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<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/parsing/" rel="canonical"><link href=../object_order/ 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="../../assets/external/fonts.googleapis.com/css.61a430c9.css"><style>:root{--md-text-font:"Roboto";--md-code-font:"JetBrains Mono"}</style><link rel=stylesheet href=../../css/custom.css><script>__md_scope=new URL("../..",location),__md_hash=e=>[...e].reduce(((e,_)=>(e<<5)-e+_.charCodeAt(0)),0),__md_get=(e,_=localStorage,t=__md_scope)=>JSON.parse(_.getItem(t.pathname+"."+e)),__md_set=(e,_,t=localStorage,a=__md_scope)=>{try{t.setItem(a.pathname+"."+e,JSON.stringify(_))}catch(e){}}</script></head> <body dir=ltr data-md-color-scheme=default data-md-color-primary=indigo data-md-color-accent=indigo> <input class=md-toggle data-md-toggle=drawer type=checkbox id=__drawer autocomplete=off> <input class=md-toggle data-md-toggle=search type=checkbox id=__search autocomplete=off> <label class=md-overlay for=__drawer></label> <div data-md-component=skip> <a href=#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> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 8a3 3 0 0 0 3-3 3 3 0 0 0-3-3 3 3 0 0 0-3 3 3 3 0 0 0 3 3m0 3.54C9.64 9.35 6.5 8 3 8v11c3.5 0 6.64 1.35 9 3.54 2.36-2.19 5.5-3.54 9-3.54V8c-3.5 0-6.64 1.35-9 3.54"/></svg> </a> <label class="md-header__button md-icon" for=__drawer> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M3 6h18v2H3zm0 5h18v2H3zm0 5h18v2H3z"/></svg> </label> <div class=md-header__title data-md-component=header-title> <div class=md-header__ellipsis> <div class=md-header__topic> <span class=md-ellipsis> JSON for Modern C++ </span> </div> <div class=md-header__topic data-md-component=header-topic> <span class=md-ellipsis> 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 data-md-color-primary=indigo data-md-color-accent=indigo aria-label="Switch to dark mode" type=radio name=__palette id=__palette_0> <label class="md-header__button md-icon" title="Switch to dark mode" for=__palette_1 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 8a4 4 0 0 0-4 4 4 4 0 0 0 4 4 4 4 0 0 0 4-4 4 4 0 0 0-4-4m0 10a6 6 0 0 1-6-6 6 6 0 0 1 6-6 6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> <input class=md-option data-md-color-media="(prefers-color-scheme: dark)" data-md-color-scheme=slate data-md-color-primary=indigo data-md-color-accent=indigo aria-label="Switch to light mode" type=radio name=__palette id=__palette_1> <label class="md-header__button md-icon" title="Switch to light mode" for=__palette_0 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 18c-.89 0-1.74-.2-2.5-.55C11.56 16.5 13 14.42 13 12s-1.44-4.5-3.5-5.45C10.26 6.2 11.11 6 12 6a6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> </form> <script>var palette=__md_get("__palette");if(palette&&palette.color){if("(prefers-color-scheme)"===palette.color.media){var media=matchMedia("(prefers-color-scheme: light)"),input=document.querySelector(media.matches?"[data-md-color-media='(prefers-color-scheme: light)']":"[data-md-color-media='(prefers-color-scheme: dark)']");palette.color.media=input.getAttribute("data-md-color-media"),palette.color.scheme=input.getAttribute("data-md-color-scheme"),palette.color.primary=input.getAttribute("data-md-color-primary"),palette.color.accent=input.getAttribute("data-md-color-accent")}for(var[key,value]of Object.entries(palette.color))document.body.setAttribute("data-md-color-"+key,value)}</script> <label class="md-header__button md-icon" for=__search> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M9.5 3A6.5 6.5 0 0 1 16 9.5c0 1.61-.59 3.09-1.56 4.23l.27.27h.79l5 5-1.5 1.5-5-5v-.79l-.27-.27A6.52 6.52 0 0 1 9.5 16 6.5 6.5 0 0 1 3 9.5 6.5 6.5 0 0 1 9.5 3m0 2C7 5 5 7 5 9.5S7 14 9.5 14 14 12 14 9.5 12 5 9.5 5"/></svg> </label> <div class=md-search data-md-componLine truncated
<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>&quot;</span><span class=dl>(</span><span class=s>{&quot;happy&quot;: true, &quot;pi&quot;: 3.141}</span><span class=dl>)</span><span class=s>&quot;</span><span class=p>);</span>
<span class=n>json</span><span class=w> </span><span class=n>j</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>parse</span><span class=p>(</span><span class=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>&quot;example.json&quot;</span><span class=p>);</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. 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.</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> <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 — see the <a href=../../api/operator_gtgt/#notes><code>operator&gt;&gt;</code> notes</a> for details and examples.</p> <h2 id=sax-vs-dom-parsing><abbr title="Simple API for XML">SAX</abbr> vs. <abbr title="Document Object Model">DOM</abbr> parsing<a class=headerlink href=#sax-vs-dom-parsing title="Permanent link">&para;</a></h2> <p>The library offers two parsing models:</p> <ul> <li><strong><abbr title="Document Object Model">DOM</abbr> parsing</strong> (the default): the complete input is read and stored as an in-memory <code>basic_json</code> value that can be traversed and modified freely. This is what <a href=../../api/basic_json/parse/ ><code>parse</code></a> does, and it is the right choice for most use cases.</li> <li><strong><abbr title="Simple API for XML">SAX</abbr> parsing</strong>: instead of building a value, the parser reports events (such as "a string was read" or "an object started") to a handler that you implement. This avoids building the full value in memory and is useful for very large inputs or when you only need to extract parts of the input. See the <a href=sax_interface/ ><abbr title="Simple API for XML">SAX</abbr> interface</a> for details and <a href=../../api/basic_json/sax_parse/ ><code>sax_parse</code></a> for the <abbr title="Application Programming Interfaces">API</abbr>.</li> </ul> <p>You can influence a <abbr title="Document Object Model">DOM</abbr> parse without switching to the <abbr title="Simple API for XML">SAX</abbr> interface by passing a <a href=parser_callbacks/ >parser callback</a>, which is called during parsing and can, for example, discard parts of the input.</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/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 ObjectLine truncated
