136 lines
12 KiB
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136 lines
12 KiB
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<title>std::unordered_map<Key,T,Hash,KeyEqual,Allocator>::emplace</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_container_unordered_map_emplace skin-cppreference2 action-view cpp-page-template cpp-navbar">
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::unordered_map<Key,T,Hash,KeyEqual,Allocator>::</span>emplace</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-container.html">container</a>‎ | <a href="cpp-container-unordered_map.html">unordered map</a></span></div>
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<div class="t-page-template" style="">
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<p></p>
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</div>
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<table class="t-dcl-begin">
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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">Args</span> <span class="sy1">></span><br />
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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>iterator,<span class="kw4">bool</span><span class="sy1">></span> emplace<span class="br0">(</span> Args<span class="sy3">&&</span>... <span class="me1">args</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></td>
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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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<p>若容器中无拥有该关键的元素，则插入以给定的 <code>args</code> 原位构造的新元素到容器。</p>
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<p>细心地使用 <code>emplace</code> 允许在构造新元素的同时避免不必要的复制或移动操作。 准确地以与提供给 <code>emplace</code> 者相同的参数，通过 <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>Args<span class="sy1">></span><span class="br0">(</span>args<span class="br0">)</span>...</span></span> 转发调用新元素（即 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-utility-pair.html"><span class="kw1096">std::<span class="me2">pair</span></span></a><span class="sy1"><</span><span class="kw4">const</span> Key, T<span class="sy1">></span></span></span> ）的构造函数。 即使容器中已有拥有该关键的元素，也可能构造元素，该情况下新构造的元素将被立即销毁。</p>
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<p>若因插入发生重哈希，则所有迭代器都被非法化。否则迭代器不受影响。引用不被非法化。重哈希仅若新元素数量大于 <span class="t-c"><span class="mw-geshi cpp source-cpp">max_load_factor<span class="br0">(</span><span class="br0">)</span><span class="sy2">*</span>bucket_count<span class="br0">(</span><span class="br0">)</span></span></span> 才发生。</p>
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<h3><span class="mw-headline" id=".E5.8F.82.E6.95.B0">参数</span></h3>
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<table class="t-par-begin">
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<tr class="t-par">
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<td>args</td>
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<td>-</td>
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<td>要转发给元素构造函数的参数</td>
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</tr>
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</table>
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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>返回由指向被插入元素，或若不发生插入则为既存元素的迭代器，和指代插入是否发生的 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span></span></span> （若发生插入则为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> ，否则为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">false</span></span></span> ）。</p>
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<h3><span class="mw-headline" id=".E5.BC.82.E5.B8.B8">异常</span></h3>
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<p>若任何操作抛出异常，则此函数无效果。</p>
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<h3><span class="mw-headline" id=".E5.A4.8D.E6.9D.82.E5.BA.A6">复杂度</span></h3>
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<p>平均为均摊常数，最坏情况与容器大小成线性。</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="t-example">
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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 <utility></span>
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<span class="co2">#include <string></span>
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<span class="co2">#include <unordered_map></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-unordered_map.html"><span class="kw1293">std::<span class="me2">unordered_map</span></span></a><span class="sy1"><</span><a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a>, <a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a><span class="sy1">></span> m<span class="sy4">;</span>
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<span class="co1">// 使用 pair 的移动构造函数</span>
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m.<span class="me1">emplace</span><span class="br0">(</span><a href="cpp-utility-pair-make_pair.html"><span class="kw1097">std::<span class="me2">make_pair</span></span></a><span class="br0">(</span><a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a><span class="br0">(</span><span class="st0">"a"</span><span class="br0">)</span>, <a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a><span class="br0">(</span><span class="st0">"a"</span><span class="br0">)</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="co1">// 使用 pair 的转换移动构造函数</span>
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m.<span class="me1">emplace</span><span class="br0">(</span><a href="cpp-utility-pair-make_pair.html"><span class="kw1097">std::<span class="me2">make_pair</span></span></a><span class="br0">(</span><span class="st0">"b"</span>, <span class="st0">"abcd"</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="co1">// 使用 pair 的模板构造函数</span>
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m.<span class="me1">emplace</span><span class="br0">(</span><span class="st0">"d"</span>, <span class="st0">"ddd"</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="co1">// 使用 pair 的逐片构造函数</span>
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m.<span class="me1">emplace</span><span class="br0">(</span><a href="cpp-utility-piecewise_construct.html"><span class="kw1106">std::<span class="me2">piecewise_construct</span></span></a>,
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<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="st0">"c"</span><span class="br0">)</span>,
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<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">'c'</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="co1">// C++17 起，能使用 m.try_emplace("c", 10, 'c');</span>
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<span class="kw1">for</span> <span class="br0">(</span><span class="kw4">const</span> <span class="kw4">auto</span> <span class="sy3">&</span>p <span class="sy4">:</span> m<span class="br0">)</span> <span class="br0">{</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> p.<span class="me1">first</span> <span class="sy1"><<</span> <span class="st0">" => "</span> <span class="sy1"><<</span> p.<span class="me1">second</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>
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<span class="br0">}</span></pre></div>
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</div>
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<p>可能的输出：</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">a => a
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b => abcd
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c => cccccccccc
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d => ddd</pre></div>
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</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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<tr class="t-dsc">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-container-unordered_map-emplace_hint.html"><span class="t-lines"><span>emplace_hint</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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</div>
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</td>
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<td>使用提示原位构造元素<br />
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<span class="t-mark">(公开成员函数)</span></td>
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</tr>
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<tr class="t-dsc">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-container-unordered_map-try_emplace.html"><span class="t-lines"><span>try_emplace</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>若键不存在则原位插入，若键存在则不做任何事<br />
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<span class="t-mark">(公开成员函数)</span></td>
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</tr>
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<tr class="t-dsc">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-container-unordered_map-insert.html"><span class="t-lines"><span>insert</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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</div>
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</td>
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<td>插入元素<span class="t-rev-inl t-since-cxx17"><span>或结点</span> <span><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></span></span><br />
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<span class="t-mark">(公开成员函数)</span></td>
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</tr>
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</table>
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</div>
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<div class="visualClear"></div>
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</div>
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</div>
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</div>
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</body>
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