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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::ranges::</span>for_each_n, <span style="font-size:0.7em; line-height:130%">std::ranges::</span>for_each_n_result</h1>
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<div id="contentSub"><span class="subpages">&lt; <a href="cpp.html">cpp</a>&lrm; | <a href="cpp-algorithm.html">algorithm</a>&lrm; | <a href="cpp-algorithm-ranges.html">ranges</a></span></div>
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<div>&#23450;&#20041;&#20110;&#22836;&#25991;&#20214; <code><a href="cpp-header-algorithm.html">&lt;algorithm&gt;</a></code></div>
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<div>&#35843;&#29992;&#31614;&#21517;</div>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1">&lt;</span> std<span class="sy4">::</span><span class="me2">input_iterator</span> I, <span class="kw1">class</span> Proj <span class="sy1">=</span> identity,<br /></span>
<p>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; std<span class="sy4">::</span><span class="me2">indirectly_unary_invocable</span><span class="sy1">&lt;</span>std<span class="sy4">::</span><span class="me2">projected</span><span class="sy1">&lt;</span>I, Proj<span class="sy1">&gt;&gt;</span> Fun <span class="sy1">&gt;</span><br />
<span class="kw4">constexpr</span> for_each_n_result<span class="sy1">&lt;</span>I, Fun<span class="sy1">&gt;</span><br /></p>
&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; for_each_n<span class="br0">(</span> I first, std<span class="sy4">::</span><span class="me2">iter_difference_t</span><span class="sy1">&lt;</span>I<span class="sy1">&gt;</span> n, Fun f, Proj proj <span class="sy1">=</span> <span class="br0">{</span><span class="br0">}</span><span class="br0">)</span><span class="sy4">;</span></div>
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<td>(1)</td>
<td><span class="t-mark-rev t-since-cxx20">(C++20 &#36215;)</span></td>
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<div>&#36741;&#21161;&#31867;&#22411;</div>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1">&lt;</span> <span class="kw1">class</span> I, <span class="kw1">class</span> F <span class="sy1">&gt;</span><br />
<span class="kw1">using</span> for_each_n_result <span class="sy1">=</span> ranges<span class="sy4">::</span><span class="me2">in_fun_result</span><span class="sy1">&lt;</span>I, F<span class="sy1">&gt;</span><span class="sy4">;</span></span></div>
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<td>(2)</td>
<td><span class="t-mark-rev t-since-cxx20">(C++20 &#36215;)</span></td>
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<div class="t-li1"><span class="t-li">1)</span> &#25353;&#39034;&#24207;&#24212;&#29992;&#32473;&#23450;&#30340;&#20989;&#25968;&#23545;&#35937; <code>f</code> &#21040;&#29992; <code>proj</code> &#25237;&#24433;&#30340;&#35299;&#24341;&#29992;&#33539;&#22260; <code>[first, first + n)</code> &#20013;&#30340;&#27599;&#20010;&#36845;&#20195;&#22120;&#30340;&#32467;&#26524;&#12290;</div>
<p>&#33509;&#36845;&#20195;&#22120;&#31867;&#22411;&#21487;&#21464;&#65292;&#21017; <code>f</code> &#21487;&#33021;&#36890;&#36807;&#35299;&#24341;&#29992;&#30340;&#36845;&#20195;&#22120;&#20462;&#25913;&#33539;&#22260;&#20013;&#30340;&#21248;&#36895;&#12290;&#33509; <code>f</code> &#36820;&#22238;&#32467;&#26524;&#65292;&#21017;&#24573;&#30053;&#32467;&#26524;&#65292;&#33509; <code>n</code> &#23567;&#20110;&#38646;&#65292;&#21017;&#34892;&#20026;&#26410;&#23450;&#20041;&#12290; &#27492;&#39029;&#38754;&#19978;&#25551;&#36848;&#30340;&#20223;&#20989;&#25968;&#23454;&#20307;&#26159; <i>niebloid</i> &#65292;&#21363;&#65306;</p>
