266 lines
22 KiB
HTML
266 lines
22 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::views::transform, std::ranges::transform_view</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_ranges_transform_view 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><span style="font-size:0.7em; line-height:130%">views::</span>transform, <span style="font-size:0.7em; line-height:130%">std::ranges::</span>transform_view</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-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-ranges.html"><ranges></a></code></div>
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</td>
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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> ranges<span class="sy4">::</span><span class="me2">input_range</span> V,<br /></span>
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<p> std<span class="sy4">::</span><span class="me2">copy_constructible</span> F <span class="sy1">></span><br />
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requires ranges<span class="sy4">::</span><span class="me2">view</span><span class="sy1"><</span>V<span class="sy1">></span> <span class="sy3">&&</span><br />
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<a href="cpp-types-is_object.html"><span class="kw487">std::<span class="me2">is_object_v</span></span></a><span class="sy1"><</span>F<span class="sy1">></span> <span class="sy3">&&</span><br />
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std<span class="sy4">::</span><span class="me2">regular_invocable</span><span class="sy1"><</span>F<span class="sy3">&</span>, ranges<span class="sy4">::</span><span class="me2">range_reference_t</span><span class="sy1"><</span>V<span class="sy1">>></span> <span class="sy3">&&</span><br />
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<span class="coMULTI">/* invoke_result_t<F&, range_reference_t<V>>& 是合法类型 */</span><br /></p>
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<span class="kw1">class</span> transform_view <span class="sy4">:</span> <span class="kw1">public</span> ranges<span class="sy4">::</span><span class="me2">view_interface</span><span class="sy1"><</span>transform_view<span class="sy1"><</span>V, F<span class="sy1">>></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><span class="mw-geshi cpp source-cpp"><span class="kw1">namespace</span> views <span class="br0">{</span><br /></span>
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<p> <span class="kw1">inline</span> <span class="kw4">constexpr</span> <span class="coMULTI">/*unspecified*/</span> transform <span class="sy1">=</span> <span class="coMULTI">/*unspecified*/</span><span class="sy4">;</span><br /></p>
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<span class="br0">}</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-cxx20">(C++20 起)</span></td>
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</tr>
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<tr class="t-dcl-sep">
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<div class="t-li1"><span class="t-li">1)</span> 表示底层序列应用变换函数到每个元素后的 <a href="cpp-ranges-view.html"><code>view</code></a> 的范围适配器。</div>
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<div class="t-li1"><span class="t-li">2)</span> <a href="cpp-ranges.html#.E8.8C.83.E5.9B.B4.E9.80.82.E9.85.8D.E5.99.A8.E5.AF.B9.E8.B1.A1">范围适配器对象</a>。对每个适合的表达式 <span class="t-c"><span class="mw-geshi cpp source-cpp">e</span></span> 与 <span class="t-c"><span class="mw-geshi cpp source-cpp">f</span></span> ，表达式 <span class="t-c"><span class="mw-geshi cpp source-cpp">views<span class="sy4">::</span><span class="me2">transform</span><span class="br0">(</span>e, f<span class="br0">)</span></span></span> <i>表达式等价</i>于 <span class="t-c"><span class="mw-geshi cpp source-cpp">transform_view<span class="br0">(</span>e, f<span class="br0">)</span></span></span> 。</div>
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<p><code>transform_view</code> 分别在底层视图 <span class="t-c"><span class="mw-geshi cpp source-cpp">V</span></span> 实现 <a href="cpp-ranges-random_access_range.html"><code>random_access_range</code></a>、 <a href="cpp-ranges-bidirectional_range.html"><code>bidirectional_range</code></a>、 <a href="cpp-ranges-forward_range.html"><code>forward_range</code></a>、 <a href="cpp-ranges-input_range.html"><code>input_range</code></a>、 <a href="cpp-ranges-common_range.html"><code>common_range</code></a> 与 <a href="cpp-ranges-sized_range.html"><code>sized_range</code></a> 时实现同一概念。</p>
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<h3><span class="mw-headline" id=".E8.A1.A8.E8.BE.BE.E5.BC.8F.E7.AD.89.E4.BB.B7">表达式等价</span></h3>
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<p>表达式 <span class="t-c"><span class="mw-geshi cpp source-cpp">e</span></span> <i>表达式等价</i>于表达式 <span class="t-c"><span class="mw-geshi cpp source-cpp">f</span></span> ，若 <span class="t-c"><span class="mw-geshi cpp source-cpp">e</span></span> 与 <span class="t-c"><span class="mw-geshi cpp source-cpp">f</span></span> 拥有相同效果，均为<a href="cpp-language-noexcept_spec.html">潜在抛出</a>或均非潜在抛出（即 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">noexcept</span><span class="br0">(</span>e<span class="br0">)</span> <span class="sy1">==</span> <span class="kw1">noexcept</span><span class="br0">(</span>f<span class="br0">)</span></span></span> ），且均为<a href="cpp-language-constant_expression.html">常量子表达式</a>或均非常量子表达式。</p>
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<h3><span class="mw-headline" id=".E6.88.90.E5.91.98.E5.87.BD.E6.95.B0">成员函数</span></h3>
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<table class="t-dsc-begin">
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<div class="t-dsc-member-div t-dsc-member-nobold-div">
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<div><a href="cpp-ranges-transform_view-transform_view.html"><span class="t-lines"><span>(构造函数)</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>构造 <code>transform_view</code><br />
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<span class="t-mark">(公开成员函数)</span></td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-ranges-transform_view-base.html"><span class="t-lines"><span>base</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>返回底层（适配的）视图的副本<br />
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<span class="t-mark">(公开成员函数)</span></td>
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</tr>
