158 lines
10 KiB
HTML
158 lines
10 KiB
HTML
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<title>std::prev</title>
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::</span>prev</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-iterator.html">iterator</a></span></div>
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<div>定义于头文件 <code><a href="cpp-header-iterator.html"><iterator></a></code></div>
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<td></td>
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<td></td>
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<tr class="t-dcl t-since-cxx11 t-until-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> BidirIt <span class="sy1">></span><br /></span>
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<p>BidirIt prev<span class="br0">(</span><br />
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BidirIt it,<br /></p>
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<span class="kw1">typename</span> <a href="cpp-iterator-iterator_traits.html"><span class="kw651">std::<span class="me2">iterator_traits</span></span></a><span class="sy1"><</span>BidirIt<span class="sy1">></span><span class="sy4">::</span><span class="me2">difference_type</span> n <span class="sy1">=</span> <span class="nu0">1</span> <span class="br0">)</span><span class="sy4">;</span></div>
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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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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1"><</span> <span class="kw1">class</span> BidirIt <span class="sy1">></span><br /></span>
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<p><span class="kw4">constexpr</span> BidirIt prev<span class="br0">(</span><br />
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BidirIt it,<br /></p>
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<span class="kw1">typename</span> <a href="cpp-iterator-iterator_traits.html"><span class="kw651">std::<span class="me2">iterator_traits</span></span></a><span class="sy1"><</span>BidirIt<span class="sy1">></span><span class="sy4">::</span><span class="me2">difference_type</span> n <span class="sy1">=</span> <span class="nu0">1</span> <span class="br0">)</span><span class="sy4">;</span></div>
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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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<td></td>
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<p>返回迭代器 <code>it</code> 的第 <code><i>n</i></code> 个前驱。</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>it</td>
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<td>-</td>
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<td>迭代器</td>
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<tr class="t-par">
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<td>n</td>
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<td>-</td>
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<td><code>it</code> 要被减少的次数</td>
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<td colspan="3">类型要求</td>
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<td colspan="3">-
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<code>BidirIt</code> 必须满足<a href="cpp-named_req-BidirectionalIterator.html"><span style="font-style:italic">老式双向迭代器</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(LegacyBidirectionalIterator)</span></a> 的要求。</td>
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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>迭代器 <code>it</code> 的第 <code><i>n</i></code> 个前驱。</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>线性。</p>
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<p>然而，若 <code>BidirIt</code> 还满足<a href="cpp-named_req-RandomAccessIterator.html"><span style="font-style:italic">老式随机访问迭代器</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(LegacyRandomAccessIterator)</span></a> 的要求，则复杂度为常数。</p>
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<h3><span class="mw-headline" id=".E5.8F.AF.E8.83.BD.E7.9A.84.E5.AE.9E.E7.8E.B0">可能的实现</span></h3>
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<table class="eq-fun-cpp-table">
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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="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> BidirIt<span class="sy1">></span>
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<span class="kw4">constexpr</span> <span class="co1">// C++17 起</span>
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BidirIt prev<span class="br0">(</span>BidirIt it, <span class="kw1">typename</span> <a href="cpp-iterator-iterator_traits.html"><span class="kw651">std::<span class="me2">iterator_traits</span></span></a><span class="sy1"><</span>BidirIt<span class="sy1">></span><span class="sy4">::</span><span class="me2">difference_type</span> n <span class="sy1">=</span> <span class="nu0">1</span><span class="br0">)</span>
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<span class="br0">{</span>
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<a href="cpp-iterator-advance.html"><span class="kw672">std::<span class="me2">advance</span></span></a><span class="br0">(</span>it, <span class="sy2">-</span>n<span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">return</span> it<span class="sy4">;</span>
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<span class="br0">}</span></pre></div>
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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>尽管表达式 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">--</span>c.<span class="me1">end</span><span class="br0">(</span><span class="br0">)</span></span></span> 通常能编译，然而不保证会这么做： <span class="t-c"><span class="mw-geshi cpp source-cpp">c.<span class="me1">end</span><span class="br0">(</span><span class="br0">)</span></span></span> 是右值表达式，而且无迭代器要求指定右值的自减保证进行。尤其是当迭代器实现为指针或其 <code>operator--</code> 为左值引用限定时， <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">--</span>c.<span class="me1">end</span><span class="br0">(</span><span class="br0">)</span></span></span> 不能编译，而 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">prev</span><span class="br0">(</span>c.<span class="me1">end</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span></span></span> 可以。</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 <iterator></span>
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<span class="co2">#include <vector></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-vector.html"><span class="kw1268">std::<span class="me2">vector</span></span></a><span class="sy1"><</span><span class="kw4">int</span><span class="sy1">></span> v<span class="br0">{</span> <span class="nu0">3</span>, <span class="nu0">1</span>, <span class="nu0">4</span> <span class="br0">}</span><span class="sy4">;</span>
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<span class="kw4">auto</span> it <span class="sy1">=</span> v.<span class="me1">end</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw4">auto</span> pv <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">prev</span><span class="br0">(</span>it, <span class="nu0">2</span><span class="br0">)</span><span class="sy4">;</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="sy2">*</span>pv <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></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">1</pre></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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<div class="t-dsc-member-div">
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<div><a href="cpp-iterator-next.html"><span class="t-lines"><span>next</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>令迭代器自增<br />
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<span class="t-mark">(函数模板)</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-iterator-advance.html"><span class="t-lines"><span>advance</span></span></a></div>
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</div>
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</td>
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<td>令迭代器前进给定的距离<br />
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<span class="t-mark">(函数模板)</span></td>
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</tr>
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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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