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@@ -89,3 +89,13 @@ non-existing, an exception is thrown.
| access to valid array index | reference to existing value is returned | const reference to existing value is returned |
| access to non-existing object key | `basic_json::out_of_range` exception is thrown | `basic_json::out_of_range` exception is thrown |
| access to invalid array index | `basic_json::out_of_range` exception is thrown | `basic_json::out_of_range` exception is thrown |
<p class="nj-table-caption" markdown>Result of [`at`](../../api/basic_json/at.md) in each scenario, for a non-const and a const value.</p>
## See also
- [Element Access](index.md) - overview of all ways to access elements
- [Unchecked access: operator[]](unchecked_access.md) - access without range checking
- [Access with default value: value](default_value.md) - fall back to a default instead of throwing
- [`at`](../../api/basic_json/at.md) - checked access by reference
- [JSON Pointer](../json_pointer.md) - access nested values with `at` and a pointer
@@ -1,4 +1,4 @@
<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/element_access/checked_access/" rel="canonical"><link href=../unchecked_access/ rel=prev><link href=../default_value/ rel=next><link rel=icon href=../../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>Checked access: at - 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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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 14Line truncated
<!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/element_access/checked_access/" rel="canonical"><link href=../unchecked_access/ rel=prev><link href=../default_value/ rel=next><link rel=icon href=../../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>Checked access: at - 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=../../../css/custom.css><link rel=stylesheet href=../../../css/brand.css><link rel=stylesheet href=../../../css/fig-sax.css><link rel=stylesheet href=../../../css/fig-jsonview.css><link rel=stylesheet href=../../../css/fig-exceptions.css><link rel=stylesheet href=../../../css/fig-pointer.css><link rel=stylesheet href=../../../css/fig-charts.css><link rel=stylesheet href=../../../css/fig-integration.css><link rel=stylesheet href=../../../css/fig-macros.css><script>__md_scope=new URL("../../..",location),__md_hash=e=>[...e].reduce(((e,_)=>(e<<5)-e+_.charCodeAt(0)),0),__md_get=(e,_=localStorage,t=__md_scope)=>JSON.parse(_.getItem(t.pathname+"."+e)),__md_set=(e,_,t=localStorage,a=__md_scope)=>{try{t.setItem(a.pathname+"."+e,JSON.stringify(_))}catch(e){}}</script></head> <body dir=ltr data-md-color-scheme=default data-md-color-primary=custom data-md-color-accent=custom> <input class=md-toggle data-md-toggle=drawer type=checkbox id=__drawer autocomplete=off> <input class=md-toggle data-md-toggle=search type=checkbox id=__search autocomplete=off> <label class=md-overlay for=__drawer></label> <div data-md-component=skip> <a href=#checked-access-at class=md-skip> Skip to content </a> </div> <div data-md-component=announce> </div> <header class=md-header data-md-component=header> <nav class="md-header__inner md-grid" aria-label=Header> <a href=../../.. title="JSON for Modern C++" class="md-header__button md-logo" aria-label="JSON for Modern C++" data-md-component=logo> <img src=../../../images/wordmark-header.svg alt=logo> </a> <label class="md-header__button md-icon" for=__drawer> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M3 6h18v2H3zm0 5h18v2H3zm0 5h18v2H3z"/></svg> </label> <div class=md-header__title data-md-component=header-title> <div class=md-header__ellipsis> <div class=md-header__topic> <span class=md-ellipsis> JSON for Modern C++ </span> </div> <div class=md-header__topic data-md-component=header-topic> <span class=md-ellipsis> Checked access: at </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=custom data-md-color-accent=custom 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=custom data-md-color-accent=custom 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 viewboLine truncated
<span class=w> </span><span class=nt>"name"</span><span class=p>:</span><span class=w> </span><span class=s2>"Mary Smith"</span><span class=p>,</span>
<span class=w> </span><span class=nt>"age"</span><span class=p>:</span><span class=w> </span><span class=mi>42</span><span class=p>,</span>
<span class=w> </span><span class=nt>"hobbies"</span><span class=p>:</span><span class=w> </span><span class=p>[</span><span class=s2>"hiking"</span><span class=p>,</span><span class=w> </span><span class=s2>"reading"</span><span class=p>]</span>
@@ -12,4 +12,4 @@
</code></pre></div> </details> <p>When accessing an invalid index (i.e., an index greater than or equal to the array size) or the passed object key is non-existing, an exception is thrown.</p> <details class=example> <summary>Example: access via invalid index or missing key</summary> <div class=highlight><pre><span></span><code><span class=n>j</span><span class=p>.</span><span class=n>at</span><span class=p>(</span><span class=s>"hobbies"</span><span class=p>).</span><span class=n>at</span><span class=p>(</span><span class=mi>3</span><span class=p>)</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=s>"cooking"</span><span class=p>;</span>
