json/api/basic_json/items/index.html

50 lines
65 KiB
HTML
Raw Normal View History

<!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/api/basic_json/items/ rel=canonical><link rel=icon href=../../../assets/images/favicon.png><meta name=generator content="mkdocs-1.4.2, mkdocs-material-8.5.10"><title>items - JSON for Modern C++</title><link rel=stylesheet href=../../../assets/stylesheets/main.975780f9.min.css><link rel=stylesheet href=../../../assets/stylesheets/palette.2505c338.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=#nlohmannbasic_jsonitems 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> items </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-.55C11.56 16.5 13 14.42 13 12c0-2.42-1.44-4.5-3.5-5.45C10.26 6.2 11.11 6 12 6a6 6 0 0 1 6 6 6 6 0 0
<span class=n>iteration_proxy</span><span class=o>&lt;</span><span class=n>const_iterator</span><span class=o>&gt;</span><span class=w> </span><span class=n>items</span><span class=p>()</span><span class=w> </span><span class=k>const</span><span class=w> </span><span class=k>noexcept</span><span class=p>;</span><span class=w></span>
</code></pre></div> <p>This function allows accessing <code>iterator::key()</code> and <code>iterator::value()</code> during range-based for loops. In these loops, a reference to the JSON values is returned, so there is no access to the underlying iterator.</p> <p>For loop without <code>items()</code> function:</p> <div class=highlight><pre><span></span><code><span class=k>for</span><span class=w> </span><span class=p>(</span><span class=k>auto</span><span class=w> </span><span class=n>it</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>j_object</span><span class=p>.</span><span class=n>begin</span><span class=p>();</span><span class=w> </span><span class=n>it</span><span class=w> </span><span class=o>!=</span><span class=w> </span><span class=n>j_object</span><span class=p>.</span><span class=n>end</span><span class=p>();</span><span class=w> </span><span class=o>++</span><span class=n>it</span><span class=p>)</span><span class=w></span>
<span class=p>{</span><span class=w></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;key: &quot;</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>it</span><span class=p>.</span><span class=n>key</span><span class=p>()</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=s>&quot;, value:&quot;</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>it</span><span class=p>.</span><span class=n>value</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=p>}</span><span class=w></span>
</code></pre></div> <p>Range-based for loop without <code>items()</code> function:</p> <div class=highlight><pre><span></span><code><span class=k>for</span><span class=w> </span><span class=p>(</span><span class=k>auto</span><span class=w> </span><span class=n>it</span><span class=w> </span><span class=o>:</span><span class=w> </span><span class=n>j_object</span><span class=p>)</span><span class=w></span>
<span class=p>{</span><span class=w></span>
<span class=w> </span><span class=c1>// &quot;it&quot; is of type json::reference and has no key() member</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;value: &quot;</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>it</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=p>}</span><span class=w></span>
</code></pre></div> <p>Range-based for loop with <code>items()</code> function:</p> <div class=highlight><pre><span></span><code><span class=k>for</span><span class=w> </span><span class=p>(</span><span class=k>auto</span><span class=o>&amp;</span><span class=w> </span><span class=n>el</span><span class=w> </span><span class=o>:</span><span class=w> </span><span class=n>j_object</span><span class=p>.</span><span class=n>items</span><span class=p>())</span><span class=w></span>
<span class=p>{</span><span class=w></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;key: &quot;</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>el</span><span class=p>.</span><span class=n>key</span><span class=p>()</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=s>&quot;, value:&quot;</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>el</span><span class=p>.</span><span class=n>value</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=p>}</span><span class=w></span>
</code></pre></div> <p>The <code>items()</code> function also allows using <a href=https://en.cppreference.com/w/cpp/language/structured_binding>structured bindings</a> (C++17):</p> <div class=highlight><pre><span></span><code><span class=k>for</span><span class=w> </span><span class=p>(</span><span class=k>auto</span><span class=o>&amp;</span><span class=w> </span><span class=p>[</span><span class=n>key</span><span class=p>,</span><span class=w> </span><span class=n>val</span><span class=p>]</span><span class=w> </span><span class=o>:</span><span class=w> </span><span class=n>j_object</span><span class=p>.</span><span class=n>items</span><span class=p>())</span><span class=w></span>
