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@@ -6,7 +6,7 @@ The [`at`](../../api/basic_json/at.md) member function performs checked access;
desired value if it exists and throws a [`basic_json::out_of_range` exception](../../home/exceptions.md#out-of-range)
otherwise.
??? example "Read access"
??? example "Example: read access"
Consider the following JSON value:
@@ -31,7 +31,7 @@ otherwise.
The return value is a reference, so it can be used to modify the original value.
??? example "Write access"
??? example "Example: write access"
```cpp
j.at("name") = "John Smith";
@@ -50,7 +50,7 @@ The return value is a reference, so it can be used to modify the original value.
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.
??? example "Accessing via invalid index or missing key"
??? example "Example: access via invalid index or missing key"
```cpp
j.at("hobbies").at(3) = "cooking";
@@ -1,15 +1,15 @@
<!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 URL("../../..",location),__md_hash=e=>[...e].reduce(((e,_)=>(e<<5)-e+_.charCodeAt(0)),0),__md_get=(e,_=localStorage,t=__md_scope)=>JSON.parse(_.getItem(t.pathname+"."+e)),__md_set=(e,_,t=localStorage,a=__md_scope)=>{try{t.setItem(a.pathname+"."+e,JSON.stringify(_))}catch(e){}}</script></head> <body dir=ltr data-md-color-scheme=default data-md-color-primary=indigo data-md-color-accent=indigo> <input class=md-toggle data-md-toggle=drawer type=checkbox id=__drawer autocomplete=off> <input class=md-toggle data-md-toggle=search type=checkbox id=__search autocomplete=off> <label class=md-overlay for=__drawer></label> <div data-md-component=skip> <a href=#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> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 8a3 3 0 0 0 3-3 3 3 0 0 0-3-3 3 3 0 0 0-3 3 3 3 0 0 0 3 3m0 3.54C9.64 9.35 6.5 8 3 8v11c3.5 0 6.64 1.35 9 3.54 2.36-2.19 5.5-3.54 9-3.54V8c-3.5 0-6.64 1.35-9 3.54"/></svg> </a> <label class="md-header__button md-icon" for=__drawer> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M3 6h18v2H3zm0 5h18v2H3zm0 5h18v2H3z"/></svg> </label> <div class=md-header__title data-md-component=header-title> <div class=md-header__ellipsis> <div class=md-header__topic> <span class=md-ellipsis> JSON for Modern C++ </span> </div> <div class=md-header__topic data-md-component=header-topic> <span class=md-ellipsis> 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=indigo data-md-color-accent=indigo aria-label="Switch to dark mode" type=radio name=__palette id=__palette_0> <label class="md-header__button md-icon" title="Switch to dark mode" for=__palette_1 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 8a4 4 0 0 0-4 4 4 4 0 0 0 4 4 4 4 0 0 0 4-4 4 4 0 0 0-4-4m0 10a6 6 0 0 1-6-6 6 6 0 0 1 6-6 6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> <input class=md-option data-md-color-media="(prefers-color-scheme: dark)" data-md-color-scheme=slate data-md-color-primary=indigo data-md-color-accent=indigo aria-label="Switch to light mode" type=radio name=__palette id=__palette_1> <label class="md-header__button md-icon" title="Switch to light mode" for=__palette_0 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 18c-.89 0-1.74-.2-2.5-.55C11.56 16.5 13 14.42 13 12s-1.44-4.5-3.5-5.45C10.26 6.2 11.11 6 12 6a6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> </form> <script>var palette=__md_get("__palette");if(palette&&palette.color){if("(prefers-color-scheme)"===palette.color.media){var media=matchMedia("(prefers-color-scheme: light)"),input=document.querySelector(media.matches?"[data-md-color-media='(prefers-color-scheme: light)']":"[data-md-color-media='(prefers-color-scheme: dark)']");palette.color.media=input.getAttribute("data-md-color-media"),palette.color.scheme=input.getAttribute("data-md-color-scheme"),palette.color.primary=input.getAttribute("data-md-color-primary"),palette.color.accent=input.getAttribute("data-md-color-accent")}for(var[key,value]of Object.entries(palette.color))document.body.setAttribute("data-md-color-"+key,value)}</script> <label class="md-header__button md-icon" for=__search> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M9.5 3A6.5 6.5 0 0 1 16 9.5c0 1.61-.59 3.09-1.56 4.23l.27.27h.79l5 5-1.5 1.5-5-5v-.79l-.27-.27A6.52 6.52 0 0 1 9.5 16 6.5 6.5 0 0 1 3 9.5 6.5 6.5 0 0 1 9.5 3m0 2C7 5 5 7 5 9.5S7 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=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 URL("../../..",location),__md_hash=e=>[...e].reduce(((e,_)=>(e<<5)-e+_.charCodeAt(0)),0),__md_get=(e,_=localStorage,t=__md_scope)=>JSON.parse(_.getItem(t.pathname+"."