170 lines
14 KiB
HTML
170 lines
14 KiB
HTML
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<title>std::deque<T,Allocator>::emplace_back</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_container_deque_emplace_back 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::deque<T,Allocator>::</span>emplace_back</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-deque.html">deque</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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<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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<tbody></tbody>
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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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<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">Args</span> <span class="sy1">></span><br />
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<span class="kw4">void</span> emplace_back<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><br />
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<span class="t-mark-rev t-until-cxx17">(C++17 前)</span></td>
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</tr>
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<tr class="t-dcl t-since-cxx17">
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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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reference emplace_back<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-cxx17">(C++17 起)</span></td>
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</tbody>
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<tbody>
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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>添加新元素到容器尾。元素通过 <span class="t-lc"><a href="cpp-memory-allocator_traits-construct.html">std::allocator_traits::construct</a></span> 构造，它典型地用布置 new 于容器所提供的位置原位构造元素。参数 <code>args...</code> 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>Args<span class="sy1">></span><span class="br0">(</span>args<span class="br0">)</span>...</span></span> 转发到构造函数。</p>
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<p>所有迭代器，包含尾后迭代器，都被非法化。没有引用被非法化。</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>转发到元素构造函数的参数</td>
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</tr>
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<tr class="t-par-hitem">
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<td colspan="3">类型要求</td>
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<tr class="t-par-req">
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<td colspan="3">-
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<div class="spacer"></div>
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<code>value_type</code> 必须满足<a href="cpp-named_req-EmplaceConstructible.html"><span style="font-style:italic">可就位构造</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(EmplaceConstructible)</span></a> 的要求。</td>
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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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<table class="t-rev-begin">
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<tr class="t-rev t-until-cxx17">
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<td>
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<p>（无）</p>
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</td>
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<td><span class="t-mark-rev t-until-cxx17">(C++17 前)</span></td>
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</tr>
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<tr class="t-rev t-since-cxx17">
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<td>
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<p>到被插入元素的引用。</p>
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</td>
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<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></td>
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</tr>
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</table>
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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>常数。</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>若抛出异常，则此函数无效果（强异常保证）。</p>
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<p><br /></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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<p>下列代码用 <code>emplace_back</code> 后附 <code>President</code> 类型对象到 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-container-deque.html"><span class="kw1271">std::<span class="me2">deque</span></span></a></span></span> 。它演示 <code>emplace_back</code> 如何转发参数到 <code>President</code> 的构造函数，并展示如何用 <code>emplace_back</code> 避免用 <code>push_back</code> 时的额外复制或移动操作。</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 <deque></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="kw1">struct</span> President
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<span class="br0">{</span>
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<a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a> name<span class="sy4">;</span>
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<a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a> country<span class="sy4">;</span>
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<span class="kw4">int</span> year<span class="sy4">;</span>
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President<span class="br0">(</span><a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a> p_name, <a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a> p_country, <span class="kw4">int</span> p_year<span class="br0">)</span>
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<span class="sy4">:</span> name<span class="br0">(</span>std<span class="sy4">::</span><span class="me2">move</span><span class="br0">(</span>p_name<span class="br0">)</span><span class="br0">)</span>, country<span class="br0">(</span>std<span class="sy4">::</span><span class="me2">move</span><span class="br0">(</span>p_country<span class="br0">)</span><span class="br0">)</span>, year<span class="br0">(</span>p_year<span class="br0">)</span>
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<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> <span class="st0">"I am being constructed.<span class="es1">\n</span>"</span><span class="sy4">;</span>
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<span class="br0">}</span>
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President<span class="br0">(</span>President<span class="sy3">&&</span> other<span class="br0">)</span>
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<span class="sy4">:</span> name<span class="br0">(</span>std<span class="sy4">::</span><span class="me2">move</span><span class="br0">(</span>other.<span class="me1">name</span><span class="br0">)</span><span class="br0">)</span>, country<span class="br0">(</span>std<span class="sy4">::</span><span class="me2">move</span><span class="br0">(</span>other.<span class="me1">country</span><span class="br0">)</span><span class="br0">)</span>, year<span class="br0">(</span>other.<span class="me1">year</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="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"I am being moved.<span class="es1">\n</span>"</span><span class="sy4">;</span>
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<span class="br0">}</span>
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President<span class="sy3">&</span> operator<span class="sy1">=</span><span class="br0">(</span><span class="kw4">const</span> President<span class="sy3">&</span> other<span class="br0">)</span> <span class="sy1">=</span> <span class="kw1">default</span><span class="sy4">;</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-container-deque.html"><span class="kw1271">std::<span class="me2">deque</span></span></a><span class="sy1"><</span>President<span class="sy1">></span> elections<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">"emplace_back:<span class="es1">\n</span>"</span><span class="sy4">;</span>
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elections.<span class="me1">emplace_back</span><span class="br0">(</span><span class="st0">"Nelson Mandela"</span>, <span class="st0">"South Africa"</span>, <span class="nu0">1994</span><span class="br0">)</span><span class="sy4">;</span>
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<a href="cpp-container-deque.html"><span class="kw1271">std::<span class="me2">deque</span></span></a><span class="sy1"><</span>President<span class="sy1">></span> reElections<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">"<span class="es1">\n</span>push_back:<span class="es1">\n</span>"</span><span class="sy4">;</span>
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reElections.<span class="me1">push_back</span><span class="br0">(</span>President<span class="br0">(</span><span class="st0">"Franklin Delano Roosevelt"</span>, <span class="st0">"the USA"</span>, <span class="nu0">1936</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">"<span class="es1">\n</span>Contents:<span class="es1">\n</span>"</span><span class="sy4">;</span>
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<span class="kw1">for</span> <span class="br0">(</span>President <span class="kw4">const</span><span class="sy3">&</span> president<span class="sy4">:</span> elections<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> president.<span class="me1">name</span> <span class="sy1"><<</span> <span class="st0">" was elected president of "</span>
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<span class="sy1"><<</span> president.<span class="me1">country</span> <span class="sy1"><<</span> <span class="st0">" in "</span> <span class="sy1"><<</span> president.<span class="me1">year</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="kw1">for</span> <span class="br0">(</span>President <span class="kw4">const</span><span class="sy3">&</span> president<span class="sy4">:</span> reElections<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> president.<span class="me1">name</span> <span class="sy1"><<</span> <span class="st0">" was re-elected president of "</span>
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<span class="sy1"><<</span> president.<span class="me1">country</span> <span class="sy1"><<</span> <span class="st0">" in "</span> <span class="sy1"><<</span> president.<span class="me1">year</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>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">emplace_back:
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I am being constructed.
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push_back:
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I am being constructed.
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I am being moved.
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Contents:
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Nelson Mandela was elected president of South Africa in 1994.
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Franklin Delano Roosevelt was re-elected president of the USA in 1936.</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-deque-push_back.html"><span class="t-lines"><span>push_back</span></span></a></div>
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</div>
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</td>
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<td>将元素添加到容器末尾<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 class="visualClear"></div>
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