302 lines
26 KiB
HTML
302 lines
26 KiB
HTML
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<title>std::transform</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_algorithm_transform skin-cppreference2 action-view cpp-navbar">
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<div id="cpp-content-base">
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::</span>transform</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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<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> OutputIt, <span class="kw1">class</span> UnaryOperation <span class="sy1">></span><br /></span>
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<p>OutputIt transform<span class="br0">(</span> InputIt first1, InputIt last1, OutputIt d_first,<br /></p>
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UnaryOperation unary_op <span class="br0">)</span><span class="sy4">;</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 class="t-dcl t-since-cxx20">
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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> OutputIt, <span class="kw1">class</span> UnaryOperation <span class="sy1">></span><br /></span>
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<p><span class="kw4">constexpr</span> OutputIt transform<span class="br0">(</span> InputIt first1, InputIt last1, OutputIt d_first,<br /></p>
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UnaryOperation unary_op <span class="br0">)</span><span class="sy4">;</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 class="t-dcl t-since-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> ExecutionPolicy, <span class="kw1">class</span> ForwardIt1, <span class="kw1">class</span> ForwardIt2, <span class="kw1">class</span> UnaryOperation <span class="sy1">></span><br /></span>
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<p>ForwardIt2 transform<span class="br0">(</span> ExecutionPolicy<span class="sy3">&&</span> policy, ForwardIt1 first1, ForwardIt1 last1,<br /></p>
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ForwardIt2 d_first, UnaryOperation unary_op <span class="br0">)</span><span class="sy4">;</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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<tbody class="t-dcl-rev t-dcl-rev-num">
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<td></td>
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<td rowspan="142">(3)</td>
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<td></td>
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<tr class="t-dcl t-until-cxx20">
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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> InputIt1, <span class="kw1">class</span> InputIt2, <span class="kw1">class</span> OutputIt, <span class="kw1">class</span> BinaryOperation <span class="sy1">></span><br /></span>
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<p>OutputIt transform<span class="br0">(</span> InputIt1 first1, InputIt1 last1, InputIt2 first2,<br /></p>
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OutputIt d_first, BinaryOperation binary_op <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-until-cxx20">(C++20 前)</span></td>
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<tr class="t-dcl t-since-cxx20">
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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> InputIt1, <span class="kw1">class</span> InputIt2, <span class="kw1">class</span> OutputIt, <span class="kw1">class</span> BinaryOperation <span class="sy1">></span><br /></span>
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<p><span class="kw4">constexpr</span> OutputIt transform<span class="br0">(</span> InputIt1 first1, InputIt1 last1, InputIt2 first2,<br /></p>
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OutputIt d_first, BinaryOperation binary_op <span class="br0">)</span><span class="sy4">;</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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</tbody>
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<tbody>
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<tr class="t-dcl t-since-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> ExecutionPolicy, <span class="kw1">class</span> ForwardIt1, <span class="kw1">class</span> ForwardIt2, <span class="kw1">class</span> ForwardIt3, <span class="kw1">class</span> BinaryOperation <span class="sy1">></span><br /></span>
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<p>ForwardIt3 transform<span class="br0">(</span> ExecutionPolicy<span class="sy3">&&</span> policy, ForwardIt1 first1, ForwardIt1 last1,<br /></p>
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ForwardIt2 first2, ForwardIt3 d_first, BinaryOperation binary_op <span class="br0">)</span><span class="sy4">;</span></div>
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</td>
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<td>(4)</td>
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<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></td>
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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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<p><code>std::transform</code> 应用给定的函数到范围并存储结果于始于 <code>d_first</code> 的另一范围。</p>
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<div class="t-li1"><span class="t-li">1)</span> 应用一元函数 <code>unary_op</code> 到 <code>[first1, last1)</code> 所定义的范围。</div>
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<div class="t-li1"><span class="t-li">3)</span> 应用二元函数 <code>binary_op</code> 到来自二个范围的元素对：一个以 <code>[first1, last1)</code> 定义，而另一个始于 <code>first2</code> 。</div>
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<div class="t-li1"><span class="t-li">2,4)</span> 同 <span class="t-v">(1,3)</span> ，但按照 <code>policy</code> 执行。这些重载仅若 <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。
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<p><code>unary_op</code> 和 <code>binary_op</code> 必须无副作用。</p>
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<td><span class="t-mark-rev t-until-cxx11">(C++11 前)</span></td>
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<tr class="t-rev t-since-cxx11">
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<p><code>unary_op</code> 和 <code>binary_op</code> 必须不非法化所涉及范围的任何迭代器，包含尾迭代器，或修改该范围的任何元素。</p>
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</td>
