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<!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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<title>std::for_each</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_algorithm_for_each 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::</span>for_each</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-algorithm.html">algorithm</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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<tbody>
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<tr class="t-dsc-header">
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<td>
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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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<td></td>
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</tr>
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<tbody class="t-dcl-rev t-dcl-rev-num">
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<tr class="t-dcl-rev-aux">
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<td></td>
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<td rowspan="142">(1)</td>
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<td></td>
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</tr>
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<tr class="t-dcl t-until-cxx20">
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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> InputIt, <span class="kw1">class</span> UnaryFunction <span class="sy1">></span><br />
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UnaryFunction for_each<span class="br0">(</span> InputIt first, InputIt last, UnaryFunction f <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-until-cxx20">(C++20 前)</span></td>
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</tr>
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<tr class="t-dcl t-since-cxx20">
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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> InputIt, <span class="kw1">class</span> UnaryFunction <span class="sy1">></span><br />
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<span class="kw4">constexpr</span> UnaryFunction for_each<span class="br0">(</span> InputIt first, InputIt last, UnaryFunction f <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-cxx20">(C++20 起)</span></td>
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</tr>
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</tbody>
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<tbody>
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<tr class="t-dcl t-since-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> ExecutionPolicy, <span class="kw1">class</span> ForwardIt, <span class="kw1">class</span> UnaryFunction2 <span class="sy1">></span><br />
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<span class="kw4">void</span> for_each<span class="br0">(</span> ExecutionPolicy<span class="sy3">&&</span> policy, ForwardIt first, ForwardIt last, UnaryFunction2 f <span class="br0">)</span><span class="sy4">;</span></span></div>
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</td>
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<td>(2)</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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<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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<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> 到解引用范围 <code>[first, last)</code> 中每个迭代器的结果。</div>
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<div class="t-li1"><span class="t-li">2)</span> 应用给定的函数对象 <code>f</code> 到解引用范围 <code>[first, last)</code> 中每个迭代器的结果（不必按顺序）。按照 <code>policy</code> 执行算法。此重载仅若 <span class="t-rev-inl t-until-cxx20"><span><span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-algorithm-is_execution_policy.html"><span class="kw1564">std::<span class="me2">is_execution_policy_v</span></span></a><span class="sy1"><</span><a href="cpp-types-decay.html"><span class="kw629">std::<span class="me2">decay_t</span></span></a><span class="sy1"><</span>ExecutionPolicy<span class="sy1">>></span></span></span></span> <span><span class="t-mark-rev t-until-cxx20">(C++20 前)</span></span></span><span class="t-rev-inl t-since-cxx20"><span><span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-algorithm-is_execution_policy.html"><span class="kw1564">std::<span class="me2">is_execution_policy_v</span></span></a><span class="sy1"><</span>std<span class="sy4">::</span><span class="me2">remove_cvref_t</span><span class="sy1"><</span>ExecutionPolicy<span class="sy1">>></span></span></span></span> <span><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></span></span> 为 true 才参与重载决议。</div>
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<p>对于两个重载，若迭代器类型为可变，则 <code>f</code> 可以通过解引用后的迭代器修改范围的元素。若 <code>f</code> 返回结果，则忽略结果。</p>
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<p>不同于其余并行算法，不允许 <code>for_each</code> 复制序列中的元素，即使它们为可平凡复制。</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, last</td>
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<td>-</td>
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<td>要应用函数到的范围</td>
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</tr>
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<tr class="t-par">
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<td>policy</td>
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<td>-</td>
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<td>所用的执行策略。细节见<a href="cpp-algorithm-execution_policy_tag_t.html">执行策略</a>。</td>
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</tr>
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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>函数对象，要应用于解引用范围 <code>[first, last)</code> 中每个迭代器结果的函数<br />
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<p>函数签名应当等价于如下者：</p>
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<p><span class="t-cc"><span class="mw-geshi cpp source-cpp"> <span class="kw4">void</span> fun<span class="br0">(</span><span class="kw4">const</span> Type <span class="sy3">&</span>a<span class="br0">)</span><span class="sy4">;</span></span></span></p>
