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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/sax_interface/ rel=canonical><link href=../parser_callbacks/ rel=prev><link href=../../assertions/ rel=next><link rel=icon href=../../../assets/images/favicon.png><meta name=generator content="mkdocs-1.5.3, mkdocs-material-9.4.7"><title>SAX Interface - JSON for Modern C++</title><link rel=stylesheet href=../../../assets/stylesheets/main.4b4a2bd9.min.css><link rel=stylesheet href=../../../assets/stylesheets/palette.356b1318.min.css><link rel=preconnect href=https://fonts.gstatic.com crossorigin><link rel=stylesheet href="https://fonts.googleapis.com/css?family=Roboto:300,300i,400,400i,700,700i%7CJetBrains+Mono:400,400i,700,700i&display=fallback"><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> <script>var palette=__md_get("__palette");if(palette&&"object"==typeof palette.color)for(var key of Object.keys(palette.color))document.body.setAttribute("data-md-color-"+key,palette.color[key])</script> <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=#sax-interface 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 3.54Z"/></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 6h18v2H3V6m0 5h18v2H3v-2m0 5h18v2H3v-2Z"/></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> SAX Interface </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_1> <label class="md-header__button md-icon" title="Switch to dark mode" for=__palette_2 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 12 20 8.69Z"/></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_2> <label class="md-header__button md-icon" title="Switch to light mode" for=__palette_1 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-.55C1
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<span class=kt>bool</span><span class=w> </span><span class=nf>null</span><span class=p>();</span>
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<span class=c1>// called when a boolean is parsed; value is passed</span>
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<span class=kt>bool</span><span class=w> </span><span class=nf>boolean</span><span class=p>(</span><span class=kt>bool</span><span class=w> </span><span class=n>val</span><span class=p>);</span>
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<span class=c1>// called when a signed or unsigned integer number is parsed; value is passed</span>
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<span class=kt>bool</span><span class=w> </span><span class=nf>number_integer</span><span class=p>(</span><span class=n>number_integer_t</span><span class=w> </span><span class=n>val</span><span class=p>);</span>
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<span class=kt>bool</span><span class=w> </span><span class=nf>number_unsigned</span><span class=p>(</span><span class=n>number_unsigned_t</span><span class=w> </span><span class=n>val</span><span class=p>);</span>
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<span class=c1>// called when a floating-point number is parsed; value and original string is passed</span>
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<span class=kt>bool</span><span class=w> </span><span class=nf>number_float</span><span class=p>(</span><span class=n>number_float_t</span><span class=w> </span><span class=n>val</span><span class=p>,</span><span class=w> </span><span class=k>const</span><span class=w> </span><span class=n>string_t</span><span class=o>&</span><span class=w> </span><span class=n>s</span><span class=p>);</span>
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<span class=c1>// called when a string is parsed; value is passed and can be safely moved away</span>
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<span class=kt>bool</span><span class=w> </span><span class=nf>string</span><span class=p>(</span><span class=n>string_t</span><span class=o>&</span><span class=w> </span><span class=n>val</span><span class=p>);</span>
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<span class=c1>// called when a binary value is parsed; value is passed and can be safely moved away</span>
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<span class=kt>bool</span><span class=w> </span><span class=nf>binary</span><span class=p>(</span><span class=n>binary</span><span class=o>&</span><span class=w> </span><span class=n>val</span><span class=p>);</span>
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<span class=c1>// called when an object or array begins or ends, resp. The number of elements is passed (or -1 if not known)</span>
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<span class=kt>bool</span><span class=w> </span><span class=nf>start_object</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>elements</span><span class=p>);</span>
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<span class=kt>bool</span><span class=w> </span><span class=nf>end_object</span><span class=p>();</span>
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<span class=kt>bool</span><span class=w> </span><span class=nf>start_array</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>elements</span><span class=p>);</span>
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<span class=kt>bool</span><span class=w> </span><span class=nf>end_array</span><span class=p>();</span>
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<span class=c1>// called when an object key is parsed; value is passed and can be safely moved away</span>
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<span class=kt>bool</span><span class=w> </span><span class=nf>key</span><span class=p>(</span><span class=n>string_t</span><span class=o>&</span><span class=w> </span><span class=n>val</span><span class=p>);</span>
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<span class=c1>// called when a parse error occurs; byte position, the last token, and an exception is passed</span>
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<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>&</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>&</span><span class=w> </span><span class=n>ex</span><span class=p>);</span>
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</code></pre></div> <p>The return value of each function determines whether parsing should proceed.</p> <p>To implement your own SAX handler, proceed as follows:</p> <ol> <li>Implement the SAX interface in a class. You can use class <code>nlohmann::json_sax<json></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 SAX 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>&</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 SAX 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 SAX event. It does not return <code>json</code> value - it is up to you to decide what to do with the SAX 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 SAX interface is used for the DOM 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">¶</a></h2> <ul> <li><a href=../../../api/json_sax/ >json_sax</a> - documentation of the SAX interface</li> <li><a href=../../../api/basic_json/sax_parse/ >sax_parse</a> - SAX parser</li> </ul> <hr> <div class=md-source-file> <small> Last update: <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date">May 1, 2022</span> </small> </div> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var tab,labels=set.querySelector(".tabbed-labels");for(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> </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 8v12Z"/></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 © 2013 - 2023 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 0 496 512"><!-- Font Awesome Free 6.4.2 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 2023 Fonticons, Inc.--><path d="M165.9 397.4c0 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 3.6zm-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 2.3zm44.2-1.7c-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-.3-1.9-3-3.2-5.9-2.9zM244.8 8C106.1 8 0 113.3 0 252c0 110.9 69.8 205.8 169.5 239.2 12.8 2.3 17.3-5.6 17.3-12.1 0-6.2-.3-40.4-.3-61.4 0 0-70 15-84.7-29.8 0 0-11.4-29.1-27.8-36.6 0 0-22.9-15.7 1.6-15.4 0 0 24.9 2 38.6 25.8 21.9 38.6 58.6 27.5 72.9 20.9 2.3-16 8.8-27.1 16-33.7-55.9-6.2-112.3-14.3-112.3-110.5 0-27.5 7.6-41.3 23.6-58.9-2.6-6.5-11.1-33.3 2.6-67.9 20.9-6.5 69 27 69 27 20-5.6 41.5-8.5 62.8-8.5s42.8 2.9 62.8 8.5c0 0 48.1-33.6 69-27 13.7 34.7 5.2 61.4 2.6 67.9 16 17.7 25.8 31.5 25.8 58.9 0 96.5-58.9 104.2-114.8 110.5 9.2 7.9 17 22.9 17 46.4 0 33.
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