225 lines
20 KiB
HTML
225 lines
20 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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<title>std::ranges::for_each_n, std::ranges::for_each_n_result</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_algorithm_ranges_for_each_n skin-cppreference2 action-view cpp-navbar">
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::ranges::</span>for_each_n, <span style="font-size:0.7em; line-height:130%">std::ranges::</span>for_each_n_result</h1>
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<div id="contentSub"><span class="subpages">< <a href="cpp.html">cpp</a>‎ | <a href="cpp-algorithm.html">algorithm</a>‎ | <a href="cpp-algorithm-ranges.html">ranges</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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<table class="t-dcl-begin">
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<div>定义于头文件 <code><a href="cpp-header-algorithm.html"><algorithm></a></code></div>
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</td>
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<td></td>
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<div>调用签名</div>
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<td></td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1"><</span> std<span class="sy4">::</span><span class="me2">input_iterator</span> I, <span class="kw1">class</span> Proj <span class="sy1">=</span> identity,<br /></span>
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<p> std<span class="sy4">::</span><span class="me2">indirectly_unary_invocable</span><span class="sy1"><</span>std<span class="sy4">::</span><span class="me2">projected</span><span class="sy1"><</span>I, Proj<span class="sy1">>></span> Fun <span class="sy1">></span><br />
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<span class="kw4">constexpr</span> for_each_n_result<span class="sy1"><</span>I, Fun<span class="sy1">></span><br /></p>
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for_each_n<span class="br0">(</span> I first, std<span class="sy4">::</span><span class="me2">iter_difference_t</span><span class="sy1"><</span>I<span class="sy1">></span> n, Fun f, Proj proj <span class="sy1">=</span> <span class="br0">{</span><span class="br0">}</span><span class="br0">)</span><span class="sy4">;</span></div>
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</td>
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<td>(1)</td>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
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<div>辅助类型</div>
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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> I, <span class="kw1">class</span> F <span class="sy1">></span><br />
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<span class="kw1">using</span> for_each_n_result <span class="sy1">=</span> ranges<span class="sy4">::</span><span class="me2">in_fun_result</span><span class="sy1"><</span>I, F<span class="sy1">></span><span class="sy4">;</span></span></div>
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<td>(2)</td>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
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<div class="t-li1"><span class="t-li">1)</span> 按顺序应用给定的函数对象 <code>f</code> 到用 <code>proj</code> 投影的解引用范围 <code>[first, first + n)</code> 中的每个迭代器的结果。</div>
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<p>若迭代器类型可变,则 <code>f</code> 可能通过解引用的迭代器修改范围中的匀速。若 <code>f</code> 返回结果,则忽略结果,若 <code>n</code> 小于零,则行为未定义。 此页面上描述的仿函数实体是 <i>niebloid</i> ,即:</p>
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<ul>
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<li>调用它们中的任何一者时不可指定显式模板实参。</li>
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<li>它们无一对<a href="cpp-language-adl.html">实参依赖查找</a>可见。</li>
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<li>在<a href="cpp-language-unqualified_lookup.html">通常无限定查找</a>寻找函数调用运算符左侧的名称找到它们之一时,它禁止<a href="cpp-language-adl.html">实参依赖查找</a>。</li>
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</ul>
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<p>实际上,它们能以函数对象,或以某些特殊编译器扩展实现。</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>first</td>
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<td>-</td>
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<td>代表要应用函数到的范围起始的迭代器</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>要应用函数到的元素数</td>
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<tr class="t-par">
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<td>f</td>
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<td>-</td>
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<td>应用到投影后范围 <code>[first, first + n)</code> 的函数</td>
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<tr class="t-par">
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<td>proj</td>
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<td>-</td>
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<td>应用到元素的投影</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>对象 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">{</span>first <span class="sy2">+</span> n, std<span class="sy4">::</span><span class="me2">move</span><span class="br0">(</span>f<span class="br0">)</span><span class="br0">}</span></span></span> ,其中 <code>first + n</code> 可能求值为 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">ranges</span><span class="sy4">::</span><span class="me2">next</span><span class="br0">(</span>std<span class="sy4">::</span><span class="me2">move</span><span class="br0">(</span>first<span class="br0">)</span>, n<span class="br0">)</span></span></span> ,取决于迭代器类别。</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>准确应用 <code>n</code> 次 <code>f</code> 与 <code>proj</code> 。</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><code>namespace::ranges</code> 中的重载要求 <code>Fun</code> 实现 <a href="cpp-concepts-copy_constructible.html"><code>copy_constructible</code></a> 。</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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<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">struct</span> for_each_n_fn <span class="br0">{</span>
