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<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=description content="JSON for Modern C++ is a header-only C++11 library that makes JSON a first-class data type."><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/parsing/" rel="canonical"><link href=../performance/ rel=prev><link href=json_lines/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.8.0"><title>Parsing - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.57a0d79d.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.fb2fffe2.min.css><link rel=stylesheet href=../../css/custom.css><link rel=stylesheet href=../../css/brand.css><link rel=stylesheet href=../../css/fig-sax.css><link rel=stylesheet href=../../css/fig-jsonview.css><link rel=stylesheet 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data-md-component=announce> </div> <header class=md-header data-md-component=header> <nav class="md-header__inner md-grid" aria-label=Header> <a href=../.. title="JSON for Modern C++" class="md-header__button md-logo" aria-label="JSON for Modern C++" data-md-component=logo> <img src=../../images/wordmark-header.svg alt=logo> </a> <label class="md-header__button md-icon" for=__drawer> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M3 6h18v2H3zm0 5h18v2H3zm0 5h18v2H3z"/></svg> </label> <div class=md-header__title data-md-component=header-title> <div class=md-header__ellipsis> <div class=md-header__topic> <span class=md-ellipsis> JSON for Modern C++ </span> </div> <div class=md-header__topic data-md-component=header-topic> <span class=md-ellipsis> Parsing </span> </div> </div> </div> <form class=md-header__option data-md-component=palette> <input class=md-option data-md-color-media="(prefers-color-scheme: light)" data-md-color-scheme=default 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<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=description content="JSON for Modern C++ is a header-only C++11 library that makes JSON a first-class data type."><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/parsing/" rel="canonical"><link href=../performance/ rel=prev><link href=json_lines/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.8.0"><title>Parsing - JSON for Modern C++</title><link rel=stylesheet href=../../assets/stylesheets/main.57a0d79d.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.fb2fffe2.min.css><link rel=stylesheet href=../../css/custom.css><link rel=stylesheet href=../../css/brand.css><link rel=stylesheet href=../../css/fig-sax.css><link rel=stylesheet href=../../css/fig-jsonview.css><link rel=stylesheet href=../../css/fig-exceptions.css><link rel=stylesheet href=../../css/fig-pointer.css><link rel=stylesheet href=../../css/fig-charts.css><link rel=stylesheet href=../../css/fig-integration.css><link rel=stylesheet href=../../css/fig-macros.css><script>__md_scope=new URL("../..",location),__md_hash=e=>[...e].reduce(((e,_)=>(e<<5)-e+_.charCodeAt(0)),0),__md_get=(e,_=localStorage,t=__md_scope)=>JSON.parse(_.getItem(t.pathname+"."+e)),__md_set=(e,_,t=localStorage,a=__md_scope)=>{try{t.setItem(a.pathname+"."+e,JSON.stringify(_))}catch(e){}}</script></head> <body dir=ltr data-md-color-scheme=default data-md-color-primary=custom data-md-color-accent=custom> <input class=md-toggle data-md-toggle=drawer type=checkbox id=__drawer autocomplete=off> <input class=md-toggle data-md-toggle=search type=checkbox id=__search autocomplete=off> <label class=md-overlay for=__drawer></label> <div data-md-component=skip> <a href=#parsing class=md-skip> Skip to content </a> </div> <div data-md-component=announce> </div> <header class=md-header data-md-component=header> <nav class="md-header__inner md-grid" aria-label=Header> <a href=../.. title="JSON for Modern C++" class="md-header__button md-logo" aria-label="JSON for Modern C++" data-md-component=logo> <img src=../../images/wordmark-header.svg alt=logo> </a> <label class="md-header__button md-icon" for=__drawer> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M3 6h18v2H3zm0 5h18v2H3zm0 5h18v2H3z"/></svg> </label> <div class=md-header__title data-md-component=header-title> <div class=md-header__ellipsis> <div class=md-header__topic> <span class=md-ellipsis> JSON for Modern C++ </span> </div> <div class=md-header__topic data-md-component=header-topic> <span class=md-ellipsis> Parsing </span> </div> </div> </div> <form class=md-header__option data-md-component=palette> <input class=md-option data-md-color-media="(prefers-color-scheme: light)" data-md-color-scheme=default 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12s-1.44-4.5-3.5-5.45C10.26 6.2 11.11 6 12 6a6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> </form> <script>var palette=__md_get("__palette");if(palette&&palette.color){if("(prefers-color-scheme)"===palette.color.media){var media=matchMedia("(prefers-color-scheme: light)"),input=document.querySelector(media.matches?"[data-md-color-media='(prefers-color-scheme: light)']":"[data-md-color-media='(prefers-color-scheme: dark)']");palette.color.media=input.getAttribute("data-md-color-media"),palette.color.scheme=input.getAttribute("data-md-color-scheme"),palette.color.primary=input.getAttribute("data-md-color-primary"),palette.color.accent=input.getAttribute("data-md-color-accent")}for(var[key,value]of Object.entries(palette.color))document.body.setAttribute("data-md-color-"+key,value)}</script> <label class="md-header__button md-icon" for=__search> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 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()"]
