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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/api/basic_json_view/operator%5B%5D/" rel="canonical"><link href=../operator_ltlt/ rel=prev><link href=../operator_eq/ rel=next><link rel=icon href=../../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>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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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> 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=indigo data-md-color-accent=indigo aria-label="Switch to dark 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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/api/basic_json_view/operator%5B%5D/" rel="canonical"><link href=../operator_ltlt/ rel=prev><link href=../operator_eq/ rel=next><link rel=icon href=../../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.7.7"><title>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 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=#nlohmannbasic_json_viewoperator 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> 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=indigo data-md-color-accent=indigo aria-label="Switch to dark 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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 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<span class=n>basic_json_view</span><span class=w> </span><span class=k>operator</span><span class=p>[](</span><span class=n>string_view_t</span><span class=w> </span><span class=n>key</span><span class=p>)</span><span class=w> </span><span class=k>const</span><span class=p>;</span>
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<span class=n>basic_json_view</span><span class=w> </span><span class=k>operator</span><span class=p>[](</span><span class=k>const</span><span class=w> </span><span class=kt>char</span><span class=o>*</span><span class=w> </span><span class=n>key</span><span class=p>)</span><span class=w> </span><span class=k>const</span><span class=p>;</span>
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<span class=n>basic_json_view</span><span class=w> </span><span class=k>operator</span><span class=p>[](</span><span class=k>const</span><span class=w> </span><span class=n>string_t</span><span class=o>&</span><span class=w> </span><span class=n>key</span><span class=p>)</span><span class=w> </span><span class=k>const</span><span class=p>;</span>
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@@ -13,7 +13,7 @@
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</code></pre></div> <ol> <li>Returns the value of the object member with key <code>key</code> -- the first one, should the key occur more than once (see the <a href=#notes>Notes</a> below) -- or a <a href=../is_discarded/ >discarded</a> view if there is no such member.</li> <li>Returns the array element at index <code>idx</code>, or a <a href=../is_discarded/ >discarded</a> view if <code>idx</code> is out of range. The template accepts every integer type except <code class=highlight><span class=kt>bool</span></code> and <code class=highlight><span class=n>std</span><span class=o>::</span><span class=kt>size_t</span></code> (<code class=highlight><span class=kt>int</span></code>, <code class=highlight><span class=kt>unsigned</span></code>, <code class=highlight><span class=kt>long</span></code>, <code class=highlight><span class=n>std</span><span class=o>::</span><span class=kt>int64_t</span></code>, ...) and forwards to the <code>size_type</code> overload, so that an integer argument is not ambiguous between that overload and 1; a negative <code>idx</code> is out of range.</li> <li>Returns the value a <abbr title="JavaScript Object Notation">JSON</abbr> pointer <code>ptr</code> refers to, starting at this value, or a <a href=../is_discarded/ >discarded</a> view wherever resolving it further is not possible without inserting into or extending the document (see <a href=#return-value>Return value</a> and <a href=#exceptions>Exceptions</a> below).</li> </ol> <h2 id=parameters>Parameters<a class=headerlink href=#parameters title="Permanent link">¶</a></h2> <dl> <dt><code>key</code> (in)</dt> <dd>object key of the element to access</dd> <dt><code>idx</code> (in)</dt> <dd>index of the element to access</dd> <dt><code>ptr</code> (in)</dt> <dd><abbr title="JavaScript Object Notation">JSON</abbr> pointer to the element to access</dd> </dl> <h2 id=return-value>Return value<a class=headerlink href=#return-value title="Permanent link">¶</a></h2> <ol> <li>the value of the first member with key <code>key</code>, or a discarded view if no member has this key (or if this view is <a href=../is_discarded/ >discarded</a>)</li> <li>the element at index <code>idx</code>, or a discarded view if <code class=highlight><span class=n>idx</span><span class=w> </span><span class=o>>=</span><span class=w> </span><span class=n>size</span><span class=p>()</span></code> or <code>idx</code> is negative (or if this view is <a href=../is_discarded/ >discarded</a>)</li> <li>the value <code>ptr</code> resolves to, starting at this value, or a discarded view (also if this view is <a href=../is_discarded/ >discarded</a>) for exactly the reference tokens where the <strong>const</strong> overload of <a href=../../basic_json/operator%5B%5D/ ><code>BasicJsonType::operator[]</code></a> invokes undefined behavior for the same pointer and the same document: an object member that does not exist, or an array index that is out of range</li> </ol> <h2 id=exception-safety>Exception safety<a class=headerlink href=#exception-safety title="Permanent link">¶</a></h2> <p>Strong guarantee: there are no changes to the view or the document it refers to.