</code></pre></div> <p>The input must be encoded in <abbr title="Unicode Transformation Format">UTF</abbr>-8; other encodings are not supported. 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.</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> <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 — see the <a href=../../api/operator_gtgt/#notes><code>operator&gt;&gt;</code> notes</a> for details and examples.</p> <h2 id=sax-vs-dom-parsing><abbr title="Simple API for XML">SAX</abbr> vs. <abbr title="Document Object Model">DOM</abbr> parsing<a class=headerlink href=#sax-vs-dom-parsing title="Permanent link">&para;</a></h2> <p>The library offers two parsing models:</p> <ul> <li><strong><abbr title="Document Object Model">DOM</abbr> parsing</strong> (the default): the complete input is read and stored as an in-memory <code>basic_json</code> value that can be traversed and modified freely. This is what <a href=../../api/basic_json/parse/ ><code>parse</code></a> does, and it is the right choice for most use cases.</li> <li><strong><abbr title="Simple API for XML">SAX</abbr> parsing</strong>: instead of building a value, the parser reports events (such as "a string was read" or "an object started") to a handler that you implement. This avoids building the full value in memory and is useful for very large inputs or when you only need to extract parts of the input. See the <a href=sax_interface/ ><abbr title="Simple API for XML">SAX</abbr> interface</a> for details and <a href=../../api/basic_json/sax_parse/ ><code>sax_parse</code></a> for the <abbr title="Application Programming Interfaces">API</abbr>.</li> </ul> <p>You can influence a <abbr title="Document Object Model">DOM</abbr> parse without switching to the <abbr title="Simple API for XML">SAX</abbr> interface by passing a <a href=parser_callbacks/ >parser callback</a>, which is called during parsing and can, for example, discard parts of the input.</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/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 ObjectLine truncated
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<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/parsing/json_lines/" rel="canonical"><link href=../ rel=prev><link href=../parse_exceptions/ 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 URL("../../..",location),__md_hash=e=>[...e].reduce(((e,_)=>(e<<5)-e+_.charCodeAt(0)),0),__md_get=(e,_=localStorage,t=__md_scope)=>JSON.parse(_.getItem(t.pathname+"."+e)),__md_set=(e,_,t=localStorage,a=__md_scope)=>{try{t.setItem(a.pathname+"."+e,JSON.stringify(_))}catch(e){}}</script></head> <body dir=ltr data-md-color-scheme=default data-md-color-primary=indigo data-md-color-accent=indigo> <input class=md-toggle data-md-toggle=drawer type=checkbox id=__drawer autocomplete=off> <input class=md-toggle data-md-toggle=search type=checkbox id=__search autocomplete=off> <label class=md-overlay for=__drawer></label> <div data-md-component=skip> <a href=#json-lines class=md-skip> Skip to content </a> </div> <div data-md-component=announce> </div> <header class=md-header data-md-component=header> <nav class="md-header__inner md-grid" aria-label=Header> <a href=../../.. title="JSON for Modern C++" class="md-header__button md-logo" aria-label="JSON for Modern C++" data-md-component=logo> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 8a3 3 0 0 0 3-3 3 3 0 0 0-3-3 3 3 0 0 0-3 3 3 3 0 0 0 3 3m0 3.54C9.64 9.35 6.5 8 3 8v11c3.5 0 6.64 1.35 9 3.54 2.36-2.19 5.5-3.54 9-3.54V8c-3.5 0-6.64 1.35-9 3.54"/></svg> </a> <label class="md-header__button md-icon" for=__drawer> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M3 6h18v2H3zm0 5h18v2H3zm0 5h18v2H3z"/></svg> </label> <div class=md-header__title data-md-component=header-title> <div class=md-header__ellipsis> <div class=md-header__topic> <span class=md-ellipsis> JSON for Modern C++ </span> </div> <div class=md-header__topic data-md-component=header-topic> <span class=md-ellipsis> JSON Lines </span> </div> </div> </div> <form class=md-header__option data-md-component=palette> <input class=md-option data-md-color-media="(prefers-color-scheme: light)" data-md-color-scheme=default data-md-color-primary=indigo data-md-color-accent=indigo aria-label="Switch to dark mode" type=radio name=__palette id=__palette_0> <label class="md-header__button md-icon" title="Switch to dark mode" for=__palette_1 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 8a4 4 0 0 0-4 4 4 4 0 0 0 4 4 4 4 0 0 0 4-4 4 4 0 0 0-4-4m0 10a6 6 0 0 1-6-6 6 6 0 0 1 6-6 6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> <input class=md-option data-md-color-media="(prefers-color-scheme: dark)" data-md-color-scheme=slate data-md-color-primary=indigo data-md-color-accent=indigo aria-label="Switch to light mode" type=radio name=__palette id=__palette_1> <label class="md-header__button md-icon" title="Switch to light mode" for=__palette_0 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 18c-.89 0-1.74-.2-2.5-.55C11.56 16.5 13 14.42 13 12s-1.44-4.5-3.5-5.45C10.26 6.2 11.11 6 12 6a6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> </form> <script>var palette=__md_get("__palette");if(palette&&palette.color){if("(prefers-color-scheme)"===palette.color.media){var media=matchMedia("(prefers-color-scheme: light)"),input=document.querySelector(media.matches?"[data-md-color-media='(prefers-color-scheme: light)']":"[data-md-color-media='(prefers-color-scheme: dark)']");palette.color.media=input.getAttribute("data-md-color-media"),palette.color.scheme=input.getAttribute("data-md-color-scheme"),palette.color.primary=input.getAttribute("data-md-color-primary"),palette.color.accent=input.getAttribute("data-md-color-accent")}for(var[key,value]of Object.entries(palette.color))document.body.setAttribute("data-md-color-"+key,value)}</script> <label class="md-header__button md-icon" for=__search> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M9.5 3A6.5 6.5 0 0 1 16 9.5c0 1.61-.59 3.09-1.56 4.23l.27.27h.79l5 5-1.5 1.5-5-5v-.79l-.27-.27A6.52 6.52 0 0 1 9.5 16 6.5 6.5 0 0 1 3 9.5 6.5 6.5 0 0 1 9.5 3m0 2C7 5 5 7 5 9.5S7 14 9.5 14 14 12 14 9.5 12 5 9.5 5"/></svg> </labelLine truncated
<span class=p>{</span><span class=nt>&quot;name&quot;</span><span class=p>:</span><span class=w> </span><span class=s2>&quot;Alexa&quot;</span><span class=p>,</span><span class=w> </span><span class=nt>&quot;wins&quot;</span><span class=p>:</span><span class=w> </span><span class=p>[[</span><span class=s2>&quot;two pair&quot;</span><span class=p>,</span><span class=w> </span><span class=s2>&quot;4♠&quot;</span><span class=p>],</span><span class=w> </span><span class=p>[</span><span class=s2>&quot;two pair&quot;</span><span class=p>,</span><span class=w> </span><span class=s2>&quot;9♠&quot;</span><span class=p>]]}</span>
<span class=p>{</span><span class=nt>&quot;name&quot;</span><span class=p>:</span><span class=w> </span><span class=s2>&quot;May&quot;</span><span class=p>,</span><span class=w> </span><span class=nt>&quot;wins&quot;</span><span class=p>:</span><span class=w> </span><span class=p>[]}</span>
<span class=p>{</span><span class=nt>&quot;name&quot;</span><span class=p>:</span><span class=w> </span><span class=s2>&quot;Deloise&quot;</span><span class=p>,</span><span class=w> </span><span class=nt>&quot;wins&quot;</span><span class=p>:</span><span class=w> </span><span class=p>[[</span><span class=s2>&quot;three of a kind&quot;</span><span class=p>,</span><span class=w> </span><span class=s2>&quot;5♣&quot;</span><span class=p>]]}</span>