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<p>签名不必拥有 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">const</span> <span class="sy3">&</span></span></span> 。<br />
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类型 <span class="t-c"><span class="mw-geshi cpp source-cpp">Type</span></span> 必须使得 <span class="t-c"><span class="mw-geshi cpp source-cpp">InputIt</span></span> 类型对象能在解引用后隐式转换到 <span class="t-c"><span class="mw-geshi cpp source-cpp">Type</span></span> 。</p>
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<p>​</p>
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</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>
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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>InputIt</code> 必须满足<a href="cpp-named_req-InputIterator.html"><span style="font-style:italic">老式输入迭代器</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(LegacyInputIterator)</span></a> 的要求。</td>
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</tr>
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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>ForwardIt</code> 必须满足<a href="cpp-named_req-ForwardIterator.html"><span style="font-style:italic">老式向前迭代器</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(LegacyForwardIterator)</span></a> 的要求。</td>
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</tr>
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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>UnaryFunction</code> 必须满足<a href="cpp-named_req-MoveConstructible.html"><span style="font-style:italic">可移动构造</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(MoveConstructible)</span></a> 的要求。不必为 <a href="cpp-named_req-CopyConstructible.html"><span style="font-style:italic">可复制构造</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(CopyConstructible)</span></a> 。</td>
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</tr>
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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>UnaryFunction2</code> 必须满足<a href="cpp-named_req-CopyConstructible.html"><span style="font-style:italic">可复制构造</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(CopyConstructible)</span></a> 的要求。</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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<div class="t-li1"><span class="t-li">1)</span> <span class="t-rev-inl t-until-cxx11"><span><code>f</code></span> <span><span class="t-mark-rev t-until-cxx11">(C++11 前)</span></span></span> <span class="t-rev-inl t-since-cxx11"><span><span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">move</span><span class="br0">(</span>f<span class="br0">)</span></span></span></span> <span><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></span></span></div>
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<div class="t-li1"><span class="t-li">2)</span> （无）</div>
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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>last</code> - <code>first</code> 次 <code>f</code> 。</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>拥有名为 <code>ExecutionPolicy</code> 的模板形参的重载按下列方式报告错误：</p>
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<ul>
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<li>若作为算法一部分调用的函数的执行抛出异常，且 <code>ExecutionPolicy</code> 为<a href="cpp-algorithm-execution_policy_tag_t.html">标准策略</a>之一，则调用 <span class="t-lc"><a href="cpp-error-terminate.html">std::terminate</a></span> 。对于任何其他 <code>ExecutionPolicy</code> ，行为是实现定义的。</li>
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<li>若算法无法分配内存，则抛出 <span class="t-lc"><a href="cpp-memory-new-bad_alloc.html">std::bad_alloc</a></span> 。</li>
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</ul>
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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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<p>参阅 <a rel="nofollow" class="external text" href="https://github.com/gcc-mirror/gcc/blob/d9375e490072d1aae73a93949aa158fcd2a27018/libstdc%2B%2B-v3/include/bits/stl_algo.h#L3858">libstdc++</a> 、 <a rel="nofollow" class="external text" href="https://github.com/llvm-mirror/libcxx/blob/a12cb9d211019d99b5875b6d8034617cbc24c2cc/include/algorithm#L880">libc++</a> 及 <a rel="nofollow" class="external text" href="https://github.com/microsoft/STL/blob/ff83542af4b683fb2f2dea1423fd6c50fe3e13b0/stl/inc/algorithm#L229">MSVC stdlib</a> 中的实现。</p>
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<table class="eq-fun-cpp-table">
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<tr>
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<td>
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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> InputIt, <span class="kw1">class</span> UnaryFunction<span class="sy1">></span>
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UnaryFunction for_each<span class="br0">(</span>InputIt first, InputIt last, UnaryFunction f<span class="br0">)</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> first <span class="sy3">!</span><span class="sy1">=</span> last<span class="sy4">;</span> <span class="sy2">++</span>first<span class="br0">)</span> <span class="br0">{</span>
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f<span class="br0">(</span><span class="sy2">*</span>first<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> f<span class="sy4">;</span> <span class="co1">// C++11 起隐式移动</span>
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<span class="br0">}</span></pre></div>
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</div>
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</td>
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</tr>
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</table>
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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 <a href="cpp-language-lambda.html">lambda 函数</a>自增 vector 的所有元素，然后用函数对象中的重载 <code>operator()</code> 计算其和。注意推荐用更适合的算法 <span class="t-lc"><a href="cpp-algorithm-accumulate.html">std::accumulate</a></span> 计算和。</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 <vector></span>