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<li>&#35843;&#29992;&#23427;&#20204;&#20013;&#30340;&#20219;&#20309;&#19968;&#32773;&#26102;&#19981;&#21487;&#25351;&#23450;&#26174;&#24335;&#27169;&#26495;&#23454;&#21442;&#12290;</li>
<li>&#23427;&#20204;&#26080;&#19968;&#23545;<a href="cpp-language-adl.html">&#23454;&#21442;&#20381;&#36182;&#26597;&#25214;</a>&#21487;&#35265;&#12290;</li>
<li>&#22312;<a href="cpp-language-unqualified_lookup.html">&#36890;&#24120;&#26080;&#38480;&#23450;&#26597;&#25214;</a>&#23547;&#25214;&#20989;&#25968;&#35843;&#29992;&#36816;&#31639;&#31526;&#24038;&#20391;&#30340;&#21517;&#31216;&#25214;&#21040;&#23427;&#20204;&#20043;&#19968;&#26102;&#65292;&#23427;&#31105;&#27490;<a href="cpp-language-adl.html">&#23454;&#21442;&#20381;&#36182;&#26597;&#25214;</a>&#12290;</li>
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<p>&#23454;&#38469;&#19978;&#65292;&#23427;&#20204;&#33021;&#20197;&#20989;&#25968;&#23545;&#35937;&#65292;&#25110;&#20197;&#26576;&#20123;&#29305;&#27530;&#32534;&#35793;&#22120;&#25193;&#23637;&#23454;&#29616;&#12290;</p>
<h3><span class="mw-headline" id=".E5.8F.82.E6.95.B0">&#21442;&#25968;</span></h3>
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<td>first</td>
<td>-</td>
<td>&#20195;&#34920;&#35201;&#24212;&#29992;&#20989;&#25968;&#21040;&#30340;&#33539;&#22260;&#36215;&#22987;&#30340;&#36845;&#20195;&#22120;</td>
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<td>n</td>
<td>-</td>
<td>&#35201;&#24212;&#29992;&#20989;&#25968;&#21040;&#30340;&#20803;&#32032;&#25968;</td>
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<td>f</td>
<td>-</td>
<td>&#24212;&#29992;&#21040;&#25237;&#24433;&#21518;&#33539;&#22260; <code>[first, first + n)</code> &#30340;&#20989;&#25968;</td>
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<td>proj</td>
<td>-</td>
<td>&#24212;&#29992;&#21040;&#20803;&#32032;&#30340;&#25237;&#24433;</td>
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<h3><span class="mw-headline" id=".E8.BF.94.E5.9B.9E.E5.80.BC">&#36820;&#22238;&#20540;</span></h3>
<p>&#23545;&#35937; <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">{</span>first <span class="sy2">+</span> n, std<span class="sy4">::</span><span class="me2">move</span><span class="br0">(</span>f<span class="br0">)</span><span class="br0">}</span></span></span> &#65292;&#20854;&#20013; <code>first + n</code> &#21487;&#33021;&#27714;&#20540;&#20026; <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">ranges</span><span class="sy4">::</span><span class="me2">next</span><span class="br0">(</span>std<span class="sy4">::</span><span class="me2">move</span><span class="br0">(</span>first<span class="br0">)</span>, n<span class="br0">)</span></span></span> &#65292;&#21462;&#20915;&#20110;&#36845;&#20195;&#22120;&#31867;&#21035;&#12290;</p>
<h3><span class="mw-headline" id=".E5.A4.8D.E6.9D.82.E5.BA.A6">&#22797;&#26434;&#24230;</span></h3>
<p>&#20934;&#30830;&#24212;&#29992; <code>n</code> &#27425; <code>f</code> &#19982; <code>proj</code> &#12290;</p>
<h3><span class="mw-headline" id=".E6.B3.A8.E8.A7.A3">&#27880;&#35299;</span></h3>
<p><code>namespace::ranges</code> &#20013;&#30340;&#37325;&#36733;&#35201;&#27714; <code>Fun</code> &#23454;&#29616; <a href="cpp-concepts-copy_constructible.html"><code>copy_constructible</code></a> &#12290;</p>
<h3><span class="mw-headline" id=".E5.8F.AF.E8.83.BD.E7.9A.84.E5.AE.9E.E7.8E.B0">&#21487;&#33021;&#30340;&#23454;&#29616;</span></h3>
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<div class="cpp source-cpp">