<!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=../parse_exceptions/ 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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xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 8a3 3 0 0 0 3-3 3 3 0 0 0-3-3 3 3 0 0 0-3 3 3 3 0 0 0 3 3m0 3.54C9.64 9.35 6.5 8 3 8v11c3.5 0 6.64 1.35 9 3.54 2.36-2.19 5.5-3.54 9-3.54V8c-3.5 0-6.64 1.35-9 3.54"/></svg> </a> <label class="md-header__button md-icon" for=__drawer> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M3 6h18v2H3zm0 5h18v2H3zm0 5h18v2H3z"/></svg> </label> <div class=md-header__title data-md-component=header-title> <div class=md-header__ellipsis> <div class=md-header__topic> <span class=md-ellipsis> JSON for Modern C++ </span> </div> <div class=md-header__topic data-md-component=header-topic> <span class=md-ellipsis> JSON Lines </span> </div> </div> </div> <form class=md-header__option data-md-component=palette> <input class=md-option data-md-color-media="(prefers-color-scheme: light)" data-md-color-scheme=default data-md-color-primary=indigo data-md-color-accent=indigo aria-label="Switch to dark mode" type=radio 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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>
</code></pre></div> </div> <p><abbr title="JavaScript Object Notation">JSON</abbr> Lines input with more than one value is treated as invalid <abbr title="JavaScript Object Notation">JSON</abbr> by the <a href=../../../api/basic_json/parse/ ><code>parse</code></a> or <a href=../../../api/basic_json/accept/ ><code>accept</code></a> functions. To process it line by line, functions like <a href="https://en.cppreference.com/w/cpp/string/basic_string/getline"><code>std::getline</code></a> can be used:</p> <div class="admonition example"> <p class=admonition-title>Example: Parse <abbr title="JavaScript Object Notation">JSON</abbr> Text input line by line</p> <p>The example below demonstrates how <abbr title="JavaScript Object Notation">JSON</abbr> Lines can be processed.</p> <div class=highlight><pre><span></span><code><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;iostream&gt;</span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;nlohmann/json.hpp&gt;</span>
@@ -12,11 +12,11 @@
<span class=p>{</span>
<span class=w> </span><span class=c1>// JSON Lines (see https://jsonlines.org)</span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>stringstream</span><span class=w> </span><span class=n>input</span><span class=p>;</span>
<span class=w> </span><span class=n>input</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=sa>R</span><span class=s>&quot;</span><span class=dl>(</span><span class=s>{&quot;name&quot;: &quot;Gilbert&quot;, &quot;wins&quot;: [[&quot;straight&quot;, &quot;7♣&quot;], [&quot;one pair&quot;, &quot;10♥&quot;]]}</span>
<span class=s>{&quot;name&quot;: &quot;Alexa&quot;, &quot;wins&quot;: [[&quot;two pair&quot;, &quot;4♠&quot;], [&quot;two pair&quot;, &quot;9♠&quot;]]}</span>
<span class=s>{&quot;name&quot;: &quot;May&quot;, &quot;wins&quot;: []}</span>
<span class=s>{&quot;name&quot;: &quot;Deloise&quot;, &quot;wins&quot;: [[&quot;three of a kind&quot;, &quot;5♣&quot;]]}</span>
<span class=dl>)</span><span class=s>&quot;</span><span class=p>;</span>
<span class=w> </span><span class=n>input</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=sa>R</span><span class=s>"</span><span class=dl>(</span><span class=s>{"name": "Gilbert", "wins": [["straight", "7♣"], ["one pair", "10♥"]]}</span>
<span class=s>{"name": "Alexa", "wins": [["two pair", "4♠"], ["two pair", "9♠"]]}</span>
<span class=s>{"name": "May", "wins": []}</span>
<span class=s>{"name": "Deloise", "wins": [["three of a kind", "5♣"]]}</span>
<span class=dl>)</span><span class=s>"</span><span class=p>;</span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>string</span><span class=w> </span><span class=n>line</span><span class=p>;</span>
<span class=w> </span><span class=k>while</span><span class=w> </span><span class=p>(</span><span class=n>std</span><span class=o>::</span><span class=n>getline</span><span class=p>(</span><span class=n>input</span><span class=p>,</span><span class=w> </span><span class=n>line</span><span class=p>))</span>
@@ -24,13 +24,13 @@
<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>json</span><span class=o>::</span><span class=n>parse</span><span class=p>(</span><span class=n>line</span><span class=p>)</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=w> </span><span class=p>}</span>
<span class=p>}</span>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code><span class=p>{</span><span class=nt>&quot;name&quot;</span><span class=p>:</span><span class=s2>&quot;Gilbert&quot;</span><span class=p>,</span><span class=nt>&quot;wins&quot;</span><span class=p>:[[</span><span class=s2>&quot;straight&quot;</span><span class=p>,</span><span class=s2>&quot;7♣&quot;</span><span class=p>],[</span><span class=s2>&quot;one pair&quot;</span><span class=p>,</span><span class=s2>&quot;10♥&quot;</span><span class=p>]]}</span>
<span class=p>{</span><span class=nt>&quot;name&quot;</span><span class=p>:</span><span class=s2>&quot;Alexa&quot;</span><span class=p>,</span><span class=nt>&quot;wins&quot;</span><span class=p>:[[</span><span class=s2>&quot;two pair&quot;</span><span class=p>,</span><span class=s2>&quot;4♠&quot;</span><span class=p>],[</span><span class=s2>&quot;two pair&quot;</span><span class=p>,</span><span class=s2>&quot;9♠&quot;</span><span class=p>]]}</span>
<span class=p>{</span><span class=nt>&quot;name&quot;</span><span class=p>:</span><span class=s2>&quot;May&quot;</span><span class=p>,</span><span class=nt>&quot;wins&quot;</span><span class=p>:[]}</span>
<span class=p>{</span><span class=nt>&quot;name&quot;</span><span class=p>:</span><span class=s2>&quot;Deloise&quot;</span><span class=p>,</span><span class=nt>&quot;wins&quot;</span><span class=p>:[[</span><span class=s2>&quot;three of a kind&quot;</span><span class=p>,</span><span class=s2>&quot;5♣&quot;</span><span class=p>]]}</span>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code><span class=p>{</span><span class=nt>"name"</span><span class=p>:</span><span class=s2>"Gilbert"</span><span class=p>,</span><span class=nt>"wins"</span><span class=p>:[[</span><span class=s2>"straight"</span><span class=p>,</span><span class=s2>"7♣"</span><span class=p>],[</span><span class=s2>"one pair"</span><span class=p>,</span><span class=s2>"10♥"</span><span class=p>]]}</span>
<span class=p>{</span><span class=nt>"name"</span><span class=p>:</span><span class=s2>"Alexa"</span><span class=p>,</span><span class=nt>"wins"</span><span class=p>:[[</span><span class=s2>"two pair"</span><span class=p>,</span><span class=s2>"4♠"</span><span class=p>],[</span><span class=s2>"two pair"</span><span class=p>,</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=s2>"May"</span><span class=p>,</span><span class=nt>"wins"</span><span class=p>:[]}</span>
<span class=p>{</span><span class=nt>"name"</span><span class=p>:</span><span class=s2>"Deloise"</span><span class=p>,</span><span class=nt>"wins"</span><span class=p>:[[</span><span class=s2>"three of a kind"</span><span class=p>,</span><span class=s2>"5♣"</span><span class=p>]]}</span>
</code></pre></div> </div> <div class="admonition warning"> <p class=admonition-title>Note</p> <p>Using <a href=../../../api/operator_gtgt/ ><code>operator&gt;&gt;</code></a> like</p> <div class=highlight><pre><span></span><code><span class=n>json</span><span class=w> </span><span class=n>j</span><span class=p>;</span>