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<div class="t-dsc-member-div">
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<div><a href="cpp-ranges-transform_view-begin.html"><span class="t-lines"><span>begin</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>返回指向起始的迭代器<br />
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<span class="t-mark">(公开成员函数)</span></td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-ranges-transform_view-end.html"><span class="t-lines"><span>end</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>返回指向末尾的迭代器或哨位<br />
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<span class="t-mark">(公开成员函数)</span></td>
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</tr>
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<tr class="t-dsc">
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<div class="t-dsc-member-div">
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<div><a href="cpp-ranges-transform_view-size.html"><span class="t-lines"><span>size</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>返回元素数，只要底层（适配的）范围满足 <a href="cpp-ranges-sized_range.html"><code>sized_range</code></a><br />
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<span class="t-mark">(公开成员函数)</span></td>
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</tr>
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<tr>
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<td colspan="2">
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<h5><span class="mw-headline" id=".E7.BB.A7.E6.89.BF.E8.87.AA_std::ranges::view_interface">继承自 <span class="t-lc">std::ranges::view_interface</span></span></h5>
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<div class="t-dsc-member-div">
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<div><a href="cpp-ranges-view_interface-empty.html"><span class="t-lines"><span>empty</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>返回视图是否为空。若视图满足 <a href="cpp-ranges-forward_range.html"><code>forward_range</code></a> 则提供。<br />
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<span class="t-mark">(<code>std::ranges::view_interface<D></code> 的公开成员函数)</span></td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-ranges-view_interface-operator_bool.html"><span class="t-lines"><span>operator bool</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>返回派生视图是否为非空。若 <span class="t-lc">ranges::empty</span> 可应用于它则提供。<br />
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<span class="t-mark">(<code>std::ranges::view_interface<D></code> 的公开成员函数)</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-ranges-view_interface-front.html"><span class="t-lines"><span>front</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>返回派生视图中的首元素。若视图满足 <a href="cpp-ranges-forward_range.html"><code>forward_range</code></a> 则提供。<br />
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<span class="t-mark">(<code>std::ranges::view_interface<D></code> 的公开成员函数)</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-ranges-view_interface-back.html"><span class="t-lines"><span>back</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>返回派生视图中的末元素。若视图满足 <a href="cpp-ranges-bidirectional_range.html"><code>bidirectional_range</code></a> 与 <a href="cpp-ranges-common_range.html"><code>common_range</code></a> 则提供。<br />
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<span class="t-mark">(<code>std::ranges::view_interface<D></code> 的公开成员函数)</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-ranges-view_interface-operator_at.html"><span class="t-lines"><span>operator[]</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>返回派生视图中的第 n 个元素。若视图满足 <a href="cpp-ranges-random_access_range.html"><code>random_access_range</code></a> 则提供。<br />
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<span class="t-mark">(<code>std::ranges::view_interface<D></code> 的公开成员函数)</span></td>
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</tr>
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</table>
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<h3><span class="mw-headline" id=".E6.8E.A8.E5.AF.BC.E6.8C.87.E5.BC.95"><a href="cpp-ranges-transform_view-deduction_guides.html">推导指引</a></span></h3>
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<h3><span class="mw-headline" id=".E5.B5.8C.E5.A5.97.E7.B1.BB">嵌套类</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-ranges-transform_view-iterator.html"><i><span class="t-lines"><span>iterator</span></span></i></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>迭代器类型<br />
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<span class="t-mark">(仅用于阐释的成员类模板)</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-ranges-transform_view-sentinel.html"><i><span class="t-lines"><span>sentinel</span></span></i></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>哨位类型<br />
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<span class="t-mark">(仅用于阐释的成员类模板)</span></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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<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 <array></span>
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<span class="co2">#include <cstdio></span>
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<span class="co2">#include <ranges></span>
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<span class="co2">#include <string></span>
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<span class="kw1">class</span> Transcoder <span class="br0">{</span>
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<span class="kw4">static</span> <span class="kw4">constexpr</span> <a href="cpp-container-array.html"><span class="kw1268">std::<span class="me2">array</span></span></a> r<span class="br0">{</span>
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<span class="nu12">0x02</span>,<span class="sy2">-</span><span class="nu12">0x02</span>,<span class="sy2">-</span><span class="nu12">0x42</span>, <span class="nu12">0x05</span>, <span class="nu12">0x04</span>, <span class="nu12">0x05</span>, <span class="nu12">0x04</span>,<span class="sy2">-</span><span class="nu12">0x02</span>,
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<span class="nu12">0x00</span>,<span class="sy2">-</span><span class="nu12">0x0a</span>, <span class="nu12">0x06</span>,<span class="sy2">-</span><span class="nu12">0x04</span>, <span class="nu12">0x00</span>, <span class="nu12">0x0c</span>, <span class="nu12">0x03</span>,<span class="sy2">-</span><span class="nu12">0x06</span>,