</code></pre></div> <p>This code produces the following exception:</p> <div class=highlight><pre><span></span><code>[json.exception.out_of_range.401] array index 3 is out of range
</code></pre></div> <p>When <a href=../../../home/exceptions/#extended-diagnostic-messages>extended diagnostic messages</a> are enabled by defining <a href=../../../api/macros/json_diagnostics/ ><code>JSON_DIAGNOSTICS</code></a>, the exception further gives information where the key or index is missing or out of range.</p> <div class=highlight><pre><span></span><code>[json.exception.out_of_range.401] (/hobbies) array index 3 is out of range
</code></pre></div> </details> <h2 id=notes>Notes<a class=headerlink href=#notes title="Permanent link">&para;</a></h2> <div class="admonition failure"> <p class=admonition-title>Exceptions</p> <ul> <li><a href=../../../api/basic_json/at/ ><code>at</code></a> can only be used with objects (with a string argument) or with arrays (with a numeric argument). For other types, a <a href=../../../home/exceptions/#jsonexceptiontype_error304><code>basic_json::type_error</code></a> is thrown.</li> <li><a href=../../../home/exceptions/#out-of-range><code>basic_json::out_of_range</code> exception</a> exceptions are thrown if the provided key is not found in an object or the provided index is invalid.</li> </ul> </div> <h2 id=summary>Summary<a class=headerlink href=#summary title="Permanent link">&para;</a></h2> <table> <thead> <tr> <th>scenario</th> <th>non-const value</th> <th>const value</th> </tr> </thead> <tbody> <tr> <td>access to existing object key</td> <td>reference to existing value is returned</td> <td>const reference to existing value is returned</td> </tr> <tr> <td>access to valid array index</td> <td>reference to existing value is returned</td> <td>const reference to existing value is returned</td> </tr> <tr> <td>access to non-existing object key</td> <td><code>basic_json::out_of_range</code> exception is thrown</td> <td><code>basic_json::out_of_range</code> exception is thrown</td> </tr> <tr> <td>access to invalid array index</td> <td><code>basic_json::out_of_range</code> exception is thrown</td> <td><code>basic_json::out_of_range</code> exception is thrown</td> </tr> </tbody> </table> <!-- 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="October 2, 2026 09:32:15 UTC">October 2, 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 23.6-58.9-2.6-6.5-11.1-33.3 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 33.7-.3 75.4-.3 83.6 0 6.5 4.6 14.4 17.3 12.1C436.2 457.8 504 362.9 504 252 504 113.3 391.5 8 252.8 8M105.2 352.9c-1.3 1-1 3.3.7 5.2 1.6 1.6 3.9 2.3 5.2 1 1.3-1 1-3.3-.7-5.2-1.6-1.6-3.9-2.3-5.2-1m-10.8-8.1c-.7 1.3.3 2.9 2.3 3.9 1.6 1 3.6.7 4.3-.7.7-1.3-.3-2.9-2.3-3.9-2-.6-3.6-.3-4.3.7m32.4 35.6c-1.6 1.3-1 4.3 1.3 6.2 2.3 2.3 5.2 2.6 6.5 1 1.3-1.3.7-4.3-1.3-6.2-2.2-2.3-5.2-2.6-6.5-1m-11.4-14.7c-1.Line truncated
</code></pre></div> </details> <h2 id=notes>Notes<a class=headerlink href=#notes title="Permanent link">&para;</a></h2> <div class="admonition failure"> <p class=admonition-title>Exceptions</p> <ul> <li><a href=../../../api/basic_json/at/ ><code>at</code></a> can only be used with objects (with a string argument) or with arrays (with a numeric argument). For other types, a <a href=../../../home/exceptions/#jsonexceptiontype_error304><code>basic_json::type_error</code></a> is thrown.</li> <li><a href=../../../home/exceptions/#out-of-range><code>basic_json::out_of_range</code> exception</a> exceptions are thrown if the provided key is not found in an object or the provided index is invalid.</li> </ul> </div> <h2 id=summary>Summary<a class=headerlink href=#summary title="Permanent link">&para;</a></h2> <table> <thead> <tr> <th>scenario</th> <th>non-const value</th> <th>const value</th> </tr> </thead> <tbody> <tr> <td>access to existing object key</td> <td>reference to existing value is returned</td> <td>const reference to existing value is returned</td> </tr> <tr> <td>access to valid array index</td> <td>reference to existing value is returned</td> <td>const reference to existing value is returned</td> </tr> <tr> <td>access to non-existing object key</td> <td><code>basic_json::out_of_range</code> exception is thrown</td> <td><code>basic_json::out_of_range</code> exception is thrown</td> </tr> <tr> <td>access to invalid array index</td> <td><code>basic_json::out_of_range</code> exception is thrown</td> <td><code>basic_json::out_of_range</code> exception is thrown</td> </tr> </tbody> </table> <p class=nj-table-caption>Result of <a href=../../../api/basic_json/at/ ><code>at</code></a> in each scenario, for a non-const and a const value.</p> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../ >Element Access</a> - overview of all ways to access elements</li> <li><a href=../unchecked_access/ >Unchecked access: operator[]</a> - access without range checking</li> <li><a href=../default_value/ >Access with default value: value</a> - fall back to a default instead of throwing</li> <li><a href=../../../api/basic_json/at/ ><code>at</code></a> - checked access by reference</li> <li><a href=../../json_pointer/ ><abbr title="JavaScript Object Notation">JSON</abbr> Pointer</a> - access nested values with <code>at</code> and a pointer</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="October 27, 2026 06:41:42 UTC">October 27, 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=../unchecked_access/ class="md-footer__link md-footer__link--prev" aria-label="Previous: Unchecked access: operator[]"> <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 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> Unchecked access: operator[] </div> </div> </a> <a href=../default_value/ class="md-footer__link md-footer__link--next" aria-label="Next: Access with default value: value"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> Access with default value: value </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 11z"/></svg> </div> </a> </nav> <div clasLine truncated
