-s-wait" d="M352.2 226 l-5 8 h10 z"/> <text class="nj-sax-status nj-sax-s-wait" x=420 y=211>error at byte 31 = size + 1</text> <text class="fg-note fg-note--l" x=420 y=227>incomplete: wait for more</text> <text class="fg-note fg-note--l" x=16 y=267>chunk 5</text> <rect class=nj-sax-old x=93 y=250 width=255 height=24 rx=4 /> <rect class=nj-sax-new x=348 y=250 width=45 height=24 rx=4 /> <text class=nj-sax-buf x=96 y=267 xml:space=preserve>{&quot;name&quot;: &quot;Mary&quot;, &quot;list&quot;: [1, 2, 3]}</text> <text class="nj-sax-status nj-sax-s-ok" x=420 y=261>parsed, size 35</text> <text class="fg-note fg-note--l" x=420 y=277>complete: one value</text> <path class=nj-sax-sep d="M16 290 H684"/> <text class="fg-note fg-note--l" x=16 y=325>other input</text> <rect class=nj-sax-new x=96 y=308 width=45 height=24 rx=4 /> <text class=nj-sax-buf x=96 y=325 xml:space=preserve>[1, ]</text> <path class="nj-sax-caret nj-sax-s-err" d="M133.8 334 l-5 8 h10 z"/> <text class="nj-sax-status nj-sax-s-err" x=420 y=319>error at byte 5, size 5</text> <text class="fg-note fg-note--l" x=420 y=335>before the end: a real error</text> <text class="fg-note fg-note--l" x=16 y=366>Bytes parsed: 4 + 12 + 19 + 30 + 35 = 100 for a value of 35 bytes.</text> <text class="fg-note fg-note--l" x=16 y=386>Every attempt starts again at byte 1, so the total work grows</text> <text class="fg-note fg-note--l" x=16 y=406>quadratically with the size of the value.</text> </svg></p> </div> <p class=nj-caption>Each row is one parse attempt on the growing buffer: an error at the byte after the end of the buffer means the value is incomplete, an error before the end is a real syntax error. The last line adds up the bytes parsed over all attempts.</p> <div class="admonition warning"> <p class=admonition-title>Limits of buffering</p> <ul> <li>The buffer must contain exactly one value. Data that bLine truncated
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;sstream&gt;</span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;nlohmann/json.hpp&gt;</span>
@@ -222,4 +222,4 @@ result: []
<span class=p>}</span>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code>parse completed: false
id: 42
</code></pre></div> </details> <p>Note that <code>sax_parse</code> returns <code class=highlight><span class=nb>false</span></code> both when a handler stopped the parse and when the input is invalid, so record the reason in the handler (here: <code>id</code>) if you need to tell the two apart.</p> <h3 id=read-parts-of-a-large-document-with-a-view>Read parts of a large document with a view<a class=headerlink href=#read-parts-of-a-large-document-with-a-view title="Permanent link">&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> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 25, 2026 11:07:13 UTC">October 25, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &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 27.5 72.9 20.9 2.3-16 8.8-27.1 16-33.7-55.9-6.2-112.3-14.3-112.3-110.5 0-27.5 7.6-41.3 23.6-58.9-2.6-6.5-11.1-33.3 2.6-67.9 20.9-6.5 69 27 69 27 20-5.6 41.5-8.5 62.8-8.5s42.8 2.9 62.8 8.5c0 0 48.1-33.6 69-27 13.Line truncated
</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
@@ -14,6 +14,8 @@ flowchart TD
H -->|"yes"| I["stop early with SAX, or read through a view"]
```
Questions about the input and the approach this page describes for each; an input can match more than one question.
## Several values in one stream
For newline-delimited input, read line by line and parse each line, as described in [JSON Lines](https://json.nlohmann.me/features/parsing/json_lines/index.md). For values that are simply concatenated, [`operator>>`](https://json.nlohmann.me/api/operator_gtgt/index.md) reads one value and leaves the stream positioned after it. Check for the end of the stream before each read, because a read at the end throws:
@@ -162,6 +164,10 @@ chunk , 3]} -> complete
The example feeds a document in five chunks. The first four are reported as incomplete, and a syntax error is reported as such, not as missing data.
The chunks of the example, and the bytes each attempt parses again:
Each row is one parse attempt on the growing buffer: an error at the byte after the end of the buffer means the value is incomplete, an error before the end is a real syntax error. The last line adds up the bytes parsed over all attempts.
Limits of buffering
- The buffer must contain exactly one value. Data that belongs to the next value is a syntax error. Use framing if values follow each other without a separator.
@@ -172,6 +178,10 @@ Limits of buffering
The input does not have to become one huge `json` value. The two tools below process it piece by piece.
How the memory use develops while a large array of records is parsed (illustrative, not a measurement):
Memory use over the number of records parsed: `parse()` keeps the whole tree, `json_document` the text plus its index, a discarding callback one record at a time, and SAX only what the handler keeps.
### Handle and discard elements with a callback
A [parser callback](https://json.nlohmann.me/features/parsing/parser_callbacks/index.md) is called for every finished value. If it returns `false`, the value is discarded and its memory is released at once. For a file that consists of one huge array (or one huge object) of records, handle each record at depth 1 and discard it:
@@ -330,3 +340,8 @@ If the whole text is available and you only need to read some of it, a [`json_do
- [SAX interface](https://json.nlohmann.me/features/parsing/sax_interface/index.md) - event-based parsing
- [Zero-copy JSON views](https://json.nlohmann.me/features/json_view/index.md) - read parts of a large document without building a tree
- [Untrusted input](https://json.nlohmann.me/features/parsing/untrusted_input/index.md) - limits for input you do not control
- [`operator>>`](https://json.nlohmann.me/api/operator_gtgt/index.md) - deserialize from stream
- [`parse`](https://json.nlohmann.me/api/basic_json/parse/index.md) - deserialize from a compatible input
- [`sax_parse`](https://json.nlohmann.me/api/basic_json/sax_parse/index.md) - generate SAX events
- [`JSON_PRECISE_STREAM_POSITION`](https://json.nlohmann.me/api/macros/json_precise_stream_position/index.md) - leave the stream positioned right after a parsed number
- [`JSON_DISABLE_STREAM_BULK_SCAN`](https://json.nlohmann.me/api/macros/json_disable_stream_bulk_scan/index.md) - read a `std::istream` one character at a time
+11
View File
@@ -280,3 +280,14 @@ acceptable, for example for configuration files. Note that `start_pos()` is a 0-
For binary formats such as CBOR, there are no lines; `byte` is the position in the byte sequence. To find out how many
bytes a successfully parsed value used, see
[Several values in one stream](large_and_partial_input.md#several-values-in-one-stream).