@@ -10,8 +10,8 @@
<span class=c1>// parse from a file</span>
<span class=n>std</span><span class=o>::</span><span class=n>ifstream</span><span class=w> </span><span class=n>f</span><span class=p>(</span><span class=s>"example.json"</span><span class=p>);</span>
<span class=n>json</span><span class=w> </span><span class=n>data</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>parse</span><span class=p>(</span><span class=n>f</span><span class=p>);</span>
</code></pre></div> <p>See <a href=../files/ >Reading and writing files</a> for checking that the file was opened and for writing files safely.</p> <p>The input must be encoded in <abbr title="Unicode Transformation Format">UTF</abbr>-8; other encodings are not supported. The exception is wide-character input, which is read as <abbr title="Unicode Transformation Format">UTF</abbr>-16 or <abbr title="Unicode Transformation Format">UTF</abbr>-32 — depending on the character type — and converted to <abbr title="Unicode Transformation Format">UTF</abbr>-8; see the <a href=../../api/basic_json/parse/ ><code>parse</code></a> page for an example. A single input may contain only one <abbr title="JavaScript Object Notation">JSON</abbr> value. Inputs consisting of multiple values separated by newlines are handled by the <a href=json_lines/ ><abbr title="JavaScript Object Notation">JSON</abbr> Lines</a> format; input that arrives in chunks or contains several values is covered in <a href=large_and_partial_input/ >Large, streamed, and partial input</a>.</p> <p>By default, the library rejects comments and trailing commas. Both can be enabled with parameters of the <code>parse</code> function — see <a href=../comments/ >comments</a> and <a href=../trailing_commas/ >trailing commas</a>.</p> <p><a href=../../api/basic_json_document/parse/ ><code>basic_json_document::parse</code></a> accepts the same inputs with the same options, but keeps the result in a document rather than in a <code>basic_json</code> value; see <a href=#documents-and-views>Documents and views</a> below.</p> <h2 id=strictness-and-trailing-data>Strictness and trailing data<a class=headerlink href=#strictness-and-trailing-data title="Permanent link">&para;</a></h2> <p><a href=../../api/basic_json/parse/ ><code>parse</code></a> reads a single <abbr title="JavaScript Object Notation">JSON</abbr> value and requires the whole input to be consumed: any non-whitespace data after the value is reported as a parse error. Use it when you want to guarantee that an input is exactly one complete <abbr title="JavaScript Object Notation">JSON</abbr> document.</p> <p><a href=../../api/operator_gtgt/ ><code>operator&gt;&gt;</code></a> follows relaxed <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>istream</span></code> semantics instead: it parses one <abbr title="JavaScript Object Notation">JSON</abbr> value and leaves the stream positioned right after it, without requiring the rest of the stream to be consumed. This is what makes it possible to read several concatenated values from the same stream, but it also means that "a valid document followed by trailing bytes" is accepted rather than rejected. If you are validating conformance, or need to reject any input that is not exactly one <abbr title="JavaScript Object Notation">JSON</abbr> document, prefer <code>parse</code>.</p> <p>When using <code>operator&gt;&gt;</code> to read several concatenated values this way, a value that is a number must be followed by whitespace, because <code>operator&gt;&gt;</code> consumes the character that terminates a number, unless <a href=../../api/macros/json_precise_stream_position/ ><code>JSON_PRECISE_STREAM_POSITION</code></a> is defined to <code>1</code> — see the <a href=../../api/operator_gtgt/#notes><code>operator&gt;&gt;</code> notes</a> for details and examples.</p> <h2 id=input-sources-and-performance>Input sources and performance<a class=headerlink href=#input-sources-and-performance title="Permanent link">&para;</a></h2> <p>All inputs produce the same values and the same exceptions, but they are read in different ways, which makes some of them faster than others:</p> <table> <thead> <tr> <th>input</th> <th>how it is read</th> </tr> </thead> <tbody> <tr> <td>strings, string views, string literals, contiguous containers, pointer or iterator ranges over contiguous memory</td> <td>the whole input is scanned in bulk: runs of string characters, numbers, and indentation are recognized several bytes at a time</td> </tr> <tr> <td><code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>istream</span></code> (<code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>ifstream</span></code>, <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>istringstream</span></code>, ...)</td> <td>the characters the stream buffer holds are scanned in bulk like a string; the stream buffer is asked for more when they are used up</td> </tr> <tr> <td><code class=highlight><span class=kt>FILE</span><span class=o>*</span></code></td> <td>with glibc and the Apple C library, the characters the <code>FILE</code> holds in its buffer are scanned in bulk like a stream's; elsewhere one character at a time; the file is 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</spanLine truncated