+e)),__md_set=(e,_,t=localStorage,a=__md_scope)=>{try{t.setItem(a.pathname+"."+e,JSON.stringify(_))}catch(e){}}</script></head> <body dir=ltr data-md-color-scheme=default data-md-color-primary=indigo data-md-color-accent=indigo> <input class=md-toggle data-md-toggle=drawer type=checkbox id=__drawer autocomplete=off> <input class=md-toggle data-md-toggle=search type=checkbox id=__search autocomplete=off> <label class=md-overlay for=__drawer></label> <div data-md-component=skip> <a href=#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> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 8a3 3 0 0 0 3-3 3 3 0 0 0-3-3 3 3 0 0 0-3 3 3 3 0 0 0 3 3m0 3.54C9.64 9.35 6.5 8 3 8v11c3.5 0 6.64 1.35 9 3.54 2.36-2.19 5.5-3.54 9-3.54V8c-3.5 0-6.64 1.35-9 3.54"/></svg> </a> <label class="md-header__button md-icon" for=__drawer> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M3 6h18v2H3zm0 5h18v2H3zm0 5h18v2H3z"/></svg> </label> <div class=md-header__title data-md-component=header-title> <div class=md-header__ellipsis> <div class=md-header__topic> <span class=md-ellipsis> JSON for Modern C++ </span> </div> <div class=md-header__topic data-md-component=header-topic> <span class=md-ellipsis> 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=indigo data-md-color-accent=indigo aria-label="Switch to dark mode" type=radio name=__palette id=__palette_0> <label class="md-header__button md-icon" title="Switch to dark mode" for=__palette_1 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 8a4 4 0 0 0-4 4 4 4 0 0 0 4 4 4 4 0 0 0 4-4 4 4 0 0 0-4-4m0 10a6 6 0 0 1-6-6 6 6 0 0 1 6-6 6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> <input class=md-option data-md-color-media="(prefers-color-scheme: dark)" data-md-color-scheme=slate data-md-color-primary=indigo data-md-color-accent=indigo aria-label="Switch to light mode" type=radio name=__palette id=__palette_1> <label class="md-header__button md-icon" title="Switch to light mode" for=__palette_0 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 18c-.89 0-1.74-.2-2.5-.55C11.56 16.5 13 14.42 13 12s-1.44-4.5-3.5-5.45C10.26 6.2 11.11 6 12 6a6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> </form> <script>var palette=__md_get("__palette");if(palette&&palette.color){if("(prefers-color-scheme)"===palette.color.media){var media=matchMedia("(prefers-color-scheme: light)"),input=document.querySelector(media.matches?"[data-md-color-media='(prefers-color-scheme: light)']":"[data-md-color-media='(prefers-color-scheme: dark)']");palette.color.media=input.getAttribute("data-md-color-media"),palette.color.scheme=input.getAttribute("data-md-color-scheme"),palette.color.primary=input.getAttribute("data-md-color-primary"),palette.color.accent=input.getAttribute("data-md-color-accent")}for(var[key,value]of Object.entries(palette.color))document.body.setAttribute("data-md-color-"+key,value)}</script> <label class="md-header__button md-icon" for=__search> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M9.5 3A6.5 6.5 0 0 1 16 9.5c0 1.61-.59 3.09-1.56 4.23l.27.27h.79l5 5-1.5 1.5-5-5v-.79l-.27-.27A6.52 6.52 0 0 1 9.5 16 6.5 6.5 0 0 1 3 9.5 6.5 6.5 0 0 1 9.5 3m0 2C7 5 5 7 5 9.5S7 14Line 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>
<span class=p>}</span>
</code></pre></div> <p>Assume the value is parsed to a <code>json</code> variable <code>j</code>.</p> <table> <thead> <tr> <th>expression</th> <th>value</th> </tr> </thead> <tbody> <tr> <td><code class=highlight><span class=n>j</span></code></td> <td><code class=highlight><span class=p>{</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></code></td> </tr> <tr> <td><code class=highlight><span class=n>j</span><span class=p>.</span><span class=n>at</span><span class=p>(</span><span class=s>"name"</span><span class=p>)</span></code></td> <td><code class=highlight><span class=s2>"Mary Smith"</span></code></td> </tr> <tr> <td><code class=highlight><span class=n>j</span><span class=p>.