@@ -80,3 +80,13 @@ Exceptions
| access to valid array index | reference to existing value is returned | const reference to existing value is returned |
| access to non-existing object key | `basic_json::out_of_range` exception is thrown | `basic_json::out_of_range` exception is thrown |
| access to invalid array index | `basic_json::out_of_range` exception is thrown | `basic_json::out_of_range` exception is thrown |
Result of [`at`](https://json.nlohmann.me/api/basic_json/at/index.md) in each scenario, for a non-const and a const value.
## See also
- [Element Access](https://json.nlohmann.me/features/element_access/index.md) - overview of all ways to access elements
- [Unchecked access: operator[]](https://json.nlohmann.me/features/element_access/unchecked_access/index.md) - access without range checking
- [Access with default value: value](https://json.nlohmann.me/features/element_access/default_value/index.md) - fall back to a default instead of throwing
- [`at`](https://json.nlohmann.me/api/basic_json/at/index.md) - checked access by reference
- [JSON Pointer](https://json.nlohmann.me/features/json_pointer/index.md) - access nested values with `at` and a pointer
+7 -5
View File
@@ -145,8 +145,10 @@ containing `.` need a different separator.
## See also
- [`value`](../../api/basic_json/value.md) for access with default value
- documentation on [checked access](checked_access.md)
- documentation on [JSON Pointer](../json_pointer.md)
- [`contains`](../../api/basic_json/contains.md) to check whether a key or JSON Pointer exists
- [`json_pointer::operator/=`](../../api/json_pointer/operator_slasheq.md) to build a pointer token by token
- [`value`](../../api/basic_json/value.md) - for access with default value
- [Checked access: at](checked_access.md) - access with bounds checking
- [JSON Pointer](../json_pointer.md) - addressing values by path
- [`contains`](../../api/basic_json/contains.md) - to check whether a key or JSON Pointer exists
- [`json_pointer::operator/=`](../../api/json_pointer/operator_slasheq.md) - to build a pointer token by token
- [Unchecked access: operator[]](unchecked_access.md) - access without range checking
- [Element Access](index.md) - overview of all ways to access elements
@@ -1,4 +1,4 @@
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href=#access-with-default-value-value class=md-skip> Skip to content </a> </div> <div data-md-component=announce> </div> <header class=md-header data-md-component=header> <nav class="md-header__inner md-grid" aria-label=Header> <a href=../../.. title="JSON for Modern C++" class="md-header__button md-logo" aria-label="JSON for Modern C++" data-md-component=logo> <img src=../../../images/wordmark-header.svg alt=logo> </a> <label class="md-header__button md-icon" for=__drawer> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M3 6h18v2H3zm0 5h18v2H3zm0 5h18v2H3z"/></svg> </label> <div class=md-header__title data-md-component=header-title> <div class=md-header__ellipsis> <div class=md-header__topic> <span class=md-ellipsis> JSON for Modern C++ </span> </div> <div class=md-header__topic data-md-component=header-topic> <span class=md-ellipsis> Access with default value: value </span> </div> </div> </div> <form class=md-header__option data-md-component=palette> <input 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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 xLine truncated
<span class=w> </span><span class=nt>"logOutput"</span><span class=p>:</span><span class=w> </span><span class=s2>"result.log"</span><span class=p>,</span>
<span class=w> </span><span class=nt>"append"</span><span class=p>:</span><span class=w> </span><span class=kc>true</span>
<span class=p>}</span>
@@ -48,4 +48,4 @@
<span class=err>de</span><span class=kc>fault</span><span class=w> </span><span class=err>value</span><span class=w> </span><span class=err>(i</span><span class=kc>nt</span><span class=err>)</span><span class=p>:</span><span class=w> </span><span class=mi>-1</span>
<span class=err>de</span><span class=kc>fault</span><span class=w> </span><span class=err>value</span><span class=w> </span><span class=err>(ui</span><span class=kc>nt</span><span class=mi>64</span><span class=err>_</span><span class=kc>t</span><span class=err>)</span><span class=p>:</span><span class=w> </span><span class=mi>18446744073709551615</span>
<span class=err>explici</span><span class=kc>t</span><span class=w> </span><span class=err>re</span><span class=kc>turn</span><span class=w> </span><span class=err>value</span><span class=w> </span><span class=kc>t</span><span class=err>ype</span><span class=p>:</span><span class=w> </span><span class=mi>18446744073709551615</span>