</code></pre></div> <p>See <a href=../files/ >Reading and writing files</a> for checking that the file was opened and for writing files safely.</p> <p>The input must be encoded in <abbr title="Unicode Transformation Format">UTF</abbr>-8; other encodings are not supported. The exception is wide-character input, which is read as <abbr title="Unicode Transformation Format">UTF</abbr>-16 or <abbr title="Unicode Transformation Format">UTF</abbr>-32 — depending on the character type — and converted to <abbr title="Unicode Transformation Format">UTF</abbr>-8; see the <a href=../../api/basic_json/parse/ ><code>parse</code></a> page for an example. A single input may contain only one <abbr title="JavaScript Object Notation">JSON</abbr> value. Inputs consisting of multiple values separated by newlines are handled by the <a href=json_lines/ ><abbr title="JavaScript Object Notation">JSON</abbr> Lines</a> format; input that arrives in chunks or contains several values is covered in <a href=large_and_partial_input/ >Large, streamed, and partial input</a>.</p> <p>By default, the library rejects comments and trailing commas. Both can be enabled with parameters of the <code>parse</code> function — see <a href=../comments/ >comments</a> and <a href=../trailing_commas/ >trailing commas</a>.</p> <p><a href=../../api/basic_json_document/parse/ ><code>basic_json_document::parse</code></a> accepts the same inputs with the same options, but keeps the result in a document rather than in a <code>basic_json</code> value; see <a href=#documents-and-views>Documents and views</a> below.</p> <h2 id=strictness-and-trailing-data>Strictness and trailing data<a class=headerlink href=#strictness-and-trailing-data title="Permanent link">&para;</a></h2> <p><a href=../../api/basic_json/parse/ ><code>parse</code></a> reads a single <abbr title="JavaScript Object Notation">JSON</abbr> value and requires the whole input to be consumed: any non-whitespace data after the value is reported as a parse error. Use it when you want to guarantee that an input is exactly one complete <abbr title="JavaScript Object Notation">JSON</abbr> document.</p> <p><a href=../../api/operator_gtgt/ ><code>operator&gt;&gt;</code></a> follows relaxed <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>istream</span></code> semantics instead: it parses one <abbr title="JavaScript Object Notation">JSON</abbr> value and leaves the stream positioned right after it, without requiring the rest of the stream to be consumed. This is what makes it possible to read several concatenated values from the same stream, but it also means that "a valid document followed by trailing bytes" is accepted rather than rejected. If you are validating conformance, or need to reject any input that is not exactly one <abbr title="JavaScript Object Notation">JSON</abbr> document, prefer <code>parse</code>.</p> <p>When using <code>operator&gt;&gt;</code> to read several concatenated values this way, a value that is a number must be followed by whitespace, because <code>operator&gt;&gt;</code> consumes the character that terminates a number, unless <a href=../../api/macros/json_precise_stream_position/ ><code>JSON_PRECISE_STREAM_POSITION</code></a> is defined to <code>1</code> — see the <a href=../../api/operator_gtgt/#notes><code>operator&gt;&gt;</code> notes</a> for details and examples.</p> <h2 id=input-sources-and-performance>Input sources and performance<a class=headerlink href=#input-sources-and-performance title="Permanent link">&para;</a></h2> <p>All inputs produce the same values and the same exceptions, but they are read in different ways, which makes some of them faster than others:</p> <table> <thead> <tr> <th>input</th> <th>how it is read</th> </tr> </thead> <tbody> <tr> <td>strings, string views, string literals, contiguous containers, pointer or iterator ranges over contiguous memory</td> <td>the whole input is scanned in bulk: runs of string characters, numbers, and indentation are recognized several bytes at a time</td> </tr> <tr> <td><code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>istream</span></code> (<code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>ifstream</span></code>, <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>istringstream</span></code>, ...)</td> <td>the characters the stream buffer holds are scanned in bulk like a string; the stream buffer is asked for more when they are used up</td> </tr> <tr> <td><code class=highlight><span class=kt>FILE</span><span class=o>*</span></code></td> <td>with glibc and the Apple C library, the characters the <code>FILE</code> holds in its buffer are scanned in bulk like a stream's; elsewhere one character at a time; where the C library has unlocked stdio functions, the file is locked once for the whole parse</td> </tr> <tr> <td>other iterator ranges (e.g., <code class=highlight><span class=n>std</spLine truncated
<span class=n>json_document</span><span class=w> </span><span class=n>doc</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>json_document</span><span class=o>::</span><span class=n>parse</span><span class=p>(</span><span class=sa>R</span><span class=s>"</span><span class=dl>(</span><span class=s>{"happy": true, "pi": 3.141}</span><span class=dl>)</span><span class=s>"</span><span class=p>);</span>
<span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>doc</span><span class=p>.</span><span class=n>root</span><span class=p>()[</span><span class=s>"happy"</span><span class=p>].</span><span class=n>get</span><span class=o>&lt;</span><span class=kt>bool</span><span class=o>&gt;</span><span class=p>()</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=sc>'\n'</span><span class=p>;</span>