<span class=p>{</span><span class=w></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;key: &quot;</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>key</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=s>&quot;, value:&quot;</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>val</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=p>}</span><span class=w></span>
</code></pre></div> <h2 id=return-value>Return value<a class=headerlink href=#return-value title="Permanent link">&para;</a></h2> <p>iteration proxy object wrapping the current value with an interface to use in range-based for loops</p> <h2 id=exception-safety>Exception safety<a class=headerlink href=#exception-safety title="Permanent link">&para;</a></h2> <p>Strong guarantee: if an exception is thrown, there are no changes in the JSON value.</p> <h2 id=complexity>Complexity<a class=headerlink href=#complexity title="Permanent link">&para;</a></h2> <p>Constant.</p> <h2 id=notes>Notes<a class=headerlink href=#notes title="Permanent link">&para;</a></h2> <p>When iterating over an array, <code>key()</code> will return the index of the element as string (see example). For primitive types (e.g., numbers), <code>key()</code> returns an empty string.</p> <div class="admonition danger"> <p class=admonition-title>Lifetime issues</p> <p>Using <code>items()</code> on temporary objects is dangerous. Make sure the object's lifetime exceeds the iteration. See <a href=https://github.com/nlohmann/json/issues/2040>https://github.com/nlohmann/json/issues/2040</a> for more information.</p> </div> <h2 id=examples>Examples<a class=headerlink href=#examples title="Permanent link">&para;</a></h2> <details class=example> <summary>Example</summary> <p>The following code shows an example for <code>items()</code>.</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></span>
<span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;nlohmann/json.hpp&gt;</span><span class=cp></span>
<span class=k>using</span><span class=w> </span><span class=n>json</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>nlohmann</span><span class=o>::</span><span class=n>json</span><span class=p>;</span><span class=w></span>
<span class=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</span><span class=w></span>
<span class=p>{</span><span class=w></span>
<span class=w> </span><span class=c1>// create JSON values</span>
<span class=w> </span><span class=n>json</span><span class=w> </span><span class=n>j_object</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=p>{{</span><span class=s>&quot;one&quot;</span><span class=p>,</span><span class=w> </span><span class=mi>1</span><span class=p>},</span><span class=w> </span><span class=p>{</span><span class=s>&quot;two&quot;</span><span class=p>,</span><span class=w> </span><span class=mi>2</span><span class=p>}};</span><span class=w></span>
<span class=w> </span><span class=n>json</span><span class=w> </span><span class=n>j_array</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=p>{</span><span class=mi>1</span><span class=p>,</span><span class=w> </span><span class=mi>2</span><span class=p>,</span><span class=w> </span><span class=mi>4</span><span class=p>,</span><span class=w> </span><span class=mi>8</span><span class=p>,</span><span class=w> </span><span class=mi>16</span><span class=p>};</span><span class=w></span>
<span class=w> </span><span class=c1>// example for an object</span>
<span class=w> </span><span class=k>for</span><span class=w> </span><span class=p>(</span><span class=k>auto</span><span class=o>&amp;</span><span class=w> </span><span class=n>x</span><span class=w> </span><span class=o>:</span><span class=w> </span><span class=n>j_object</span><span class=p>.</span><span class=n>items</span><span class=p>())</span><span class=w></span>
<span class=w> </span><span class=p>{</span><span class=w></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;key: &quot;</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>x</span><span class=p>.</span><span class=n>key</span><span class=p>()</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=s>&quot;, value: &quot;</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>x</span><span class=p>.</span><span class=n>value</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=w> </span><span class=p>}</span><span class=w></span>
<span class=w> </span><span class=c1>// example for an array</span>
<span class=w> </span><span class=k>for</span><span class=w> </span><span class=p>(</span><span class=k>auto</span><span class=o>&amp;</span><span class=w> </span><span class=n>x</span><span class=w> </span><span class=o>:</span><span class=w> </span><span class=n>j_array</span><span class=p>.</span><span class=n>items</span><span class=p>())</span><span class=w></span>
