204 lines
15 KiB
HTML
204 lines
15 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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<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
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<title>std::ranges::generate_n</title>
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<link rel="stylesheet" href="ext.css" />
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<link rel="stylesheet" href="site_modules.css" />
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_algorithm_ranges_generate_n skin-cppreference2 action-view cpp-navbar">
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<div id="cpp-content-base">
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<div id="content"><a id="top"></a>
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::ranges::</span>generate_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>‎ | <a href="cpp-algorithm-ranges.html">ranges</a></span></div>
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<table class="t-dcl-begin">
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<div>定义于头文件 <code><a href="cpp-header-algorithm.html"><algorithm></a></code></div>
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<td></td>
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<td></td>
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<div>调用签名</div>
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<td></td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1"><</span>std<span class="sy4">::</span><span class="me2">input_or_output_iterator</span> O, std<span class="sy4">::</span><span class="me2">copy_constructible</span> F<span class="sy1">></span><br /></span>
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<p> requires std<span class="sy4">::</span><span class="me2">invocable</span><span class="sy1"><</span>F<span class="sy3">&</span><span class="sy1">></span> <span class="sy3">&&</span> std<span class="sy4">::</span><span class="me2">indirectly_writable</span><span class="sy1"><</span>O, std<span class="sy4">::</span><span class="me2">invoke_result_t</span><span class="sy1"><</span>F<span class="sy3">&</span><span class="sy1">>></span><br /></p>
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<span class="kw4">constexpr</span> O generate_n<span class="br0">(</span> O first, std<span class="sy4">::</span><span class="me2">iter_difference_t</span><span class="sy1"><</span>O<span class="sy1">></span> n, F gen <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-since-cxx20">(C++20 起)</span></td>
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<td></td>
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<td></td>
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<td></td>
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<p>若 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="nu0">0</span> <span class="sy1"><</span> n</span></span> 则对范围 <code>[first, first + n)</code> 中的每个元素赋值<i>连续</i>调用函数对象 <code>gen</code> 的结果，否则不做任何事。</p>
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<p>此页面上描述的仿函数实体是 <i>niebloid</i> ，即：</p>
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<ul>
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<li>调用它们中的任何一者时不可指定显式模板实参。</li>
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<li>它们无一对<a href="cpp-language-adl.html">实参依赖查找</a>可见。</li>
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<li>在<a href="cpp-language-unqualified_lookup.html">通常无限定查找</a>寻找函数调用运算符左侧的名称找到它们之一时，它禁止<a href="cpp-language-adl.html">实参依赖查找</a>。</li>
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</ul>
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<p>实际上，它们能以函数对象，或以某些特殊编译器扩展实现。</p>
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<h3><span class="mw-headline" id=".E5.8F.82.E6.95.B0">参数</span></h3>
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<table class="t-par-begin">
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<td>first</td>
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<td>-</td>
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<td>要修改的元素范围起始</td>
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<tr class="t-par">
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<td>n</td>
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<td>-</td>
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<td>要修改的元素数</td>
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<tr class="t-par">
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<td>gen</td>
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<td>-</td>
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<td>生成器函数对象</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>若 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="nu0">0</span> <span class="sy1"><</span> count</span></span> 则为末元素后一位置迭代器，否则为 <code>first</code> 。</p>
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<h3><span class="mw-headline" id=".E5.A4.8D.E6.9D.82.E5.BA.A6">复杂度</span></h3>
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<p>准确调用 <span class="t-c"><span class="mw-geshi cpp source-cpp">n</span></span> 次 <code>gen()</code> 以及赋值。</p>
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<h3><span class="mw-headline" id=".E5.8F.AF.E8.83.BD.E7.9A.84.E5.AE.9E.E7.8E.B0">可能的实现</span></h3>
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<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">struct</span> generate_n_fn <span class="br0">{</span>
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<span class="kw1">template</span><span class="sy1"><</span>std<span class="sy4">::</span><span class="me2">input_or_output_iterator</span> O, std<span class="sy4">::</span><span class="me2">copy_constructible</span> F<span class="sy1">></span>
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requires std<span class="sy4">::</span><span class="me2">invocable</span><span class="sy1"><</span>F<span class="sy3">&</span><span class="sy1">></span> <span class="sy3">&&</span> std<span class="sy4">::</span><span class="me2">indirectly_writable</span><span class="sy1"><</span>O, std<span class="sy4">::</span><span class="me2">invoke_result_t</span><span class="sy1"><</span>F<span class="sy3">&</span><span class="sy1">>></span>
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<span class="kw4">constexpr</span> O operator<span class="br0">(</span><span class="br0">)</span><span class="br0">(</span> O first, std<span class="sy4">::</span><span class="me2">iter_difference_t</span><span class="sy1"><</span>O<span class="sy1">></span> n, F gen <span class="br0">)</span> <span class="kw4">const</span> <span class="br0">{</span>
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<span class="kw1">for</span> <span class="br0">(</span><span class="sy4">;</span> n<span class="sy2">--</span> <span class="sy1">></span> <span class="nu0">0</span><span class="sy4">;</span> <span class="sy2">*</span>first <span class="sy1">=</span> <a href="cpp-utility-functional-invoke.html"><span class="kw1058">std::<span class="me2">invoke</span></span></a><span class="br0">(</span>gen<span class="br0">)</span>, <span class="sy2">++</span>first<span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">return</span> first<span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="br0">}</span><span class="sy4">;</span>
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<span class="kw1">inline</span> <span class="kw4">constexpr</span> generate_n_fn generate_n<span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span></pre></div>
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<h3><span class="mw-headline" id=".E7.A4.BA.E4.BE.8B">示例</span></h3>