## See also
- [`parse`](../../api/basic_json/parse.md) - deserialize from a compatible input
- [`accept`](../../api/basic_json/accept.md) - check if the input is valid JSON
- [`sax_parse`](../../api/basic_json/sax_parse.md) - generate SAX events
- [`parse_error`](../../api/basic_json/parse_error.md) - exception thrown on a parse error
- [`JSON_DIAGNOSTIC_POSITIONS`](../../api/macros/json_diagnostic_positions.md) - record start and end positions of parsed values
- [Working Without Exceptions](../no_exceptions.md) - error handling when exceptions are disabled
- [Exceptions](../../home/exceptions.md) - list of exceptions and error codes
- [Parsing Untrusted Input](untrusted_input.md) - limits and checks for input you do not control
+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=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/parsing/parse_exceptions/" rel="canonical"><link href=../large_and_partial_input/ rel=prev><link href=../parser_callbacks/ rel=next><link rel=icon href=../../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>Parsing and Exceptions - JSON for Modern C++</title><link rel=stylesheet href=../../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../../assets/stylesheets/palette.ab4e12ef.min.css><link rel="stylesheet" href="../../../assets/external/fonts.googleapis.com/css.61a430c9.css"><style>:root{--md-text-font:"Roboto";--md-code-font:"JetBrains Mono"}</style><link rel=stylesheet href=../../../css/custom.css><script>__md_scope=new 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<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=description content="JSON for Modern C++ is a header-only C++11 library that makes JSON a first-class data type."><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/parsing/parse_exceptions/" rel="canonical"><link href=../large_and_partial_input/ rel=prev><link href=../parser_callbacks/ rel=next><link rel=icon href=../../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>Parsing and Exceptions - JSON for Modern C++</title><link rel=stylesheet href=../../../assets/stylesheets/main.ec1eaa64.min.css><link rel=stylesheet href=../../../assets/stylesheets/palette.ab4e12ef.min.css><link rel=stylesheet href=../../../css/custom.css><link rel=stylesheet href=../../../css/brand.css><link rel=stylesheet href=../../../css/fig-sax.css><link rel=stylesheet 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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> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 25, 2026 11:07:13 UTC">October 25, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &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 27.5 72.9 20.9 2.3-16 8.8-27.1 16-33.7-55.9-6.2-112.3-14.3-112.3-110.5 0-27.5 7.6-41.3 23.6-58.9-2.6-6.5-11.1-33.3 2.6-67.9 20.9-6.5 69 27 69 27 20-5.6 41.5-8.5 62.8-8.5s42.8 2.9 62.8 8.5c0 0 48.1-33.6 69-27 13.7 34.7 5.2 61.4 2.6 67.9 16 17.7 25.8 31.5 25.8 58.9 0 96.5-58.9 104.2-114.8 110.5 9.2 7.9 17 22.9 17 46.4 0 33.7-.3 75.4-.3 83.6 0 6.5 4.6 14.4 17.3 12.1C436.2 457.8 504 362.9 504 252 504 113.3 391.5 8 252.8 8M105.2 352.9c-1.3 1-1 3.3.7 5.2 1.6 1.6 3.9 2.3 5.2 1 1.3-1 1-3.3-.7-5.2-1.6-1.6-3.9-2.3-5.2-1m-10.8-8.1c-.7 1.3.3 2.9 2.3 3.9 1.6 1 3.6.7 4.3-.7.7-1.3-.3-2.9-2.3-3.9-2-.6-3.6-.3-4.3.7m32.4 35.6c-1.6 1.3-1 4.3 1.3 6.2 2.3 2.3 5.2 2.6 6.5 1 1.3-1.3.7-4.3-1.3-6.2-2.2-2.3-5.2-2.6-6.5-1m-11.4-14.7c-1.6 1-1.6 3.6 0 5.9s4.3 3.3 5.6 2.3c1.6-1.3 1.6-3.9 0-6.2-1.4-2.3-4-3.3-5.6-2"/></svg> </a> <a href="https://www.linkedin.com/in/nielslohmann/" target="_blank" rel="noopener" title="www.linkedin.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 448 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2025 Fonticons, Inc.--><path d="M416 32H31.9C14.3 32 0 46.5 0 64.3v383.4C0 465.5 14.3 480 31.9 480H416c17.6 0 32-14.5 32-32.3V64.3c0-17.8-14.4-32.3-32-32.3M135.4 416H69V202.2h66.5V416zM102.2 96a38.5 38.5 0 1 1 0 77 38.5 38.5 0 1 1 0-77m282.1 320h-66.4V312c0-24.8-.5-56.7-34.5-56.7-34.6 0-39.9 27-39.9 54.9V416h-66.4V202.2hLine truncated
</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
@@ -249,3 +249,14 @@ the value starts at line 3, column 10
Positions are only available for values created by [`parse`](https://json.nlohmann.me/api/basic_json/parse/index.md), 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 `start_pos()` is a 0-based index into the input, whereas `byte` of a `parse_error` starts at 1.
For binary formats such as CBOR, there are no lines; `byte` is the position in the byte sequence. To find out how many bytes a successfully parsed value used, see [Several values in one stream](https://json.nlohmann.me/features/parsing/large_and_partial_input/#several-values-in-one-stream).
## See also
- [`parse`](https://json.nlohmann.me/api/basic_json/parse/index.md) - deserialize from a compatible input
- [`accept`](https://json.nlohmann.me/api/basic_json/accept/index.md) - check if the input is valid JSON
- [`sax_parse`](https://json.nlohmann.me/api/basic_json/sax_parse/index.md) - generate SAX events
- [`parse_error`](https://json.nlohmann.me/api/basic_json/parse_error/index.md) - exception thrown on a parse error
- [`JSON_DIAGNOSTIC_POSITIONS`](https://json.nlohmann.me/api/macros/json_diagnostic_positions/index.md) - record start and end positions of parsed values
- [Working Without Exceptions](https://json.nlohmann.me/features/no_exceptions/index.md) - error handling when exceptions are disabled
- [Exceptions](https://json.nlohmann.me/home/exceptions/index.md) - list of exceptions and error codes
- [Parsing Untrusted Input](https://json.nlohmann.me/features/parsing/untrusted_input/index.md) - limits and checks for input you do not control
+56 -2
View File
@@ -31,6 +31,52 @@ table describes the values of the parameters `depth`, `event`, and `parsed`.
| `parse_event_t::array_end` | the parser read `]` and finished processing a JSON array | depth of the parent of the JSON array | the parsed JSON array |
| `parse_event_t::value` | the parser finished reading a JSON value | depth of the value | the parsed JSON value |
<div class="nj-fig-wrap" markdown>
<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" x="344" y="194">array_end</text>
<circle class="cb-v" cx="327" cy="219" r="6"/>
<text class="fg-code fg-leg" x="344" y="224">value</text>
</svg>
</div>
<p class="nj-caption" markdown>Each coloured dot marks where the parser reports a `parse_event_t` event while reading
the JSON text on the left; the legend on the right names the event per colour.</p>
??? example "Example: sequence of callback events"
When parsing the following JSON text,
@@ -177,7 +223,7 @@ was called:
The JSON specification leaves the handling of objects with repeated keys up to the implementation. As described in
[`object_t`](../../api/basic_json/object_t.md#behavior), it is unspecified which value for a repeated key ends up in
the resulting `#!c json` value -- once parsing has produced that value, the duplicate is already gone, because object
the resulting `#!c json` value — once parsing has produced that value, the duplicate is already gone, because object
storage maps each key to a single value. If duplicate keys should instead be treated as an error, a parser callback
can detect them while the object is still being read, before that ambiguity ever applies.