<pre class="de1"><span class="kw1">struct</span> for_each_n_fn <span class="br0">{</span>
<span class="kw1">template</span><span class="sy1">&lt;</span>std<span class="sy4">::</span><span class="me2">input_iterator</span> I, <span class="kw1">class</span> Proj <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">identity</span>,
std<span class="sy4">::</span><span class="me2">indirectly_unary_invocable</span><span class="sy1">&lt;</span>std<span class="sy4">::</span><span class="me2">projected</span><span class="sy1">&lt;</span>I, Proj<span class="sy1">&gt;&gt;</span> Fun<span class="sy1">&gt;</span>
<span class="kw4">constexpr</span> for_each_n_result<span class="sy1">&lt;</span>I, Fun<span class="sy1">&gt;</span>
operator<span class="br0">(</span><span class="br0">)</span><span class="br0">(</span>I first, std<span class="sy4">::</span><span class="me2">iter_difference_t</span><span class="sy1">&lt;</span>I<span class="sy1">&gt;</span> n, Fun fun, Proj proj <span class="sy1">=</span> Proj<span class="br0">{</span><span class="br0">}</span><span class="br0">)</span> <span class="kw4">const</span>
<span class="br0">{</span>
<span class="kw1">for</span> <span class="br0">(</span><span class="sy4">;</span> n<span class="sy2">--</span> <span class="sy1">&gt;</span> <span class="nu0">0</span><span class="sy4">;</span> <span class="sy2">++</span>first<span class="br0">)</span> <span class="br0">{</span>
<a href="cpp-utility-functional-invoke.html"><span class="kw1058">std::<span class="me2">invoke</span></span></a><span class="br0">(</span>fun, <a href="cpp-utility-functional-invoke.html"><span class="kw1058">std::<span class="me2">invoke</span></span></a><span class="br0">(</span>proj, <span class="sy2">*</span>first<span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span>
<span class="kw1">return</span> <span class="br0">{</span>std<span class="sy4">::</span><span class="me2">move</span><span class="br0">(</span>first<span class="br0">)</span>, std<span class="sy4">::</span><span class="me2">move</span><span class="br0">(</span>fun<span class="br0">)</span><span class="br0">}</span><span class="sy4">;</span>
<span class="br0">}</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">inline</span> <span class="kw4">constexpr</span> for_each_n_fn for_each_n<span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span></pre></div>
</div>
<h3><span class="mw-headline" id=".E7.A4.BA.E4.BE.8B">&#31034;&#20363;</span></h3>
<div class="t-example">
<div class="t-example-live-link"></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;">
<div class="cpp source-cpp">
<pre class="de1"><span class="co2">#include &lt;algorithm&gt;</span>
<span class="co2">#include &lt;array&gt;</span>
<span class="co2">#include &lt;iostream&gt;</span>
<span class="co2">#include &lt;ranges&gt;</span>
<span class="co2">#include &lt;string_view&gt;</span>
<span class="kw1">struct</span> P <span class="br0">{</span>
<span class="kw4">int</span> first<span class="sy4">;</span> <span class="kw4">char</span> second<span class="sy4">;</span>
<span class="kw1">friend</span> <a href="cpp-io-basic_ostream.html"><span class="kw1729">std::<span class="me2">ostream</span></span></a><span class="sy3">&amp;</span> operator<span class="sy1">&lt;&lt;</span> <span class="br0">(</span><a href="cpp-io-basic_ostream.html"><span class="kw1729">std::<span class="me2">ostream</span></span></a><span class="sy3">&amp;</span> os, <span class="kw4">const</span> P<span class="sy3">&amp;</span> p<span class="br0">)</span> <span class="br0">{</span>