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<span class="co2">#include <algorithm></span>
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<span class="co2">#include <iostream></span>
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<span class="kw1">struct</span> Sum
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<span class="br0">{</span>
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<span class="kw4">void</span> operator<span class="br0">(</span><span class="br0">)</span><span class="br0">(</span><span class="kw4">int</span> n<span class="br0">)</span> <span class="br0">{</span> sum <span class="sy2">+</span><span class="sy1">=</span> n<span class="sy4">;</span> <span class="br0">}</span>
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<span class="kw4">int</span> sum<span class="br0">{</span><span class="nu0">0</span><span class="br0">}</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-vector.html"><span class="kw1269">std::<span class="me2">vector</span></span></a><span class="sy1"><</span><span class="kw4">int</span><span class="sy1">></span> nums<span class="br0">{</span><span class="nu0">3</span>, <span class="nu0">4</span>, <span class="nu0">2</span>, <span class="nu0">8</span>, <span class="nu0">15</span>, <span class="nu0">267</span><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><span class="kw4">const</span> <span class="kw4">int</span><span class="sy3">&</span> n<span class="br0">)</span> <span class="br0">{</span> <a href="cpp-io-cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">" "</span> <span class="sy1"><<</span> n<span class="sy4">;</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="st0">"before:"</span><span class="sy4">;</span>
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std<span class="sy4">::</span><span class="me2">for_each</span><span class="br0">(</span>nums.<span class="me1">cbegin</span><span class="br0">(</span><span class="br0">)</span>, nums.<span class="me1">cend</span><span class="br0">(</span><span class="br0">)</span>, print<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="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
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||||
std<span class="sy4">::</span><span class="me2">for_each</span><span class="br0">(</span>nums.<span class="me1">begin</span><span class="br0">(</span><span class="br0">)</span>, nums.<span class="me1">end</span><span class="br0">(</span><span class="br0">)</span>, <span class="br0">[</span><span class="br0">]</span><span class="br0">(</span><span class="kw4">int</span> <span class="sy3">&</span>n<span class="br0">)</span><span class="br0">{</span> n<span class="sy2">++</span><span class="sy4">;</span> <span class="br0">}</span><span class="br0">)</span><span class="sy4">;</span>
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||||
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||||
<span class="co1">// 对每个数调用 Sum::operator()</span>
|
||||
Sum s <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">for_each</span><span class="br0">(</span>nums.<span class="me1">begin</span><span class="br0">(</span><span class="br0">)</span>, nums.<span class="me1">end</span><span class="br0">(</span><span class="br0">)</span>, Sum<span class="br0">(</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="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"after: "</span><span class="sy4">;</span>
|
||||
std<span class="sy4">::</span><span class="me2">for_each</span><span class="br0">(</span>nums.<span class="me1">cbegin</span><span class="br0">(</span><span class="br0">)</span>, nums.<span class="me1">cend</span><span class="br0">(</span><span class="br0">)</span>, print<span class="br0">)</span><span class="sy4">;</span>
|
||||
<a href="cpp-io-cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
|
||||
<a href="cpp-io-cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"sum: "</span> <span class="sy1"><<</span> s.<span class="me1">sum</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
|
||||
<span class="br0">}</span></pre></div>
|
||||
</div>
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||||
<p>输出：</p>
|
||||
<div dir="ltr" class="mw-geshi" style="text-align: left;">
|
||||
<div class="text source-text">
|
||||
<pre class="de1">before: 3 4 2 8 15 267
|
||||
after: 4 5 3 9 16 268
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||||
sum: 305</pre></div>
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||||
</div>
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||||
</div>
|
||||
<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">参阅</span></h3>
|
||||
<table class="t-dsc-begin">
|
||||
<tr class="t-dsc">
|
||||
<td>
|
||||
<div class="t-dsc-member-div">
|
||||
<div><a href="cpp-algorithm-transform.html"><span class="t-lines"><span>transform</span></span></a></div>
|
||||
</div>
|
||||
</td>
|
||||
<td>将一个函数应用于某一范围的各个元素，并在目标范围存储结果<br />
|
||||
<span class="t-mark">(函数模板)</span></td>
|
||||
</tr>
|
||||
<tr class="t-dsc">
|
||||
<td>
|
||||
<div class="t-dsc-member-div">
|
||||
<div><a href="cpp-algorithm-for_each_n.html"><span class="t-lines"><span>for_each_n</span></span></a></div>
|
||||
<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span></span></div>
|
||||
</div>
|
||||
</td>
|
||||
<td>应用一个函数对象到序列的前 n 个元素<br />
|
||||
<span class="t-mark">(函数模板)</span></td>
|
||||
</tr>
|
||||
<tr class="t-dsc">
|
||||
<td>
|
||||
<div class="t-dsc-member-div">
|
||||
<div><a href="cpp-algorithm-ranges-for_each.html"><span class="t-lines"><span>ranges::for_each</span></span></a></div>
|
||||
<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span></span></div>
|
||||
</div>
|
||||
</td>
|
||||
<td>应用函数到范围中的元素<br />
|
||||
<span class="t-mark">(niebloid)</span></td>
|
||||
</tr>
|
||||
<tr class="t-dsc">
|
||||
<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>
|
||||
<td>执行范围上的循环</td>
|
||||
</tr>
|
||||
</table>
|
||||
</div>
|
||||
<div class="visualClear"></div>
|
||||
</div>
|
||||
</div>
|
||||
</div>
|
||||
</body>
|
||||
</html>
|
||||
Reference in New Issue
Block a user