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<title>std::variant<Types...>::index</title>
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::variant<Types...>::</span>index</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><span class="mw-geshi cpp source-cpp"><span class="kw4">constexpr</span> <a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> index<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 起)</span></td>
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<p>返回 <code>variant</code> 当前所保有的可选项的零基下标。</p>
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<p>若 <code>variant</code> 因异常无值（ <a href="cpp-utility-variant-valueless_by_exception.html">valueless_by_exception</a> ），则返回 <a href="cpp-utility-variant-variant_npos.html">variant_npos</a> 。</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 <string></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>, <a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a><span class="sy1">></span> v <span class="sy1">=</span> <span class="st0">"abc"</span><span class="sy4">;</span>
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<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"v.index = "</span> <span class="sy1"><<</span> v.<span class="me1">index</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
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v <span class="sy1">=</span> <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>
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<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"v.index = "</span> <span class="sy1"><<</span> v.<span class="me1">index</span><span class="br0">(</span><span class="br0">)</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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<div class="text source-text">
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<pre class="de1">v.index = 1
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v.index = 0</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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<div><a href="cpp-utility-variant-holds_alternative.html"><span class="t-lines"><span>holds_alternative</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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</div>
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<td>检查某个 <code>variant</code> 是否当前持有某个给定类型<br />
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<span class="t-mark">(函数模板)</span></td>
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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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</div>
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</td>
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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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