<span class="kw1">return</span> os <span class="sy1">&lt;&lt;</span> <span class="st0">&#39;{&#39;</span> <span class="sy1">&lt;&lt;</span> p.<span class="me1">first</span> <span class="sy1">&lt;&lt;</span> <span class="st0">&quot;,&#39;&quot;</span> <span class="sy1">&lt;&lt;</span> p.<span class="me1">second</span> <span class="sy1">&lt;&lt;</span> <span class="st0">&quot;&#39;}&quot;</span><span class="sy4">;</span>
<span class="br0">}</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="kw4">auto</span> print <span class="sy1">=</span> <span class="br0">[</span><span class="br0">]</span><span class="br0">(</span><a href="cpp-string-basic_string_view.html"><span class="kw1248">std::<span class="me2">string_view</span></span></a> name, <span class="kw4">auto</span> <span class="kw4">const</span><span class="sy3">&amp;</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">&lt;&lt;</span> name <span class="sy1">&lt;&lt;</span> <span class="st0">&quot;: &quot;</span><span class="sy4">;</span>
<span class="kw1">for</span> <span class="br0">(</span><span class="kw4">auto</span> n <span class="sy1">=</span> v.<span class="me1">size</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="kw4">const</span> <span class="kw4">auto</span><span class="sy3">&amp;</span> e<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">&lt;&lt;</span> e <span class="sy1">&lt;&lt;</span> <span class="br0">(</span><span class="sy2">--</span>n <span class="sy4">?</span> <span class="st0">&quot;, &quot;</span> <span class="sy4">:</span> <span class="st0">&quot;<span class="es1">\n</span>&quot;</span><span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
<a href="cpp-container-array.html"><span class="kw1268">std::<span class="me2">array</span></span></a> a<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>
print<span class="br0">(</span><span class="st0">&quot;a&quot;</span>, a<span class="br0">)</span><span class="sy4">;</span>
<span class="co1">// &#23545;&#21069;&#19977;&#20010;&#25968;&#21462;&#30456;&#21453;&#25968;&#65306;</span>
std<span class="sy4">::</span><span class="me2">ranges</span><span class="sy4">::</span><span class="me2">for_each_n</span><span class="br0">(</span>a.<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">&amp;</span> n<span class="br0">)</span> <span class="br0">{</span> n <span class="sy2">*</span><span class="sy1">=</span> <span class="sy2">-</span><span class="nu0">1</span><span class="sy4">;</span> <span class="br0">}</span><span class="br0">)</span><span class="sy4">;</span>
print<span class="br0">(</span><span class="st0">&quot;a&quot;</span>, a<span class="br0">)</span><span class="sy4">;</span>
<a href="cpp-container-array.html"><span class="kw1268">std::<span class="me2">array</span></span></a> s<span class="br0">{</span> P<span class="br0">{</span><span class="nu0">1</span>,<span class="st0">&#39;a&#39;</span><span class="br0">}</span>, P<span class="br0">{</span><span class="nu0">2</span>, <span class="st0">&#39;b&#39;</span><span class="br0">}</span>, P<span class="br0">{</span><span class="nu0">3</span>, <span class="st0">&#39;c&#39;</span><span class="br0">}</span>, P<span class="br0">{</span><span class="nu0">4</span>, <span class="st0">&#39;d&#39;</span><span class="br0">}</span> <span class="br0">}</span><span class="sy4">;</span>
print<span class="br0">(</span><span class="st0">&quot;s&quot;</span>, s<span class="br0">)</span><span class="sy4">;</span>
<span class="co1">// &#29992;&#25237;&#24433;&#23545;&#25968;&#25454;&#25104;&#21592; &#39;pair::first&#39; &#21462;&#30456;&#21453;&#25968;&#65306;</span>
