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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 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=#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> <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 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<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>
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<span class=w> </span><span class=nt>"append"</span><span class=p>:</span><span class=w> </span><span class=kc>true</span>
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<span class=p>}</span>
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@@ -33,7 +33,7 @@
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</code></pre></div> </div> <div class="admonition warning"> <p class=admonition-title>Return type</p> <p>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 <a href=../../../api/basic_json/operator%5B%5D/ ><code>operator[]</code></a>. However, when we call <code>value</code> and provide <code class=highlight><span class=mi>0</span></code> as default value, then <code class=highlight><span class=mi>-1</span></code> is returned. This occurs, because <code class=highlight><span class=mi>0</span></code> has type <code class=highlight><span class=kt>int</span></code> which overflows when handling the value <code class=highlight><span class=mi>18446744073709551615</span></code>.</p> <p>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 value is not used as the return value.</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.hpp></span>
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<span class=k>using</span><span class=w> </span><span class=n>json</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=n>nlohmann</span><span class=o>::</span><span class=n>json</span><span class=p>;</span>
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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</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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@@ -41,7 +41,7 @@
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<span class=w> </span><span class=n>std</span><span class=o>::</span><span class=n>cout</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=s>"operator[]: "</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>"uint64"</span><span class=p>]</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=sc>'\n'</span>
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<span class=w> </span><span class=o><<</span><span class=w> </span><span class=s>"default value (int): "</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=n>j</span><span class=p>.</span><span class=n>value</span><span class=p>(</span><span class=s>"uint64"</span><span class=p>,</span><span class=w> </span><span class=mi>0</span><span class=p>)</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=sc>'\n'</span>
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<span class=w> </span><span class=o><<</span><span class=w> </span><span class=s>"default value (uint64_t): "</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=n>j</span><span class=p>.</span><span class=n>value</span><span class=p>(</span><span class=s>"uint64"</span><span class=p>,</span><span class=w> </span><span class=n>std</span><span class=o>::</span><span class=kt>uint64_t</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=sc>'\n'</span>
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<span class=w> </span><span class=o><<</span><span class=w> </span><span class=s>"default value (uint64_t): "</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=n>j</span><span class=p>.</span><span class=n>value</span><span class=p>(</span><span class=s>"uint64"</span><span class=p>,</span><span class=w> </span><span class=k>static_cast</span><span class=o><</span><span class=n>std</span><span class=o>::</span><span class=kt>uint64_t</span><span class=o>></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=sc>'\n'</span>
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<span class=w> </span><span class=o><<</span><span class=w> </span><span class=s>"explicit return value type: "</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=n>j</span><span class=p>.</span><span class=n>value</span><span class=o><</span><span class=n>std</span><span class=o>::</span><span class=kt>uint64_t</span><span class=o>></span><span class=p>(</span><span class=s>"uint64"</span><span class=p>,</span><span class=w> </span><span class=mi>0</span><span class=p>)</span><span class=w> </span><span class=o><<</span><span class=w> </span><span class=sc>'\n'</span><span class=p>;</span>
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<span class=p>}</span>
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</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code><span class=err>opera</span><span class=kc>t</span><span class=err>or</span><span class=p>[]:</span><span class=w> </span><span class=mi>18446744073709551615</span>
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