</code></pre></div> <p>See <a href=../json_view/ >Zero-copy <abbr title="JavaScript Object Notation">JSON</abbr> views</a> for the concept and the trade-offs.</p> <h2 id=exceptions>Exceptions<a class=headerlink href=#exceptions title="Permanent link">&para;</a></h2> <p>When the input is not valid <abbr title="JavaScript Object Notation">JSON</abbr>, the <code>parse</code> function throws an exception by default. If exceptions are undesired or unavailable, the parser can instead return a discarded value, or <a href=../../api/basic_json/accept/ ><code>accept</code></a> can be used to only check whether an input is valid <abbr title="JavaScript Object Notation">JSON</abbr>. See <a href=parse_exceptions/ >parsing and exceptions</a> for the available options.</p> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../../api/basic_json/parse/ ><code>parse</code></a> - deserialize from a compatible input</li> <li><a href=../../api/basic_json_document/parse/ ><code>basic_json_document::parse</code></a> - deserialize into a document instead of a value</li> <li><a href=../../api/basic_json/accept/ ><code>accept</code></a> - check if the input is valid <abbr title="JavaScript Object Notation">JSON</abbr></li> <li><a href=../../api/basic_json/sax_parse/ ><code>sax_parse</code></a> - generate <abbr title="Simple API for XML">SAX</abbr> events</li> <li><a href=json_lines/ ><abbr title="JavaScript Object Notation">JSON</abbr> Lines</a> - parse newline-delimited <abbr title="JavaScript Object Notation">JSON</abbr></li> <li><a href=large_and_partial_input/ >Large, streamed, and partial input</a> - chunked, concatenated, and very large input</li> <li><a href=parser_callbacks/ >parser callbacks</a> - influence the parsing by a callback function</li> <li><a href=sax_interface/ ><abbr title="Simple API for XML">SAX</abbr> interface</a> - implement a custom <abbr title="Simple API for XML">SAX</abbr> handler</li> <li><a href=../json_view/ >Zero-copy <abbr title="JavaScript Object Notation">JSON</abbr> views</a> - parse into a read-only document and read it through views</li> <li><a href=parse_exceptions/ >parsing and exceptions</a> - control error handling</li> <li><a href=untrusted_input/ >parsing untrusted input</a> - what to consider when parsing input from untrusted sources</li> <li><a href=../../api/operator_gtgt/ ><code>operator&gt;&gt;</code></a> - deserialize from stream</li> <li><a href=../../api/json_sax/ ><code>json_sax</code></a> - documentation of the <abbr title="Simple API for XML">SAX</abbr> interface</li> <li><a href=../../api/basic_json/parse_error/ ><code>parse_error</code></a> - exception thrown on a parse error</li> <li><a href=../comments/ >Comments</a> - ignore <code>//</code> and <code>/* */</code> comments while parsing</li> <li><a href=../trailing_commas/ >Trailing Commas</a> - accept trailing commas in arrays and objects</li> <li><a href=../serialization/ >Serialization</a> - pretty-printing, <abbr title="Unicode Transformation Format">UTF</abbr>-8 handling, and number output</li> <li><a href=../no_exceptions/ >Working Without Exceptions</a> - error handling when exceptions are disabled</li> <li><a href=../../home/exceptions/ >Exceptions</a> - list of exceptions and error codes</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="November 10, 2026 06:08:44 UTC">November 10, 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--pLine truncated
</code></pre></div> <p>See <a href=../json_view/ >Zero-copy <abbr title="JavaScript Object Notation">JSON</abbr> views</a> for the concept and the trade-offs.</p> <h2 id=exceptions>Exceptions<a class=headerlink href=#exceptions title="Permanent link">&para;</a></h2> <p>When the input is not valid <abbr title="JavaScript Object Notation">JSON</abbr>, the <code>parse</code> function throws an exception by default. If exceptions are undesired or unavailable, the parser can instead return a discarded value, or <a href=../../api/basic_json/accept/ ><code>accept</code></a> can be used to only check whether an input is valid <abbr title="JavaScript Object Notation">JSON</abbr>. See <a href=parse_exceptions/ >parsing and exceptions</a> for the available options.</p> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../../api/basic_json/parse/ ><code>parse</code></a> - deserialize from a compatible input</li> <li><a href=../../api/basic_json_document/parse/ ><code>basic_json_document::parse</code></a> - deserialize into a document instead of a value</li> <li><a href=../../api/basic_json/accept/ ><code>accept</code></a> - check if the input is valid <abbr title="JavaScript Object Notation">JSON</abbr></li> <li><a href=../../api/basic_json/sax_parse/ ><code>sax_parse</code></a> - generate <abbr title="Simple API for XML">SAX</abbr> events</li> <li><a href=json_lines/ ><abbr title="JavaScript Object Notation">JSON</abbr> Lines</a> - parse newline-delimited <abbr title="JavaScript Object Notation">JSON</abbr></li> <li><a href=large_and_partial_input/ >Large, streamed, and partial input</a> - chunked, concatenated, and very large input</li> <li><a href=parser_callbacks/ >parser callbacks</a> - influence the parsing by a callback function</li> <li><a href=sax_interface/ ><abbr title="Simple API for XML">SAX</abbr> interface</a> - implement a custom <abbr title="Simple API for XML">SAX</abbr> handler</li> <li><a href=../json_view/ >Zero-copy <abbr title="JavaScript Object Notation">JSON</abbr> views</a> - parse into a read-only document and read it through views</li> <li><a href=parse_exceptions/ >parsing and exceptions</a> - control error handling</li> <li><a href=untrusted_input/ >parsing untrusted input</a> - what to consider when parsing input from untrusted sources</li> <li><a