@@ -267,7 +313,7 @@ This approach has three limitations:
maintaining a stack of the enclosing keys and array indices in the callback as well.
A [SAX interface](sax_interface.md) does not lift the position limitation: its `key` function receives no position
either -- only `parse_error` is passed the byte position.
either — only `parse_error` is passed the byte position.
## Recipe: streaming a large homogeneous array
@@ -316,3 +362,11 @@ tracking the maximum `depth` seen and throwing once it is exceeded:
json::parse(input, callback);
```
## See also
- [Parsing](index.md) - overview of the parsing functions
- [SAX Interface](sax_interface.md) - guide to event-based parsing
- [`parse`](../../api/basic_json/parse.md) - deserialize from a compatible input
- [`parser_callback_t`](../../api/basic_json/parser_callback_t.md) - callback function type for the parser
- [`parse_event_t`](../../api/basic_json/parse_event_t.md) - enumeration of parser events
+5 -5
View File
@@ -1,7 +1,7 @@
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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 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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> <details class=example> <summary>Example: sequence of callback events</summary> <p>When parsing the following <abbr title="JavaScript Object Notation">JSON</abbr> text,</p> <div class=highlight><pre><span></span><code><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
<span class=w> </span><span class=nt>"name"</span><span class=p>:</span><span class=w> </span><span class=s2>"Berlin"</span><span class=p>,</span>
<span class=w> </span><span class=nt>"location"</span><span class=p>:</span><span class=w> </span><span class=p>[</span>
<span class=w> </span><span class=mf>52.519444</span><span class=p>,</span>
@@ -90,7 +90,7 @@
<span class=w> </span><span class=nt>"Width"</span><span class=p>:</span><span class=w> </span><span class=mi>800</span>
<span class=w> </span><span class=p>}</span>
<span class=p>}</span>
</code></pre></div> </details> <h2 id=recipe-rejecting-duplicate-object-keys>Recipe: rejecting duplicate object keys<a class=headerlink href=#recipe-rejecting-duplicate-object-keys title="Permanent link">&para;</a></h2> <p>The <abbr title="JavaScript Object Notation">JSON</abbr> specification leaves the handling of objects with repeated keys up to the implementation. As described in <a href=../../../api/basic_json/object_t/#behavior><code>object_t</code></a>, it is unspecified which value for a repeated key ends up in the resulting <code class=highlight><span class=n>json</span></code> value -- once parsing has produced that value, the duplicate is already gone, because object storage maps each key to a single value. If duplicate keys should instead be treated as an error, a parser callback can detect them while the object is still being read, before that ambiguity ever applies.</p> <details class=example> <summary>Example: reject duplicate object keys</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> </details> <h2 id=recipe-rejecting-duplicate-object-keys>Recipe: rejecting duplicate object keys<a class=headerlink href=#recipe-rejecting-duplicate-object-keys title="Permanent link">&para;</a></h2> <p>The <abbr title="JavaScript Object Notation">JSON</abbr> specification leaves the handling of objects with repeated keys up to the implementation. As described in <a href=../../../api/basic_json/object_t/#behavior><code>object_t</code></a>, it is unspecified which value for a repeated key ends up in the resulting <code class=highlight><span class=n>json</span></code> value — once parsing has produced that value, the duplicate is already gone, because object storage maps each key to a single value. If duplicate keys should instead be treated as an error, a parser callback can detect them while the object is still being read, before that ambiguity ever applies.</p> <details class=example> <summary>Example: reject duplicate object keys</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=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>
@@ -153,7 +153,7 @@
<span class=p>}</span>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code><span class=p>{</span><span class=nt>"one"</span><span class=p>:</span><span class=mi>1</span><span class=p>,</span><span class=nt>"two"</span><span class=p>:</span><span class=mi>2</span><span class=p>}</span>
<span class=err>duplica</span><span class=kc>te</span><span class=w> </span><span class=err>JSON</span><span class=w> </span><span class=err>objec</span><span class=kc>t</span><span class=w> </span><span class=err>key</span><span class=p>:</span><span class=w> </span><span class=err>o</span><span class=kc>ne</span>
</code></pre></div> </details> <p>This approach has three limitations:</p> <ul> <li>The depth-indexed bookkeeping must account for the fact that <code>object_start</code> reports the depth of the <em>parent</em> of the object, while the <code>key</code> events inside that object are reported one depth deeper (see the event table above); it is easy to get this off by one for nested objects.</li> <li>The thrown exception cannot carry a <code>parse_error</code>-style byte offset, because position tracking only exists inside the parser and lexer, not at the callback layer.</li> <li>The exception only names the repeated key, not where it occurs in the document. Reporting its full path requires maintaining a stack of the enclosing keys and array indices in the callback as well.</li> </ul> <p>A <a href=../sax_interface/ ><abbr title="Simple API for XML">SAX</abbr> interface</a> does not lift the position limitation: its <code>key</code> function receives no position either -- only <code>parse_error</code> is passed the byte position.</p> <h2 id=recipe-streaming-a-large-homogeneous-array>Recipe: streaming a large homogeneous array<a class=headerlink href=#recipe-streaming-a-large-homogeneous-array title="Permanent link">&para;</a></h2> <p>A common use case is a huge top-level array of many similarly-shaped objects, too large to hold entirely in memory as a <code class=highlight><span class=n>json</span></code> value. A parser callback can hand off each completed element to a user function and then discard it, so memory usage stays bounded by a single element (plus the not-yet-parsed tail of the input) rather than the whole document. Since the top-level array's <code>array_start</code>/<code>array_end</code> are reported at <code>depth == 0</code> (its parent is the document root), the object elements it contains are reported at <code>depth == 1</code>:</p> <details class=example> <summary>Example: stream a large top-level array</summary> <div class=highlight><pre><span></span><code><span class=n>std</span><span class=o>::</span><span class=n>ifstream</span><span class=w> </span><span class=n>input</span><span class=p>(</span><span class=s>"large_array.json"</span><span class=p>);</span>