std<span class="sy4">::</span><span class="me2">ranges</span><span class="sy4">::</span><span class="me2">for_each_n</span><span class="br0">(</span>s.<span class="me1">begin</span><span class="br0">(</span><span class="br0">)</span>, <span class="nu0">2</span>, <span class="br0">[</span><span class="br0">]</span><span class="br0">(</span><span class="kw4">auto</span><span class="sy3">&amp;</span> x<span class="br0">)</span> <span class="br0">{</span> x <span class="sy2">*</span><span class="sy1">=</span> <span class="sy2">-</span><span class="nu0">1</span><span class="sy4">;</span> <span class="br0">}</span>, <span class="sy3">&amp;</span>P<span class="sy4">::</span><span class="me2">first</span><span class="br0">)</span><span class="sy4">;</span>
print<span class="br0">(</span><span class="st0">&quot;s&quot;</span>, s<span class="br0">)</span><span class="sy4">;</span>
<span class="co1">// &#29992;&#25237;&#24433;&#22823;&#20889;&#21270;&#25968;&#25454;&#25104;&#21592; &#39;pair::second&#39; &#65306;</span>
std<span class="sy4">::</span><span class="me2">ranges</span><span class="sy4">::</span><span class="me2">for_each_n</span><span class="br0">(</span>s.<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">&amp;</span> c<span class="br0">)</span> <span class="br0">{</span> c <span class="sy2">-</span><span class="sy1">=</span> <span class="st0">&#39;a&#39;</span><span class="sy2">-</span><span class="st0">&#39;A&#39;</span><span class="sy4">;</span> <span class="br0">}</span>, <span class="sy3">&amp;</span>P<span class="sy4">::</span><span class="me2">second</span><span class="br0">)</span><span class="sy4">;</span>
print<span class="br0">(</span><span class="st0">&quot;s&quot;</span>, s<span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span></pre></div>
</div>
<p>&#36755;&#20986;&#65306;</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;">
<div class="text source-text">
<pre class="de1">a: 1, 2, 3, 4, 5
a: -1, -2, -3, 4, 5
s: {1,&#39;a&#39;}, {2,&#39;b&#39;}, {3,&#39;c&#39;}, {4,&#39;d&#39;}
s: {-1,&#39;a&#39;}, {-2,&#39;b&#39;}, {3,&#39;c&#39;}, {4,&#39;d&#39;}
s: {-1,&#39;A&#39;}, {-2,&#39;B&#39;}, {3,&#39;C&#39;}, {4,&#39;d&#39;}</pre></div>
</div>
</div>
<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">&#21442;&#38405;</span></h3>
<table class="t-dsc-begin">
<tr class="t-dsc">
<td><a href="cpp-language-range-for.html">&#33539;&#22260; <code>for</code> &#24490;&#29615;</a><span class="t-mark-rev t-since-cxx11">(C++11)</span></td>
<td>&#25191;&#34892;&#33539;&#22260;&#19978;&#30340;&#24490;&#29615;</td>
</tr>
<tr class="t-dsc">
<td>
<div class="t-dsc-member-div">
<div><a href="cpp-algorithm-ranges-for_each.html"><span class="t-lines"><span>ranges::for_each</span></span></a></div>
<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span></span></div>
</div>
</td>
<td>&#24212;&#29992;&#20989;&#25968;&#21040;&#33539;&#22260;&#20013;&#30340;&#20803;&#32032;<br />
<span class="t-mark">(niebloid)</span></td>
</tr>
<tr class="t-dsc">
<td>
<div class="t-dsc-member-div">
<div><a href="cpp-algorithm-for_each_n.html"><span class="t-lines"><span>for_each_n</span></span></a></div>
<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span></span></div>
</div>
</td>
<td>&#24212;&#29992;&#19968;&#20010;&#20989;&#25968;&#23545;&#35937;&#21040;&#24207;&#21015;&#30340;&#21069; n &#20010;&#20803;&#32032;<br />
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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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<td>&#24212;&#29992;&#20989;&#25968;&#21040;&#33539;&#22260;&#20013;&#30340;&#20803;&#32032;<br />
<span class="t-mark">(&#20989;&#25968;&#27169;&#26495;)</span></td>
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