</p> <h2 id=exceptions>Exceptions<a class=headerlink href=#exceptions title="Permanent link">¶</a></h2> <ol> <li>Throws <a href=../../../home/exceptions/#jsonexceptiontype_error305><code>type_error.305</code></a> if the value is not an object and not <a href=../is_discarded/ >discarded</a> -- the same exception, with the same message, that the <strong>const</strong> overload of <a href=../../basic_json/operator%5B%5D/ ><code>BasicJsonType::operator[]</code></a> throws for a string argument on a non-object value.</li> <li>Throws <a href=../../../home/exceptions/#jsonexceptiontype_error305><code>type_error.305</code></a> if the value is not an array and not <a href=../is_discarded/ >discarded</a> -- the same exception, with the same message, that the <strong>const</strong> overload of <a href=../../basic_json/operator%5B%5D/ ><code>BasicJsonType::operator[]</code></a> throws for a numeric argument on a non-array value.</li> <li>Throws the same exceptions, with the same messages, that the <strong>const</strong> overload of <a href=../../basic_json/operator%5B%5D/ ><code>BasicJsonType::operator[]</code></a> throws for the same pointer and the same document:<ul> <li><a href=../../../home/exceptions/#jsonexceptionout_of_range402><code>out_of_range.402</code></a> if a reference token is <code class=highlight><span class=s>"-"</span></code> at an array.</li> <li><a href=../../../home/exceptions/#jsonexceptionout_of_range404><code>out_of_range.404</code></a> if a reference token cannot be resolved because it is used on a primitive value.</li> <li><a href=../../../home/exceptions/#jsonexceptionparse_error106><code>parse_error.106</code></a> if an array index in <code>ptr</code> begins with <code class=highlight><span class=sc>'0'</span></code>.</li> <li><a href=../../../home/exceptions/#jsonexceptionparsLine truncated
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<span class=cp>#include</span><span class=w> </span><span class=cpf><nlohmann/json_view.hpp></span>
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<span class=k>using</span><span class=w> </span><span class=n>json_document</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>nlohmann</span><span class=o>::</span><span class=n>json_document</span><span class=p>;</span>
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<span class=k>using</span><span class=w> </span><span class=n>nlohmann</span><span class=o>::</span><span class=n>json_document</span><span class=p>;</span>
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<span class=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</span>
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<span class=p>{</span>
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@@ -59,7 +59,7 @@
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</code></pre></div> </details> <details class=example> <summary>Example: (3) access specified element via <abbr title="JavaScript Object Notation">JSON</abbr> pointer</summary> <p>The example below reaches straight into one deeply nested field of a large document with a single <abbr title="JavaScript Object Notation">JSON</abbr> pointer, without ever building a tree for the rest of it, and shows the discarded-view and throwing outcomes of a pointer that cannot be fully resolved.</p> <div class=highlight><pre><span></span><code><span class=cp>#include</span><span class=w> </span><span class=cpf><iostream></span>
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<span class=cp>#include</span><span class=w> </span><span class=cpf><nlohmann/json_view.hpp></span>
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<span class=k>using</span><span class=w> </span><span class=n>json_document</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>nlohmann</span><span class=o>::</span><span class=n>json_document</span><span class=p>;</span>
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<span class=k>using</span><span class=w> </span><span class=n>nlohmann</span><span class=o>::</span><span class=n>json_document</span><span class=p>;</span>
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<span class=k>using</span><span class=w> </span><span class=n>json_pointer</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>nlohmann</span><span class=o>::</span><span class=n>json</span><span class=o>::</span><span class=n>json_pointer</span><span class=p>;</span>
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<span class=kt>int</span><span class=w> </span><span class=nf>main</span><span class=p>()</span>
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@@ -81,7 +81,7 @@ The example below reads a couple of fields out of a batch of user records withou
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#include <iostream>
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#include <nlohmann/json_view.hpp>
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using json_document = nlohmann::json_document;
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using nlohmann::json_document;
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int main()
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{
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@@ -139,7 +139,7 @@ The example below reaches straight into one deeply nested field of a large docum
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#include <iostream>
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#include <nlohmann/json_view.hpp>
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using json_document = nlohmann::json_document;
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using nlohmann::json_document;
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using json_pointer = nlohmann::json::json_pointer;
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int main()
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