</code></pre></div> </div> <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/value/ ><code>value</code></a> for access with default value</li> <li>documentation on <a href=../checked_access/ >checked access</a></li> <li>documentation on <a href=../../json_pointer/ ><abbr title="JavaScript Object Notation">JSON</abbr> Pointer</a></li> <li><a href=../../../api/basic_json/contains/ ><code>contains</code></a> to check whether a key or <abbr title="JavaScript Object Notation">JSON</abbr> Pointer exists</li> <li><a href=../../../api/json_pointer/operator_slasheq/ ><code>json_pointer::operator/=</code></a> to build a pointer token by token</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="October 11, 2026 06:22:10 UTC">October 11, 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> </div> <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/value/ ><code>value</code></a> - for access with default value</li> <li><a href=../checked_access/ >Checked access: at</a> - access with bounds checking</li> <li><a href=../../json_pointer/ ><abbr title="JavaScript Object Notation">JSON</abbr> Pointer</a> - addressing values by path</li> <li><a href=../../../api/basic_json/contains/ ><code>contains</code></a> - to check whether a key or <abbr title="JavaScript Object Notation">JSON</abbr> Pointer exists</li> <li><a href=../../../api/json_pointer/operator_slasheq/ ><code>json_pointer::operator/=</code></a> - to build a pointer token by token</li> <li><a href=../unchecked_access/ >Unchecked access: operator[]</a> - access without range checking</li> <li><a href=../ >Element Access</a> - overview of all ways to access elements</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="October 27, 2026 06:41:42 UTC">October 27, 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=../checked_access/ class="md-footer__link md-footer__link--prev" aria-label="Previous: Checked access: at"> <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 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> Checked access: at </div> </div> </a> <a href=../../iterators/ class="md-footer__link md-footer__link--next" aria-label="Next: Iterators"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> Iterators </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 11z"/></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; 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@@ -124,8 +124,10 @@ explicit return value type: 18446744073709551615
## See also
- [`value`](https://json.nlohmann.me/api/basic_json/value/index.md) for access with default value
- documentation on [checked access](https://json.nlohmann.me/features/element_access/checked_access/index.md)
- documentation on [JSON Pointer](https://json.nlohmann.me/features/json_pointer/index.md)
- [`contains`](https://json.nlohmann.me/api/basic_json/contains/index.md) to check whether a key or JSON Pointer exists
- [`json_pointer::operator/=`](https://json.nlohmann.me/api/json_pointer/operator_slasheq/index.md) to build a pointer token by token
- [`value`](https://json.nlohmann.me/api/basic_json/value/index.md) - for access with default value
- [Checked access: at](https://json.nlohmann.me/features/element_access/checked_access/index.md) - access with bounds checking
- [JSON Pointer](https://json.nlohmann.me/features/json_pointer/index.md) - addressing values by path
- [`contains`](https://json.nlohmann.me/api/basic_json/contains/index.md) - to check whether a key or JSON Pointer exists
- [`json_pointer::operator/=`](https://json.nlohmann.me/api/json_pointer/operator_slasheq/index.md) - to build a pointer token by token
- [Unchecked access: operator[]](https://json.nlohmann.me/features/element_access/unchecked_access/index.md) - access without range checking
- [Element Access](https://json.nlohmann.me/features/element_access/index.md) - overview of all ways to access elements
+11 -8
View File
@@ -1,10 +1,13 @@
<!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/element_access/" rel="canonical"><link href=../creating_values/ rel=prev><link href=unchecked_access/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>Element Access - 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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A["accessing a value"] --&gt; B{"must it exist?"}
B --&gt;|"yes, missing is an error"| C["at() -- throws"]
B --&gt;|"yes, but checking is my job"| D["operator[] -- unchecked"]
B --&gt;|"no, a fallback is fine"| E["value() -- default value"]
A --&gt; F{"just testing first?"}
F --&gt;|"yes"| G["contains() / find()"]</code></pre> <h2 id=does-this-key-exist>Does this key exist?<a class=headerlink href=#does-this-key-exist title="Permanent link">&para;</a></h2> <p>Use <a href=../../api/basic_json/contains/ ><code>contains</code></a> to test for a key; it does not modify the value and does not throw:</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=p>{{</span><span class=s>"name"</span><span class=p>,</span><span class=w> </span><span class=s>"Mary"</span><span class=p>},</span><span class=w> </span><span class=p>{</span><span class=s>"address"</span><span class=p>,</span><span class=w> </span><span class=p>{{</span><span class=s>"city"</span><span class=p>,</span><span class=w> </span><span class=s>"Berlin"</span><span class=p>}}}};</span>