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<span class="br0">}</span><span class="sy4">;</span>
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decltype<span class="br0">(</span>r.<span class="me1">size</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span> p<span class="br0">{</span><span class="br0">}</span>, q<span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>
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<span class="kw1">public</span><span class="sy4">:</span>
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<span class="kw4">char</span> operator<span class="br0">(</span><span class="br0">)</span> <span class="br0">(</span><span class="kw4">int</span> x<span class="br0">)</span> <span class="br0">{</span>
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<span class="sy3">!</span> <span class="br0">(</span>p<span class="sy3">|</span>q<span class="br0">)</span>
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<span class="sy4">?</span> <span class="br0">(</span><span class="nu0">99</span> <span class="sy1">==</span> x <span class="sy4">?</span> x <span class="sy2">+</span><span class="sy1">=</span> r<span class="br0">[</span>p<span class="sy2">++</span><span class="br0">]</span> <span class="sy4">:</span> x <span class="sy2">-</span><span class="sy1">=</span> r<span class="br0">[</span>q<span class="sy2">++</span><span class="br0">]</span><span class="br0">)</span>
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<span class="sy4">:</span> <span class="br0">(</span><span class="nu0">0</span> <span class="sy1"><</span> p <span class="sy3">&&</span> p <span class="sy1"><</span> r.<span class="me1">size</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span> <span class="sy4">?</span> x <span class="sy2">+</span><span class="sy1">=</span> r<span class="br0">[</span>p<span class="sy2">++</span><span class="br0">]</span>
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<span class="sy4">:</span> <span class="br0">(</span><span class="nu0">0</span> <span class="sy1"><</span> q <span class="sy3">&&</span> q <span class="sy1"><</span> r.<span class="me1">size</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span> <span class="sy4">?</span> x <span class="sy2">-</span><span class="sy1">=</span> r<span class="br0">[</span>q<span class="sy2">++</span><span class="br0">]</span>
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<span class="sy4">:</span> <span class="br0">(</span><span class="nu0">0</span><span class="br0">)</span>
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<span class="sy4">;</span>
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<span class="kw1">return</span> x<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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<span class="kw4">auto</span> show <span class="sy1">=</span> <span class="br0">[</span><span class="br0">]</span><span class="br0">(</span><span class="kw4">const</span> <span class="kw4">char</span> x<span class="br0">)</span> <span class="br0">{</span> <a href="cpp-io-c-putchar.html"><span class="kw1829">std::<span class="me2">putchar</span></span></a><span class="br0">(</span>x<span class="br0">)</span><span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span>
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<a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a> in<span class="br0">{</span> <span class="st0">"cppreference.com<span class="es1">\n</span>"</span> <span class="br0">}</span><span class="sy4">;</span>
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std<span class="sy4">::</span><span class="me2">ranges</span><span class="sy4">::</span><span class="me2">for_each</span><span class="br0">(</span>in, show<span class="br0">)</span><span class="sy4">;</span>
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<a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a> out<span class="sy4">;</span>
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std<span class="sy4">::</span><span class="me2">ranges</span><span class="sy4">::</span><span class="me2">copy</span><span class="br0">(</span>
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std<span class="sy4">::</span><span class="me2">ranges</span><span class="sy4">::</span><span class="me2">views</span><span class="sy4">::</span><span class="me2">transform</span><span class="br0">(</span>in, Transcoder<span class="br0">{</span><span class="br0">}</span><span class="br0">)</span>,
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std<span class="sy4">::</span><span class="me2">ranges</span><span class="sy4">::</span><span class="me2">back_inserter</span><span class="br0">(</span>out<span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
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std<span class="sy4">::</span><span class="me2">ranges</span><span class="sy4">::</span><span class="me2">for_each</span><span class="br0">(</span>out, show<span class="br0">)</span><span class="sy4">;</span>
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<span class="kw4">auto</span> view <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">transform_view</span><span class="br0">{</span> out, Transcoder<span class="br0">{</span><span class="br0">}</span> <span class="br0">}</span><span class="sy4">;</span>
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std<span class="sy4">::</span><span class="me2">ranges</span><span class="sy4">::</span><span class="me2">for_each</span><span class="br0">(</span>view, show<span class="br0">)</span><span class="sy4">;</span>
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std<span class="sy4">::</span><span class="me2">ranges</span><span class="sy4">::</span><span class="me2">for_each</span><span class="br0">(</span>view, show<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">cppreference.com
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en.wikipedia.org
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cppreference.com
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en.wikipedia.org</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-ranges-transform.html"><span class="t-lines"><span>ranges::transform</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>将一个函数应用于某一范围的各个元素<br />
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<span class="t-mark">(niebloid)</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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