209 lines
17 KiB
HTML
209 lines
17 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::for_each_n</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_algorithm_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::</span>for_each_n</h1>
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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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<div>定义于头文件 <code><a href="cpp-header-algorithm.html"><algorithm></a></code></div>
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<td></td>
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<td></td>
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<td rowspan="142">(1)</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> Size, <span class="kw1">class</span> UnaryFunction <span class="sy1">></span><br />
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InputIt for_each_n<span class="br0">(</span> InputIt first, Size n, 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-cxx17">(C++17 起)</span><br />
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<span class="t-mark-rev t-until-cxx20">(C++20 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> InputIt, <span class="kw1">class</span> Size, <span class="kw1">class</span> UnaryFunction <span class="sy1">></span><br />
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<span class="kw4">constexpr</span> InputIt for_each_n<span class="br0">(</span> InputIt first, Size n, UnaryFunction f <span class="br0">)</span><span class="sy4">;</span></span></div>
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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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<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> Size, <span class="kw1">class</span> UnaryFunction2 <span class="sy1">></span><br />
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ForwardIt for_each_n<span class="br0">(</span> ExecutionPolicy<span class="sy3">&&</span> policy, ForwardIt first, Size n, 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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<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, first + n)</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, first + n)</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>对于两种重载，若迭代器是可变的，则 <code>f</code> 可以通过解引用的迭代器修改范围中的元素。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。</p>
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<p>不同于其余并行算法，不允许 <code>for_each_n</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</td>
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<td>-</td>
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<td>要应用函数的范围起始</td>
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</tr>
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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>要应用函数的元素个数</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， 应用到范围 <code>[first, first + n)</code> 中每个迭代器解引用结果<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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<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>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 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 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 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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<p><span class="t-c"><span class="mw-geshi cpp source-cpp">first <span class="sy2">+</span> n</span></span></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>正好 <code>n</code> 次应用 <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/pstl/algorithm_impl.h#L82">libstdc++</a> 与 <a rel="nofollow" class="external text" href="https://github.com/llvm-mirror/libcxx/blob/a12cb9d211019d99b5875b6d8034617cbc24c2cc/include/algorithm#L896">libc++</a> 中的实现。</p>
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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> InputIt, <span class="kw1">class</span> Size, <span class="kw1">class</span> UnaryFunction<span class="sy1">></span>
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InputIt for_each_n<span class="br0">(</span>InputIt first, Size n, 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>Size i <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span> i <span class="sy1"><</span> n<span class="sy4">;</span> <span class="sy2">++</span>first, <span class="br0">(</span><span class="kw4">void</span><span class="br0">)</span> <span class="sy2">++</span>i<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> first<span class="sy4">;</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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</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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<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 <iostream></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="kw1269">std::<span class="me2">vector</span></span></a><span class="sy1"><</span><span class="kw4">int</span><span class="sy1">></span> ns<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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<span class="kw1">for</span> <span class="br0">(</span><span class="kw4">auto</span> n<span class="sy4">:</span> ns<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> n <span class="sy1"><<</span> <span class="st0">", "</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_n</span><span class="br0">(</span>ns.<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="nu0">2</span><span class="sy4">;</span> <span class="br0">}</span><span class="br0">)</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="sy4">:</span> ns<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> n <span class="sy1"><<</span> <span class="st0">", "</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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<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, 2, 3, 4, 5,
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2, 4, 6, 4, 5,</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-algorithm-transform.html"><span class="t-lines"><span>transform</span></span></a></div>
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</div>
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</td>
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<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 />
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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><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-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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</html>
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