href=../../api/operator_gtgt/ ><code>operator&gt;&gt;</code></a> - deserialize from stream</li> <li><a href=../../api/json_sax/ ><code>json_sax</code></a> - documentation of the <abbr title="Simple API for XML">SAX</abbr> interface</li> <li><a href=../../api/basic_json/parse_error/ ><code>parse_error</code></a> - exception thrown on a parse error</li> <li><a href=../comments/ >Comments</a> - ignore <code>//</code> and <code>/* */</code> comments while parsing</li> <li><a href=../trailing_commas/ >Trailing Commas</a> - accept trailing commas in arrays and objects</li> <li><a href=../serialization/ >Serialization</a> - pretty-printing, <abbr title="Unicode Transformation Format">UTF</abbr>-8 handling, and number output</li> <li><a href=../no_exceptions/ >Working Without Exceptions</a> - error handling when exceptions are disabled</li> <li><a href=../../home/exceptions/ >Exceptions</a> - list of exceptions and error codes</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="November 12, 2026 07:40:05 UTC">November 12, 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--pLine truncated
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@@ -54,12 +54,14 @@ When using `operator>>` to read several concatenated values this way, a value th
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:
| input | how it is read |
| ---------------------------------------------------------------------------------------------------------------- | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| strings, string views, string literals, contiguous containers, pointer or iterator ranges over contiguous memory | the whole input is scanned in bulk: runs of string characters, numbers, and indentation are recognized several bytes at a time |
| `std::istream` (`std::ifstream`, `std::istringstream`, ...) | 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 |
| `FILE*` | with glibc and the Apple C library, the characters the `FILE` holds in its buffer are scanned in bulk like a stream's; elsewhere one character at a time; the file is locked once for the whole parse |
| other iterator ranges (e.g., `std::list<char>`, `std::istreambuf_iterator<char>`), wide strings | one character at a time |
| input | how it is read |
| ---------------------------------------------------------------------------------------------------------------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| strings, string views, string literals, contiguous containers, pointer or iterator ranges over contiguous memory | the whole input is scanned in bulk: runs of string characters, numbers, and indentation are recognized several bytes at a time |
| `std::istream` (`std::ifstream`, `std::istringstream`, ...) | 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 |
| `FILE*` | with glibc and the Apple C library, the characters the `FILE` holds in its buffer are scanned in bulk like a stream's; elsewhere one character at a time; where the C library has unlocked stdio functions, the file is locked once for the whole parse |
| other iterator ranges (e.g., `std::list<char>`, `std::istreambuf_iterator<char>`), wide strings | one character at a time |
Defining [`JSON_DISABLE_STREAM_BULK_SCAN`](https://json.nlohmann.me/api/macros/json_disable_stream_bulk_scan/index.md) to `1` turns off the bulk scan of a stream buffer, so a `std::istream` is read one character at a time. Defining [`JSON_DISABLE_UNLOCKED_FILE_IO`](https://json.nlohmann.me/api/macros/json_disable_unlocked_file_io/index.md) to `1` makes a `FILE*` read every character with `std::fgetc`, without locking the file for the whole parse and without the bulk scan of its buffer (which `JSON_DISABLE_STREAM_BULK_SCAN` does not affect). Neither changes any result.
Input in one block of memory is read even faster with the default options (no [parser callback](https://json.nlohmann.me/api/basic_json/parser_callback_t/index.md), no comments, no trailing commas, and for `sax_parse()` `strict = true`): `parse()`, `accept()`, and `sax_parse()` then read the grammar and the tokens in one loop over the memory. Only after an error do `parse()` and `sax_parse()` read the input again, to report the error exactly as before. This faster path is not used when [`JSON_DIAGNOSTIC_POSITIONS`](https://json.nlohmann.me/api/macros/json_diagnostic_positions/index.md) is enabled, because the positions are recorded by the default parser: the input is then parsed as before, at the same speed as before.
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@@ -1,4 +1,4 @@
<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=description content="JSON for Modern C++ is a header-only C++11 library that makes JSON a first-class data type."><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/parsing/json_lines/" rel="canonical"><link href=../ rel=prev><link href=../large_and_partial_input/ rel=next><link rel=icon href=../../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.8.0"><title>JSON Lines - JSON for Modern C++</title><link rel=stylesheet href=../../../assets/stylesheets/main.57a0d79d.min.css><link rel=stylesheet href=../../../assets/stylesheets/palette.fb2fffe2.min.css><link rel=stylesheet href=../../../css/custom.css><link rel=stylesheet href=../../../css/brand.css><link rel=stylesheet href=../../../css/fig-sax.css><link rel=stylesheet href=../../../css/fig-jsonview.css><link 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<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=description content="JSON for Modern C++ is a header-only C++11 library that makes JSON a first-class data type."