123 lines
7.6 KiB
HTML
123 lines
7.6 KiB
HTML
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<title>std::piecewise_construct_t</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_utility_piecewise_construct_t 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>piecewise_construct_t</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></span></div>
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<div>定义于头文件 <code><a href="cpp-header-utility.html"><utility></a></code></div>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">struct</span> piecewise_construct_t <span class="br0">{</span> <span class="kw1">explicit</span> piecewise_construct_t<span class="br0">(</span><span class="br0">)</span> <span class="sy1">=</span> <span class="kw1">default</span><span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span></span></div>
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<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
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<p><code>std::piecewise_construct_t</code> 是用于在接收二个 <code>tuple</code> 参数的不同函数间消歧义的空类标签类型。</p>
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<p>不使用 <code>std::piecewise_construct_t</code> 的重载假设每个 <code>tuple</code> 参数各变成一个 <code>pair</code> 的元素。使用 <code>std::piecewise_construct_t</code> 的重载假设每个 <code>tuple</code> 参数用于逐块构造一个指定类型的新对象，而它将成为 pair 的元素。</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 <iostream></span>
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<span class="co2">#include <utility></span>
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<span class="co2">#include <tuple></span>
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<span class="kw1">struct</span> Foo <span class="br0">{</span>
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Foo<span class="br0">(</span><a href="cpp-utility-tuple.html"><span class="kw1098">std::<span class="me2">tuple</span></span></a><span class="sy1"><</span><span class="kw4">int</span>, <span class="kw4">float</span><span class="sy1">></span><span class="br0">)</span>
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<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">"Constructed a Foo from a tuple<span class="es1">\n</span>"</span><span class="sy4">;</span>
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<span class="br0">}</span>
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Foo<span class="br0">(</span><span class="kw4">int</span>, <span class="kw4">float</span><span class="br0">)</span>
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<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">"Constructed a Foo from an int and a float<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><span class="sy4">;</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-tuple.html"><span class="kw1098">std::<span class="me2">tuple</span></span></a><span class="sy1"><</span><span class="kw4">int</span>, <span class="kw4">float</span><span class="sy1">></span> t<span class="br0">(</span><span class="nu0">1</span>, <span class="nu16">3.14</span><span class="br0">)</span><span class="sy4">;</span>
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<a href="cpp-utility-pair.html"><span class="kw1096">std::<span class="me2">pair</span></span></a><span class="sy1"><</span>Foo, Foo<span class="sy1">></span> p1<span class="br0">(</span>t, t<span class="br0">)</span><span class="sy4">;</span>
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<a href="cpp-utility-pair.html"><span class="kw1096">std::<span class="me2">pair</span></span></a><span class="sy1"><</span>Foo, Foo<span class="sy1">></span> p2<span class="br0">(</span><a href="cpp-utility-piecewise_construct.html"><span class="kw1105">std::<span class="me2">piecewise_construct</span></span></a>, t, t<span class="br0">)</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 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">Constructed a Foo from a tuple
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Constructed a Foo from a tuple
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Constructed a Foo from an int and a float
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Constructed a Foo from an int and a float</pre></div>
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<h3><span class="mw-headline" id=".E7.BC.BA.E9.99.B7.E6.8A.A5.E5.91.8A">缺陷报告</span></h3>
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<p>下列更改行为的缺陷报告追溯地应用于以前出版的 C++ 标准。</p>
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<table class="dsctable" style="font-size:0.8em">
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<th>DR</th>
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<th>应用于</th>
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<th>出版时的行为</th>
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<th>正确行为</th>
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<td><a rel="nofollow" class="external text" href="https://cplusplus.github.io/LWG/issue2510">LWG 2510</a></td>
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<td>C++11</td>
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<td>默认构造函数为非 explicit ，能导致歧义</td>
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<td>使之为 explicit</td>
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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-piecewise_construct.html"><span class="t-lines"><span>piecewise_construct</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div>
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<td>用于为逐段构造的函数消歧义的 <code>piecewise_construct_t</code> 类型的对象<br />
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<span class="t-mark">(常量)</span></td>
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<div class="t-dsc-member-div t-dsc-member-nobold-div">
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<div><a href="cpp-utility-pair-pair.html"><span class="t-lines"><span>(构造函数)</span></span></a></div>
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</div>
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</td>
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<td>构造新的 pair<br />
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<span class="t-mark">(<code>std::pair<T1,T2></code> 的公开成员函数)</span></td>
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