116 lines
6.5 KiB
HTML
116 lines
6.5 KiB
HTML
<!DOCTYPE html>
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<html lang="zh" dir="ltr" class="client-nojs" xmlns="http://www.w3.org/1999/xhtml" xml:lang="zh">
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<title>std::ignore</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_utility_tuple_ignore skin-cppreference2 action-view cpp-navbar">
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<div id="cpp-content-base">
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<div id="content"><a id="top"></a>
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::</span>ignore</h1>
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<div id="bodyContent">
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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-tuple.html">tuple</a></span></div>
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<div>定义于头文件 <code><a href="cpp-header-tuple.html"><tuple></a></code></div>
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<td></td>
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<td></td>
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<tbody class="t-dcl-rev">
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<tr class="t-dcl t-since-cxx11 t-until-cxx17">
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">const</span> <span class="coMULTI">/*unspecified*/</span> ignore<span class="sy4">;</span></span></div>
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<td class="t-dcl-nopad"></td>
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<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span><br />
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<span class="t-mark-rev t-until-cxx17">(C++17 前)</span></td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">inline</span> <span class="kw4">constexpr</span> <span class="coMULTI">/*unspecified*/</span> ignore<span class="sy4">;</span></span></div>
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<td class="t-dcl-nopad"></td>
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<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></td>
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<p>任何值均可赋给而无效果的未指定类型的对象。目的是令 <span class="t-lc"><a href="cpp-utility-tuple-tie.html">std::tie</a></span> 在解包 <span class="t-lc"><a href="cpp-utility-tuple.html">std::tuple</a></span> 时作为不使用的参数的占位符使用。</p>
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<h3><span class="mw-headline" id=".E5.8F.AF.E8.83.BD.E7.9A.84.E5.AE.9E.E7.8E.B0">可能的实现</span></h3>
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<table class="eq-fun-cpp-table">
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<div dir="ltr" class="mw-geshi" style="text-align: left;">
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<div class="cpp source-cpp">
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<pre class="de1"><span class="kw1">namespace</span> detail <span class="br0">{</span>
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<span class="kw1">struct</span> ignore_t <span class="br0">{</span>
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<span class="kw1">template</span> <span class="sy1"><</span><span class="kw1">typename</span> T<span class="sy1">></span>
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<span class="kw4">constexpr</span> <span class="co1">// C++14 起要求</span>
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<span class="kw4">void</span> operator<span class="sy1">=</span><span class="br0">(</span>T<span class="sy3">&&</span><span class="br0">)</span> <span class="kw4">const</span> <span class="kw1">noexcept</span> <span class="br0">{</span><span class="br0">}</span>
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<span class="br0">}</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="kw1">inline</span> <span class="kw4">constexpr</span> detail<span class="sy4">::</span><span class="me2">ignore_t</span> ignore<span class="sy4">;</span> <span class="co1">// C++17 前仅 'const'</span></pre></div>
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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="t-example">
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<p>解包 set.insert() 所返回的 pair ，但只保存布尔值。</p>
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<div class="t-example-live-link"></div>
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<div dir="ltr" class="mw-geshi" style="text-align: left;">
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<div class="cpp source-cpp">
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<pre class="de1"><span class="co2">#include <iostream></span>
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<span class="co2">#include <string></span>
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<span class="co2">#include <set></span>
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<span class="co2">#include <tuple></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-container-set.html"><span class="kw1276">std::<span class="me2">set</span></span></a><span class="sy1"><</span><a href="cpp-string-basic_string.html"><span class="kw1229">std::<span class="me2">string</span></span></a><span class="sy1">></span> set_of_str<span class="sy4">;</span>
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<span class="kw4">bool</span> inserted <span class="sy1">=</span> <span class="kw2">false</span><span class="sy4">;</span>
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<a href="cpp-utility-tuple-tie.html"><span class="kw1113">std::<span class="me2">tie</span></span></a><span class="br0">(</span>std<span class="sy4">::</span><span class="me2">ignore</span>, inserted<span class="br0">)</span> <span class="sy1">=</span> set_of_str.<span class="me1">insert</span><span class="br0">(</span><span class="st0">"Test"</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">if</span> <span class="br0">(</span>inserted<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">"Value was inserted successfully<span class="es1">\n</span>"</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="br0">}</span></pre></div>
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</div>
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<p>输出：</p>
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<div dir="ltr" class="mw-geshi" style="text-align: left;">
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<div class="text source-text">
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<pre class="de1">Value was inserted successfully</pre></div>
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</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-tuple-tie.html"><span class="t-lines"><span>tie</span></span></a></div>
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</div>
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</td>
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<td>创建左值引用的 <code>tuple</code>，或将 tuple 解包为独立对象<br />
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<span class="t-mark">(函数模板)</span></td>
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</div>
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<div class="visualClear"></div>
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</div>
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