><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/parsing/json_lines/" rel="canonical"><link href=../ rel=prev><link href=../large_and_partial_input/ rel=next><link rel=icon href=../../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.8.0"><title>JSON Lines - JSON for Modern C++</title><link rel=stylesheet href=../../../assets/stylesheets/main.57a0d79d.min.css><link rel=stylesheet href=../../../assets/stylesheets/palette.fb2fffe2.min.css><link rel=stylesheet href=../../../css/custom.css><link rel=stylesheet href=../../../css/brand.css><link rel=stylesheet href=../../../css/fig-sax.css><link rel=stylesheet href=../../../css/fig-jsonview.css><link 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<span class=p>{</span><span class=nt>"name"</span><span class=p>:</span><span class=w> </span><span class=s2>"Alexa"</span><span class=p>,</span><span class=w> </span><span class=nt>"wins"</span><span class=p>:</span><span class=w> </span><span class=p>[[</span><span class=s2>"two pair"</span><span class=p>,</span><span class=w> </span><span class=s2>"4♠"</span><span class=p>],</span><span class=w> </span><span class=p>[</span><span class=s2>"two pair"</span><span class=p>,</span><span class=w> </span><span class=s2>"9♠"</span><span class=p>]]}</span>
<span class=p>{</span><span class=nt>"name"</span><span class=p>:</span><span class=w> </span><span class=s2>"May"</span><span class=p>,</span><span class=w> </span><span class=nt>"wins"</span><span class=p>:</span><span class=w> </span><span class=p>[]}</span>
<span class=p>{</span><span class=nt>"name"</span><span class=p>:</span><span class=w> </span><span class=s2>"Deloise"</span><span class=p>,</span><span class=w> </span><span class=nt>"wins"</span><span class=p>:</span><span class=w> </span><span class=p>[[</span><span class=s2>"three of a kind"</span><span class=p>,</span><span class=w> </span><span class=s2>"5♣"</span><span class=p>]]}</span>
@@ -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)?"}
+1 -1
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@@ -1,4 +1,4 @@
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<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>
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<span class=k>using</span><span class=w> </span><span class=n>parser_callback_t</span><span class=w> </span><span class=o>=</span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>function</span><span class=o>&lt;</span><span class=kt>bool</span><span class=p>(</span><span class=kt>int</span><span class=w> </span><span class=n>depth</span><span class=p>,</span><span class=w> </span><span class=n>parse_event_t</span><span class=w> </span><span class=n>event</span><span class=p>,</span><span class=w> </span><span class=n>BasicJsonType</span><span class=o>&amp;</span><span class=w> </span><span class=n>parsed</span><span class=p>)</span><span class=o>&gt;</span><span class=p>;</span>
</code></pre></div> <h2 id=callback-event-types>Callback event types<a class=headerlink href=#callback-event-types title="Permanent link">&para;</a></h2> <p>We distinguish six scenarios (determined by the event type) in which the callback function can be called. The following table describes the values of the parameters <code>depth</code>, <code>event</code>, and <code>parsed</code>.</p> <table> <thead> <tr> <th>parameter <code>event</code></th> <th>description</th> <th>parameter <code>depth</code></th> <th>parameter <code>parsed</code></th> </tr> </thead> <tbody> <tr> <td><code>parse_event_t::object_start</code></td> <td>the parser read <code>{</code> and started to process a <abbr title="JavaScript Object Notation">JSON</abbr> object</td> <td>depth of the parent of the <abbr title="JavaScript Object Notation">JSON</abbr> object</td> <td>a <abbr title="JavaScript Object Notation">JSON</abbr> value with type discarded</td> </tr> <tr> <td><code>parse_event_t::key</code></td> <td>the parser read a key of a value in an object</td> <td>depth of the currently parsed <abbr title="JavaScript Object Notation">JSON</abbr> object</td> <td>a <abbr title="JavaScript Object Notation">JSON</abbr> string containing the key</td> </tr> <tr> <td><code>parse_event_t::object_end</code></td> <td>the parser read <code>}</code> and finished processing a <abbr title="JavaScript Object Notation">JSON</abbr> object</td> <td>depth of the parent of the <abbr title="JavaScript Object Notation">JSON</abbr> object</td> <td>the parsed <abbr title="JavaScript Object Notation">JSON</abbr> object</td> </tr> <tr> <td><code>parse_event_t::array_start</code></td> <td>the parser read <code>[</code> and started to process a <abbr title="JavaScript Object Notation">JSON</abbr> array</td> <td>depth of the parent of the <abbr title="JavaScript Object Notation">JSON</abbr> array</td> <td>a <abbr title="JavaScript Object Notation">JSON</abbr> value with type discarded</td> </tr> <tr> <td><code>parse_event_t::array_end</code></td> <td>the parser read <code>]</code> and finished processing a <abbr title="JavaScript Object Notation">JSON</abbr> array</td> <td>depth of the parent of the <abbr title="JavaScript Object Notation">JSON</abbr> array</td> <td>the parsed <abbr title="JavaScript Object Notation">JSON</abbr> array</td> </tr> <tr> <td><code>parse_event_t::value</code></td> <td>the parser finished reading a <abbr title="JavaScript Object Notation">JSON</abbr> value</td> <td>depth of the value</td> <td>the parsed <abbr title="JavaScript Object Notation">JSON</abbr> value</td> </tr> </tbody> </table> <div class=nj-fig-wrap> <p><svg class="nj-fig nj-fig--cb" viewbox="0 0 480 262" role=img aria-labelledby=fig-cb-title xmlns=http://www.w3.org/2000/svg> <title id=fig-cb-title>Example of the parse events a callback sees for the J&#8203;SON text {&quot;name&quot;: &quot;Berlin&quot;, &quot;location&quot;: [52.519444, 13.406667]}: object_start after the opening brace, key after each key, array_start after the opening bracket, value after each value, array_end after the closing bracket, and object_end after the closing brace. Every array and object also produces a value event when it is complete.