95 lines
5.7 KiB
HTML
95 lines
5.7 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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<head><meta http-equiv="x-ua-compatible" content="ie=edge">
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
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<title>std::deque<T,Allocator>::front</title>
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<link rel="stylesheet" href="ext.css" />
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<link rel="stylesheet" href="site_modules.css" />
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</head>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_container_deque_front skin-cppreference2 action-view cpp-page-template 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::deque<T,Allocator>::</span>front</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>
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<tr class="t-dcl t-since-cxx98">
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<td class="t-dcl-nopad">
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<div><span class="mw-geshi cpp source-cpp">reference front<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 class="t-dcl-nopad"></td>
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</tr>
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<tr class="t-dcl t-since-cxx98">
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<td class="t-dcl-nopad">
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<div><span class="mw-geshi cpp source-cpp">const_reference front<span class="br0">(</span><span class="br0">)</span> <span class="kw4">const</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 class="t-dcl-nopad"></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>返回到容器首元素的引用。</p>
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<p>在空容器上对 <code>front</code> 的调用是未定义的。</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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<p>（无）</p>
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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>到首元素的引用</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>常数</p>
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<h3><span class="mw-headline" id=".E6.B3.A8.E8.A7.A3">注解</span></h3>
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<p>对于容器 <code>c</code> ，表达式 <span class="t-c"><span class="mw-geshi cpp source-cpp">c.<span class="me1">front</span><span class="br0">(</span><span class="br0">)</span></span></span> 等价于 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>c.<span class="me1">begin</span><span class="br0">(</span><span class="br0">)</span></span></span> 。</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>front</code> 显示 <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 class="sy1"><</span><span class="kw4">char</span><span class="sy1">></span></span></span> 的首元素：</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 <iostream></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><span class="kw4">char</span><span class="sy1">></span> letters <span class="br0">{</span><span class="st0">'o'</span>, <span class="st0">'m'</span>, <span class="st0">'g'</span>, <span class="st0">'w'</span>, <span class="st0">'t'</span>, <span class="st0">'f'</span><span class="br0">}</span><span class="sy4">;</span>
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<span class="kw1">if</span> <span class="br0">(</span><span class="sy3">!</span>letters.<span class="me1">empty</span><span class="br0">(</span><span class="br0">)</span><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> <span class="st0">"The first character is: "</span> <span class="sy1"><<</span> letters.<span class="me1">front</span><span class="br0">(</span><span class="br0">)</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">The first character is o</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-back.html"><span class="t-lines"><span>back</span></span></a></div>
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</div>
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</td>
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<td>访问最后一个元素<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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</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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</html>
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