<span class=k>while</span><span class=w> </span><span class=p>(</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=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>with a <abbr title="JavaScript Object Notation">JSON</abbr> Lines input does not work, because the parser will try to parse one value after the last one.</p> <p>This is different from parsing a stream of <em>concatenated</em> (non-newline-delimited) <abbr title="JavaScript Object Notation">JSON</abbr> values, for which <code>operator&gt;&gt;</code> does work, provided that a value that is a number is followed by whitespace -- see its <a href=../../../api/operator_gtgt/#notes>notes</a> 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="August 7, 2026 17:29:58 UTC">August 7, 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>with a <abbr title="JavaScript Object Notation">JSON</abbr> Lines input does not work, because the parser will try to parse one value after the last one.</p> <p>This is different from parsing a stream of <em>concatenated</em> (non-newline-delimited) <abbr title="JavaScript Object Notation">JSON</abbr> values, for which <code>operator&gt;&gt;</code> does work, provided that a value that is a number is followed by whitespace -- see its <a href=../../../api/operator_gtgt/#notes>notes</a> 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="August 19, 2026 18:19:47 UTC">August 19, 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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+12 -12
View File
@@ -5,16 +5,16 @@
<span class=p>}</span>
<span class=k>catch</span><span class=w> </span><span class=p>(</span><span class=n>json</span><span class=o>::</span><span class=n>parse_error</span><span class=o>&amp;</span><span class=w> </span><span class=n>ex</span><span class=p>)</span>
<span class=p>{</span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cerr</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=s>&quot;parse error at byte &quot;</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>ex</span><span class=p>.</span><span class=n>byte</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=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cerr</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=s>"parse error at byte "</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>ex</span><span class=p>.</span><span class=kt>byte</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>In case exceptions are undesired or not supported by the environment, there are different ways to proceed:</p> <h2 id=switch-off-exceptions>Switch off exceptions<a class=headerlink href=#switch-off-exceptions title="Permanent link">&para;</a></h2> <p>The <code>parse()</code> function accepts a <code class=highlight><span class=kt>bool</span></code> parameter <code>allow_exceptions</code> which controls whether an exception is thrown when a parse error occurs (<code class=highlight><span class=nb>true</span></code>, default) or whether a discarded value should be returned (<code class=highlight><span class=nb>false</span></code>).</p> <div class=highlight><pre><span></span><code><span class=n>json</span><span class=w> </span><span class=n>j</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=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><span class=w> </span><span class=nb>nullptr</span><span class=p>,</span><span class=w> </span><span class=nb>false</span><span class=p>);</span>
<span class=k>if</span><span class=w> </span><span class=p>(</span><span class=n>j</span><span class=p>.</span><span class=n>is_discarded</span><span class=p>())</span>
<span class=p>{</span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cerr</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=s>&quot;parse error&quot;</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=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cerr</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=s>"parse error"</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>Note there is no diagnostic information available in this scenario.</p> <h2 id=use-accept-function>Use accept() function<a class=headerlink href=#use-accept-function title="Permanent link">&para;</a></h2> <p>Alternatively, function <code>accept()</code> can be used which does not return a <code>json</code> value, but a <code class=highlight><span class=kt>bool</span></code> indicating whether the input is valid <abbr title="JavaScript Object Notation">JSON</abbr>.</p> <div class=highlight><pre><span></span><code><span class=k>if</span><span class=w> </span><span class=p>(</span><span class=o>!</span><span class=n>json</span><span class=o>::</span><span class=n>accept</span><span class=p>(</span><span class=n>my_input</span><span class=p>))</span>
<span class=p>{</span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cerr</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=s>&quot;parse error&quot;</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=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cerr</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=s>"parse error"</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>Again, there is no diagnostic information available.</p> <h2 id=user-defined-sax-interface>User-defined <abbr title="Simple API for XML">SAX</abbr> interface<a class=headerlink href=#user-defined-sax-interface title="Permanent link">&para;</a></h2> <p>Finally, you can implement the <a href=../sax_interface/ ><abbr title="Simple API for XML">SAX</abbr> interface</a> and decide what should happen in case of a parse error.</p> <p>This function has the following interface:</p> <div class=highlight><pre><span></span><code><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>
@@ -35,9 +35,9 @@
<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>
<span class=w> </span><span class=p>{</span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cerr</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=s>&quot;parse error at input byte &quot;</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>position</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=s>&quot;</span><span class=se>\n</span><span class=s>&quot;</span>
<span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>ex</span><span class=p>.</span><span class=n>what</span><span class=p>()</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=s>&quot;</span><span class=se>\n</span><span class=s>&quot;</span>
<span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=s>&quot;last read: </span><span class=se>\&quot;</span><span class=s>&quot;</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>last_token</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=s>&quot;</span><span class=se>\&quot;</span><span class=s>&quot;</span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cerr</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=s>"parse error at input byte "</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>position</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=s>"</span><span class=se>\n</span><span class=s>"</span>