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<div class="t-example">
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<div class="t-example-live-link"></div>
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<div dir="ltr" class="mw-geshi" style="text-align: left;">
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<div class="cpp source-cpp">
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<pre class="de1"><span class="co2">#include <algorithm></span>
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<span class="co2">#include <array></span>
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<span class="co2">#include <iostream></span>
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<span class="co2">#include <random></span>
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<span class="co2">#include <string_view></span>
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<span class="kw4">auto</span> dice<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span>
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<span class="kw4">static</span> <a href="cpp-numeric-random-uniform_int_distribution.html"><span class="kw1502">std::<span class="me2">uniform_int_distribution</span></span></a><span class="sy1"><</span><span class="kw4">int</span><span class="sy1">></span> distr<span class="br0">{</span><span class="nu0">1</span>, <span class="nu0">6</span><span class="br0">}</span><span class="sy4">;</span>
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<span class="kw4">static</span> <a href="cpp-numeric-random-random_device.html"><span class="kw1486">std::<span class="me2">random_device</span></span></a> engine<span class="sy4">;</span>
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<span class="kw4">static</span> <a href="cpp-numeric-random-mersenne_twister_engine.html"><span class="kw1489">std::<span class="me2">mt19937</span></span></a> noise<span class="br0">{</span>engine<span class="br0">(</span><span class="br0">)</span><span class="br0">}</span><span class="sy4">;</span>
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<span class="kw1">return</span> distr<span class="br0">(</span>noise<span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="kw4">void</span> print<span class="br0">(</span><span class="kw4">const</span> <span class="kw4">auto</span><span class="sy3">&</span> v, <a href="cpp-string-basic_string_view.html"><span class="kw1248">std::<span class="me2">string_view</span></span></a> comment<span class="br0">)</span> <span class="br0">{</span>
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<span class="kw1">for</span> <span class="br0">(</span><span class="kw4">int</span> i <span class="sy4">:</span> v<span class="br0">)</span> <span class="br0">{</span> <a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> i <span class="sy1"><<</span> <span class="st0">' '</span><span class="sy4">;</span> <span class="br0">}</span>
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<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"("</span> <span class="sy1"><<</span> comment <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>
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<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
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<span class="br0">{</span>
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<a href="cpp-container-array.html"><span class="kw1268">std::<span class="me2">array</span></span></a><span class="sy1"><</span><span class="kw4">int</span>, <span class="nu0">8</span><span class="sy1">></span> v<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">generate_n</span><span class="br0">(</span>v.<span class="me1">begin</span><span class="br0">(</span><span class="br0">)</span>, v.<span class="me1">size</span><span class="br0">(</span><span class="br0">)</span>, dice<span class="br0">)</span><span class="sy4">;</span>
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print<span class="br0">(</span>v, <span class="st0">"dice"</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">generate_n</span><span class="br0">(</span>v.<span class="me1">begin</span><span class="br0">(</span><span class="br0">)</span>, v.<span class="me1">size</span><span class="br0">(</span><span class="br0">)</span>, <span class="br0">[</span>n<span class="br0">{</span><span class="nu0">0</span><span class="br0">}</span><span class="br0">]</span> <span class="br0">(</span><span class="br0">)</span> mutable <span class="br0">{</span> <span class="kw1">return</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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<span class="co1">// 与 std::iota(v.begin(), v.end(), 0); 效果相同</span>
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print<span class="br0">(</span>v, <span class="st0">"iota"</span><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>可能的输出：</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">5 5 2 2 6 6 3 5 (dice)
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0 1 2 3 4 5 6 7 (iota)</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-generate.html"><span class="t-lines"><span>ranges::generate</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>保存函数结果到一个范围中<br />
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<span class="t-mark">(niebloid)</span></td>
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</tr>
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<tr class="t-dsc">
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<div class="t-dsc-member-div">
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<div><a href="cpp-algorithm-ranges-fill.html"><span class="t-lines"><span>ranges::fill</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>将一个给定值复制赋值给一个范围内的每个元素<br />
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<span class="t-mark">(niebloid)</span></td>
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</tr>
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<tr class="t-dsc">
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<div class="t-dsc-member-div">
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<div><a href="cpp-algorithm-ranges-fill_n.html"><span class="t-lines"><span>ranges::fill_n</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>将一个值复制赋值给一定量的元素<br />
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<span class="t-mark">(niebloid)</span></td>
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</tr>
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<tr class="t-dsc">
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<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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<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-generate_n.html"><span class="t-lines"><span>generate_n</span></span></a></div>
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</div>
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</td>
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<td>将相继的函数调用结果赋值给一个范围中的 N 个元素<br />
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<span class="t-mark">(函数模板)</span></td>
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</tr>
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</table>
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</div>
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<div class="visualClear"></div>
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</div>
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</body>
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