</title> <text class=fg-h x=16 y=28>J&#8203;SON to parse</text> <text class=fg-h x=320 y=28>callback events</text> <text class=fg-code x=16 y=74 xml:space=preserve>{</text> <circle class=cb-os cx=35.6 cy=69 r=5.5 /> <text class=fg-code x=35.2 y=102 xml:space=preserve>&quot;name&quot;</text> <circle class=cb-k cx=102.8 cy=97 r=5.5 /> <text class=fg-code x=112.8 y=102 xml:space=preserve>: &quot;Berlin&quot;</text> <circle class=cb-v cx=218.8 cy=97 r=5.5 /> <text class=fg-code x=228.8 y=102 xml:space=preserve>,</text> <text class=fg-code x=35.2 y=130 xml:space=preserve>&quot;location&quot;</text> <circle class=cb-k cx=141.2 cy=125 r=5.5 /> <text class=fg-code x=151.2 y=130 xml:space=preserve>: [</text> <circle class=cb-as cx=190 cy=125 r=5.5 /> <text class=fg-code x=54.4 y=158 xml:space=preserve>52.519444</text> <circle class=cb-v cx=150.8 cy=153 r=5.5 /> <text class=fg-code x=160.8 y=158 xml:space=preserve>,</text> <text class=fg-code x=54.4 y=186 xml:space=preserve>13.406667</text> <circle class=cb-v cx=150.8 cy=181 r=5.5 /> <text class=fg-code x=35.2 y=214 xml:space=preserve>]</text> <circle class=cb-ae cx=54.8 cy=209 r=5.5 /> <circle class=cb-v cx=74.8 cy=209 r=5.5 /> <text class=fg-code x=16 y=242 xml:space=preserve>}</text> <circle class=cb-oe cx=35.6 cy=237 r=5.5 /> <circle class=cb-v cx=55.6 cy=237 r=5.5 /> <circle class=cb-os cx=327 cy=69 r=6 /> <text class="fg-code fg-leg" x=344 y=74>object_start</text> <circle class=cb-k cx=327 cy=99 r=6 /> <text class="fg-code fg-leg" x=344 y=104>key</text> <circle class=cb-oe cx=327 cy=129 r=6 /> <text class="fg-code fg-leg" x=344 y=134>object_end</text> <circle class=cb-as cx=327 cy=159 r=6 /> <text class="fg-code fg-leg" x=344 y=164>array_start</text> <circle class=cb-ae cx=327 cy=189 r=6 /> <text class="fg-code fg-leg" xLine truncated
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class sax_t ["json::sax_t"] {
&lt;&lt;interface&gt;&gt;
+37 -9
View File
@@ -54,10 +54,32 @@ Once a value has been parsed, operations that walk it recursively — serializin
[`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
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.
[`merge_diff`](../../api/basic_json/merge_diff.md), converting it to another `basic_json` type, destroying it, and the
binary writers (`to_cbor`, `to_msgpack`, ...) — descend at most
[`JSON_RECURSION_DEPTH_LIMIT`](../../api/macros/json_recursion_depth_limit.md) levels on the call stack, 128 by default,
and continue below that with an explicit stack on the heap. So the call stack does not grow with the nesting depth of the
value, however deeply it is nested. ([`flatten`](../../api/basic_json/flatten.md),
[`unflatten`](../../api/basic_json/unflatten.md), and [`to_bson`](../../api/basic_json/to_bson.md) never use the call
stack for the nesting.)
The call stack still needs room for the levels that are recursed into, and the default limit assumes a stack of ordinary
size. The stack that an operation needs is roughly proportional to the limit; with the default of 128 levels, the most
demanding operation (`dump` in an unoptimized build) needed up to 288 KiB, and an optimized build needed 48 KiB or less,
in measurements on macOS with arm64. The most demanding operation needed this much stack at the limits `0`, `32`,
`128`, and `254`:
| Stack needed, in KiB | limit 0 | limit 32 | limit 128 (default) | limit 254 |
|---|---:|---:|---:|---:|
| unoptimized build (`-O0`) | 16 or less | 80 | 288 | 576 |
| optimized build (`-O2`) | 16 or less | 16 or less | 48 | 112 |
A thread with a smaller stack (a task of a real-time operating system, a fiber, or a thread of the musl C library, whose
default stack is 128 KiB) can still overflow its stack on deeply nested input, which is why the limit can be lowered:
define `JSON_RECURSION_DEPTH_LIMIT` to a smaller value, down to `0`, before including the library. Such a limit changes
only the stack that is needed and the speed (values nested more deeply than the limit are walked more slowly), never a
result. See the [documentation of the macro](../../api/macros/json_recursion_depth_limit.md) for the complete tables,
the speed, and the rules for choosing a value. Destroying a value (its destructor) recurses over the same number of
levels and then continues without the call stack, and needs 32 KiB or less at every limit.
## Input size
@@ -79,15 +101,19 @@ For the binary formats, an announced size is never trusted outright:
## Strings
Invalid UTF-8 is rejected while parsing, not just while serializing:
Invalid UTF-8 is rejected while parsing JSON text, not just while serializing:
- In JSON text, an ill-formed UTF-8 byte in a string is a
[`parse_error.101`](../../home/exceptions.md#jsonexceptionparse_error101) ("invalid string: ill-formed UTF-8 byte").
- In UTF-16 or UTF-32 input, an ill-formed sequence is a
[`parse_error.101`](../../home/exceptions.md#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`](../../home/exceptions.md#jsonexceptionparse_error113).
