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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::</span>get_if</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-variant.html">variant</a></span></div>
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<div>&#23450;&#20041;&#20110;&#22836;&#25991;&#20214; <code><a href="cpp-header-variant.html">&lt;variant&gt;</a></code></div>
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<td rowspan="142">(1)</td>
<td rowspan="142"><span class="t-mark-rev t-since-cxx17">(C++17 &#36215;)</span></td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1">&lt;</span> <a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> I, <span class="kw1">class</span>... <span class="me1">Types</span> <span class="sy1">&gt;</span><br /></span>
<p><span class="kw4">constexpr</span> <a href="cpp-types-add_pointer.html"><span class="kw615">std::<span class="me2">add_pointer_t</span></span></a><span class="sy1">&lt;</span><a href="cpp-utility-variant-variant_alternative.html"><span class="kw1093">std::<span class="me2">variant_alternative_t</span></span></a><span class="sy1">&lt;</span>I, <a href="cpp-utility-variant.html"><span class="kw1085">std::<span class="me2">variant</span></span></a><span class="sy1">&lt;</span>Types...<span class="sy1">&gt;&gt;&gt;</span><br /></p>
&nbsp; &nbsp; get_if<span class="br0">(</span> <a href="cpp-utility-variant.html"><span class="kw1085">std::<span class="me2">variant</span></span></a><span class="sy1">&lt;</span>Types...<span class="sy1">&gt;</span><span class="sy2">*</span> pv <span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></div>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1">&lt;</span> <a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> I, <span class="kw1">class</span>... <span class="me1">Types</span> <span class="sy1">&gt;</span><br /></span>
<p><span class="kw4">constexpr</span> <a href="cpp-types-add_pointer.html"><span class="kw615">std::<span class="me2">add_pointer_t</span></span></a><span class="sy1">&lt;</span><span class="kw4">const</span> <a href="cpp-utility-variant-variant_alternative.html"><span class="kw1093">std::<span class="me2">variant_alternative_t</span></span></a><span class="sy1">&lt;</span>I, variant<span class="sy1">&lt;</span>Types...<span class="sy1">&gt;&gt;&gt;</span><br /></p>
&nbsp; &nbsp; get_if<span class="br0">(</span> <span class="kw4">const</span> <a href="cpp-utility-variant.html"><span class="kw1085">std::<span class="me2">variant</span></span></a><span class="sy1">&lt;</span>Types...<span class="sy1">&gt;</span><span class="sy2">*</span> pv <span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></div>
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<td rowspan="142">(2)</td>
<td rowspan="142"><span class="t-mark-rev t-since-cxx17">(C++17 &#36215;)</span></td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1">&lt;</span> <span class="kw1">class</span> T, <span class="kw1">class</span>... <span class="me1">Types</span> <span class="sy1">&gt;</span><br />
<span class="kw4">constexpr</span> <a href="cpp-types-add_pointer.html"><span class="kw615">std::<span class="me2">add_pointer_t</span></span></a><span class="sy1">&lt;</span>T<span class="sy1">&gt;</span> get_if<span class="br0">(</span> <a href="cpp-utility-variant.html"><span class="kw1085">std::<span class="me2">variant</span></span></a><span class="sy1">&lt;</span>Types...<span class="sy1">&gt;</span><span class="sy2">*</span> pv <span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1">&lt;</span> <span class="kw1">class</span> T, <span class="kw1">class</span>... <span class="me1">Types</span> <span class="sy1">&gt;</span><br />
<span class="kw4">constexpr</span> <a href="cpp-types-add_pointer.html"><span class="kw615">std::<span class="me2">add_pointer_t</span></span></a><span class="sy1">&lt;</span><span class="kw4">const</span> T<span class="sy1">&gt;</span> get_if<span class="br0">(</span> <span class="kw4">const</span> <a href="cpp-utility-variant.html"><span class="kw1085">std::<span class="me2">variant</span></span></a><span class="sy1">&lt;</span>Types...<span class="sy1">&gt;</span><span class="sy2">*</span> pv <span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div>