</span><span class=n>at</span><span class=p>(</span><span class=s>"age"</span><span class=p>)</span></code></td> <td><code class=highlight><span class=mi>42</span></code></td> </tr> <tr> <td><code class=highlight><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></code></td> <td><code class=highlight><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></code></td> </tr> <tr> <td><code class=highlight><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>0</span><span class=p>)</span></code></td> <td><code class=highlight><span class=s2>"hiking"</span></code></td> </tr> <tr> <td><code class=highlight><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>1</span><span class=p>)</span></code></td> <td><code class=highlight><span class=s2>"reading"</span></code></td> </tr> </tbody> </table> </details> <p>The return value is a reference, so it can be used to modify the original value.</p> <details class=example> <summary>Write access</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>"name"</span><span class=p>)</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=s>"John Smith"</span><span class=p>;</span>
</code></pre></div> <p>Assume the value is parsed to a <code>json</code> variable <code>j</code>.</p> <table> <thead> <tr> <th>expression</th> <th>value</th> </tr> </thead> <tbody> <tr> <td><code class=highlight><span class=n>j</span></code></td> <td><code class=highlight><span class=p>{</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></code></td> </tr> <tr> <td><code class=highlight><span class=n>j</span><span class=p>.</span><span class=n>at</span><span class=p>(</span><span class=s>"name"</span><span class=p>)</span></code></td> <td><code class=highlight><span class=s2>"Mary Smith"</span></code></td> </tr> <tr> <td><code class=highlight><span class=n>j</span><span class=p>.</span><span class=n>at</span><span class=p>(</span><span class=s>"age"</span><span class=p>)</span></code></td> <td><code class=highlight><span class=mi>42</span></code></td> </tr> <tr> <td><code class=highlight><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></code></td> <td><code class=highlight><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></code></td> </tr> <tr> <td><code class=highlight><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>0</span><span class=p>)</span></code></td> <td><code class=highlight><span class=s2>"hiking"</span></code></td> </tr> <tr> <td><code class=highlight><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>1</span><span class=p>)</span></code></td> <td><code class=highlight><span class=s2>"reading"</span></code></td> </tr> </tbody> </table> </details> <p>The return value is a reference, so it can be used to modify the original value.</p> <details class=example> <summary>Example: write access</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>"name"</span><span class=p>)</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=s>"John Smith"</span><span class=p>;</span>
</code></pre></div> <p>This code produces the following <abbr title="JavaScript Object Notation">JSON</abbr> value:</p> <div class=highlight><pre><span></span><code><span class=p>{</span>
<span class=w> </span><span class=nt>"name"</span><span class=p>:</span><span class=w> </span><span class=s2>"John 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>
<span class=p>}</span>
</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>Accessing 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> </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 09:23:55 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> <!-- 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
@@ -4,7 +4,7 @@
The [`at`](https://json.nlohmann.me/api/basic_json/at/index.md) member function performs checked access; that is, it returns a reference to the desired value if it exists and throws a [`basic_json::out_of_range` exception](https://json.nlohmann.me/home/exceptions/#out-of-range) otherwise.
Read access
Example: read access
Consider the following JSON value:
@@ -29,7 +29,7 @@ Assume the value is parsed to a `json` variable `j`.
The return value is a reference, so it can be used to modify the original value.
Write access
Example: write access
```
j.at("name") = "John Smith";
@@ -47,7 +47,7 @@ This code produces the following JSON value:
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.