</code></pre></div> </details> <p>This approach has three limitations:</p> <ul> <li>The depth-indexed bookkeeping must account for the fact that <code>object_start</code> reports the depth of the <em>parent</em> of the object, while the <code>key</code> events inside that object are reported one depth deeper (see the event table above); it is easy to get this off by one for nested objects.</li> <li>The thrown exception cannot carry a <code>parse_error</code>-style byte offset, because position tracking only exists inside the parser and lexer, not at the callback layer.</li> <li>The exception only names the repeated key, not where it occurs in the document. Reporting its full path requires maintaining a stack of the enclosing keys and array indices in the callback as well.</li> </ul> <p>A <a href=../sax_interface/ ><abbr title="Simple API for XML">SAX</abbr> interface</a> does not lift the position limitation: its <code>key</code> function receives no position either — only <code>parse_error</code> is passed the byte position.</p> <h2 id=recipe-streaming-a-large-homogeneous-array>Recipe: streaming a large homogeneous array<a class=headerlink href=#recipe-streaming-a-large-homogeneous-array title="Permanent link">&para;</a></h2> <p>A common use case is a huge top-level array of many similarly-shaped objects, too large to hold entirely in memory as a <code class=highlight><span class=n>json</span></code> value. A parser callback can hand off each completed element to a user function and then discard it, so memory usage stays bounded by a single element (plus the not-yet-parsed tail of the input) rather than the whole document. Since the top-level array's <code>array_start</code>/<code>array_end</code> are reported at <code>depth == 0</code> (its parent is the document root), the object elements it contains are reported at <code>depth == 1</code>:</p> <details class=example> <summary>Example: stream a large top-level array</summary> <div class=highlight><pre><span></span><code><span class=n>std</span><span class=o>::</span><span class=n>ifstream</span><span class=w> </span><span class=n>input</span><span class=p>(</span><span class=s>"large_array.json"</span><span class=p>);</span>
<span class=k>auto</span><span class=w> </span><span class=n>callback</span><span class=w> </span><span class=o>=</span><span class=w> </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>json</span><span class=o>::</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>json</span><span class=o>&amp;</span><span class=w> </span><span class=n>parsed</span><span class=p>)</span><span class=w> </span><span class=o>-&gt;</span><span class=w> </span><span class=kt>bool</span><span class=w> </span><span class=p>{</span>
<span class=w> </span><span class=k>if</span><span class=w> </span><span class=p>(</span><span class=n>depth</span><span class=w> </span><span class=o>==</span><span class=w> </span><span class=mi>1</span><span class=w> </span><span class=o>&amp;&amp;</span><span class=w> </span><span class=n>event</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_event_t</span><span class=o>::</span><span class=n>object_end</span><span class=p>)</span><span class=w> </span><span class=p>{</span>
@@ -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> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="October 2, 2026 09:32:15 UTC">October 2, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &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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+12 -2
View File
@@ -25,6 +25,8 @@ We distinguish six scenarios (determined by the event type) in which the callbac
| `parse_event_t::array_end` | the parser read `]` and finished processing a JSON array | depth of the parent of the JSON array | the parsed JSON array |
| `parse_event_t::value` | the parser finished reading a JSON value | depth of the value | the parsed JSON value |
Each coloured dot marks where the parser reports a `parse_event_t` event while reading the JSON text on the left; the legend on the right names the event per colour.
Example: sequence of callback events
When parsing the following JSON text,
@@ -160,7 +162,7 @@ Output:
## Recipe: rejecting duplicate object keys
The JSON specification leaves the handling of objects with repeated keys up to the implementation. As described in [`object_t`](https://json.nlohmann.me/api/basic_json/object_t/#behavior), it is unspecified which value for a repeated key ends up in the resulting `json` value -- once parsing has produced that value, the duplicate is already gone, because object storage maps each key to a single value. If duplicate keys should instead be treated as an error, a parser callback can detect them while the object is still being read, before that ambiguity ever applies.
The JSON specification leaves the handling of objects with repeated keys up to the implementation. As described in [`object_t`](https://json.nlohmann.me/api/basic_json/object_t/#behavior), it is unspecified which value for a repeated key ends up in the resulting `json` value — once parsing has produced that value, the duplicate is already gone, because object storage maps each key to a single value. If duplicate keys should instead be treated as an error, a parser callback can detect them while the object is still being read, before that ambiguity ever applies.
Example: reject duplicate object keys
@@ -241,7 +243,7 @@ This approach has three limitations:
- The thrown exception cannot carry a `parse_error`-style byte offset, because position tracking only exists inside the parser and lexer, not at the callback layer.
- The exception only names the repeated key, not where it occurs in the document. Reporting its full path requires maintaining a stack of the enclosing keys and array indices in the callback as well.
A [SAX interface](https://json.nlohmann.me/features/parsing/sax_interface/index.md) does not lift the position limitation: its `key` function receives no position either -- only `parse_error` is passed the byte position.
A [SAX interface](https://json.nlohmann.me/features/parsing/sax_interface/index.md) does not lift the position limitation: its `key` function receives no position either — only `parse_error` is passed the byte position.
## Recipe: streaming a large homogeneous array
@@ -283,3 +285,11 @@ auto callback = [](int depth, json::parse_event_t /*event*/, json& /*parsed*/) -
json::parse(input, callback);
```
## See also
- [Parsing](https://json.nlohmann.me/features/parsing/index.md) - overview of the parsing functions
- [SAX Interface](https://json.nlohmann.me/features/parsing/sax_interface/index.md) - guide to event-based parsing
- [`parse`](https://json.nlohmann.me/api/basic_json/parse/index.md) - deserialize from a compatible input
- [`parser_callback_t`](https://json.nlohmann.me/api/basic_json/parser_callback_t/index.md) - callback function type for the parser
- [`parse_event_t`](https://json.nlohmann.me/api/basic_json/parse_event_t/index.md) - enumeration of parser events
+36 -2
View File
@@ -29,6 +29,8 @@ class sax_t ["json::sax_t"] {
}
```
<p class="nj-caption" markdown>The interface `json::sax_t`: one function per event, each returning `bool`; the functions marked with `*` are pure virtual.</p>
```cpp
// called when null is parsed
bool null();
@@ -62,6 +64,34 @@ bool parse_error(std::size_t position, const std::string& last_token, const json
The return value of each function determines whether parsing should proceed.
