149 lines
11 KiB
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149 lines
11 KiB
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<title>std::get_if</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_utility_variant_get_if skin-cppreference2 action-view cpp-navbar">
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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">< <a href="cpp.html">cpp</a>‎ | <a href="cpp-utility.html">utility</a>‎ | <a href="cpp-utility-variant.html">variant</a></span></div>
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<div>定义于头文件 <code><a href="cpp-header-variant.html"><variant></a></code></div>
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<td rowspan="142">(1)</td>
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<td rowspan="142"><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1"><</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">></span><br /></span>
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<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"><</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"><</span>I, <a href="cpp-utility-variant.html"><span class="kw1085">std::<span class="me2">variant</span></span></a><span class="sy1"><</span>Types...<span class="sy1">>>></span><br /></p>
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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"><</span>Types...<span class="sy1">></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"><</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">></span><br /></span>
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<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"><</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"><</span>I, variant<span class="sy1"><</span>Types...<span class="sy1">>>></span><br /></p>
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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"><</span>Types...<span class="sy1">></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>
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<td rowspan="142"><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1"><</span> <span class="kw1">class</span> T, <span class="kw1">class</span>... <span class="me1">Types</span> <span class="sy1">></span><br />
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<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"><</span>T<span class="sy1">></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"><</span>Types...<span class="sy1">></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"><</span> <span class="kw1">class</span> T, <span class="kw1">class</span>... <span class="me1">Types</span> <span class="sy1">></span><br />
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<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"><</span><span class="kw4">const</span> T<span class="sy1">></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"><</span>Types...<span class="sy1">></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> 基于下标的不抛出访问器：若 <code>pv</code> 不是空指针且 <span class="t-c"><span class="mw-geshi cpp source-cpp">pv<span class="sy2">-</span><span class="sy1">></span>index<span class="br0">(</span><span class="br0">)</span> <span class="sy1">==</span> I</span></span> ，则返回指向存储于 <code>pv</code> 所指向的 <code>variant</code> 中的值的指针。否则，返回空指针值。若 <code>I</code> 不是 <code>variant</code> 的合法下标，则此调用为病式。</div>
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<div class="t-li1"><span class="t-li">2)</span> 基于类型的不抛出访问器：等价于 <span class="t-v">(1)</span> ，其中 <code>I</code> 是 <code>T</code> 在 <code>Types...</code> 中的零基下标。若 <code>T</code> 不是 <code>Types...</code> 中的唯一存在的元素，则此调用为病式。</div>
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<h3><span class="mw-headline" id=".E5.8F.82.E6.95.B0">参数</span></h3>
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<td>I</td>
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<td>-</td>
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<td>要查找的下标</td>
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<td>Type</td>
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<td>要查找的类型</td>
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<td>pv</td>
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<td>-</td>
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<td>指向 <code>variant</code> 的指针</td>
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<h3><span class="mw-headline" id=".E8.BF.94.E5.9B.9E.E5.80.BC">返回值</span></h3>
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<p>指向存储于被指向的 <code>variant</code> 中值的指针，错误时为空指针。</p>
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<h3><span class="mw-headline" id=".E7.A4.BA.E4.BE.8B">示例</span></h3>
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<div class="cpp source-cpp">
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<pre class="de1"><span class="co2">#include <variant></span>
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<span class="co2">#include <iostream></span>
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<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
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<span class="br0">{</span>
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<a href="cpp-utility-variant.html"><span class="kw1085">std::<span class="me2">variant</span></span></a><span class="sy1"><</span><span class="kw4">int</span>, <span class="kw4">float</span><span class="sy1">></span> v<span class="br0">{</span><span class="nu0">12</span><span class="br0">}</span><span class="sy4">;</span>
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<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"><</span><span class="kw4">int</span><span class="sy1">></span><span class="br0">(</span><span class="sy3">&</span>v<span class="br0">)</span><span class="br0">)</span>
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<a href="cpp-io-cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"variant value: "</span> <span class="sy1"><<</span> <span class="sy2">*</span>pval <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
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<span class="kw1">else</span>
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<a href="cpp-io-cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"failed to get value!"</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
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<span class="br0">}</span></pre></div>
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<p>输出：</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">参阅</span></h3>
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<table class="t-dsc-begin">
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<div class="t-dsc-member-div">
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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>
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<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>以给定索引或类型（若类型唯一）读取 <code>variant</code> 的值，错误时抛出异常<br />
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<span class="t-mark">(函数模板)</span></td>
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