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<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
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</table>
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</div>
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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>first1, last1</td>
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<td>-</td>
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<td>要变换的第一元素范围</td>
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</tr>
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<tr class="t-par">
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<td>first2</td>
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<td>-</td>
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<td>要变换的第二元素范围的起始</td>
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</tr>
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<tr class="t-par">
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<td>d_first</td>
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<td>-</td>
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<td>目标范围的起始，可以等于 <code>first1</code> 或 <code>first2</code></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>unary_op</td>
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<td>-</td>
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<td>将要应用的一元算符函数。<br />
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函数签名应等价于如下者：
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<p><span class="t-cc"><span class="mw-geshi cpp source-cpp"> Ret 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> 。 类型 <span class="t-c"><span class="mw-geshi cpp source-cpp">Ret</span></span> 必须使得 <span class="t-c"><span class="mw-geshi cpp source-cpp">OutputIt</span></span> 类型对象能被解引用并能被赋 <span class="t-c"><span class="mw-geshi cpp source-cpp">Ret</span></span> 类型值。 ​</p>
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</td>
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</tr>
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<tr class="t-par">
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<td>binary_op</td>
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<td>-</td>
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<td>被使用的二元函数对象。<br />
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<p>该函数的签名应当等价于：</p>
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<p><span class="t-cc"><span class="mw-geshi cpp source-cpp"> Ret fun<span class="br0">(</span><span class="kw4">const</span> Type1 <span class="sy3">&</span>a, <span class="kw4">const</span> Type2 <span class="sy3">&</span>b<span class="br0">)</span><span class="sy4">;</span></span></span></p>
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<p>签名中并不需要有 <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">Type1</span></span> 与 <span class="t-c"><span class="mw-geshi cpp source-cpp">Type2</span></span> 必须使得 <span class="t-c"><span class="mw-geshi cpp source-cpp">InputIt1</span></span> 与 <span class="t-c"><span class="mw-geshi cpp source-cpp">InputIt2</span></span> 类型的对象在解引用后分别能隐式转换到 <span class="t-c"><span class="mw-geshi cpp source-cpp">Type1</span></span> 与 <span class="t-c"><span class="mw-geshi cpp source-cpp">Type2</span></span> 。 类型 <span class="t-c"><span class="mw-geshi cpp source-cpp">Ret</span></span> 必须使得 <span class="t-c"><span class="mw-geshi cpp source-cpp">OutputIt</span></span> 类型对象能被解引用并能被赋 <span class="t-c"><span class="mw-geshi cpp source-cpp">Ret</span></span> 类型值。 ​</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, InputIt1, InputIt2</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>OutputIt</code> 必须满足<a href="cpp-named_req-OutputIterator.html"><span style="font-style:italic">老式输出迭代器</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(LegacyOutputIterator)</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>ForwardIt1, ForwardIt2, ForwardIt3</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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</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>指向最后一个变换的元素的输出迭代器。</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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<div class="t-li1"><span class="t-li">1-2)</span> 准确应用 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-iterator-distance.html"><span class="kw673">std::<span class="me2">distance</span></span></a><span class="br0">(</span>first1, last1<span class="br0">)</span></span></span> 次 <code>unary_op</code> 。</div>
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<div class="t-li1"><span class="t-li">3-4)</span> 准确应用 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-iterator-distance.html"><span class="kw673">std::<span class="me2">distance</span></span></a><span class="br0">(</span>first1, last1<span class="br0">)</span></span></span> 次 <code>binary_op</code> 。</div>
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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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<table class="eq-fun-cpp-table">
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<tr>
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<th>版本一</th>
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</tr>
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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> OutputIt, <span class="kw1">class</span> UnaryOperation<span class="sy1">></span>
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OutputIt transform<span class="br0">(</span>InputIt first1, InputIt last1, OutputIt d_first,
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UnaryOperation unary_op<span class="br0">)</span>
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<span class="br0">{</span>
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<span class="kw1">while</span> <span class="br0">(</span>first1 <span class="sy3">!</span><span class="sy1">=</span> last1<span class="br0">)</span> <span class="br0">{</span>
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<span class="sy2">*</span>d_first<span class="sy2">++</span> <span class="sy1">=</span> unary_op<span class="br0">(</span><span class="sy2">*</span>first1<span class="sy2">++</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> d_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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</tr>
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<tr>
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<th>版本二</th>
