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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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<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
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<title>std::pair<T1,T2>::pair</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_utility_pair_pair 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::pair<T1,T2>::</span>pair</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-pair.html">pair</a></span></div>
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<div id="mw-content-text" lang="zh" dir="ltr" class="mw-content-ltr" xml:lang="zh">
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<td rowspan="142">(1)</td>
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<td></td>
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<tr class="t-dcl t-until-cxx11">
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<td>
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<div><span class="mw-geshi cpp source-cpp">pair<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span></span></div>
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</td>
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<td class="t-dcl-nopad"></td>
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<td><span class="t-mark-rev t-until-cxx11">(C++11 前)</span></td>
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</tr>
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<tr class="t-dcl t-since-cxx11">
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">constexpr</span> pair<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span></span></div>
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</td>
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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">(条件性 <code>explicit</code>)</span></td>
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</tr>
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</tbody>
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<tbody></tbody>
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<tbody class="t-dcl-rev t-dcl-rev-num">
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<tr class="t-dcl-rev-aux">
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<td></td>
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<td rowspan="142">(2)</td>
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<td></td>
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</tr>
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<tr class="t-dcl t-until-cxx11">
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<td>
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<div><span class="mw-geshi cpp source-cpp">pair<span class="br0">(</span> <span class="kw4">const</span> T1<span class="sy3">&</span> x, <span class="kw4">const</span> T2<span class="sy3">&</span> y <span class="br0">)</span><span class="sy4">;</span></span></div>
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</td>
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<td class="t-dcl-nopad"></td>
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<td><span class="t-mark-rev t-until-cxx11">(C++11 前)</span></td>
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</tr>
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<tr class="t-dcl t-since-cxx11 t-until-cxx14">
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<td>
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<div><span class="mw-geshi cpp source-cpp">pair<span class="br0">(</span> <span class="kw4">const</span> T1<span class="sy3">&</span> x, <span class="kw4">const</span> T2<span class="sy3">&</span> y <span class="br0">)</span><span class="sy4">;</span></span></div>
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</td>
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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-cxx14">(C++14 前)</span><br />
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<span class="t-mark">(条件性 <code>explicit</code>)</span></td>
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</tr>
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<tr class="t-dcl t-since-cxx14">
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<td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">constexpr</span> pair<span class="br0">(</span> <span class="kw4">const</span> T1<span class="sy3">&</span> x, <span class="kw4">const</span> T2<span class="sy3">&</span> y <span class="br0">)</span><span class="sy4">;</span></span></div>
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</td>
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<td class="t-dcl-nopad"></td>
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<td><span class="t-mark-rev t-since-cxx14">(C++14 起)</span><br />
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<span class="t-mark">(条件性 <code>explicit</code>)</span></td>
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</tr>
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</tbody>
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<tbody></tbody>
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<tbody class="t-dcl-rev t-dcl-rev-num">
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<tr class="t-dcl-rev-aux">
