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+<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=description content="JSON for Modern C++ is a header-only C++11 library that makes JSON a first-class data type."><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/json_view/reading/" rel="canonical"><link href=../differences/ rel=prev><link href=../generic_code/ rel=next><link rel=icon href=../../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.8.0"><title>Reading values - JSON for Modern C++</title><link rel=stylesheet href=../../../assets/stylesheets/main.57a0d79d.min.css><link rel=stylesheet href=../../../assets/stylesheets/palette.fb2fffe2.min.css><link rel=stylesheet href=../../../css/custom.css><link rel=stylesheet href=../../../css/brand.css><link rel=stylesheet href=../../../css/fig-sax.css><link rel=stylesheet href=../../../css/fig-jsonview.css><link 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</a> </div> <div data-md-component=announce> </div> <header class=md-header data-md-component=header> <nav class="md-header__inner md-grid" aria-label=Header> <a href=../../.. title="JSON for Modern C++" class="md-header__button md-logo" aria-label="JSON for Modern C++" data-md-component=logo> <img src=../../../images/wordmark-header.svg alt=logo> </a> <label class="md-header__button md-icon" for=__drawer> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M3 6h18v2H3zm0 5h18v2H3zm0 5h18v2H3z"/></svg> </label> <div class=md-header__title data-md-component=header-title> <div class=md-header__ellipsis> <div class=md-header__topic> <span class=md-ellipsis> JSON for Modern C++ </span> </div> <div class=md-header__topic data-md-component=header-topic> <span class=md-ellipsis> Reading values </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)" 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viewbox="0 0 24 24"><path d="M9.5 3Line truncated
+<span class=p>{</span>
+<span class=w>    </span><span class=n>std</span><span class=o>::</span><span class=n>string_view</span><span class=w> </span><span class=n>name</span><span class=p>;</span>
+<span class=w>    </span><span class=kt>int</span><span class=w> </span><span class=n>count</span><span class=p>;</span>
+<span class=w>    </span><span class=n>NLOHMANN_DEFINE_TYPE_INTRUSIVE</span><span class=p>(</span><span class=n>record</span><span class=p>,</span><span class=w> </span><span class=n>name</span><span class=p>,</span><span class=w> </span><span class=n>count</span><span class=p>)</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>string</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>{"name": "widget", "count": 3}</span><span class=dl>)</span><span class=s>"</span><span class=p>;</span>
+<span class=k>const</span><span class=w> </span><span class=n>json_document</span><span class=w> </span><span class=n>doc</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>json_document</span><span class=o>::</span><span class=n>parse</span><span class=p>(</span><span class=n>text</span><span class=p>);</span><span class=w>  </span><span class=c1>// borrows text</span>
+<span class=k>const</span><span class=w> </span><span class=n>record</span><span class=w> </span><span class=n>r</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>doc</span><span class=p>.</span><span class=n>root</span><span class=p>().</span><span class=n>get</span><span class=o>&lt;</span><span class=n>record</span><span class=o>&gt;</span><span class=p>();</span><span class=w>             </span><span class=c1>// r.name points into text, valid while text and doc are</span>
+</code></pre></div> <div class="admonition warning"> <p class=admonition-title>Decoding into a <code>string_view</code> from a <code>json</code> value dangles at once</p> <p>The same struct also compiles with <code class=highlight><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=n>get</span><span class=o>&lt;</span><span class=n>record</span><span class=o>&gt;</span><span class=p>()</span></code>, but there <code>name</code> points into the temporary <code>json</code> value, which is destroyed at the end of the statement. Use a named <a href=../../../api/json_document/ ><code>json_document</code></a>, as above, to decode such a struct.</p> </div> <h2 id=writing-a-view-back>Writing a view back<a class=headerlink href=#writing-a-view-back title="Permanent link">&para;</a></h2> <p><a href=../../../api/basic_json_view/dump/ ><code>dump()</code></a> serializes a view directly from the flat index, without ever building a <code>basic_json</code> value. It writes object members in document order and every occurrence of a repeated key, so it can differ from <code class=highlight><span class=n>materialize</span><span class=p>().</span><span class=n>dump</span><span class=p>()</span></code>; see <a href=../differences/#serialization>Serialization</a>.</p> <p>By default, numbers are written the way <a href=../../../api/basic_json/dump/ ><code>basic_json::dump()</code></a> would. <a href=../../../api/basic_json_view/number_format/ ><code>number_format::source</code></a> instead copies every number exactly as it was written in the source text — a price like <code class=highlight><span class=mf>19.90</span></code>, a long order or account ID with more digits than any number type holds, or a high-precision coordinate — something <code>basic_json</code> cannot do at all, since parsing already reduces a number to its parsed <code class=highlight><span class=kt>double</span></code>/<code class=highlight><span class=kt>int64_t</span></code> value. Together with an <a href=../editing/ >editable document</a>, this lets you patch a file without reformatting the rest of it.</p> <p><a href=../../../api/basic_json_view/operator_ltlt/ ><code>operator&lt;&lt;</code></a> writes a view to a stream the way <code>basic_json</code>'s does, using the stream's <code>width</code>/<code>fill</code> for indentation.</p> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../ >Zero-copy <abbr title="JavaScript Object Notation">JSON</abbr> views</a> - overview</li> <li><a href=../../../api/basic_json_view/get/ ><code>get</code></a> - convert a view</li> <li><a href=../../../api/basic_json_view/dump/ ><code>dump</code></a> - serialize a view</li> <li><a href=../generic_code/ >Writing generic code</a> - functions and <code>from_json()</code> overloads that accept views</li> </ul> <h2 id=version-history>Version history<a class=headerlink href=#version-history title="Permanent link">&para;</a></h2> <ul> <li>Added in version 3.13.0 <span class=unreleased-version title="Not part of a release yet; the latest release is 3.12.0.">unreleased</span>.</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="November 9, 2026 07:12:00 UTC">November 9, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../differences/ class="md-footer__link md-footer__link--prev" aria-label="Previous: Differences from basic_json"> <div class="md-footeLine truncated