<span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>ex</span><span class=p>.</span><span class=n>what</span><span class=p>()</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=s>"</span><span class=se>\n</span><span class=s>"</span>
<span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=s>"last read: </span><span class=se>\"</span><span class=s>"</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>last_token</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=s>"</span><span class=se>\"</span><span class=s>"</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=w> </span><span class=k>return</span><span class=w> </span><span class=nb>false</span><span class=p>;</span>
<span class=w> </span><span class=p>}</span>
@@ -45,7 +45,7 @@
<span class=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</span>
<span class=p>{</span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>string</span><span class=w> </span><span class=n>myinput</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=s>&quot;[1,2,3,]&quot;</span><span class=p>;</span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>string</span><span class=w> </span><span class=n>myinput</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=s>"[1,2,3,]"</span><span class=p>;</span>
<span class=w> </span><span class=n>json</span><span class=w> </span><span class=n>result</span><span class=p>;</span>
<span class=w> </span><span class=n>sax_no_exception</span><span class=w> </span><span class=n>sax</span><span class=p>(</span><span class=n>result</span><span class=p>);</span>
@@ -53,14 +53,14 @@
<span class=w> </span><span class=kt>bool</span><span class=w> </span><span class=n>parse_result</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>sax_parse</span><span class=p>(</span><span class=n>myinput</span><span class=p>,</span><span class=w> </span><span class=o>&amp;</span><span class=n>sax</span><span class=p>);</span>
<span class=w> </span><span class=k>if</span><span class=w> </span><span class=p>(</span><span class=o>!</span><span class=n>parse_result</span><span class=p>)</span>
<span class=w> </span><span class=p>{</span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cerr</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=s>&quot;parsing unsuccessful!&quot;</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=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cerr</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=s>"parsing unsuccessful!"</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=w> </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=s>&quot;parsed value: &quot;</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>result</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=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=s>"parsed value: "</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>result</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>endl</span><span class=p>;</span>
<span class=p>}</span>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code>parse error at input byte 8
[json.exception.parse_error.101] parse error at line 1, column 8: syntax error while parsing value - unexpected &#39;]&#39;; expected &#39;[&#39;, &#39;{&#39;, or a literal
last read: &quot;3,]&quot;
[json.exception.parse_error.101] parse error at line 1, column 8: syntax error while parsing value - unexpected ']'; expected '[', '{', or a literal
last read: "3,]"
parsing unsuccessful!
parsed value: [1,2,3]
</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="August 7, 2026 17:29:58 UTC">August 7, 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> <!-- 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="August 19, 2026 18:19:47 UTC">August 19, 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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+44 -44
View File
@@ -2,13 +2,13 @@
<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</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>
<span class=w> </span><span class=nt>&quot;name&quot;</span><span class=p>:</span><span class=w> </span><span class=s2>&quot;Berlin&quot;</span><span class=p>,</span>
<span class=w> </span><span class=nt>&quot;location&quot;</span><span class=p>:</span><span class=w> </span><span class=p>[</span>
<span class=w> </span><span class=nt>"name"</span><span class=p>:</span><span class=w> </span><span class=s2>"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>
<span class=w> </span><span class=mf>13.406667</span>
<span class=w> </span><span class=p>]</span>
<span class=p>}</span>
</code></pre></div> <p>these calls are made to the callback function:</p> <table> <thead> <tr> <th>event</th> <th>depth</th> <th>parsed</th> </tr> </thead> <tbody> <tr> <td><code>object_start</code></td> <td>0</td> <td><em>discarded</em></td> </tr> <tr> <td><code>key</code></td> <td>1</td> <td><code class=highlight><span class=s2>&quot;name&quot;</span></code></td> </tr> <tr> <td><code>value</code></td> <td>1</td> <td><code class=highlight><span class=s2>&quot;Berlin&quot;</span></code></td> </tr> <tr> <td><code>key</code></td> <td>1</td> <td><code class=highlight><span class=s2>&quot;location&quot;</span></code></td> </tr> <tr> <td><code>array_start</code></td> <td>1</td> <td><em>discarded</em></td> </tr> <tr> <td><code>value</code></td> <td>2</td> <td><code class=highlight><span class=mf>52.519444</span></code></td> </tr> <tr> <td><code>value</code></td> <td>2</td> <td><code class=highlight><span class=mf>13.406667</span></code></td> </tr> <tr> <td><code>array_end</code></td> <td>1</td> <td><code class=highlight><span class=p>[</span><span class=mf>52.519444</span><span class=p>,</span><span class=mf>13.406667</span><span class=p>]</span></code></td> </tr> <tr> <td><code>object_end</code></td> <td>0</td> <td><code class=highlight><span class=p>{</span><span class=nt>&quot;location&quot;</span><span class=p>:[</span><span class=mf>52.519444</span><span class=p>,</span><span class=mf>13.406667</span><span class=p>],</span><span class=nt>&quot;name&quot;</span><span class=p>:</span><span class=s2>&quot;Berlin&quot;</span><span class=p>}</span></code></td> </tr> </tbody> </table> </details> <div class="admonition note"> <p class=admonition-title>No built-in nesting depth limit</p> <p>The library has no built-in limit on recursion/nesting depth while parsing. A parser callback can only <em>discard</em> content it has already parsed (by returning <code class=highlight><span class=nb>false</span></code>); it cannot make parsing fail once a nesting limit is exceeded partway through reading a deeply nested value. If you need to reject over-deep untrusted input outright, track <code>depth</code> in a callback and <code>throw</code> from it once your limit is exceeded (a thrown exception propagates out of <code>parse()</code> as usual).</p> </div> <h2 id=return-value>Return value<a class=headerlink href=#return-value title="Permanent link">&para;</a></h2> <p>Discarding a value (i.e., returning <code class=highlight><span class=nb>false</span></code>) has different effects depending on the context in which the function was called:</p> <ul> <li>Discarded values in structured types are skipped. That is, the parser will behave as if the discarded value was never read.</li> <li>In case a value outside a structured type is skipped, it is replaced with <code class=highlight><span class=kc>null</span></code>. This case happens if the top-level element is skipped.</li> </ul> <details class=example> <summary>Example</summary> <p>The example below demonstrates the <code>parse()</code> function with and without callback function.</p> <div class=highlight><pre><span></span><code><span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;iostream&gt;</span>