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<td></td>
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<td rowspan="142">(3)</td>
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<td></td>
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</tr>
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<tr class="t-dcl t-since-cxx11 t-until-cxx14">
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<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> U1, <span class="kw1">class</span> U2 <span class="sy1">></span><br />
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pair<span class="br0">(</span> U1<span class="sy3">&&</span> x, U2<span class="sy3">&&</span> y <span class="br0">)</span><span class="sy4">;</span></span></div>
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</td>
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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-cxx14">(C++14 前)</span><br />
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<span class="t-mark">(条件性 <code>explicit</code>)</span></td>
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</tr>
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<tr class="t-dcl t-since-cxx14 t-until-cxx23">
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<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> U1, <span class="kw1">class</span> U2 <span class="sy1">></span><br />
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<span class="kw4">constexpr</span> pair<span class="br0">(</span> U1<span class="sy3">&&</span> x, U2<span class="sy3">&&</span> y <span class="br0">)</span><span class="sy4">;</span></span></div>
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</td>
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<td class="t-dcl-nopad"></td>
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<td><span class="t-mark-rev t-since-cxx14">(C++14 起)</span><br />
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<span class="t-mark-rev t-until-cxx23">(C++23 前)</span><br />
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<span class="t-mark">(条件性 <code>explicit</code>)</span></td>
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</tr>
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<tr class="t-dcl t-since-cxx23">
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<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> U1 <span class="sy1">=</span> T1, <span class="kw1">class</span> U2 <span class="sy1">=</span> T2 <span class="sy1">></span><br />
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<span class="kw4">constexpr</span> pair<span class="br0">(</span> U1<span class="sy3">&&</span> x, U2<span class="sy3">&&</span> y <span class="br0">)</span><span class="sy4">;</span></span></div>
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</td>
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<td class="t-dcl-nopad"></td>
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<td><span class="t-mark-rev t-since-cxx23">(C++23 起)</span><br />
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<span class="t-mark">(条件性 <code>explicit</code>)</span></td>
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</tr>
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</tbody>
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<tbody></tbody>
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<tbody class="t-dcl-rev t-dcl-rev-num">
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<tr class="t-dcl-rev-aux">
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<td></td>
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<td rowspan="142">(4)</td>
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<td></td>
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</tr>
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<tr class="t-dcl t-until-cxx11">
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<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> U1, <span class="kw1">class</span> U2 <span class="sy1">></span><br />
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pair<span class="br0">(</span> <span class="kw4">const</span> pair<span class="sy1"><</span>U1, U2<span class="sy1">></span><span class="sy3">&</span> p <span class="br0">)</span><span class="sy4">;</span></span></div>
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</td>
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<td class="t-dcl-nopad"></td>
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<td><span class="t-mark-rev t-until-cxx11">(C++11 前)</span></td>
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</tr>
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<tr class="t-dcl t-since-cxx11 t-until-cxx14">
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<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> U1, <span class="kw1">class</span> U2 <span class="sy1">></span><br />
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pair<span class="br0">(</span> <span class="kw4">const</span> pair<span class="sy1"><</span>U1, U2<span class="sy1">></span><span class="sy3">&</span> p <span class="br0">)</span><span class="sy4">;</span></span></div>
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</td>