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A["getting a value out"] --&gt; P{{"nested path such as /server/port?"}}
P --&gt;|"yes"| JP["JSON Pointer: at(ptr), value(ptr, default), contains(ptr)"]
P --&gt;|"no, one key or index"| T{{"only testing whether it exists?"}}
T --&gt;|"yes"| CF["contains() / find()"]
T --&gt;|"no"| M{{"may it be missing?"}}
M --&gt;|"yes, a fallback is fine"| V["value(key, default)"]
M --&gt;|"yes, and missing is an error"| AT["at(): throws out_of_range"]
M --&gt;|"no, it exists and its type is known"| OP["operator[] and get#lt;T#gt;()"]
OP -.- W["non-const operator[] inserts null for a missing key"]</code></pre> <p class=nj-caption>Decision tree for reading a value: the labels on the arrows are the answers to the questions, and the boxes name the functions to use. The dashed line marks a side effect of the non-const <code>operator[]</code>.</p> <h2 id=does-this-key-exist>Does this key exist?<a class=headerlink href=#does-this-key-exist title="Permanent link">&para;</a></h2> <p>Use <a href=../../api/basic_json/contains/ ><code>contains</code></a> to test for a key; it does not modify the value and does not throw:</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=p>{{</span><span class=s>"name"</span><span class=p>,</span><span class=w> </span><span class=s>"Mary"</span><span class=p>},</span><span class=w> </span><span class=p>{</span><span class=s>"address"</span><span class=p>,</span><span class=w> </span><span class=p>{{</span><span class=s>"city"</span><span class=p>,</span><span class=w> </span><span class=s>"Berlin"</span><span class=p>}}}};</span>
<span class=n>j</span><span class=p>.</span><span class=n>contains</span><span class=p>(</span><span class=s>"name"</span><span class=p>);</span><span class=w> </span><span class=c1>// true</span>
<span class=n>j</span><span class=p>.</span><span class=n>contains</span><span class=p>(</span><span class=s>"age"</span><span class=p>);</span><span class=w> </span><span class=c1>// false</span>
@@ -17,4 +20,4 @@
<span class=p>}</span>
</code></pre></div> <p><code>contains</code> also returns <code>true</code> for a key whose value is <code>null</code>; use <code>j.contains("k") &amp;&amp; !j["k"].is_null()</code> if a <code>null</code> should count as missing. To read a value with a fallback instead, see <a href=default_value/ ><code>value</code></a>.</p> <div class="admonition warning"> <p class=admonition-title><code>operator[]</code> inserts missing keys</p> <p>Like <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>map</span><span class=o>::</span><span class=k>operator</span><span class=p>[]</span></code>, the non-const <a href=unchecked_access/ ><code>operator[]</code></a> adds a <code>null</code> value if the key is missing. Testing a key with it therefore <em>changes the object</em>:</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=p>{{</span><span class=s>"foo"</span><span class=p>,</span><span class=w> </span><span class=mi>1</span><span class=p>}};</span>
<span class=kt>bool</span><span class=w> </span><span class=n>absent</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>j</span><span class=p>[</span><span class=s>"bar"</span><span class=p>].</span><span class=n>is_null</span><span class=p>();</span><span class=w> </span><span class=c1>// true, but j is now {"bar": null, "foo": 1}</span>
</code></pre></div> <p>Use <code>contains</code>/<code>find</code> to test, and <a href=checked_access/ ><code>at</code></a> to access a value that must exist (it throws an <a href=../../home/exceptions/#out-of-range><code>out_of_range</code></a> exception otherwise).</p> </div> <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/operator%5B%5D/ ><strong>operator[]</strong></a> - unchecked access</li> <li><a href=../../api/basic_json/at/ ><strong>at</strong></a> - checked access</li> <li><a href=../../api/basic_json/value/ ><strong>value</strong></a> - access with default value</li> <li><a href=../../api/basic_json/contains/ ><strong>contains</strong></a> and <a href=../../api/basic_json/find/ ><strong>find</strong></a> - test for existence</li> <li><a href=../../api/basic_json/front/ ><strong>front</strong></a> and <a href=../../api/basic_json/back/ ><strong>back</strong></a> - first and last element</li> <li><a href=../../api/basic_json/items/ ><strong>items</strong></a> - iterate over keys and values</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="October 25, 2026 11:07:13 UTC">October 25, 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>Use <code>contains</code>/<code>find</code> to test, and <a href=checked_access/ ><code>at</code></a> to access a value that must exist (it throws an <a href=../../home/exceptions/#out-of-range><code>out_of_range</code></a> exception otherwise).