</code></pre></div> <p>these calls are made to the callback function:</p> <table> <thead> <tr> <th>event</th> <th>depth</th> <th>parsed</th> </tr> </thead> <tbody> <tr> <td><code>object_start</code></td> <td>0</td> <td><em>discarded</em></td> </tr> <tr> <td><code>key</code></td> <td>1</td> <td><code class=highlight><span class=s2>"name"</span></code></td> </tr> <tr> <td><code>value</code></td> <td>1</td> <td><code class=highlight><span class=s2>"Berlin"</span></code></td> </tr> <tr> <td><code>key</code></td> <td>1</td> <td><code class=highlight><span class=s2>"location"</span></code></td> </tr> <tr> <td><code>array_start</code></td> <td>1</td> <td><em>discarded</em></td> </tr> <tr> <td><code>value</code></td> <td>2</td> <td><code class=highlight><span class=mf>52.519444</span></code></td> </tr> <tr> <td><code>value</code></td> <td>2</td> <td><code class=highlight><span class=mf>13.406667</span></code></td> </tr> <tr> <td><code>array_end</code></td> <td>1</td> <td><code class=highlight><span class=p>[</span><span class=mf>52.519444</span><span class=p>,</span><span class=mf>13.406667</span><span class=p>]</span></code></td> </tr> <tr> <td><code>object_end</code></td> <td>0</td> <td><code class=highlight><span class=p>{</span><span class=nt>"location"</span><span class=p>:[</span><span class=mf>52.519444</span><span class=p>,</span><span class=mf>13.406667</span><span class=p>],</span><span class=nt>"name"</span><span class=p>:</span><span class=s2>"Berlin"</span><span class=p>}</span></code></td> </tr> </tbody> </table> </details> <div class="admonition note"> <p class=admonition-title>No built-in nesting depth limit</p> <p>The library has no built-in limit on recursion/nesting depth while parsing. A parser callback can only <em>discard</em> content it has already parsed (by returning <code class=highlight><span class=nb>false</span></code>); it cannot make parsing fail once a nesting limit is exceeded partway through reading a deeply nested value. If you need to reject over-deep untrusted input outright, track <code>depth</code> in a callback and <code>throw</code> from it once your limit is exceeded (a thrown exception propagates out of <code>parse()</code> as usual).</p> </div> <h2 id=return-value>Return value<a class=headerlink href=#return-value title="Permanent link">&para;</a></h2> <p>Discarding a value (i.e., returning <code class=highlight><span class=nb>false</span></code>) has different effects depending on the context in which the function was called:</p> <ul> <li>Discarded values in structured types are skipped. That is, the parser will behave as if the discarded value was never read.</li> <li>In case a value outside a structured type is skipped, it is replaced with <code class=highlight><span class=kc>null</span></code>. This case happens if the top-level element is skipped.</li> </ul> <details class=example> <summary>Example</summary> <p>The example below demonstrates the <code>parse()</code> function with and without callback function.</p> <div class=highlight><pre><span></span><code><span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;iostream&gt;</span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;iomanip&gt;</span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;nlohmann/json.hpp&gt;</span>
@@ -17,32 +17,32 @@
<span class=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</span>
<span class=p>{</span>
<span class=w> </span><span class=c1>// a JSON text</span>
<span class=w> </span><span class=k>auto</span><span class=w> </span><span class=n>text</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=sa>R</span><span class=s>&quot;</span><span class=dl>(</span>
<span class=w> </span><span class=k>auto</span><span class=w> </span><span class=n>text</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=sa>R</span><span class=s>"</span><span class=dl>(</span>
<span class=s> {</span>
<span class=s> &quot;Image&quot;: {</span>
<span class=s> &quot;Width&quot;: 800,</span>
<span class=s> &quot;Height&quot;: 600,</span>
<span class=s> &quot;Title&quot;: &quot;View from 15th Floor&quot;,</span>
<span class=s> &quot;Thumbnail&quot;: {</span>
<span class=s> &quot;Url&quot;: &quot;http://www.example.com/image/481989943&quot;,</span>
<span class=s> &quot;Height&quot;: 125,</span>
<span class=s> &quot;Width&quot;: 100</span>
<span class=s> "Image": {</span>
<span class=s> "Width": 800,</span>
<span class=s> "Height": 600,</span>
<span class=s> "Title": "View from 15th Floor",</span>
<span class=s> "Thumbnail": {</span>
<span class=s> "Url": "http://www.example.com/image/481989943",</span>
<span class=s> "Height": 125,</span>
<span class=s> "Width": 100</span>
<span class=s> },</span>
<span class=s> &quot;Animated&quot; : false,</span>
<span class=s> &quot;IDs&quot;: [116, 943, 234, 38793]</span>
<span class=s> "Animated" : false,</span>
<span class=s> "IDs": [116, 943, 234, 38793]</span>
<span class=s> }</span>
<span class=s> }</span>
<span class=s> </span><span class=dl>)</span><span class=s>&quot;</span><span class=p>;</span>
<span class=s> </span><span class=dl>)</span><span class=s>"</span><span class=p>;</span>
<span class=w> </span><span class=c1>// parse and serialize JSON</span>
<span class=w> </span><span class=n>json</span><span class=w> </span><span class=n>j_complete</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>text</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>std</span><span class=o>::</span><span class=n>setw</span><span class=p>(</span><span class=mi>4</span><span class=p>)</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>j_complete</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=s>&quot;</span><span class=se>\n\n</span><span class=s>&quot;</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>std</span><span class=o>::</span><span class=n>setw</span><span class=p>(</span><span class=mi>4</span><span class=p>)</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>j_complete</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=s>"</span><span class=se>\n\n</span><span class=s>"</span><span class=p>;</span>
<span class=w> </span><span class=c1>// define parser callback</span>
<span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>parser_callback_t</span><span class=w> </span><span class=n>cb</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=w> </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=p>{</span>
<span class=w> </span><span class=c1>// skip object elements with key &quot;Thumbnail&quot;</span>