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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-cxx14">(C++14 前)</span><br />
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<span class="t-mark">(条件性 <code>explicit</code>)</span></td>
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</tr>
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<tr class="t-dcl t-since-cxx14">
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<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> U1, <span class="kw1">class</span> U2 <span class="sy1">></span><br />
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<span class="kw4">constexpr</span> pair<span class="br0">(</span> <span class="kw4">const</span> pair<span class="sy1"><</span>U1, U2<span class="sy1">></span><span class="sy3">&</span> p <span class="br0">)</span><span class="sy4">;</span></span></div>
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</td>
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<td class="t-dcl-nopad"></td>
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<td><span class="t-mark-rev t-since-cxx14">(C++14 起)</span><br />
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<span class="t-mark">(条件性 <code>explicit</code>)</span></td>
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</tr>
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</tbody>
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<tbody></tbody>
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<tbody class="t-dcl-rev t-dcl-rev-num">
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<tr class="t-dcl-rev-aux">
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<td></td>
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<td rowspan="142">(5)</td>
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<td></td>
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</tr>
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<tr class="t-dcl t-since-cxx11 t-until-cxx14">
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<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> U1, <span class="kw1">class</span> U2 <span class="sy1">></span><br />
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pair<span class="br0">(</span> pair<span class="sy1"><</span>U1, U2<span class="sy1">></span><span class="sy3">&&</span> p <span class="br0">)</span><span class="sy4">;</span></span></div>
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</td>
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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-cxx14">(C++14 前)</span><br />
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<span class="t-mark">(条件性 <code>explicit</code>)</span></td>
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</tr>
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<tr class="t-dcl t-since-cxx14">
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<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> U1, <span class="kw1">class</span> U2 <span class="sy1">></span><br />
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<span class="kw4">constexpr</span> pair<span class="br0">(</span> pair<span class="sy1"><</span>U1, U2<span class="sy1">></span><span class="sy3">&&</span> p <span class="br0">)</span><span class="sy4">;</span></span></div>
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</td>
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<td class="t-dcl-nopad"></td>
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<td><span class="t-mark-rev t-since-cxx14">(C++14 起)</span><br />
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<span class="t-mark">(条件性 <code>explicit</code>)</span></td>
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</tr>
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</tbody>
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<tbody></tbody>
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<tbody class="t-dcl-rev t-dcl-rev-num">
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<tr class="t-dcl-rev-aux">
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<td></td>
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<td rowspan="142">(6)</td>
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<td></td>
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</tr>
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<tr class="t-dcl t-since-cxx11 t-until-cxx20">
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<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>... <span class="me1">Args1</span>, <span class="kw1">class</span>... <span class="me1">Args2</span> <span class="sy1">></span><br /></span>
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<p>pair<span class="br0">(</span> <a href="cpp-utility-piecewise_construct_t.html"><span class="kw1105">std::<span class="me2">piecewise_construct_t</span></span></a>,<br />
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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>Args1...<span class="sy1">></span> first_args,<br /></p>
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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>Args2...<span class="sy1">></span> second_args <span class="br0">)</span><span class="sy4">;</span></div>