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<div class="t-li1"><span class="t-li">1)</span> &#22522;&#20110;&#19979;&#26631;&#30340;&#19981;&#25243;&#20986;&#35775;&#38382;&#22120;&#65306;&#33509; <code>pv</code> &#19981;&#26159;&#31354;&#25351;&#38024;&#19988; <span class="t-c"><span class="mw-geshi cpp source-cpp">pv<span class="sy2">-</span><span class="sy1">&gt;</span>index<span class="br0">(</span><span class="br0">)</span> <span class="sy1">==</span> I</span></span> &#65292;&#21017;&#36820;&#22238;&#25351;&#21521;&#23384;&#20648;&#20110; <code>pv</code> &#25152;&#25351;&#21521;&#30340; <code>variant</code> &#20013;&#30340;&#20540;&#30340;&#25351;&#38024;&#12290;&#21542;&#21017;&#65292;&#36820;&#22238;&#31354;&#25351;&#38024;&#20540;&#12290;&#33509; <code>I</code> &#19981;&#26159; <code>variant</code> &#30340;&#21512;&#27861;&#19979;&#26631;&#65292;&#21017;&#27492;&#35843;&#29992;&#20026;&#30149;&#24335;&#12290;</div>
<div class="t-li1"><span class="t-li">2)</span> &#22522;&#20110;&#31867;&#22411;&#30340;&#19981;&#25243;&#20986;&#35775;&#38382;&#22120;&#65306;&#31561;&#20215;&#20110; <span class="t-v">(1)</span> &#65292;&#20854;&#20013; <code>I</code> &#26159; <code>T</code> &#22312; <code>Types...</code> &#20013;&#30340;&#38646;&#22522;&#19979;&#26631;&#12290;&#33509; <code>T</code> &#19981;&#26159; <code>Types...</code> &#20013;&#30340;&#21807;&#19968;&#23384;&#22312;&#30340;&#20803;&#32032;&#65292;&#21017;&#27492;&#35843;&#29992;&#20026;&#30149;&#24335;&#12290;</div>
<h3><span class="mw-headline" id=".E5.8F.82.E6.95.B0">&#21442;&#25968;</span></h3>
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<td>I</td>
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<td>&#25351;&#21521; <code>variant</code> &#30340;&#25351;&#38024;</td>
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<h3><span class="mw-headline" id=".E8.BF.94.E5.9B.9E.E5.80.BC">&#36820;&#22238;&#20540;</span></h3>
<p>&#25351;&#21521;&#23384;&#20648;&#20110;&#34987;&#25351;&#21521;&#30340; <code>variant</code> &#20013;&#20540;&#30340;&#25351;&#38024;&#65292;&#38169;&#35823;&#26102;&#20026;&#31354;&#25351;&#38024;&#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;variant&gt;</span>
<span class="co2">#include &lt;iostream&gt;</span>
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
<a href="cpp-utility-variant.html"><span class="kw1085">std::<span class="me2">variant</span></span></a><span class="sy1">&lt;</span><span class="kw4">int</span>, <span class="kw4">float</span><span class="sy1">&gt;</span> v<span class="br0">{</span><span class="nu0">12</span><span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">if</span><span class="br0">(</span><span class="kw4">auto</span> pval <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">get_if</span><span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy1">&gt;</span><span class="br0">(</span><span class="sy3">&amp;</span>v<span class="br0">)</span><span class="br0">)</span>
<a href="cpp-io-cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">&quot;variant value: &quot;</span> <span class="sy1">&lt;&lt;</span> <span class="sy2">*</span>pval <span class="sy1">&lt;&lt;</span> <span class="st0">&#39;<span class="es1">\n</span>&#39;</span><span class="sy4">;</span>
<span class="kw1">else</span>
<a href="cpp-io-cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">&quot;failed to get value!&quot;</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">variant value: 12</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-variant-get.html"><span class="t-lines"><span>std::get<span class="t-dsc-small">(std::variant)</span></span></span></a></div>
<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span></span></div>
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<td>&#20197;&#32473;&#23450;&#32034;&#24341;&#25110;&#31867;&#22411;&#65288;&#33509;&#31867;&#22411;&#21807;&#19968;&#65289;&#35835;&#21462; <code>variant</code> &#30340;&#20540;&#65292;&#38169;&#35823;&#26102;&#25243;&#20986;&#24322;&#24120;<br />
<span class="t-mark">(&#20989;&#25968;&#27169;&#26495;)</span></td>
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