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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::any::</span>has_value</h1>
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<div id="contentSub"><span class="subpages">&lt; <a href="cpp.html">cpp</a>&lrm; | <a href="cpp-utility.html">utility</a>&lrm; | <a href="cpp-utility-any.html">any</a></span></div>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span> has_value<span class="br0">(</span><span class="br0">)</span> <span class="kw4">const</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div>
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<td><span class="t-mark-rev t-since-cxx17">(C++17 &#36215;)</span></td>
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<p>&#26816;&#26597;&#23545;&#35937;&#26159;&#21542;&#21547;&#26377;&#20540;&#12290;</p>
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<p>&#65288;&#26080;&#65289;</p>
<h3><span class="mw-headline" id=".E8.BF.94.E5.9B.9E.E5.80.BC">&#36820;&#22238;&#20540;</span></h3>
<p>&#33509;&#23454;&#20363;&#21547;&#20540;&#21017;&#20026; <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> &#65292;&#21542;&#21017;&#20026; <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">false</span></span></span> &#12290;</p>
<h3><span class="mw-headline" id=".E7.A4.BA.E4.BE.8B">&#31034;&#20363;</span></h3>
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<pre class="de1"><span class="co2">#include &lt;any&gt;</span>
<span class="co2">#include &lt;iostream&gt;</span>
<span class="co2">#include &lt;string&gt;</span>
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
<a href="cpp-io-manip-boolalpha.html"><span class="kw1764">std::<span class="me2">boolalpha</span></span></a><span class="br0">(</span><a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a><span class="br0">)</span><span class="sy4">;</span>
<a href="cpp-utility-any.html"><span class="kw1071">std::<span class="me2">any</span></span></a> a0<span class="sy4">;</span>
<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">&quot;a0.has_value(): &quot;</span> <span class="sy1">&lt;&lt;</span> a0.<span class="me1">has_value</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1">&lt;&lt;</span> <span class="st0">&quot;<span class="es1">\n</span>&quot;</span><span class="sy4">;</span>
<a href="cpp-utility-any.html"><span class="kw1071">std::<span class="me2">any</span></span></a> a1 <span class="sy1">=</span> <span class="nu0">42</span><span class="sy4">;</span>
<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">&quot;a1.has_value(): &quot;</span> <span class="sy1">&lt;&lt;</span> a1.<span class="me1">has_value</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1">&lt;&lt;</span> <span class="st0">&#39;<span class="es1">\n</span>&#39;</span><span class="sy4">;</span>
<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">&quot;a1 = &quot;</span> <span class="sy1">&lt;&lt;</span> <a href="cpp-utility-any-any_cast.html"><span class="kw1072">std::<span class="me2">any_cast</span></span></a><span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy1">&gt;</span><span class="br0">(</span>a1<span class="br0">)</span> <span class="sy1">&lt;&lt;</span> <span class="st0">&#39;<span class="es1">\n</span>&#39;</span><span class="sy4">;</span>
a1.<span class="me1">reset</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">&quot;a1.has_value(): &quot;</span> <span class="sy1">&lt;&lt;</span> a1.<span class="me1">has_value</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1">&lt;&lt;</span> <span class="st0">&#39;<span class="es1">\n</span>&#39;</span><span class="sy4">;</span>
<span class="kw4">auto</span> a2 <span class="sy1">=</span> <a href="cpp-utility-any-make_any.html"><span class="kw1073">std::<span class="me2">make_any</span></span></a><span class="sy1">&lt;</span><a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a><span class="sy1">&gt;</span><span class="br0">(</span><span class="st0">&quot;Milky Way&quot;</span><span class="br0">)</span><span class="sy4">;</span>
<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">&quot;a2.has_value(): &quot;</span> <span class="sy1">&lt;&lt;</span> a2.<span class="me1">has_value</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1">&lt;&lt;</span> <span class="st0">&#39;<span class="es1">\n</span>&#39;</span><span class="sy4">;</span>
<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">&quot;a2 = <span class="es1">\&quot;</span>&quot;</span> <span class="sy1">&lt;&lt;</span> <a href="cpp-utility-any-any_cast.html"><span class="kw1072">std::<span class="me2">any_cast</span></span></a><span class="sy1">&lt;</span><a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a><span class="sy3">&amp;</span><span class="sy1">&gt;</span><span class="br0">(</span>a2<span class="br0">)</span> <span class="sy1">&lt;&lt;</span> <span class="st0">&quot;<span class="es1">\&quot;</span><span class="es1">\n</span>&quot;</span><span class="sy4">;</span>
a2.<span class="me1">reset</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">&quot;a2.has_value(): &quot;</span> <span class="sy1">&lt;&lt;</span> a2.<span class="me1">has_value</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1">&lt;&lt;</span> <span class="st0">&#39;<span class="es1">\n</span>&#39;</span><span class="sy4">;</span>
<span class="br0">}</span></pre></div>
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<p>&#36755;&#20986;&#65306;</p>
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<pre class="de1">a0.has_value(): false
a1.has_value(): true
a1 = 42
a1.has_value(): false
a2.has_value(): true
a2 = &quot;Milky Way&quot;
a2.has_value(): false</pre></div>
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<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">&#21442;&#38405;</span></h3>
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<div><a href="cpp-utility-any-reset.html"><span class="t-lines"><span>reset</span></span></a></div>
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<td>&#38144;&#27585;&#25152;&#21547;&#23545;&#35937;<br />
<span class="t-mark">(&#20844;&#24320;&#25104;&#21592;&#20989;&#25968;)</span></td>
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