## Stepping through the events
The events for a small input, step by step, and what two different handlers do with them. Press *Play* or step with *Next*:
<div class="nj-sax nj-fig" id="sax-event-stepper" data-nj-sax role="group" aria-label="SAX events for the input {&quot;a&quot;:[1,true],&quot;b&quot;:&quot;x&quot;}, and the state of a handler after each event">
<div class="nj-sax__lane nj-sax__lane--text"><p class="nj-sax__h">input</p><p class="nj-sax__text"><code class="nj-sax__src">{"a":[1,true],"b":"x"}</code></p></div>
<div class="nj-sax__lane nj-sax__lane--events"><p class="nj-sax__h">events, in the order the parser calls them</p><ol class="nj-sax__events">
<li class="nj-sax-ev nj-sax-ev--os"><code>start_object(-1)</code></li>
<li class="nj-sax-ev nj-sax-ev--k"><code>key("a")</code></li>
<li class="nj-sax-ev nj-sax-ev--as"><code>start_array(-1)</code></li>
<li class="nj-sax-ev nj-sax-ev--v"><code>number_unsigned(1)</code></li>
<li class="nj-sax-ev nj-sax-ev--v"><code>boolean(true)</code></li>
<li class="nj-sax-ev nj-sax-ev--ae"><code>end_array()</code></li>
<li class="nj-sax-ev nj-sax-ev--k"><code>key("b")</code></li>
<li class="nj-sax-ev nj-sax-ev--v"><code>string("x")</code></li>
<li class="nj-sax-ev nj-sax-ev--oe"><code>end_object()</code></li>
</ol></div>
<div class="nj-sax__lane nj-sax__lane--state"><p class="nj-sax__h">handler: build a value tree, as <code>parse</code> does</p><div class="nj-sax__state"><pre class="nj-sax__tree"><code>{
"a": [
1,
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" markdown>The three lanes show the input consumed so far, the events called so far, and the state of the selected handler (*build a tree* or *keep only /b*). Check *return false at this event* to stop the parse at the current event and see which events are no longer called.</p>
To implement your own SAX handler, proceed as follows:
1. Implement the SAX interface in a class. You can use class `nlohmann::json_sax<json>` as base class, but you can also use any class where the functions described above are implemented and public.
@@ -72,5 +102,9 @@ Note the `sax_parse` function only returns a `#!cpp bool` indicating the result
## See also
- [json_sax](../../api/json_sax/index.md) - documentation of the SAX interface
- [sax_parse](../../api/basic_json/sax_parse.md) - SAX parser
- [`json_sax`](../../api/json_sax/index.md) - documentation of the SAX interface
- [`sax_parse`](../../api/basic_json/sax_parse.md) - SAX parser
- [Parsing](index.md) - overview of the parsing functions
- [Parsing and Exceptions](parse_exceptions.md) - handling parse errors and locating them in the input
- [`parse`](../../api/basic_json/parse.md) - deserialize from a compatible input
- [Binary Formats](../binary_formats/index.md) - CBOR, MessagePack, BSON, and other formats
+9 -3
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@@ -1,4 +1,4 @@
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class sax_t ["json::sax_t"] {
&lt;&lt;interface&gt;&gt;
@@ -21,7 +21,7 @@ class sax_t ["json::sax_t"] {
+bool key(string_t&amp; val)*
+bool parse_error(std::size_t position, const std::string&amp; last_token, const json::exception&amp; ex)*
}</code></pre> <div class=highlight><pre><span></span><code><span class=c1>// called when null is parsed</span>
}</code></pre> <p class=nj-caption>The interface <code>json::sax_t</code>: one function per event, each returning <code>bool</code>; the functions marked with <code>*</code> are pure virtual.</p> <div class=highlight><pre><span></span><code><span class=c1>// called when null is parsed</span>
<span class=kt>bool</span><span class=w> </span><span class=nf>null</span><span class=p>();</span>
<span class=c1>// called when a boolean is parsed; value is passed</span>
@@ -49,4 +49,10 @@ class sax_t ["json::sax_t"] {
<span class=c1>// called when a parse error occurs; byte position, the last token, and an exception is passed</span>
<span class=kt>bool</span><span class=w> </span><span class=nf>parse_error</span><span class=p>(</span><span class=n>std</span><span class=o>::</span><span class=kt>size_t</span><span class=w> </span><span class=n>position</span><span class=p>,</span><span class=w> </span><span class=k>const</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>string</span><span class=o>&amp;</span><span class=w> </span><span class=n>last_token</span><span class=p>,</span><span class=w> </span><span class=k>const</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>exception</span><span class=o>&amp;</span><span class=w> </span><span class=n>ex</span><span class=p>);</span>
</code></pre></div> <p>The return value of each function determines whether parsing should proceed.</p> <p>To implement your own <abbr title="Simple API for XML">SAX</abbr> handler, proceed as follows:</p> <ol> <li>Implement the <abbr title="Simple API for XML">SAX</abbr> interface in a class. You can use class <code>nlohmann::json_sax&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/ >json_sax</a> - documentation of the <abbr title="Simple API for XML">SAX</abbr> interface</li> <li><a href=../../../api/basic_json/sax_parse/ >sax_parse</a> - <abbr title="Simple API for XML">SAX</abbr> parser</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="March 6, 2025 10:40:09 UTC">March 6, 2025</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &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 27.5 72.9 20.9 2.3-16 8.8-27.1 16-33.7-55.9-6.2-112.3-14.3-112.3-110.5 0-27.5 7.6-41.3 23.6-58.9-2.6Line truncated
</code></pre></div> <p>The return value of each function determines whether parsing should proceed.</p> <h2 id=stepping-through-the-events>Stepping through the events<a class=headerlink href=#stepping-through-the-events title="Permanent link">&para;</a></h2> <p>The events for a small input, step by step, and what two different handlers do with them. Press <em>Play</em> or step with <em>Next</em>:</p> <div class="nj-sax nj-fig" id=sax-event-stepper data-nj-sax role=group aria-label='SAX events for the input {"a":[1,true],"b":"x"}, and the state of a handler after each event'> <div class="nj-sax__lane nj-sax__lane--text"><p class=nj-sax__h>input</p><p class=nj-sax__text><code class=nj-sax__src>{"a":[1,true],"b":"x"}</code></p></div> <div class="nj-sax__lane nj-sax__lane--events"><p class=nj-sax__h>events, in the order the parser calls them</p><ol class=nj-sax__events> <li class="nj-sax-ev nj-sax-ev--os"><code>start_object(-1)</code></li> <li class="nj-sax-ev nj-sax-ev--k"><code>key("a")</code></li> <li class="nj-sax-ev nj-sax-ev--as"><code>start_array(-1)</code></li> <li class="nj-sax-ev nj-sax-ev--v"><code>number_unsigned(1)</code></li> <li class="nj-sax-ev nj-sax-ev--v"><code>boolean(true)</code></li> <li class="nj-sax-ev nj-sax-ev--ae"><code>end_array()</code></li> <li class="nj-sax-ev nj-sax-ev--k"><code>key("b")</code></li> <li class="nj-sax-ev nj-sax-ev--v"><code>string("x")</code></li> <li class="nj-sax-ev nj-sax-ev--oe"><code>end_object()</code></li> </ol></div> <div class="nj-sax__lane nj-sax__lane--state"><p class=nj-sax__h>handler: build a value tree, as <code>parse</code> does</p><div class=nj-sax__state><pre class=nj-sax__tree><code>{
"a": [
1,
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 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=../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
+44 -2
View File
@@ -29,6 +29,8 @@ class sax_t ["json::sax_t"] {
}
```
The interface `json::sax_t`: one function per event, each returning `bool`; the functions marked with `*` are pure virtual.