<span class=w> </span><span class=k>if</span><span class=w> </span><span class=p>(</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>key</span><span class=w> </span><span class=k>and</span><span class=w> </span><span class=n>parsed</span><span class=w> </span><span class=o>==</span><span class=w> </span><span class=n>json</span><span class=p>(</span><span class=s>&quot;Thumbnail&quot;</span><span class=p>))</span>
<span class=w> </span><span class=c1>// skip object elements with key "Thumbnail"</span>
<span class=w> </span><span class=k>if</span><span class=w> </span><span class=p>(</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>key</span><span class=w> </span><span class=k>and</span><span class=w> </span><span class=n>parsed</span><span class=w> </span><span class=o>==</span><span class=w> </span><span class=n>json</span><span class=p>(</span><span class=s>"Thumbnail"</span><span class=p>))</span>
<span class=w> </span><span class=p>{</span>
<span class=w> </span><span class=k>return</span><span class=w> </span><span class=nb>false</span><span class=p>;</span>
<span class=w> </span><span class=p>}</span>
@@ -54,40 +54,40 @@
<span class=w> </span><span class=c1>// parse (with callback) and serialize JSON</span>
<span class=w> </span><span class=n>json</span><span class=w> </span><span class=n>j_filtered</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>text</span><span class=p>,</span><span class=w> </span><span class=n>cb</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>std</span><span class=o>::</span><span class=n>setw</span><span class=p>(</span><span class=mi>4</span><span class=p>)</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>j_filtered</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=sc>&#39;\n&#39;</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>std</span><span class=o>::</span><span class=n>setw</span><span class=p>(</span><span class=mi>4</span><span class=p>)</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>j_filtered</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=sc>'\n'</span><span class=p>;</span>
<span class=p>}</span>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code><span class=p>{</span>
<span class=w> </span><span class=nt>&quot;Image&quot;</span><span class=p>:</span><span class=w> </span><span class=p>{</span>
<span class=w> </span><span class=nt>&quot;Animated&quot;</span><span class=p>:</span><span class=w> </span><span class=kc>false</span><span class=p>,</span>
<span class=w> </span><span class=nt>&quot;Height&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>600</span><span class=p>,</span>
<span class=w> </span><span class=nt>&quot;IDs&quot;</span><span class=p>:</span><span class=w> </span><span class=p>[</span>
<span class=w> </span><span class=nt>"Image"</span><span class=p>:</span><span class=w> </span><span class=p>{</span>
<span class=w> </span><span class=nt>"Animated"</span><span class=p>:</span><span class=w> </span><span class=kc>false</span><span class=p>,</span>
<span class=w> </span><span class=nt>"Height"</span><span class=p>:</span><span class=w> </span><span class=mi>600</span><span class=p>,</span>
<span class=w> </span><span class=nt>"IDs"</span><span class=p>:</span><span class=w> </span><span class=p>[</span>
<span class=w> </span><span class=mi>116</span><span class=p>,</span>
<span class=w> </span><span class=mi>943</span><span class=p>,</span>
<span class=w> </span><span class=mi>234</span><span class=p>,</span>
<span class=w> </span><span class=mi>38793</span>
<span class=w> </span><span class=p>],</span>
<span class=w> </span><span class=nt>&quot;Thumbnail&quot;</span><span class=p>:</span><span class=w> </span><span class=p>{</span>
<span class=w> </span><span class=nt>&quot;Height&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>125</span><span class=p>,</span>
<span class=w> </span><span class=nt>&quot;Url&quot;</span><span class=p>:</span><span class=w> </span><span class=s2>&quot;http://www.example.com/image/481989943&quot;</span><span class=p>,</span>
<span class=w> </span><span class=nt>&quot;Width&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>100</span>
<span class=w> </span><span class=nt>"Thumbnail"</span><span class=p>:</span><span class=w> </span><span class=p>{</span>
<span class=w> </span><span class=nt>"Height"</span><span class=p>:</span><span class=w> </span><span class=mi>125</span><span class=p>,</span>
<span class=w> </span><span class=nt>"Url"</span><span class=p>:</span><span class=w> </span><span class=s2>"http://www.example.com/image/481989943"</span><span class=p>,</span>
<span class=w> </span><span class=nt>"Width"</span><span class=p>:</span><span class=w> </span><span class=mi>100</span>
<span class=w> </span><span class=p>},</span>
<span class=w> </span><span class=nt>&quot;Title&quot;</span><span class=p>:</span><span class=w> </span><span class=s2>&quot;View from 15th Floor&quot;</span><span class=p>,</span>
<span class=w> </span><span class=nt>&quot;Width&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>800</span>
<span class=w> </span><span class=nt>"Title"</span><span class=p>:</span><span class=w> </span><span class=s2>"View from 15th Floor"</span><span class=p>,</span>
<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>
<span class=p>{</span>
<span class=w> </span><span class=nt>&quot;Image&quot;</span><span class=p>:</span><span class=w> </span><span class=p>{</span>
<span class=w> </span><span class=nt>&quot;Animated&quot;</span><span class=p>:</span><span class=w> </span><span class=kc>false</span><span class=p>,</span>
<span class=w> </span><span class=nt>&quot;Height&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>600</span><span class=p>,</span>
<span class=w> </span><span class=nt>&quot;IDs&quot;</span><span class=p>:</span><span class=w> </span><span class=p>[</span>
<span class=w> </span><span class=nt>"Image"</span><span class=p>:</span><span class=w> </span><span class=p>{</span>
<span class=w> </span><span class=nt>"Animated"</span><span class=p>:</span><span class=w> </span><span class=kc>false</span><span class=p>,</span>
<span class=w> </span><span class=nt>"Height"</span><span class=p>:</span><span class=w> </span><span class=mi>600</span><span class=p>,</span>
<span class=w> </span><span class=nt>"IDs"</span><span class=p>:</span><span class=w> </span><span class=p>[</span>
<span class=w> </span><span class=mi>116</span><span class=p>,</span>
<span class=w> </span><span class=mi>943</span><span class=p>,</span>
<span class=w> </span><span class=mi>234</span><span class=p>,</span>
<span class=w> </span><span class=mi>38793</span>
<span class=w> </span><span class=p>],</span>
<span class=w> </span><span class=nt>&quot;Title&quot;</span><span class=p>:</span><span class=w> </span><span class=s2>&quot;View from 15th Floor&quot;</span><span class=p>,</span>
<span class=w> </span><span class=nt>&quot;Width&quot;</span><span class=p>:</span><span class=w> </span><span class=mi>800</span>
<span class=w> </span><span class=nt>"Title"</span><span class=p>:</span><span class=w> </span><span class=s2>"View from 15th Floor"</span><span class=p>,</span>