Accessing via invalid index or missing key
Example: access via invalid index or missing key
```
j.at("hobbies").at(3) = "cooking";
+3 -3
View File
@@ -41,9 +41,9 @@ you want to access and a default value in case there is no value stored with tha
The value function is a template, and the return type of the function is determined by the type of the provided
default value unless otherwise specified. This can have unexpected effects. In the example below, we store a 64-bit
unsigned integer. We get exactly that value when using [`operator[]`](../../api/basic_json/operator[].md). However,
when we call `value` and provide `#!c 0` as default value, then `#!c -1` is returned. This occurs, because `#!c 0`
has type `#!c int` which overflows when handling the value `#!c 18446744073709551615`.
unsigned integer. We get exactly that value when using [`operator[]`](../../api/basic_json/operator%5B%5D.md).
However, when we call `value` and provide `#!c 0` as default value, then `#!c -1` is returned. This occurs,
because `#!c 0` has type `#!c int` which overflows when handling the value `#!c 18446744073709551615`.
To address this issue, either provide a correctly typed default value or use the template parameter to specify the
desired return type. Note that this issue occurs even when a value is stored at the provided key, and the default
@@ -1,4 +1,4 @@
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<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/element_access/default_value/" rel="canonical"><link href=../checked_access/ rel=prev><link href=../../iterators/ rel=next><link rel=icon href=../../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>Access with default value: value - 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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data-md-component=logo> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 8a3 3 0 0 0 3-3 3 3 0 0 0-3-3 3 3 0 0 0-3 3 3 3 0 0 0 3 3m0 3.54C9.64 9.35 6.5 8 3 8v11c3.5 0 6.64 1.35 9 3.54 2.36-2.19 5.5-3.54 9-3.54V8c-3.5 0-6.64 1.35-9 3.54"/></svg> </a> <label class="md-header__button md-icon" for=__drawer> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M3 6h18v2H3zm0 5h18v2H3zm0 5h18v2H3z"/></svg> </label> <div class=md-header__title data-md-component=header-title> <div class=md-header__ellipsis> <div class=md-header__topic> <span class=md-ellipsis> JSON for Modern C++ </span> </div> <div class=md-header__topic data-md-component=header-topic> <span class=md-ellipsis> Access with default value: value </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 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6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> </form> <script>var palette=__md_get("__palette");if(palette&&palette.color){if("(prefers-color-scheme)"===palette.color.media){var media=matchMedia("(prefers-color-scheme: light)"),input=document.querySelector(media.matches?"[data-md-color-media='(prefers-color-scheme: light)']":"[data-md-color-media='(prefers-color-scheme: dark)']");palette.color.media=input.getAttribute("data-md-color-media"),palette.color.scheme=input.getAttribute("data-md-color-scheme"),palette.color.primary=input.getAttribute("data-md-color-primary"),palette.color.accent=input.getAttribute("data-md-color-accent")}for(var[key,value]of Object.entries(palette.color))document.body.setAttribute("data-md-color-"+key,value)}</script> <label class="md-header__button md-icon" for=__search> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M9.5 3A6.5 6.5 0 0 1 16 9.5c0 1.61-.59 3.09-1.56 4.23l.27.27h.79l5 5-1.5 1.5-5-5v-.79l-.27-.27A6.52 6.52 0 0 1 9.5 16 6.5 6.5 0 0 1 3 9.5 6.5 Line 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>
@@ -20,4 +20,4 @@
<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> </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 2, 2026 09:23:55 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> </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> </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 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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+7 -1
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@@ -1 +1,7 @@
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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> <!-- 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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+14 -2
View File
@@ -5,5 +5,17 @@ There are many ways elements in a JSON value can be accessed:
- unchecked access via [`operator[]`](https://json.nlohmann.me/features/element_access/unchecked_access/index.md)
- checked access via [`at`](https://json.nlohmann.me/features/element_access/checked_access/index.md)
- access with default value via [`value`](https://json.nlohmann.me/features/element_access/default_value/index.md)
- iterators
- JSON pointers
- [iterators](https://json.nlohmann.me/features/iterators/index.md)
- [JSON pointers](https://json.nlohmann.me/features/json_pointer/index.md)
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).
```
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()"]
```
+8 -7
View File
@@ -5,7 +5,7 @@
Elements in a JSON object and a JSON array can be accessed via [`operator[]`](../../api/basic_json/operator%5B%5D.md)
similar to a `#!cpp std::map` and a `#!cpp std::vector`, respectively.