```
// called when null is parsed
bool null();
@@ -62,6 +64,42 @@ bool parse_error(std::size_t position, const std::string& last_token, const json
The return value of each function determines whether parsing should proceed.
## Stepping through the events
The events for a small input, step by step, and what two different handlers do with them. Press *Play* or step with *Next*:
input
`{"a":[1,true],"b":"x"}`
events, in the order the parser calls them
1. `start_object(-1)`
1. `key("a")`
1. `start_array(-1)`
1. `number_unsigned(1)`
1. `boolean(true)`
1. `end_array()`
1. `key("b")`
1. `string("x")`
1. `end_object()`
handler: build a value tree, as `parse` does
```
{
"a": [
1,
true
],
"b": "x"
}
```
After `end_object()` 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.
The three lanes show the input consumed so far, the events called so far, and the state of the selected handler (*build a tree* or *keep only /b*). Check *return false at this event* to stop the parse at the current event and see which events are no longer called.
To implement your own SAX handler, proceed as follows:
1. Implement the SAX interface in a class. You can use class `nlohmann::json_sax<json>` as base class, but you can also use any class where the functions described above are implemented and public.
@@ -72,5 +110,9 @@ Note the `sax_parse` function only returns a `bool` indicating the result of the
## See also
- [json_sax](https://json.nlohmann.me/api/json_sax/index.md) - documentation of the SAX interface
- [sax_parse](https://json.nlohmann.me/api/basic_json/sax_parse/index.md) - SAX parser
- [`json_sax`](https://json.nlohmann.me/api/json_sax/index.md) - documentation of the SAX interface
- [`sax_parse`](https://json.nlohmann.me/api/basic_json/sax_parse/index.md) - SAX parser
- [Parsing](https://json.nlohmann.me/features/parsing/index.md) - overview of the parsing functions
- [Parsing and Exceptions](https://json.nlohmann.me/features/parsing/parse_exceptions/index.md) - handling parse errors and locating them in the input
- [`parse`](https://json.nlohmann.me/api/basic_json/parse/index.md) - deserialize from a compatible input
- [Binary Formats](https://json.nlohmann.me/features/binary_formats/index.md) - CBOR, MessagePack, BSON, and other formats
+15 -9
View File
@@ -1,7 +1,7 @@
# Parsing Untrusted Input
This page is for applications that parse JSON -- or one of the supported [binary formats](../binary_formats/index.md)
(BJData, BON8, BSON, CBOR, MessagePack, UBJSON) -- from a source they do not fully control, such as a network
This page is for applications that parse JSON — or one of the supported [binary formats](../binary_formats/index.md)
(BJData, BON8, BSON, CBOR, MessagePack, UBJSON) — from a source they do not fully control, such as a network
connection, an uploaded file, or another process. It summarizes what the library already does for such input and what
remains the caller's responsibility, linking to the pages that cover each aspect in detail rather than repeating them.
@@ -50,12 +50,12 @@ stack, however deeply it is nested.
[SAX handler](sax_interface.md) (for the JSON parser or a binary format alike) and throwing once your limit is
exceeded.
Once a value has been parsed, operations that walk it recursively -- serializing it with
Once a value has been parsed, operations that walk it recursively — serializing it with
[`dump`](../../api/basic_json/dump.md), hashing it, copying it, comparing two values with `#!cpp ==`, `#!cpp <`, or (in
C++20) `#!cpp <=>`, merging with [`update`](../../api/basic_json/update.md), applying a
[`merge_patch`](../../api/basic_json/merge_patch.md), creating a patch with [`diff`](../../api/basic_json/diff.md) or
[`merge_diff`](../../api/basic_json/merge_diff.md), [`flatten`](../../api/basic_json/flatten.md), and the binary
writers (`to_cbor`, `to_msgpack`, ...) -- descend at most 128 levels on the call stack and continue
writers (`to_cbor`, `to_msgpack`, ...) — descend at most 128 levels on the call stack and continue
below that with an explicit stack instead, so none of them can exhaust the stack either, however deeply the value is
nested. Destroying a value (its destructor) never recurses at all, regardless of nesting depth, for the same reason.
@@ -63,12 +63,12 @@ nested. Destroying a value (its destructor) never recurses at all, regardless of
The library does not limit the overall size of a JSON text; a value nested or wide enough will use memory
proportional to the input. If you parse untrusted input of unbounded size, check the size of the file or stream
yourself before -- or while -- handing it to `parse()`.
yourself before — or while — handing it to `parse()`.
For the binary formats, an announced size is never trusted outright:
- Reading a string or binary value copies the input in bounded 4096-byte chunks and grows the result as bytes are
actually consumed, rather than allocating the announced length up front -- a truncated input runs out of bytes
actually consumed, rather than allocating the announced length up front — a truncated input runs out of bytes
(reported as a parse error) instead of triggering an oversized allocation.
- When an array announces its number of elements and the array container supports `reserve()` (as `#!cpp std::vector`,
the default, does), the library reserves storage for at most 16384 of them upfront, regardless of how large the
@@ -91,7 +91,7 @@ Invalid UTF-8 is rejected while parsing, not just while serializing:
A `#!cpp '\0'` (NUL) byte *inside* a quoted JSON string is always rejected (it must be escaped as `\u0000`). A NUL byte
*outside* of a string is different: by default it is silently treated as the end of the input, so trailing bytes after
it -- including further, otherwise well-formed JSON -- are silently ignored rather than rejected. Since untrusted input
it — including further, otherwise well-formed JSON — are silently ignored rather than rejected. Since untrusted input
that happens to embed a NUL is a way to make part of it disappear without a parse error, see the
[FAQ entry](../../home/faq.md#nul-bytes-in-the-input) and consider defining
[`JSON_STRICT_NUL_HANDLING`](../../api/macros/json_strict_nul_handling.md) to `1` to reject a NUL byte like any other
@@ -116,7 +116,7 @@ silently resolving them one way or another, see the
## Numbers
A number whose value cannot be represented -- for instance `1E1000`, which overflows `double` -- is rejected while
A number whose value cannot be represented — for instance `1E1000`, which overflows `double` — is rejected while
parsing as [`out_of_range.406`](../../home/exceptions.md#jsonexceptionout_of_range406) rather than silently becoming
infinity. An integer that is syntactically valid but does not fit the 64-bit integer types is not rejected; it is
instead stored as a `double`, which may lose precision for very large values. See
@@ -139,7 +139,7 @@ conforming JSON consumers is exactly what the default rejects.