</p> </div> <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/operator%5B%5D/ ><code>operator[]</code></a> - unchecked access</li> <li><a href=../../api/basic_json/at/ ><code>at</code></a> - checked access</li> <li><a href=../../api/basic_json/value/ ><code>value</code></a> - access with default value</li> <li><a href=../../api/basic_json/contains/ ><code>contains</code></a> - and <a href=../../api/basic_json/find/ ><code>find</code></a> - test for existence</li> <li><a href=../../api/basic_json/front/ ><code>front</code></a> - and <a href=../../api/basic_json/back/ ><code>back</code></a> - first and last element</li> <li><a href=../../api/basic_json/items/ ><code>items</code></a> - iterate over keys and values</li> <li><a href=checked_access/ >Checked access: at</a> - access with range checking</li> <li><a href=unchecked_access/ >Unchecked access: operator[]</a> - access without range checking</li> <li><a href=default_value/ >Access with default value: value</a> - fall back to a default instead of throwing</li> <li><a href=../iterators/ >Iterators</a> - visit all elements of arrays and objects</li> <li><a href=../json_pointer/ ><abbr title="JavaScript Object Notation">JSON</abbr> Pointer</a> - address nested values with a single string</li> <li><a href=../modifying_values/ >Modifying values</a> - add, merge and remove elements</li> <li><a href=../../api/basic_json/ ><code>basic_json</code></a> - all member functions</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="October 27, 2026 06:41:42 UTC">October 27, 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=../creating_values/ class="md-footer__link md-footer__link--prev" aria-label="Previous: Creating JSON values"> <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 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> Creating JSON values </div> </div> </a> <a href=unchecked_access/ class="md-footer__link md-footer__link--next" aria-label="Next: Unchecked access: operator[]"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> Unchecked access: operator[] </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 11z"/></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; 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+26 -12
View File
@@ -10,16 +10,23 @@ There are many ways elements in a JSON value can be accessed:
Testing whether a key or index exists before accessing it is also possible, with [`contains`](https://json.nlohmann.me/api/basic_json/contains/index.md) or [`find`](https://json.nlohmann.me/api/basic_json/find/index.md) (which returns an iterator to the value, or `end()` if it is not found).
How do I get a value out? Start at the top and follow the first question that applies:
```
flowchart TD
A["accessing a value"] --> B{"must it exist?"}
B -->|"yes, missing is an error"| C["at() -- throws"]
B -->|"yes, but checking is my job"| D["operator[] -- unchecked"]
B -->|"no, a fallback is fine"| E["value() -- default value"]
A --> F{"just testing first?"}
F -->|"yes"| G["contains() / find()"]
A["getting a value out"] --> P{{"nested path such as /server/port?"}}
P -->|"yes"| JP["JSON Pointer: at(ptr), value(ptr, default), contains(ptr)"]
P -->|"no, one key or index"| T{{"only testing whether it exists?"}}
T -->|"yes"| CF["contains() / find()"]
T -->|"no"| M{{"may it be missing?"}}
M -->|"yes, a fallback is fine"| V["value(key, default)"]
M -->|"yes, and missing is an error"| AT["at(): throws out_of_range"]
M -->|"no, it exists and its type is known"| OP["operator[] and get#lt;T#gt;()"]
OP -.- W["non-const operator[] inserts null for a missing key"]
```
Decision tree for reading a value: the labels on the arrows are the answers to the questions, and the boxes name the functions to use. The dashed line marks a side effect of the non-const `operator[]`.
## Does this key exist?
Use [`contains`](https://json.nlohmann.me/api/basic_json/contains/index.md) to test for a key; it does not modify the value and does not throw:
@@ -58,9 +65,16 @@ Use `contains`/`find` to test, and [`at`](https://json.nlohmann.me/features/elem
## See also
- [**operator[]**](https://json.nlohmann.me/api/basic_json/operator%5B%5D/index.md) - unchecked access
- [**at**](https://json.nlohmann.me/api/basic_json/at/index.md) - checked access
- [**value**](https://json.nlohmann.me/api/basic_json/value/index.md) - access with default value
- [**contains**](https://json.nlohmann.me/api/basic_json/contains/index.md) and [**find**](https://json.nlohmann.me/api/basic_json/find/index.md) - test for existence
- [**front**](https://json.nlohmann.me/api/basic_json/front/index.md) and [**back**](https://json.nlohmann.me/api/basic_json/back/index.md) - first and last element
- [**items**](https://json.nlohmann.me/api/basic_json/items/index.md) - iterate over keys and values
- [`operator[]`](https://json.nlohmann.me/api/basic_json/operator%5B%5D/index.md) - unchecked access
- [`at`](https://json.nlohmann.me/api/basic_json/at/index.md) - checked access
- [`value`](https://json.nlohmann.me/api/basic_json/value/index.md) - access with default value
- [`contains`](https://json.nlohmann.me/api/basic_json/contains/index.md) - and [`find`](https://json.nlohmann.me/api/basic_json/find/index.md) - test for existence
- [`front`](https://json.nlohmann.me/api/basic_json/front/index.md) - and [`back`](https://json.nlohmann.me/api/basic_json/back/index.md) - first and last element
- [`items`](https://json.nlohmann.me/api/basic_json/items/index.md) - iterate over keys and values
- [Checked access: at](https://json.nlohmann.me/features/element_access/checked_access/index.md) - access with range checking
- [Unchecked access: operator[]](https://json.nlohmann.me/features/element_access/unchecked_access/index.md) - access without range checking
- [Access with default value: value](https://json.nlohmann.me/features/element_access/default_value/index.md) - fall back to a default instead of throwing