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</tr>
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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> InputIt1, <span class="kw1">class</span> InputIt2,
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<span class="kw1">class</span> OutputIt, <span class="kw1">class</span> BinaryOperation<span class="sy1">></span>
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OutputIt transform<span class="br0">(</span>InputIt1 first1, InputIt1 last1, InputIt2 first2,
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OutputIt d_first, BinaryOperation binary_op<span class="br0">)</span>
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<span class="br0">{</span>
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<span class="kw1">while</span> <span class="br0">(</span>first1 <span class="sy3">!</span><span class="sy1">=</span> last1<span class="br0">)</span> <span class="br0">{</span>
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<span class="sy2">*</span>d_first<span class="sy2">++</span> <span class="sy1">=</span> binary_op<span class="br0">(</span><span class="sy2">*</span>first1<span class="sy2">++</span>, <span class="sy2">*</span>first2<span class="sy2">++</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> d_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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</tr>
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</table>
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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>std::transform</code> 不保证按顺序应用 <code>unary_op</code> 或 <code>binary_op</code> 。为按顺序应用函数到数列，或应用修改序列元素的函数，应使用 <span class="t-lc"><a href="cpp-algorithm-for_each.html">std::for_each</a></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>transform</code> ，以 <code>toupper</code> 函数原地转换字符串为大写，然后转换每个 <code>char</code> 为其序数值：</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 <algorithm></span>
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<span class="co2">#include <cctype></span>
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<span class="co2">#include <iostream></span>
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<span class="co2">#include <string></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-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a> s<span class="br0">(</span><span class="st0">"hello"</span><span class="br0">)</span><span class="sy4">;</span>
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std<span class="sy4">::</span><span class="me2">transform</span><span class="br0">(</span>s.<span class="me1">begin</span><span class="br0">(</span><span class="br0">)</span>, s.<span class="me1">end</span><span class="br0">(</span><span class="br0">)</span>, s.<span class="me1">begin</span><span class="br0">(</span><span class="br0">)</span>,
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<span class="br0">[</span><span class="br0">]</span><span class="br0">(</span><span class="kw4">unsigned</span> <span class="kw4">char</span> c<span class="br0">)</span> <span class="sy2">-</span><span class="sy1">></span> <span class="kw4">unsigned</span> <span class="kw4">char</span> <span class="br0">{</span> <span class="kw1">return</span> std<span class="sy4">::</span><span class="me2">toupper</span><span class="br0">(</span>c<span class="br0">)</span><span class="sy4">;</span> <span class="br0">}</span><span class="br0">)</span><span class="sy4">;</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><a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a><span class="sy1">></span> ordinals<span class="sy4">;</span>
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std<span class="sy4">::</span><span class="me2">transform</span><span class="br0">(</span>s.<span class="me1">begin</span><span class="br0">(</span><span class="br0">)</span>, s.<span class="me1">end</span><span class="br0">(</span><span class="br0">)</span>, <a href="cpp-iterator-back_inserter.html"><span class="kw665">std::<span class="me2">back_inserter</span></span></a><span class="br0">(</span>ordinals<span class="br0">)</span>,
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<span class="br0">[</span><span class="br0">]</span><span class="br0">(</span><span class="kw4">unsigned</span> <span class="kw4">char</span> c<span class="br0">)</span> <span class="sy2">-</span><span class="sy1">></span> <a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> <span class="br0">{</span> <span class="kw1">return</span> c<span class="sy4">;</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> s <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> ord <span class="sy4">:</span> ordinals<span class="br0">)</span> <span class="br0">{</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> <span class="sy1"><<</span> ord<span class="sy4">;</span>
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<span class="br0">}</span>
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std<span class="sy4">::</span><span class="me2">transform</span><span class="br0">(</span>ordinals.<span class="me1">cbegin</span><span class="br0">(</span><span class="br0">)</span>, ordinals.<span class="me1">cend</span><span class="br0">(</span><span class="br0">)</span>, ordinals.<span class="me1">cbegin</span><span class="br0">(</span><span class="br0">)</span>,
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ordinals.<span class="me1">begin</span><span class="br0">(</span><span class="br0">)</span>, <a href="cpp-utility-functional-plus.html"><span class="kw1029">std::<span class="me2">plus</span></span></a><span class="sy1"><></span><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">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
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<span class="kw1">for</span> <span class="br0">(</span><span class="kw4">auto</span> ord <span class="sy4">:</span> ordinals<span class="br0">)</span> <span class="br0">{</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> ord <span class="sy1"><<</span> <span class="st0">' '</span><span class="sy4">;</span>
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<span class="br0">}</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">HELLO: 72 69 76 76 79
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144 138 152 152 158</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-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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