[SAX handler](sax_interface.md); the library bounds the call stack but not memory use.
- Bound the size of the input itself before parsing, independent of the library's own bounded allocations for binary
format lengths.
- Decide up front how you want a string with invalid UTF-8 -- from any source, not only parsing -- to be serialized
- Decide up front how you want a string with invalid UTF-8 — from any source, not only parsing — to be serialized
(`strict`, `replace`, or `ignore`), rather than discovering it from an uncaught `type_error.316`.
- If a stray NUL byte silently truncating trailing input is a problem for your input format, define
`JSON_STRICT_NUL_HANDLING`.
@@ -160,3 +160,9 @@ report a security issue in the library itself, follow the [security policy](../.
- [Number handling](../types/number_handling.md) - number ranges and overflow behavior
- [Assurance case](../../community/assurance_case.md) - the library's threat model and countermeasures
- [Security policy](../../community/security_policy.md) - how to report a vulnerability
- [`parse`](../../api/basic_json/parse.md) - deserialize from a compatible input
- [`accept`](../../api/basic_json/accept.md) - check if the input is valid JSON
- [`sax_parse`](../../api/basic_json/sax_parse.md) - generate SAX events
- [`parser_callback_t`](../../api/basic_json/parser_callback_t.md) - callback function type for the parser
- [`JSON_STRICT_NUL_HANDLING`](../../api/macros/json_strict_nul_handling.md) - opt in to rejecting a NUL byte in the input
- [`JSON_DIAGNOSTIC_POSITIONS`](../../api/macros/json_diagnostic_positions.md) - record start and end positions of parsed values
+1 -1
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@@ -1 +1 @@
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# Parsing Untrusted Input
This page is for applications that parse JSON -- or one of the supported [binary formats](https://json.nlohmann.me/features/binary_formats/index.md) (BJData, BON8, BSON, CBOR, MessagePack, UBJSON) -- from a source they do not fully control, such as a network connection, an uploaded file, or another process. It summarizes what the library already does for such input and what remains the caller's responsibility, linking to the pages that cover each aspect in detail rather than repeating them.
This page is for applications that parse JSON — or one of the supported [binary formats](https://json.nlohmann.me/features/binary_formats/index.md) (BJData, BON8, BSON, CBOR, MessagePack, UBJSON) — from a source they do not fully control, such as a network connection, an uploaded file, or another process. It summarizes what the library already does for such input and what remains the caller's responsibility, linking to the pages that cover each aspect in detail rather than repeating them.
For the project's threat model and the countermeasures behind these behaviors, see the [assurance case](https://json.nlohmann.me/community/assurance_case/index.md); to report a vulnerability, see the [security policy](https://json.nlohmann.me/community/security_policy/index.md).
@@ -22,15 +22,15 @@ No built-in depth limit while parsing
Neither the parser nor the binary readers impose a limit on how deep the input may nest. An attacker can still exhaust memory (though not the call stack) with a sufficiently deep document. If you need to reject over-deep untrusted input outright, track the depth yourself, either with a [parser callback](https://json.nlohmann.me/features/parsing/parser_callbacks/#recipe-max-nesting-depth-via-a-callback) for the JSON parser, or by counting `start_object`/`start_array` and `end_object`/`end_array` calls in a [SAX handler](https://json.nlohmann.me/features/parsing/sax_interface/index.md) (for the JSON parser or a binary format alike) and throwing once your limit is exceeded.
Once a value has been parsed, operations that walk it recursively -- serializing it with [`dump`](https://json.nlohmann.me/api/basic_json/dump/index.md), hashing it, copying it, comparing two values with `==`, `<`, or (in C++20) `<=>`, merging with [`update`](https://json.nlohmann.me/api/basic_json/update/index.md), applying a [`merge_patch`](https://json.nlohmann.me/api/basic_json/merge_patch/index.md), creating a patch with [`diff`](https://json.nlohmann.me/api/basic_json/diff/index.md) or [`merge_diff`](https://json.nlohmann.me/api/basic_json/merge_diff/index.md), [`flatten`](https://json.nlohmann.me/api/basic_json/flatten/index.md), and the binary writers (`to_cbor`, `to_msgpack`, ...) -- descend at most 128 levels on the call stack and continue below that with an explicit stack instead, so none of them can exhaust the stack either, however deeply the value is nested. Destroying a value (its destructor) never recurses at all, regardless of nesting depth, for the same reason.
Once a value has been parsed, operations that walk it recursively — serializing it with [`dump`](https://json.nlohmann.me/api/basic_json/dump/index.md), hashing it, copying it, comparing two values with `==`, `<`, or (in C++20) `<=>`, merging with [`update`](https://json.nlohmann.me/api/basic_json/update/index.md), applying a [`merge_patch`](https://json.nlohmann.me/api/basic_json/merge_patch/index.md), creating a patch with [`diff`](https://json.nlohmann.me/api/basic_json/diff/index.md) or [`merge_diff`](https://json.nlohmann.me/api/basic_json/merge_diff/index.md), [`flatten`](https://json.nlohmann.me/api/basic_json/flatten/index.md), and the binary writers (`to_cbor`, `to_msgpack`, ...) — descend at most 128 levels on the call stack and continue below that with an explicit stack instead, so none of them can exhaust the stack either, however deeply the value is nested. Destroying a value (its destructor) never recurses at all, regardless of nesting depth, for the same reason.
## Input size
The library does not limit the overall size of a JSON text; a value nested or wide enough will use memory proportional to the input. If you parse untrusted input of unbounded size, check the size of the file or stream yourself before -- or while -- handing it to `parse()`.
The library does not limit the overall size of a JSON text; a value nested or wide enough will use memory proportional to the input. If you parse untrusted input of unbounded size, check the size of the file or stream yourself before — or while — handing it to `parse()`.
For the binary formats, an announced size is never trusted outright:
- Reading a string or binary value copies the input in bounded 4096-byte chunks and grows the result as bytes are actually consumed, rather than allocating the announced length up front -- a truncated input runs out of bytes (reported as a parse error) instead of triggering an oversized allocation.
- Reading a string or binary value copies the input in bounded 4096-byte chunks and grows the result as bytes are actually consumed, rather than allocating the announced length up front — a truncated input runs out of bytes (reported as a parse error) instead of triggering an oversized allocation.
- When an array announces its number of elements and the array container supports `reserve()` (as `std::vector`, the default, does), the library reserves storage for at most 16384 of them upfront, regardless of how large the announced count is; further elements still grow the container normally as they are read.