??? example "Read access"
??? example "Example: read access"
Consider the following JSON value:
@@ -31,7 +31,7 @@ similar to a `#!cpp std::map` and a `#!cpp std::vector`, respectively.
The return value is a reference, so it can modify the original value. In case the passed object key is non-existing, a
`#!json null` value is inserted which can immediately be overwritten.
??? example "Write access"
??? example "Example: write access"
```cpp
j["name"] = "John Smith";
@@ -52,7 +52,7 @@ The return value is a reference, so it can modify the original value. In case th
When accessing an invalid index (i.e., an index greater than or equal to the array size), the JSON array is resized such
that the passed index is the new maximal index. Intermediate values are filled with `#!json null`.
??? example "Filling up arrays with `#!json null` values"
??? example "Example: filling up arrays with `#!json null` values"
```cpp
j["hobbies"][0] = "running";
@@ -94,8 +94,8 @@ that the passed index is the new maximal index. Intermediate values are filled w
- It is **undefined behavior** to access a const object with a non-existing key.
- It is **undefined behavior** to access a const array with an invalid index.
- In debug mode, an **assertion** will fire in both cases. You can disable assertions by defining the preprocessor
symbol `#!cpp NDEBUG` or redefine the macro [`JSON_ASSERT(x)`](../macros.md#json_assertx). See the documentation
on [runtime assertions](../assertions.md) for more information.
symbol `#!cpp NDEBUG` or redefine the macro [`JSON_ASSERT(x)`](../../api/macros/json_assert.md). See the
documentation on [runtime assertions](../assertions.md) for more information.
!!! failure "Exceptions"
@@ -105,8 +105,9 @@ that the passed index is the new maximal index. Intermediate values are filled w
## Performance: reserving array capacity
There is no public `reserve(count)` member on `basic_json` for pre-allocating array capacity. If you are building
a large array incrementally (e.g., via repeated `push_back()`) and know its final size ahead of time, you can
reserve capacity via `get_ref()` to access the underlying `array_t` directly:
a large array incrementally (e.g., via repeated [`push_back()`](../../api/basic_json/push_back.md)) and know its final
size ahead of time, you can reserve capacity via [`get_ref()`](../../api/basic_json/get_ref.md) to access the
underlying `array_t` directly:
```cpp
json j = json::array();
@@ -1,9 +1,9 @@
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<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=author content="Niels Lohmann"><link href="https://json.nlohmann.me/features/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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<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>
<span class=p>}</span>
</code></pre></div> <p>Assume the value is parsed to a <code>json</code> variable <code>j</code>.</p> <table> <thead> <tr> <th>expression</th> <th>value</th> </tr> </thead> <tbody> <tr> <td><code class=highlight><span class=n>j</span></code></td> <td><code class=highlight><span class=p>{</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></code></td> </tr> <tr> <td><code class=highlight><span class=n>j</span><span class=p>[</span><span class=s>"name"</span><span class=p>]</span></code></td> <td><code class=highlight><span class=s2>"Mary Smith"</span></code></td> </tr> <tr> <td><code class=highlight><span class=n>j</span><span class=p>[</span><span class=s>"age"</span><span class=p>]</span></code></td> <td><code class=highlight><span class=mi>42</span></code></td> </tr> <tr> <td><code class=highlight><span class=n>j</span><span class=p>[</span><span class=s>"hobbies"</span><span class=p>]</span></code></td> <td><code class=highlight><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></code></td> </tr> <tr> <td><code class=highlight><span class=n>j</span><span class=p>[</span><span class=s>"hobbies"</span><span class=p>][</span><span class=mi>0</span><span class=p>]</span></code></td> <td><code class=highlight><span class=s2>"hiking"</span></code></td> </tr> <tr> <td><code class=highlight><span class=n>j</span><span class=p>[</span><span class=s>"hobbies"</span><span class=p>][</span><span class=mi>1</span><span class=p>]</span></code></td> <td><code class=highlight><span class=s2>"reading"</span></code></td> </tr> </tbody> </table> </details> <p>The return value is a reference, so it can modify the original value. In case the passed object key is non-existing, a <code class=highlight><span class=kc>null</span></code> value is inserted which can immediately be overwritten.</p> <details class=example> <summary>Write access</summary> <div class=highlight><pre><span></span><code><span class=n>j</span><span class=p>[</span><span class=s>"name"</span><span class=p>]</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=s>"John Smith"</span><span class=p>;</span>