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</td>
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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-cxx20">(C++20 前)</span></td>
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</tr>
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<tr class="t-dcl t-since-cxx20">
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<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>... <span class="me1">Args1</span>, <span class="kw1">class</span>... <span class="me1">Args2</span> <span class="sy1">></span><br /></span>
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<p><span class="kw4">constexpr</span> pair<span class="br0">(</span> <a href="cpp-utility-piecewise_construct_t.html"><span class="kw1105">std::<span class="me2">piecewise_construct_t</span></span></a>,<br />
|
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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>Args1...<span class="sy1">></span> first_args,<br /></p>
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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>Args2...<span class="sy1">></span> second_args <span class="br0">)</span><span class="sy4">;</span></div>
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</td>
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<td class="t-dcl-nopad"></td>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
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</tr>
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</tbody>
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<tbody>
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<tr class="t-dcl">
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<td>
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<div><span class="mw-geshi cpp source-cpp">pair<span class="br0">(</span> <span class="kw4">const</span> pair<span class="sy3">&</span> p <span class="br0">)</span> <span class="sy1">=</span> <span class="kw1">default</span><span class="sy4">;</span></span></div>
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</td>
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<td>(7)</td>
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<td class="t-dcl-nopad"></td>
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</tr>
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<tr class="t-dcl t-since-cxx11">
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<td>
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<div><span class="mw-geshi cpp source-cpp">pair<span class="br0">(</span> pair<span class="sy3">&&</span> p <span class="br0">)</span> <span class="sy1">=</span> <span class="kw1">default</span><span class="sy4">;</span></span></div>
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</td>
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<td>(8)</td>
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<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
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</tr>
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<tr class="t-dcl-sep">
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<td></td>
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<td></td>
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<td></td>
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</tr>
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</tbody>
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</table>
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<p><br />
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构造新的 <code>pair</code> 。</p>
|
||||
<div class="t-li1"><span class="t-li">1)</span> 默认构造函数。值初始化 pair 的两个元素 <code>first</code> 和 <code>second</code> 。
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<table class="t-rev-begin">
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<tr class="t-rev t-since-cxx11">
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<td>
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<ul>
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<li>此构造函数参与重载决议，当且仅当 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_default_constructible.html"><span class="kw527">std::<span class="me2">is_default_constructible_v</span></span></a><span class="sy1"><</span>first_type<span class="sy1">></span></span></span> 与 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_default_constructible.html"><span class="kw527">std::<span class="me2">is_default_constructible_v</span></span></a><span class="sy1"><</span>second_type<span class="sy1">></span></span></span> 均为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> 。</li>
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||||
<li>此构造函数为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">explicit</span></span></span> ，当且仅当 <code>first_type</code> 或 <code>second_type</code> 不可隐式默认构造。</li>
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||||
</ul>
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||||
</td>
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<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
|
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</tr>
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</table>
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</div>