- An announced array or object size that exceeds what the target container could ever hold (its `max_size()`) is rejected immediately as [`out_of_range.408`](https://json.nlohmann.me/home/exceptions/#jsonexceptionout_of_range408), without attempting to allocate anything.
@@ -42,7 +42,7 @@ Invalid UTF-8 is rejected while parsing, not just while serializing:
- In UTF-16 or UTF-32 input, an ill-formed sequence is a [`parse_error.101`](https://json.nlohmann.me/home/exceptions/#jsonexceptionparse_error101) as well: a lone surrogate, a high surrogate that is not followed by its low surrogate, and a code point above `U+10FFFF` are all rejected.
- In a binary format, a string that is not valid UTF-8 is a [`parse_error.113`](https://json.nlohmann.me/home/exceptions/#jsonexceptionparse_error113).
A `'\0'` (NUL) byte *inside* a quoted JSON string is always rejected (it must be escaped as `\u0000`). A NUL byte *outside* of a string is different: by default it is silently treated as the end of the input, so trailing bytes after it -- including further, otherwise well-formed JSON -- are silently ignored rather than rejected. Since untrusted input that happens to embed a NUL is a way to make part of it disappear without a parse error, see the [FAQ entry](https://json.nlohmann.me/home/faq/#nul-bytes-in-the-input) and consider defining [`JSON_STRICT_NUL_HANDLING`](https://json.nlohmann.me/api/macros/json_strict_nul_handling/index.md) to `1` to reject a NUL byte like any other unexpected byte instead.
A `'\0'` (NUL) byte *inside* a quoted JSON string is always rejected (it must be escaped as `\u0000`). A NUL byte *outside* of a string is different: by default it is silently treated as the end of the input, so trailing bytes after it — including further, otherwise well-formed JSON — are silently ignored rather than rejected. Since untrusted input that happens to embed a NUL is a way to make part of it disappear without a parse error, see the [FAQ entry](https://json.nlohmann.me/home/faq/#nul-bytes-in-the-input) and consider defining [`JSON_STRICT_NUL_HANDLING`](https://json.nlohmann.me/api/macros/json_strict_nul_handling/index.md) to `1` to reject a NUL byte like any other unexpected byte instead.
Parsing is not the only place invalid UTF-8 matters: a string that reached a `json` value some other way (for example, constructed by application code, or, before JSON_STRICT_NUL_HANDLING existed, read from a binary format that does not validate strings) still has to round-trip back to JSON text. By default, [`dump()`](https://json.nlohmann.me/api/basic_json/dump/index.md) throws [`type_error.316`](https://json.nlohmann.me/home/exceptions/#jsonexceptiontype_error316) if the string is not valid UTF-8; passing [`error_handler_t::replace`](https://json.nlohmann.me/api/basic_json/error_handler_t/index.md) or `error_handler_t::ignore` avoids the exception instead of crashing an application that forgot to catch it. See [Handling invalid UTF-8](https://json.nlohmann.me/features/serialization/#handling-invalid-utf-8) for the options and an example.
@@ -52,7 +52,7 @@ The JSON specification leaves the handling of repeated keys in an object up to t
## Numbers
A number whose value cannot be represented -- for instance `1E1000`, which overflows `double` -- is rejected while parsing as [`out_of_range.406`](https://json.nlohmann.me/home/exceptions/#jsonexceptionout_of_range406) rather than silently becoming infinity. An integer that is syntactically valid but does not fit the 64-bit integer types is not rejected; it is instead stored as a `double`, which may lose precision for very large values. See [number limits](https://json.nlohmann.me/features/types/number_handling/#number-limits) for the exact ranges and an example.
A number whose value cannot be represented — for instance `1E1000`, which overflows `double` — is rejected while parsing as [`out_of_range.406`](https://json.nlohmann.me/home/exceptions/#jsonexceptionout_of_range406) rather than silently becoming infinity. An integer that is syntactically valid but does not fit the 64-bit integer types is not rejected; it is instead stored as a `double`, which may lose precision for very large values. See [number limits](https://json.nlohmann.me/features/types/number_handling/#number-limits) for the exact ranges and an example.
## Comments and trailing commas
@@ -63,7 +63,7 @@ Both [comments](https://json.nlohmann.me/features/comments/index.md) and [traili
- Wrap parsing in a `try`/`catch` block, or use `allow_exceptions=false`/`accept()` if your environment cannot use exceptions; never let `JSON_NOEXCEPTION`'s `abort()` be the first time you think about error handling.
- If the input's nesting depth matters to you, enforce your own limit with a [parser callback](https://json.nlohmann.me/features/parsing/parser_callbacks/#recipe-max-nesting-depth-via-a-callback) or a [SAX handler](https://json.nlohmann.me/features/parsing/sax_interface/index.md); the library bounds the call stack but not memory use.
- Bound the size of the input itself before parsing, independent of the library's own bounded allocations for binary format lengths.
- Decide up front how you want a string with invalid UTF-8 -- from any source, not only parsing -- to be serialized (`strict`, `replace`, or `ignore`), rather than discovering it from an uncaught `type_error.316`.
- Decide up front how you want a string with invalid UTF-8 — from any source, not only parsing — to be serialized (`strict`, `replace`, or `ignore`), rather than discovering it from an uncaught `type_error.316`.
- If a stray NUL byte silently truncating trailing input is a problem for your input format, define `JSON_STRICT_NUL_HANDLING`.
- Decide whether duplicate object keys should be an error for your application, and add a callback if so.
- Do not enable `ignore_comments` or `ignore_trailing_commas` for input that must be strictly conforming JSON.
@@ -80,3 +80,9 @@ For the broader design rationale and how it is tested (fuzzing, sanitizers, stat
- [Number handling](https://json.nlohmann.me/features/types/number_handling/index.md) - number ranges and overflow behavior
- [Assurance case](https://json.nlohmann.me/community/assurance_case/index.md) - the library's threat model and countermeasures
- [Security policy](https://json.nlohmann.me/community/security_policy/index.md) - how to report a vulnerability
- [`parse`](https://json.nlohmann.me/api/basic_json/parse/index.md) - deserialize from a compatible input
- [`accept`](https://json.nlohmann.me/api/basic_json/accept/index.md) - check if the input is valid JSON
- [`sax_parse`](https://json.nlohmann.me/api/basic_json/sax_parse/index.md) - generate SAX events
- [`parser_callback_t`](https://json.nlohmann.me/api/basic_json/parser_callback_t/index.md) - callback function type for the parser
- [`JSON_STRICT_NUL_HANDLING`](https://json.nlohmann.me/api/macros/json_strict_nul_handling/index.md) - opt in to rejecting a NUL byte in the input
- [`JSON_DIAGNOSTIC_POSITIONS`](https://json.nlohmann.me/api/macros/json_diagnostic_positions/index.md) - record start and end positions of parsed values