</code></pre></div> <p>Assume the value is parsed to a <code>json</code> variable <code>j</code>.</p> <table> <thead> <tr> <th>expression</th> <th>value</th> </tr> </thead> <tbody> <tr> <td><code class=highlight><span class=n>j</span></code></td> <td><code class=highlight><span class=p>{</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></code></td> </tr> <tr> <td><code class=highlight><span class=n>j</span><span class=p>[</span><span class=s>"name"</span><span class=p>]</span></code></td> <td><code class=highlight><span class=s2>"Mary Smith"</span></code></td> </tr> <tr> <td><code class=highlight><span class=n>j</span><span class=p>[</span><span class=s>"age"</span><span class=p>]</span></code></td> <td><code class=highlight><span class=mi>42</span></code></td> </tr> <tr> <td><code class=highlight><span class=n>j</span><span class=p>[</span><span class=s>"hobbies"</span><span class=p>]</span></code></td> <td><code class=highlight><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></code></td> </tr> <tr> <td><code class=highlight><span class=n>j</span><span class=p>[</span><span class=s>"hobbies"</span><span class=p>][</span><span class=mi>0</span><span class=p>]</span></code></td> <td><code class=highlight><span class=s2>"hiking"</span></code></td> </tr> <tr> <td><code class=highlight><span class=n>j</span><span class=p>[</span><span class=s>"hobbies"</span><span class=p>][</span><span class=mi>1</span><span class=p>]</span></code></td> <td><code class=highlight><span class=s2>"reading"</span></code></td> </tr> </tbody> </table> </details> <p>The return value is a reference, so it can modify the original value. In case the passed object key is non-existing, a <code class=highlight><span class=kc>null</span></code> value is inserted which can immediately be overwritten.</p> <details class=example> <summary>Example: write access</summary> <div class=highlight><pre><span></span><code><span class=n>j</span><span class=p>[</span><span class=s>"name"</span><span class=p>]</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=s>"John Smith"</span><span class=p>;</span>
<span class=n>j</span><span class=p>[</span><span class=s>"maidenName"</span><span class=p>]</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=s>"Jones"</span><span class=p>;</span>
</code></pre></div> <p>This code produces the following <abbr title="JavaScript Object Notation">JSON</abbr> value:</p> <div class=highlight><pre><span></span><code><span class=p>{</span>
<span class=w> </span><span class=nt>"name"</span><span class=p>:</span><span class=w> </span><span class=s2>"John Smith"</span><span class=p>,</span>
@@ -11,7 +11,7 @@
<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>
<span class=p>}</span>
</code></pre></div> </details> <p>When accessing an invalid index (i.e., an index greater than or equal to the array size), the <abbr title="JavaScript Object Notation">JSON</abbr> array is resized such that the passed index is the new maximal index. Intermediate values are filled with <code class=highlight><span class=kc>null</span></code>.</p> <details class=example> <summary>Filling up arrays with <code class=highlight><span class=kc>null</span></code> values</summary> <div class=highlight><pre><span></span><code><span class=n>j</span><span class=p>[</span><span class=s>"hobbies"</span><span class=p>][</span><span class=mi>0</span><span class=p>]</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=s>"running"</span><span class=p>;</span>
</code></pre></div> </details> <p>When accessing an invalid index (i.e., an index greater than or equal to the array size), the <abbr title="JavaScript Object Notation">JSON</abbr> array is resized such that the passed index is the new maximal index. Intermediate values are filled with <code class=highlight><span class=kc>null</span></code>.</p> <details class=example> <summary>Example: filling up arrays with <code class=highlight><span class=kc>null</span></code> values</summary> <div class=highlight><pre><span></span><code><span class=n>j</span><span class=p>[</span><span class=s>"hobbies"</span><span class=p>][</span><span class=mi>0</span><span class=p>]</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=s>"running"</span><span class=p>;</span>
<span class=n>j</span><span class=p>[</span><span class=s>"hobbies"</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 <abbr title="JavaScript Object Notation">JSON</abbr> value:</p> <div class=highlight><pre><span></span><code><span class=p>{</span>
<span class=w> </span><span class=nt>"name"</span><span class=p>:</span><span class=w> </span><span class=s2>"John Smith"</span><span class=p>,</span>
@@ -19,9 +19,9 @@