- [Iterators](https://json.nlohmann.me/features/iterators/index.md) - visit all elements of arrays and objects
- [JSON Pointer](https://json.nlohmann.me/features/json_pointer/index.md) - address nested values with a single string
- [Modifying values](https://json.nlohmann.me/features/modifying_values/index.md) - add, merge and remove elements
- [`basic_json`](https://json.nlohmann.me/api/basic_json/index.md) - all member functions
@@ -125,3 +125,13 @@ for (int i = 0; i < 1000; ++i) {
| access to valid array index | reference to existing value is returned | const reference to existing value is returned |
| access to non-existing object key | reference to newly inserted `#!json null` value is returned | **undefined behavior**; [runtime assertion](../assertions.md) in debug mode |
| access to invalid array index | reference to newly inserted `#!json null` value is returned; any index between previous maximal index and passed index are filled with `#!json null` | **undefined behavior**; [runtime assertion](../assertions.md) in debug mode |
<p class="nj-table-caption" markdown>Result of [`operator[]`](../../api/basic_json/operator%5B%5D.md) in each scenario, for a non-const and a const value.</p>
## See also
- [Element Access](index.md) - overview of all ways to access elements
- [Checked access: at](checked_access.md) - access with range checking
- [Access with default value: value](default_value.md) - fall back to a default instead of throwing
- [`operator[]`](../../api/basic_json/operator%5B%5D.md) - unchecked access by reference
- [Runtime assertions](../assertions.md) - assertions that guard `operator[]` misuse
@@ -1,4 +1,4 @@
<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/element_access/unchecked_access/" rel="canonical"><link href=../ rel=prev><link href=../checked_access/ rel=next><link rel=icon href=../../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>Unchecked access: operator[] - 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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<!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/element_access/unchecked_access/" rel="canonical"><link href=../ rel=prev><link href=../checked_access/ rel=next><link rel=icon href=../../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>Unchecked access: operator[] - 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=../../../css/custom.css><link rel=stylesheet href=../../../css/brand.css><link rel=stylesheet href=../../../css/fig-sax.css><link rel=stylesheet 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href=#unchecked-access-operator class=md-skip> Skip to content </a> </div> <div data-md-component=announce> </div> <header class=md-header data-md-component=header> <nav class="md-header__inner md-grid" aria-label=Header> <a href=../../.. title="JSON for Modern C++" class="md-header__button md-logo" aria-label="JSON for Modern C++" data-md-component=logo> <img src=../../../images/wordmark-header.svg alt=logo> </a> <label class="md-header__button md-icon" for=__drawer> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M3 6h18v2H3zm0 5h18v2H3zm0 5h18v2H3z"/></svg> </label> <div class=md-header__title data-md-component=header-title> <div class=md-header__ellipsis> <div class=md-header__topic> <span class=md-ellipsis> JSON for Modern C++ </span> </div> <div class=md-header__topic data-md-component=header-topic> <span class=md-ellipsis> Unchecked access: operator[] </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=custom data-md-color-accent=custom 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=custom data-md-color-accent=custom 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/2Line truncated
<span class=w> </span><span class=nt>"name"</span><span class=p>:</span><span class=w> </span><span class=s2>"Mary Smith"</span><span class=p>,</span>
<span class=w> </span><span class=nt>"age"</span><span class=p>:</span><span class=w> </span><span class=mi>42</span><span class=p>,</span>
<span class=w> </span><span class=nt>"hobbies"</span><span class=p>:</span><span class=w> </span><span class=p>[</span><span class=s2>"hiking"</span><span class=p>,</span><span class=w> </span><span class=s2>"reading"</span><span class=p>]</span>
@@ -24,4 +24,4 @@
<span class=k>for</span><span class=w> </span><span class=p>(</span><span class=kt>int</span><span class=w> </span><span class=n>i</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=mi>0</span><span class=p>;</span><span class=w> </span><span class=n>i</span><span class=w> </span><span class=o>&lt;</span><span class=w> </span><span class=mi>1000</span><span class=p>;</span><span class=w> </span><span class=o>++</span><span class=n>i</span><span class=p>)</span><span class=w> </span><span class=p>{</span>
<span class=w> </span><span class=n>j</span><span class=p>.</span><span class=n>push_back</span><span class=p>(</span><span class=n>i</span><span class=p>);</span>
<span class=p>}</span>