<span class=w> </span><span class=p>{</span><span class=w></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;key: &quot;</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>x</span><span class=p>.</span><span class=n>key</span><span class=p>()</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=s>&quot;, value: &quot;</span><span class=w> </span><span class=o>&lt;&lt;</span><span class=w> </span><span class=n>x</span><span class=p>.</span><span class=n>value</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=w> </span><span class=p>}</span><span class=w></span>
<span class=p>}</span><span class=w></span>
</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code><span class=err>key</span><span class=p>:</span><span class=w> </span><span class=err>o</span><span class=kc>ne</span><span class=p>,</span><span class=w> </span><span class=err>value</span><span class=p>:</span><span class=w> </span><span class=mi>1</span><span class=w></span>
<span class=err>key</span><span class=p>:</span><span class=w> </span><span class=kc>t</span><span class=err>wo</span><span class=p>,</span><span class=w> </span><span class=err>value</span><span class=p>:</span><span class=w> </span><span class=mi>2</span><span class=w></span>
<span class=err>key</span><span class=p>:</span><span class=w> </span><span class=mi>0</span><span class=p>,</span><span class=w> </span><span class=err>value</span><span class=p>:</span><span class=w> </span><span class=mi>1</span><span class=w></span>
<span class=err>key</span><span class=p>:</span><span class=w> </span><span class=mi>1</span><span class=p>,</span><span class=w> </span><span class=err>value</span><span class=p>:</span><span class=w> </span><span class=mi>2</span><span class=w></span>
<span class=err>key</span><span class=p>:</span><span class=w> </span><span class=mi>2</span><span class=p>,</span><span class=w> </span><span class=err>value</span><span class=p>:</span><span class=w> </span><span class=mi>4</span><span class=w></span>
<span class=err>key</span><span class=p>:</span><span class=w> </span><span class=mi>3</span><span class=p>,</span><span class=w> </span><span class=err>value</span><span class=p>:</span><span class=w> </span><span class=mi>8</span><span class=w></span>
<span class=err>key</span><span class=p>:</span><span class=w> </span><span class=mi>4</span><span class=p>,</span><span class=w> </span><span class=err>value</span><span class=p>:</span><span class=w> </span><span class=mi>16</span><span class=w></span>
</code></pre></div> </details> <h2 id=version-history>Version history<a class=headerlink href=#version-history title="Permanent link">&para;</a></h2> <ul> <li>Added <code>iterator_wrapper</code> in version 3.0.0.</li> <li>Added <code>items</code> and deprecated <code>iterator_wrapper</code> in version 3.1.0.</li> <li>Added structured binding support in version 3.5.0.</li> </ul> <div class="admonition warning"> <p class=admonition-title>Deprecation</p> <p>This function replaces the static function <code>iterator_wrapper</code> which was introduced in version 1.0.0, but has been deprecated in version 3.1.0. Function <code>iterator_wrapper</code> will be removed in version 4.0.0. Please replace all occurrences of <code class=highlight><span class=n>iterator_wrapper</span><span class=p>(</span><span class=n>j</span><span class=p>)</span><span class=w></span></code> with <code class=highlight><span class=n>j</span><span class=p>.</span><span class=n>items</span><span class=p>()</span><span class=w></span></code>.</p> <p>You should be warned by your compiler with a <code>-Wdeprecated-declarations</code> warning if you are using a deprecated function.</p> </div> <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> <a href=# 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 </a> </main> <footer class=md-footer> <nav class="md-footer__inner md-grid" aria-label=Footer> <a href=../is_structured/ class="md-footer__link md-footer__link--prev" aria-label="Previous: is_structured" rel=prev> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11h12Z"/></svg> </div> <div class=md-footer__title> <div class=md-ellipsis> <span class=md-footer__direction> Previous </span> is_structured </div> </div> </a> <a href=../json_base_class_t/ class="md-footer__link md-footer__link--next" aria-label="Next: json_base_class_t" rel=next> <div class=md-footer__title> <div class=md-ellipsis> <span class=md-footer__direction> Next </span> json_base_class_t </div> </div> <div class="md-footer__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11H4Z"/></svg> </div> </a> </nav> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright &copy; 2013 - 2022 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 6.2.1 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 2022 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 16-33.7-5