<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>"running"</span><span class=p>,</span><span class=w> </span><span class=s2>"reading"</span><span class=p>,</span><span class=w> </span><span class=kc>null</span><span class=p>,</span><span class=w> </span><span class=s2>"cooking"</span><span class=p>]</span>
<span class=p>}</span>
</code></pre></div> </details> <h2 id=notes>Notes<a class=headerlink href=#notes title="Permanent link">&para;</a></h2> <div class="admonition info"> <p class=admonition-title>Design rationale</p> <p>The library behaves differently to <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>vector</span></code> and <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>map</span></code>:</p> <ul> <li><code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>vector</span><span class=o>::</span><span class=k>operator</span><span class=p>[]</span></code> never inserts a new element.</li> <li><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> is not available for const values.</li> </ul> <p>The type <code class=highlight><span class=n>json</span></code> wraps all <abbr title="JavaScript Object Notation">JSON</abbr> value types. It would be impossible to remove <a href=../../../api/basic_json/operator%5B%5D/ ><code>operator[]</code></a> for const objects. At the same time, inserting elements for non-const objects is really convenient as it avoids awkward <code>insert</code> calls. To this end, we decided to have an inserting non-const behavior for both arrays and objects.</p> </div> <div class="admonition info"> <p class=admonition-title>Info</p> <p>The access is unchecked. In case the passed object key does not exist or the passed array index is invalid, no exception is thrown.</p> </div> <div class="admonition danger"> <p class=admonition-title>Danger</p> <ul> <li>It is <strong>undefined behavior</strong> to access a const object with a non-existing key.</li> <li>It is <strong>undefined behavior</strong> to access a const array with an invalid index.</li> <li>In debug mode, an <strong>assertion</strong> will fire in both cases. You can disable assertions by defining the preprocessor symbol <code class=highlight><span class=n>NDEBUG</span></code> or redefine the macro <a href=../../macros/#json_assertx><code>JSON_ASSERT(x)</code></a>. See the documentation on <a href=../../assertions/ >runtime assertions</a> for more information.</li> </ul> </div> <div class="admonition failure"> <p class=admonition-title>Exceptions</p> <p><code>operator[]</code> 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_error305><code>basic_json::type_error</code></a> is thrown.</p> </div> <h2 id=performance-reserving-array-capacity>Performance: reserving array capacity<a class=headerlink href=#performance-reserving-array-capacity title="Permanent link">&para;</a></h2> <p>There is no public <code>reserve(count)</code> member on <code>basic_json</code> for pre-allocating array capacity. If you are building a large array incrementally (e.g., via repeated <code>push_back()</code>) and know its final size ahead of time, you can reserve capacity via <code>get_ref()</code> to access the underlying <code>array_t</code> directly:</p> <div class=highlight><pre><span></span><code><span class=n>json</span><span class=w> </span><span class=n>j</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>array</span><span class=p>();</span>
</code></pre></div> </details> <h2 id=notes>Notes<a class=headerlink href=#notes title="Permanent link">&para;</a></h2> <div class="admonition info"> <p class=admonition-title>Design rationale</p> <p>The library behaves differently to <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>vector</span></code> and <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>map</span></code>:</p> <ul> <li><code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>vector</span><span class=o>::</span><span class=k>operator</span><span class=p>[]</span></code> never inserts a new element.</li> <li><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> is not available for const values.</li> </ul> <p>The type <code class=highlight><span class=n>json</span></code> wraps all <abbr title="JavaScript Object Notation">JSON</abbr> value types. It would be impossible to remove <a href=../../../api/basic_json/operator%5B%5D/ ><code>operator[]</code></a> for const objects. At the same time, inserting elements for non-const objects is really convenient as it avoids awkward <code>insert</code> calls. To this end, we decided to have an inserting non-const behavior for both arrays and objects.</p> </div> <div class="admonition info"> <p class=admonition-title>Info</p> <p>The access is unchecked. In case the passed object key does not exist or the passed array index is invalid, no exception is thrown.</p> </div> <div class="admonition danger"> <p class=admonition-title>Danger</p> <ul> <li>It is <strong>undefined behavior</strong> to access a const object with a non-existing key.