<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</summary> <div class=highlight><pre><span></span><code><span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;iostream&gt;</span>
@@ -124,7 +124,7 @@
<span class=w> </span><span class=k>const</span><span class=w> </span><span class=k>auto</span><span class=o>&amp;</span><span class=w> </span><span class=n>key</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>parsed</span><span class=p>.</span><span class=n>get_ref</span><span class=o>&lt;</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;&gt;</span><span class=p>();</span>
<span class=w> </span><span class=k>if</span><span class=w> </span><span class=p>(</span><span class=o>!</span><span class=n>seen</span><span class=p>.</span><span class=n>insert</span><span class=p>(</span><span class=n>key</span><span class=p>).</span><span class=n>second</span><span class=p>)</span>
<span class=w> </span><span class=p>{</span>
<span class=w> </span><span class=k>throw</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>runtime_error</span><span class=p>(</span><span class=s>&quot;duplicate JSON object key: &quot;</span><span class=w> </span><span class=o>+</span><span class=w> </span><span class=n>key</span><span class=p>);</span>
<span class=w> </span><span class=k>throw</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>runtime_error</span><span class=p>(</span><span class=s>"duplicate JSON object key: "</span><span class=w> </span><span class=o>+</span><span class=w> </span><span class=n>key</span><span class=p>);</span>
<span class=w> </span><span class=p>}</span>
<span class=w> </span><span class=k>return</span><span class=w> </span><span class=nb>true</span><span class=p>;</span>
<span class=w> </span><span class=p>}</span>
@@ -138,22 +138,22 @@
<span class=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</span>
<span class=p>{</span>
<span class=w> </span><span class=c1>// parsing succeeds when all keys are unique</span>
<span class=w> </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>parse_strict</span><span class=p>(</span><span class=sa>R</span><span class=s>&quot;</span><span class=dl>(</span><span class=s>{&quot;one&quot;: 1, &quot;two&quot;: 2}</span><span class=dl>)</span><span class=s>&quot;</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=sc>&#39;\n&#39;</span><span class=p>;</span>
<span class=w> </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>parse_strict</span><span class=p>(</span><span class=sa>R</span><span class=s>"</span><span class=dl>(</span><span class=s>{"one": 1, "two": 2}</span><span class=dl>)</span><span class=s>"</span><span class=p>);</span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>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=sc>'\n'</span><span class=p>;</span>
<span class=w> </span><span class=c1>// parsing throws when a key is repeated</span>
<span class=w> </span><span class=k>try</span>
<span class=w> </span><span class=p>{</span>
<span class=w> </span><span class=n>parse_strict</span><span class=p>(</span><span class=sa>R</span><span class=s>&quot;</span><span class=dl>(</span><span class=s>{&quot;one&quot;: 1, &quot;one&quot;: 2}</span><span class=dl>)</span><span class=s>&quot;</span><span class=p>);</span>
<span class=w> </span><span class=n>parse_strict</span><span class=p>(</span><span class=sa>R</span><span class=s>"</span><span class=dl>(</span><span class=s>{"one": 1, "one": 2}</span><span class=dl>)</span><span class=s>"</span><span class=p>);</span>
<span class=w> </span><span class=p>}</span>
<span class=w> </span><span class=k>catch</span><span class=w> </span><span class=p>(</span><span class=k>const</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>exception</span><span class=o>&amp;</span><span class=w> </span><span class=n>e</span><span class=p>)</span>
<span class=w> </span><span class=p>{</span>
<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>e</span><span class=p>.</span><span class=n>what</span><span class=p>()</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=sc>&#39;\n&#39;</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>e</span><span class=p>.</span><span class=n>what</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>
<span class=w> </span><span class=p>}</span>
<span class=p>}</span>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code><span class=p>{</span><span class=nt>&quot;one&quot;</span><span class=p>:</span><span class=mi>1</span><span class=p>,</span><span class=nt>&quot;two&quot;</span><span class=p>:</span><span class=mi>2</span><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 two 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> </ul> <p>For strict validation with precise error positions, implementing a <a href=../sax_interface/ ><abbr title="Simple API for XML">SAX</abbr> interface</a> instead gives access to the parser's position information directly.</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</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>&quot;large_array.json&quot;</span><span class=p>);</span>
</code></pre></div> </details> <p>This approach has two 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> </ul> <p>For strict validation with precise error positions, implementing a <a href=../sax_interface/ ><abbr title="Simple API for XML">SAX</abbr> interface</a> instead gives access to the parser's position information directly.</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</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>
@@ -168,10 +168,10 @@
<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=cm>/*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=cm>/*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>&gt;</span><span class=w> </span><span class=n>max_depth</span><span class=p>)</span><span class=w> </span><span class=p>{</span>
<span class=w> </span><span class=k>throw</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>runtime_error</span><span class=p>(</span><span class=s>&quot;maximum nesting depth exceeded&quot;</span><span class=p>);</span>
<span class=w> </span><span class=k>throw</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>runtime_error</span><span class=p>(</span><span class=s>"maximum nesting depth exceeded"</span><span class=p>);</span>
<span class=w> </span><span class=p>}</span>
<span class=w> </span><span class=k>return</span><span class=w> </span><span class=nb>true</span><span class=p>;</span>
<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="August 7, 2026 17:29:58 UTC">August 7, 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> <!-- 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="August 19, 2026 18:19:47 UTC">August 19, 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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+1 -1
View File
@@ -49,4 +49,4 @@ 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="August 7, 2026 17:29:58 UTC">August 7, 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 23.6-58.9-2Line truncated
</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="August 19, 2026 18:19:47 UTC">August 19, 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 23.6-58.9Line truncated