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<span class="kw1">template</span><span class="sy1"><</span>std<span class="sy4">::</span><span class="me2">input_iterator</span> I, <span class="kw1">class</span> Proj <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">identity</span>,
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std<span class="sy4">::</span><span class="me2">indirectly_unary_invocable</span><span class="sy1"><</span>std<span class="sy4">::</span><span class="me2">projected</span><span class="sy1"><</span>I, Proj<span class="sy1">>></span> Fun<span class="sy1">></span>
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<span class="kw4">constexpr</span> for_each_n_result<span class="sy1"><</span>I, Fun<span class="sy1">></span>
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operator<span class="br0">(</span><span class="br0">)</span><span class="br0">(</span>I first, std<span class="sy4">::</span><span class="me2">iter_difference_t</span><span class="sy1"><</span>I<span class="sy1">></span> n, Fun fun, Proj proj <span class="sy1">=</span> Proj<span class="br0">{</span><span class="br0">}</span><span class="br0">)</span> <span class="kw4">const</span>
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<span class="br0">{</span>
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<span class="kw1">for</span> <span class="br0">(</span><span class="sy4">;</span> n<span class="sy2">--</span> <span class="sy1">></span> <span class="nu0">0</span><span class="sy4">;</span> <span class="sy2">++</span>first<span class="br0">)</span> <span class="br0">{</span>
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<a href="cpp-utility-functional-invoke.html"><span class="kw1058">std::<span class="me2">invoke</span></span></a><span class="br0">(</span>fun, <a href="cpp-utility-functional-invoke.html"><span class="kw1058">std::<span class="me2">invoke</span></span></a><span class="br0">(</span>proj, <span class="sy2">*</span>first<span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="kw1">return</span> <span class="br0">{</span>std<span class="sy4">::</span><span class="me2">move</span><span class="br0">(</span>first<span class="br0">)</span>, std<span class="sy4">::</span><span class="me2">move</span><span class="br0">(</span>fun<span class="br0">)</span><span class="br0">}</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="br0">}</span><span class="sy4">;</span>
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<span class="kw1">inline</span> <span class="kw4">constexpr</span> for_each_n_fn for_each_n<span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span></pre></div>
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</div>
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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 <algorithm></span>
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<span class="co2">#include <array></span>
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<span class="co2">#include <iostream></span>
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<span class="co2">#include <ranges></span>
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<span class="co2">#include <string_view></span>
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<span class="kw1">struct</span> P <span class="br0">{</span>
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<span class="kw4">int</span> first<span class="sy4">;</span> <span class="kw4">char</span> second<span class="sy4">;</span>
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<span class="kw1">friend</span> <a href="cpp-io-basic_ostream.html"><span class="kw1729">std::<span class="me2">ostream</span></span></a><span class="sy3">&</span> operator<span class="sy1"><<</span> <span class="br0">(</span><a href="cpp-io-basic_ostream.html"><span class="kw1729">std::<span class="me2">ostream</span></span></a><span class="sy3">&</span> os, <span class="kw4">const</span> P<span class="sy3">&</span> p<span class="br0">)</span> <span class="br0">{</span>
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<span class="kw1">return</span> os <span class="sy1"><<</span> <span class="st0">'{'</span> <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><span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="br0">}</span><span class="sy4">;</span>
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<span class="kw4">auto</span> print <span class="sy1">=</span> <span class="br0">[</span><span class="br0">]</span><span class="br0">(</span><a href="cpp-string-basic_string_view.html"><span class="kw1248">std::<span class="me2">string_view</span></span></a> name, <span class="kw4">auto</span> <span class="kw4">const</span><span class="sy3">&</span> v<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> name <span class="sy1"><<</span> <span class="st0">": "</span><span class="sy4">;</span>
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<span class="kw1">for</span> <span class="br0">(</span><span class="kw4">auto</span> n <span class="sy1">=</span> v.<span class="me1">size</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="kw4">const</span> <span class="kw4">auto</span><span class="sy3">&</span> e<span class="sy4">:</span> v<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> e <span class="sy1"><<</span> <span class="br0">(</span><span class="sy2">--</span>n <span class="sy4">?</span> <span class="st0">", "</span> <span class="sy4">:</span> <span class="st0">"<span class="es1">\n</span>"</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">}</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-array.html"><span class="kw1268">std::<span class="me2">array</span></span></a> a<span class="br0">{</span><span class="nu0">1</span>, <span class="nu0">2</span>, <span class="nu0">3</span>, <span class="nu0">4</span>, <span class="nu0">5</span><span class="br0">}</span><span class="sy4">;</span>