In a binary format, a string that is not valid UTF-8 is accepted unchanged by default (none of the formats except BON8
requires a decoder to reject it). Pass [`error_handler_t::strict`](../../api/basic_json/error_handler_t.md) to the
`from_*` function to get a [`parse_error.113`](../../home/exceptions.md#jsonexceptionparse_error113) instead. BON8
always validates ([`parse_error.112`](../../home/exceptions.md#jsonexceptionparse_error112) for an ill-formed
sequence).
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
@@ -98,8 +124,8 @@ that happens to embed a NUL is a way to make part of it disappear without a pars
unexpected byte instead.
Parsing is not the only place invalid UTF-8 matters: a string that reached a `#!cpp 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,
example, constructed by application code, or read from a binary format with the default `error_handler_t::keep`)
still has to round-trip back to JSON text. By default,
[`dump()`](../../api/basic_json/dump.md) throws [`type_error.316`](../../home/exceptions.md#jsonexceptiontype_error316)
if the string is not valid UTF-8; passing
[`error_handler_t::replace`](../../api/basic_json/error_handler_t.md) or `error_handler_t::ignore` avoids the exception
@@ -152,6 +178,8 @@ indices (or the pointer as a whole) before using them, or use [`at`](../../api/b
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`.
- For untrusted input in a binary format, pass `error_handler_t::strict` to the `from_*` function if the strings must be
valid UTF-8; by default they are not checked (BON8 always checks).
- 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.
+1 -1
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@@ -1 +1 @@
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@@ -22,7 +22,16 @@ 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), converting it to another `basic_json` type, destroying it, and the binary writers (`to_cbor`, `to_msgpack`, ...) — descend at most [`JSON_RECURSION_DEPTH_LIMIT`](https://json.nlohmann.me/api/macros/json_recursion_depth_limit/index.md) levels on the call stack, 128 by default, and continue below that with an explicit stack on the heap. So the call stack does not grow with the nesting depth of the value, however deeply it is nested. ([`flatten`](https://json.nlohmann.me/api/basic_json/flatten/index.md), [`unflatten`](https://json.nlohmann.me/api/basic_json/unflatten/index.md), and [`to_bson`](https://json.nlohmann.me/api/basic_json/to_bson/index.md) never use the call stack for the nesting.)
The call stack still needs room for the levels that are recursed into, and the default limit assumes a stack of ordinary size. The stack that an operation needs is roughly proportional to the limit; with the default of 128 levels, the most demanding operation (`dump` in an unoptimized build) needed up to 288 KiB, and an optimized build needed 48 KiB or less, in measurements on macOS with arm64. The most demanding operation needed this much stack at the limits `0`, `32`, `128`, and `254`:
| Stack needed, in KiB | limit 0 | limit 32 | limit 128 (default) | limit 254 |
| ------------------------- | ---------- | ---------- | ------------------- | --------- |
| unoptimized build (`-O0`) | 16 or less | 80 | 288 | 576 |
| optimized build (`-O2`) | 16 or less | 16 or less | 48 | 112 |
A thread with a smaller stack (a task of a real-time operating system, a fiber, or a thread of the musl C library, whose default stack is 128 KiB) can still overflow its stack on deeply nested input, which is why the limit can be lowered: define `JSON_RECURSION_DEPTH_LIMIT` to a smaller value, down to `0`, before including the library. Such a limit changes only the stack that is needed and the speed (values nested more deeply than the limit are walked more slowly), never a result. See the [documentation of the macro](https://json.nlohmann.me/api/macros/json_recursion_depth_limit/index.md) for the complete tables, the speed, and the rules for choosing a value. Destroying a value (its destructor) recurses over the same number of levels and then continues without the call stack, and needs 32 KiB or less at every limit.
## Input size
@@ -36,15 +45,16 @@ For the binary formats, an announced size is never trusted outright:
## Strings
Invalid UTF-8 is rejected while parsing, not just while serializing:
Invalid UTF-8 is rejected while parsing JSON text, not just while serializing:
- In JSON text, an ill-formed UTF-8 byte in a string is a [`parse_error.101`](https://json.nlohmann.me/home/exceptions/#jsonexceptionparse_error101) ("invalid string: ill-formed UTF-8 byte").
- 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).
In a binary format, a string that is not valid UTF-8 is accepted unchanged by default (none of the formats except BON8 requires a decoder to reject it). Pass [`error_handler_t::strict`](https://json.nlohmann.me/api/basic_json/error_handler_t/index.md) to the `from_*` function to get a [`parse_error.113`](https://json.nlohmann.me/home/exceptions/#jsonexceptionparse_error113) instead. BON8 always validates ([`parse_error.112`](https://json.nlohmann.me/home/exceptions/#jsonexceptionparse_error112) for an ill-formed sequence).
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.
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 read from a binary format with the default `error_handler_t::keep`) 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.
## Duplicate object keys
@@ -68,6 +78,7 @@ A [JSON Pointer](https://json.nlohmann.me/features/json_pointer/index.md) can co
- 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`.
- For untrusted input in a binary format, pass `error_handler_t::strict` to the `from_*` function if the strings must be valid UTF-8; by default they are not checked (BON8 always checks).
- 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.