</li> <li>It is <strong>undefined behavior</strong> to access a const array with an invalid index.</li> <li>In debug mode, an <strong>assertion</strong> will fire in both cases. You can disable assertions by defining the preprocessor symbol <code class=highlight><span class=n>NDEBUG</span></code> or redefine the macro <a href=../../../api/macros/json_assert/ ><code>JSON_ASSERT(x)</code></a>. See the documentation on <a href=../../assertions/ >runtime assertions</a> for more information.</li> </ul> </div> <div class="admonition failure"> <p class=admonition-title>Exceptions</p> <p><code>operator[]</code> 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_error305><code>basic_json::type_error</code></a> is thrown.</p> </div> <h2 id=performance-reserving-array-capacity>Performance: reserving array capacity<a class=headerlink href=#performance-reserving-array-capacity title="Permanent link">&para;</a></h2> <p>There is no public <code>reserve(count)</code> member on <code>basic_json</code> for pre-allocating array capacity. If you are building a large array incrementally (e.g., via repeated <a href=../../../api/basic_json/push_back/ ><code>push_back()</code></a>) and know its final size ahead of time, you can reserve capacity via <a href=../../../api/basic_json/get_ref/ ><code>get_ref()</code></a> to access the underlying <code>array_t</code> directly:</p> <div class=highlight><pre><span></span><code><span class=n>json</span><span class=w> </span><span class=n>j</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>json</span><span class=o>::</span><span class=n>array</span><span class=p>();</span>
<span class=n>j</span><span class=p>.</span><span class=n>get_ref</span><span class=o>&lt;</span><span class=n>json</span><span class=o>::</span><span class=n>array_t</span><span class=o>&amp;&gt;</span><span class=p>().</span><span class=n>reserve</span><span class=p>(</span><span class=mi>1000</span><span class=p>);</span>
<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 09:23:55 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> <!-- 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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@@ -4,7 +4,7 @@
Elements in a JSON object and a JSON array can be accessed via [`operator[]`](https://json.nlohmann.me/api/basic_json/operator%5B%5D/index.md) similar to a `std::map` and a `std::vector`, respectively.
Read access
Example: read access
Consider the following JSON value:
@@ -29,7 +29,7 @@ Assume the value is parsed to a `json` variable `j`.
The return value is a reference, so it can modify the original value. In case the passed object key is non-existing, a `null` value is inserted which can immediately be overwritten.
Write access
Example: write access
```
j["name"] = "John Smith";
@@ -49,7 +49,7 @@ This code produces the following JSON value:
When accessing an invalid index (i.e., an index greater than or equal to the array size), the JSON array is resized such that the passed index is the new maximal index. Intermediate values are filled with `null`.
Filling up arrays with `null` values
Example: filling up arrays with `null` values
```
j["hobbies"][0] = "running";
@@ -86,7 +86,7 @@ Danger
- It is **undefined behavior** to access a const object with a non-existing key.
- It is **undefined behavior** to access a const array with an invalid index.
- In debug mode, an **assertion** will fire in both cases. You can disable assertions by defining the preprocessor symbol `NDEBUG` or redefine the macro [`JSON_ASSERT(x)`](https://json.nlohmann.me/features/macros/#json_assertx). See the documentation on [runtime assertions](https://json.nlohmann.me/features/assertions/index.md) for more information.
- In debug mode, an **assertion** will fire in both cases. You can disable assertions by defining the preprocessor symbol `NDEBUG` or redefine the macro [`JSON_ASSERT(x)`](https://json.nlohmann.me/api/macros/json_assert/index.md). See the documentation on [runtime assertions](https://json.nlohmann.me/features/assertions/index.md) for more information.
Exceptions
@@ -94,7 +94,7 @@ Exceptions
## Performance: reserving array capacity
There is no public `reserve(count)` member on `basic_json` for pre-allocating array capacity. If you are building a large array incrementally (e.g., via repeated `push_back()`) and know its final size ahead of time, you can reserve capacity via `get_ref()` to access the underlying `array_t` directly:
There is no public `reserve(count)` member on `basic_json` for pre-allocating array capacity. If you are building a large array incrementally (e.g., via repeated [`push_back()`](https://json.nlohmann.me/api/basic_json/push_back/index.md)) and know its final size ahead of time, you can reserve capacity via [`get_ref()`](https://json.nlohmann.me/api/basic_json/get_ref/index.md) to access the underlying `array_t` directly:
```
json j = json::array();