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print<span class="br0">(</span><span class="st0">"a"</span>, a<span class="br0">)</span><span class="sy4">;</span>
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<span class="co1">// 对前三个数取相反数:</span>
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std<span class="sy4">::</span><span class="me2">ranges</span><span class="sy4">::</span><span class="me2">for_each_n</span><span class="br0">(</span>a.<span class="me1">begin</span><span class="br0">(</span><span class="br0">)</span>, <span class="nu0">3</span>, <span class="br0">[</span><span class="br0">]</span><span class="br0">(</span><span class="kw4">auto</span><span class="sy3">&</span> n<span class="br0">)</span> <span class="br0">{</span> n <span class="sy2">*</span><span class="sy1">=</span> <span class="sy2">-</span><span class="nu0">1</span><span class="sy4">;</span> <span class="br0">}</span><span class="br0">)</span><span class="sy4">;</span>
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print<span class="br0">(</span><span class="st0">"a"</span>, a<span class="br0">)</span><span class="sy4">;</span>
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<a href="cpp-container-array.html"><span class="kw1268">std::<span class="me2">array</span></span></a> s<span class="br0">{</span> P<span class="br0">{</span><span class="nu0">1</span>,<span class="st0">'a'</span><span class="br0">}</span>, P<span class="br0">{</span><span class="nu0">2</span>, <span class="st0">'b'</span><span class="br0">}</span>, P<span class="br0">{</span><span class="nu0">3</span>, <span class="st0">'c'</span><span class="br0">}</span>, P<span class="br0">{</span><span class="nu0">4</span>, <span class="st0">'d'</span><span class="br0">}</span> <span class="br0">}</span><span class="sy4">;</span>
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print<span class="br0">(</span><span class="st0">"s"</span>, s<span class="br0">)</span><span class="sy4">;</span>
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<span class="co1">// 用投影对数据成员 'pair::first' 取相反数:</span>
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std<span class="sy4">::</span><span class="me2">ranges</span><span class="sy4">::</span><span class="me2">for_each_n</span><span class="br0">(</span>s.<span class="me1">begin</span><span class="br0">(</span><span class="br0">)</span>, <span class="nu0">2</span>, <span class="br0">[</span><span class="br0">]</span><span class="br0">(</span><span class="kw4">auto</span><span class="sy3">&</span> x<span class="br0">)</span> <span class="br0">{</span> x <span class="sy2">*</span><span class="sy1">=</span> <span class="sy2">-</span><span class="nu0">1</span><span class="sy4">;</span> <span class="br0">}</span>, <span class="sy3">&</span>P<span class="sy4">::</span><span class="me2">first</span><span class="br0">)</span><span class="sy4">;</span>
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print<span class="br0">(</span><span class="st0">"s"</span>, s<span class="br0">)</span><span class="sy4">;</span>
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<span class="co1">// 用投影大写化数据成员 'pair::second' :</span>
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std<span class="sy4">::</span><span class="me2">ranges</span><span class="sy4">::</span><span class="me2">for_each_n</span><span class="br0">(</span>s.<span class="me1">begin</span><span class="br0">(</span><span class="br0">)</span>, <span class="nu0">3</span>, <span class="br0">[</span><span class="br0">]</span><span class="br0">(</span><span class="kw4">auto</span><span class="sy3">&</span> c<span class="br0">)</span> <span class="br0">{</span> c <span class="sy2">-</span><span class="sy1">=</span> <span class="st0">'a'</span><span class="sy2">-</span><span class="st0">'A'</span><span class="sy4">;</span> <span class="br0">}</span>, <span class="sy3">&</span>P<span class="sy4">::</span><span class="me2">second</span><span class="br0">)</span><span class="sy4">;</span>
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print<span class="br0">(</span><span class="st0">"s"</span>, s<span class="br0">)</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">a: 1, 2, 3, 4, 5
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a: -1, -2, -3, 4, 5
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s: {1,'a'}, {2,'b'}, {3,'c'}, {4,'d'}
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s: {-1,'a'}, {-2,'b'}, {3,'c'}, {4,'d'}
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s: {-1,'A'}, {-2,'B'}, {3,'C'}, {4,'d'}</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><a href="cpp-language-range-for.html">范围 <code>for</code> 循环</a><span class="t-mark-rev t-since-cxx11">(C++11)</span></td>
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<td>执行范围上的循环</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-algorithm-ranges-for_each.html"><span class="t-lines"><span>ranges::for_each</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span></span></div>
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</div>
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</td>
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<td>应用函数到范围中的元素<br />
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<span class="t-mark">(niebloid)</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-algorithm-for_each_n.html"><span class="t-lines"><span>for_each_n</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>应用一个函数对象到序列的前 n 个元素<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-algorithm-for_each.html"><span class="t-lines"><span>for_each</span></span></a></div>
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</div>
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</td>
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<td>应用函数到范围中的元素<br />
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<span class="t-mark">(函数模板)</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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