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<div class="t-li1"><span class="t-li">2)</span> 以 <code>x</code> 初始化 <code>first</code> 并以 <code>y</code> 初始化 <code>second</code> 。
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<table class="t-rev-begin">
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||||
<tr class="t-rev t-since-cxx11">
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<td>
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<ul>
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<li>此构造函数参与重载决议，当且仅当 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_copy_constructible.html"><span class="kw533">std::<span class="me2">is_copy_constructible_v</span></span></a><span class="sy1"><</span>first_type<span class="sy1">></span></span></span> 与 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_copy_constructible.html"><span class="kw533">std::<span class="me2">is_copy_constructible_v</span></span></a><span class="sy1"><</span>second_type<span class="sy1">></span></span></span> 均为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> 。</li>
|
||||
<li>此构造函数为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">explicit</span></span></span> ，当且仅当 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_convertible.html"><span class="kw593">std::<span class="me2">is_convertible_v</span></span></a><span class="sy1"><</span><span class="kw4">const</span> first_type<span class="sy3">&</span>, first_type<span class="sy1">></span></span></span> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">false</span></span></span> 或 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_convertible.html"><span class="kw593">std::<span class="me2">is_convertible_v</span></span></a><span class="sy1"><</span><span class="kw4">const</span> second_type<span class="sy3">&</span>, second_type<span class="sy1">></span></span></span> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">false</span></span></span> 。</li>
|
||||
</ul>
|
||||
</td>
|
||||
<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
|
||||
</tr>
|
||||
</table>
|
||||
</div>
|
||||
<div class="t-li1"><span class="t-li">3)</span> 以 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-utility-forward.html"><span class="kw1117">std::<span class="me2">forward</span></span></a><span class="sy1"><</span>U1<span class="sy1">></span><span class="br0">(</span>x<span class="br0">)</span></span></span> 初始化 <code>first</code> 并以 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-utility-forward.html"><span class="kw1117">std::<span class="me2">forward</span></span></a><span class="sy1"><</span>U2<span class="sy1">></span><span class="br0">(</span>y<span class="br0">)</span></span></span> 初始化 <code>second</code> 。
|
||||
<ul>
|
||||
<li>此构造函数参与重载决议，当且仅当 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_constructible.html"><span class="kw521">std::<span class="me2">is_constructible_v</span></span></a><span class="sy1"><</span>first_type, U1<span class="sy3">&&</span><span class="sy1">></span></span></span> 和 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_constructible.html"><span class="kw521">std::<span class="me2">is_constructible_v</span></span></a><span class="sy1"><</span>second_type, U2<span class="sy3">&&</span><span class="sy1">></span></span></span> 均为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> 。</li>
|
||||
<li>此构造函数为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">explicit</span></span></span> ，当且仅当 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_convertible.html"><span class="kw593">std::<span class="me2">is_convertible_v</span></span></a><span class="sy1"><</span>U1<span class="sy3">&&</span>, first_type<span class="sy1">></span></span></span> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">false</span></span></span> 或 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_convertible.html"><span class="kw593">std::<span class="me2">is_convertible_v</span></span></a><span class="sy1"><</span>U2<span class="sy3">&&</span>, second_type<span class="sy1">></span></span></span> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">false</span></span></span> 。</li>
|
||||
</ul>
|
||||
</div>
|
||||
<div class="t-li1"><span class="t-li">4)</span> 以 <code>p.first</code> 初始化 <code>first</code> 并以 <code>p.second</code> 初始化 <code>second</code> 。
|
||||
<table class="t-rev-begin">
|
||||
<tr class="t-rev t-since-cxx11">
|
||||
<td>
|
||||
<ul>
|
||||
<li>此构造函数参与重载决议，当且仅当 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_constructible.html"><span class="kw521">std::<span class="me2">is_constructible_v</span></span></a><span class="sy1"><</span>first_type, <span class="kw4">const</span> U1<span class="sy3">&</span><span class="sy1">></span></span></span> 和 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_constructible.html"><span class="kw521">std::<span class="me2">is_constructible_v</span></span></a><span class="sy1"><</span>second_type, <span class="kw4">const</span> U2<span class="sy3">&</span><span class="sy1">></span></span></span> 均为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> 。</li>
|
||||