</code></pre></div> <h2 id=summary>Summary<a class=headerlink href=#summary title="Permanent link">&para;</a></h2> <table> <thead> <tr> <th>scenario</th> <th>non-const value</th> <th>const value</th> </tr> </thead> <tbody> <tr> <td>access to existing object key</td> <td>reference to existing value is returned</td> <td>const reference to existing value is returned</td> </tr> <tr> <td>access to valid array index</td> <td>reference to existing value is returned</td> <td>const reference to existing value is returned</td> </tr> <tr> <td>access to non-existing object key</td> <td>reference to newly inserted <code class=highlight><span class=kc>null</span></code> value is returned</td> <td><strong>undefined behavior</strong>; <a href=../../assertions/ >runtime assertion</a> in debug mode</td> </tr> <tr> <td>access to invalid array index</td> <td>reference to newly inserted <code class=highlight><span class=kc>null</span></code> value is returned; any index between previous maximal index and passed index are filled with <code class=highlight><span class=kc>null</span></code></td> <td><strong>undefined behavior</strong>; <a href=../../assertions/ >runtime assertion</a> in debug mode</td> </tr> </tbody> </table> <!-- 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="October 2, 2026 09:32:15 UTC">October 2, 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> <h2 id=summary>Summary<a class=headerlink href=#summary title="Permanent link">&para;</a></h2> <table> <thead> <tr> <th>scenario</th> <th>non-const value</th> <th>const value</th> </tr> </thead> <tbody> <tr> <td>access to existing object key</td> <td>reference to existing value is returned</td> <td>const reference to existing value is returned</td> </tr> <tr> <td>access to valid array index</td> <td>reference to existing value is returned</td> <td>const reference to existing value is returned</td> </tr> <tr> <td>access to non-existing object key</td> <td>reference to newly inserted <code class=highlight><span class=kc>null</span></code> value is returned</td> <td><strong>undefined behavior</strong>; <a href=../../assertions/ >runtime assertion</a> in debug mode</td> </tr> <tr> <td>access to invalid array index</td> <td>reference to newly inserted <code class=highlight><span class=kc>null</span></code> value is returned; any index between previous maximal index and passed index are filled with <code class=highlight><span class=kc>null</span></code></td> <td><strong>undefined behavior</strong>; <a href=../../assertions/ >runtime assertion</a> in debug mode</td> </tr> </tbody> </table> <p class=nj-table-caption>Result of <a href=../../../api/basic_json/operator%5B%5D/ ><code>operator[]</code></a> in each scenario, for a non-const and a const value.</p> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=../ >Element Access</a> - overview of all ways to access elements</li> <li><a href=../checked_access/ >Checked access: at</a> - access with range checking</li> <li><a href=../default_value/ >Access with default value: value</a> - fall back to a default instead of throwing</li> <li><a href=../../../api/basic_json/operator%5B%5D/ ><code>operator[]</code></a> - unchecked access by reference</li> <li><a href=../../assertions/ >Runtime assertions</a> - assertions that guard <code>operator[]</code> misuse</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="October 27, 2026 06:41:42 UTC">October 27, 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=../ class="md-footer__link md-footer__link--prev" aria-label="Previous: Element Access"> <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 11z"/></svg> </div> <div class=md-footer__title> <span class=md-footer__direction> Previous </span> <div class=md-ellipsis> Element Access </div> </div> </a> <a href=../checked_access/ class="md-footer__link md-footer__link--next" aria-label="Next: Checked access: at"> <div class=md-footer__title> <span class=md-footer__direction> Next </span> <div class=md-ellipsis> Checked access: at </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 11z"/></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; 2013-2026 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://fontawesomLine truncated
@@ -112,3 +112,13 @@ for (int i = 0; i < 1000; ++i) {
| access to valid array index | reference to existing value is returned | const reference to existing value is returned |
| access to non-existing object key | reference to newly inserted `null` value is returned | **undefined behavior**; [runtime assertion](https://json.nlohmann.me/features/assertions/index.md) in debug mode |
| access to invalid array index | reference to newly inserted `null` value is returned; any index between previous maximal index and passed index are filled with `null` | **undefined behavior**; [runtime assertion](https://json.nlohmann.me/features/assertions/index.md) in debug mode |
Result of [`operator[]`](https://json.nlohmann.me/api/basic_json/operator%5B%5D/index.md) in each scenario, for a non-const and a const value.
## See also
- [Element Access](https://json.nlohmann.me/features/element_access/index.md) - overview of all ways to access elements
- [Checked access: at](https://json.nlohmann.me/features/element_access/checked_access/index.md) - access with range checking
- [Access with default value: value](https://json.nlohmann.me/features/element_access/default_value/index.md) - fall back to a default instead of throwing
- [`operator[]`](https://json.nlohmann.me/api/basic_json/operator%5B%5D/index.md) - unchecked access by reference
- [Runtime assertions](https://json.nlohmann.me/features/assertions/index.md) - assertions that guard `operator[]` misuse