<li>此构造函数为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">explicit</span></span></span> ，当且仅当 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_convertible.html"><span class="kw593">std::<span class="me2">is_convertible_v</span></span></a><span class="sy1"><</span><span class="kw4">const</span> U1<span class="sy3">&</span>, first_type<span class="sy1">></span></span></span> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">false</span></span></span> 或 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_convertible.html"><span class="kw593">std::<span class="me2">is_convertible_v</span></span></a><span class="sy1"><</span><span class="kw4">const</span> U2<span class="sy3">&</span>, second_type<span class="sy1">></span></span></span> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">false</span></span></span> 。</li>
|
||||
</ul>
|
||||
</td>
|
||||
<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
|
||||
</tr>
|
||||
</table>
|
||||
</div>
|
||||
<div class="t-li1"><span class="t-li">5)</span> 以 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-utility-forward.html"><span class="kw1117">std::<span class="me2">forward</span></span></a><span class="sy1"><</span>U1<span class="sy1">></span><span class="br0">(</span>p.<span class="me1">first</span><span class="br0">)</span></span></span> 初始化 <code>first</code> 并以 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-utility-forward.html"><span class="kw1117">std::<span class="me2">forward</span></span></a><span class="sy1"><</span>U2<span class="sy1">></span><span class="br0">(</span>p.<span class="me1">second</span><span class="br0">)</span></span></span> 初始化 <code>second</code> 。
|
||||
<ul>
|
||||
<li>此构造函数参与重载决议，当且仅当 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_constructible.html"><span class="kw521">std::<span class="me2">is_constructible_v</span></span></a><span class="sy1"><</span>first_type, U1<span class="sy3">&&</span><span class="sy1">></span></span></span> 和 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_constructible.html"><span class="kw521">std::<span class="me2">is_constructible_v</span></span></a><span class="sy1"><</span>second_type, U2<span class="sy3">&&</span><span class="sy1">></span></span></span> 均为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> 。</li>
|
||||
<li>此构造函数为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">explicit</span></span></span> ，当且仅当 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_convertible.html"><span class="kw593">std::<span class="me2">is_convertible_v</span></span></a><span class="sy1"><</span>U1<span class="sy3">&&</span>, first_type<span class="sy1">></span></span></span> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">false</span></span></span> 或 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_convertible.html"><span class="kw593">std::<span class="me2">is_convertible_v</span></span></a><span class="sy1"><</span>U2<span class="sy3">&&</span>, second_type<span class="sy1">></span></span></span> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">false</span></span></span> 。</li>
|
||||
</ul>
|
||||
</div>
|
||||
<div class="t-li1"><span class="t-li">6)</span> 转发 <code>first_args</code> 的元素到 <code>first</code> 的构造函数并转发 <code>second_args</code> 的元素到 <code>second</code> 的构造函数。这是能用于构造不可复制不可移动类型的 pair 的仅有的非默认构造函数。</div>
|
||||
<div class="t-li1"><span class="t-li">7)</span> 复制构造函数为<span class="t-rev-inl t-until-cxx11"><span>隐式声明</span> <span><span class="t-mark-rev t-until-cxx11">(C++11 前)</span></span></span><span class="t-rev-inl t-since-cxx11"><span>默认化，且若两个元素的复制满足 constexpr 函数的要求则为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">constexpr</span></span></span></span> <span><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></span></span>。</div>
|
||||
<div class="t-li1"><span class="t-li">8)</span> 移动构造函数为默认化，且若两个元素的移动满足 constexpr 函数的要求则为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">constexpr</span></span></span> 。</div>
|
||||
<h3><span class="mw-headline" id=".E5.8F.82.E6.95.B0">参数</span></h3>
|
||||
<table class="t-par-begin">
|
||||
<tr class="t-par">
|
||||
<td>x</td>
|
||||
<td>-</td>
|
||||
<td>初始化此 pair 首元素的值</td>
|
||||
</tr>
|
||||
<tr class="t-par">
|
||||
<td>y</td>
|
||||
<td>-</td>
|
||||
<td>初始化此 pair 第二元素的值</td>
|
||||
</tr>
|
||||
<tr class="t-par">
|
||||
<td>p</td>
|
||||
<td>-</td>
|
||||
<td>用于初始化此 pair 两个元素的值的 pair</td>
|
||||
</tr>
|
||||
<tr class="t-par">
|
||||
<td>first_args</td>
|
||||
<td>-</td>
|
||||
<td>初始化此 pair 首元素的构造函数参数的 tuple</td>
|
||||
</tr>
|
||||
<tr class="t-par">
|
||||
<td>second_args</td>
|
||||
<td>-</td>
|
||||
<td>初始化此 pair 第二元素的构造函数参数的 tuple</td>
|
||||
</tr>
|
||||
</table>
|
||||
<h3><span class="mw-headline" id=".E5.BC.82.E5.B8.B8">异常</span></h3>
|
||||
<p>不抛异常，除非指定操作之一（如元素的构造）抛出。</p>
|
||||
<h3><span class="mw-headline" id=".E7.A4.BA.E4.BE.8B">示例</span></h3>
|
||||
<div class="t-example">
|
||||
<div class="t-example-live-link"></div>
|
||||
<div dir="ltr" class="mw-geshi" style="text-align: left;">
|
||||
<div class="cpp source-cpp">
|
||||
<pre class="de1"><span class="co2">#include <utility></span>
|
||||
<span class="co2">#include <string></span>
|
||||
<span class="co2">#include <complex></span>
|
||||
<span class="co2">#include <tuple></span>
|
||||
<span class="co2">#include <iostream></span>
|
||||
|
||||
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
|
||||
<span class="br0">{</span>
|
||||
<span class="kw4">auto</span> print <span class="sy1">=</span> <span class="br0">[</span><span class="br0">]</span><span class="br0">(</span><span class="kw4">auto</span> rem, <span class="kw4">auto</span> <span class="kw4">const</span><span class="sy3">&</span> pair<span class="br0">)</span> <span class="br0">{</span>
|
||||
<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> rem <span class="sy1"><<</span> <span class="st0">"("</span> <span class="sy1"><<</span> pair.<span class="me1">first</span> <span class="sy1"><<</span> <span class="st0">", "</span> <span class="sy1"><<</span> pair.<span class="me1">second</span> <span class="sy1"><<</span> <span class="st0">")<span class="es1">\n</span>"</span><span class="sy4">;</span>
|
||||
<span class="br0">}</span><span class="sy4">;</span>
|
||||
|
||||
<a href="cpp-utility-pair.html"><span class="kw1096">std::<span class="me2">pair</span></span></a><span class="sy1"><</span><span class="kw4">int</span>, <span class="kw4">float</span><span class="sy1">></span> p1<span class="sy4">;</span>
|
||||
print<span class="br0">(</span><span class="st0">"(1) Value-initialized: "</span>, p1<span class="br0">)</span><span class="sy4">;</span>
|
||||
|
||||
<a href="cpp-utility-pair.html"><span class="kw1096">std::<span class="me2">pair</span></span></a><span class="sy1"><</span><span class="kw4">int</span>, <span class="kw4">double</span><span class="sy1">></span> p2<span class="br0">{</span><span class="nu0">42</span>, <span class="nu16">3.1415</span><span class="br0">}</span><span class="sy4">;</span>
|
||||
print<span class="br0">(</span><span class="st0">"(2) Initialized with two values: "</span>, p2<span class="br0">)</span><span class="sy4">;</span>
|
||||
|
||||
<a href="cpp-utility-pair.html"><span class="kw1096">std::<span class="me2">pair</span></span></a><span class="sy1"><</span><span class="kw4">char</span>, <span class="kw4">int</span><span class="sy1">></span> p4<span class="br0">{</span>p2<span class="br0">}</span><span class="sy4">;</span>
|
||||
print<span class="br0">(</span><span class="st0">"(4) Implicitly converted: "</span>, p4<span class="br0">)</span><span class="sy4">;</span>
|
||||
|
||||
<a href="cpp-utility-pair.html"><span class="kw1096">std::<span class="me2">pair</span></span></a><span class="sy1"><</span><a href="cpp-numeric-complex.html"><span class="kw1472">std::<span class="me2">complex</span></span></a><span class="sy1"><</span><span class="kw4">double</span><span class="sy1">></span>, <a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a><span class="sy1">></span> p6<span class="br0">{</span>
|
||||
<a href="cpp-utility-piecewise_construct.html"><span class="kw1106">std::<span class="me2">piecewise_construct</span></span></a>,
|
||||
<a href="cpp-utility-tuple-forward_as_tuple.html"><span class="kw1114">std::<span class="me2">forward_as_tuple</span></span></a><span class="br0">(</span><span class="nu16">0.123</span>, <span class="nu16">7.7</span><span class="br0">)</span>,
|
||||
<a href="cpp-utility-tuple-forward_as_tuple.html"><span class="kw1114">std::<span class="me2">forward_as_tuple</span></span></a><span class="br0">(</span><span class="nu0">10</span>, <span class="st0">'a'</span><span class="br0">)</span><span class="br0">}</span><span class="sy4">;</span>
|
||||
print<span class="br0">(</span><span class="st0">"(6) Piecewise constructed: "</span>, p6<span class="br0">)</span><span class="sy4">;</span>
|
||||
<span class="br0">}</span></pre></div>
|
||||
</div>
|
||||
<p>可能的输出：</p>
|
||||
<div dir="ltr" class="mw-geshi" style="text-align: left;">
|
||||
<div class="text source-text">
|
||||
<pre class="de1">(1) Value-initialized: (0, 0)
|
||||
(2) Initialized with two values: (42, 3.1415)
|
||||
(4) Implicitly converted: (*, 3)
|
||||
(6) Piecewise constructed: ((0.123,7.7), aaaaaaaaaa)</pre></div>
|
||||
</div>
|
||||
</div>
|
||||
<h3><span class="mw-headline" id=".E7.BC.BA.E9.99.B7.E6.8A.A5.E5.91.8A">缺陷报告</span></h3>
|
||||
<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>
|
||||
<table class="dsctable" style="font-size:0.8em">
|
||||
<tr>
|
||||
<th>DR</th>
|
||||
<th>应用于</th>
|
||||
<th>出版时的行为</th>
|
||||
<th>正确行为</th>
|
||||
</tr>
|
||||
<tr>
|
||||
<td><a rel="nofollow" class="external text" href="https://wg21.link/N4387">N4387</a></td>
|
||||
<td>C++11</td>
|
||||
<td>某些构造函数曾为 explicit ，阻止了有用的行为</td>
|
||||
<td>使大多数构造函数为条件性 explicit</td>
|
||||
</tr>
|
||||
<tr>
|
||||
<td><a rel="nofollow" class="external text" href="https://cplusplus.github.io/LWG/issue2510">LWG 2510</a></td>
|
||||
<td>C++11</td>
|
||||
<td>默认构造函数为隐式</td>
|
||||
<td>使之为条件性 explicit</td>
|
||||
</tr>
|
||||
</table>
|
||||
<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">参阅</span></h3>
|
||||
<table class="t-dsc-begin">
|
||||
<tr class="t-dsc">
|
||||
<td>
|
||||
<div class="t-dsc-member-div">
|
||||
<div><a href="cpp-utility-pair-make_pair.html"><span class="t-lines"><span>make_pair</span></span></a></div>
|
||||
</div>
|
||||
</td>
|
||||
<td>创建一个 <code>pair</code> 对象，其类型根据各实参类型定义<br />
|
||||
<span class="t-mark">(函数模板)</span></td>
|
||||
</tr>
|
||||
</table>
|
||||
</div>
|
||||
<div class="visualClear"></div>
|
||||
</div>
|
||||
</div>
|
||||
</div>
|
||||
</body>
|
||||
</html>
|
||||
Reference in New Issue
Block a user