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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::</span>identity</h1>
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<div id="contentSub"><span class="subpages">&lt; <a href="cpp.html">cpp</a>&lrm; | <a href="cpp-utility.html">utility</a>&lrm; | <a href="cpp-utility-functional.html">functional</a></span></div>
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<div>&#23450;&#20041;&#20110;&#22836;&#25991;&#20214; <code><a href="cpp-header-functional.html">&lt;functional&gt;</a></code></div>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">struct</span> identity<span class="sy4">;</span></span></div>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 &#36215;)</span></td>
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<p><code>std::identity</code> &#26159;&#20989;&#25968;&#23545;&#35937;&#31867;&#22411;&#65292;&#20854; <span class="t-c"><span class="mw-geshi cpp source-cpp">operator<span class="br0">(</span><span class="br0">)</span></span></span> &#36820;&#22238;&#20854;&#19981;&#26356;&#25913;&#30340;&#21442;&#25968;&#12290;</p>
<h3><span class="mw-headline" id=".E6.88.90.E5.91.98.E7.B1.BB.E5.9E.8B">&#25104;&#21592;&#31867;&#22411;</span></h3>
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<td><code>is_transparent</code></td>
<td>/* &#26410;&#25351;&#23450; */</td>
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<div><strong class="selflink"><span class="t-lines"><span>operator()</span></span></strong></div>
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<td>&#36820;&#22238;&#19981;&#26356;&#25913;&#30340;&#21442;&#25968;<br />
<span class="t-mark">(&#20844;&#24320;&#25104;&#21592;&#20989;&#25968;)</span></td>
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<h2><span class="mw-headline" id="std::identity::operator.28.29"><span style="font-size:0.7em; line-height:130%">std::identity::</span>operator()</span></h2>
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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> T<span class="sy1">&gt;</span><br />
<span class="kw4">constexpr</span> T<span class="sy3">&amp;&amp;</span> operator<span class="br0">(</span><span class="br0">)</span><span class="br0">(</span> T<span class="sy3">&amp;&amp;</span> t <span class="br0">)</span> <span class="kw4">const</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div>
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<p>&#36820;&#22238; <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-utility-forward.html"><span class="kw1117">std::<span class="me2">forward</span></span></a><span class="sy1">&lt;</span>T<span class="sy1">&gt;</span><span class="br0">(</span>t<span class="br0">)</span></span></span> &#12290;</p>
<h3><span class="mw-headline" id=".E5.8F.82.E6.95.B0">&#21442;&#25968;</span></h3>
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<p><span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-utility-forward.html"><span class="kw1117">std::<span class="me2">forward</span></span></a><span class="sy1">&lt;</span>T<span class="sy1">&gt;</span><span class="br0">(</span>t<span class="br0">)</span></span></span> &#12290;</p>
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<h3><span class="mw-headline" id=".E6.B3.A8.E8.A7.A3">&#27880;&#35299;</span></h3>
<p>&#25104;&#21592;&#31867;&#22411; <code>is_transparent</code> &#25351;&#31034;&#35843;&#29992;&#26041;&#65292;&#27492;&#20989;&#25968;&#23545;&#35937;&#26159;&#19968;&#20010;<i>&#36890;&#36879;</i>&#20989;&#25968;&#23545;&#35937;&#65306;&#23427;&#25509;&#21463;&#20219;&#24847;&#31867;&#22411;&#30340;&#21442;&#25968;&#24182;&#20351;&#29992;&#23436;&#32654;&#36716;&#21457;&#65292;&#36825;&#22312;&#23558;&#20989;&#25968;&#23545;&#35937;&#22312;&#22810;&#31181;&#35821;&#22659;&#20013;&#65292;&#25110;&#20197;&#21491;&#20540;&#21442;&#25968;&#20351;&#29992;&#26102;&#65292;&#36991;&#20813;&#19981;&#38656;&#35201;&#30340;&#22797;&#21046;&#21644;&#36716;&#25442;&#12290;&#29305;&#21035;&#26159;&#65292;&#35832;&#22914; <span class="t-lc"><a href="cpp-container-set-find.html">std::set::find</a></span> &#21644; <span class="t-lc"><a href="cpp-container-set-lower_bound.html">std::set::lower_bound</a></span> &#30340;&#27169;&#26495;&#20989;&#25968;&#22312;&#20854; <code>Compare</code> &#31867;&#22411;&#19978;&#20351;&#29992;&#27492;&#31867;&#22411;&#12290;</p>
<p><code>std::identity</code> &#22312;<a href="cpp-algorithm-ranges.html">&#21463;&#32422;&#26463;&#31639;&#27861;</a>&#20013;&#25285;&#24403;&#40664;&#35748;&#25237;&#24433;&#12290;&#36890;&#24120;&#19981;&#38656;&#35201;&#30452;&#25509;&#20351;&#29992;&#23427;&#12290;</p>
<h3><span class="mw-headline" id=".E7.A4.BA.E4.BE.8B">&#31034;&#20363;</span></h3>
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<pre class="de1"><span class="co2">#include &lt;algorithm&gt;</span>
<span class="co2">#include &lt;functional&gt;</span>
<span class="co2">#include &lt;iostream&gt;</span>
<span class="co2">#include &lt;ranges&gt;</span>
<span class="co2">#include &lt;string&gt;</span>
<span class="co2">#include &lt;vector&gt;</span>
<span class="kw1">struct</span> Pair <span class="br0">{</span>
<span class="kw4">int</span> n<span class="sy4">;</span> <a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a> s<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> Pair<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">&quot;{ &quot;</span> <span class="sy1">&lt;&lt;</span> p.<span class="me1">n</span> <span class="sy1">&lt;&lt;</span> <span class="st0">&quot;, &quot;</span> <span class="sy1">&lt;&lt;</span> p.<span class="me1">s</span> <span class="sy1">&lt;&lt;</span> <span class="st0">&quot; }&quot;</span><span class="sy4">;</span>
<span class="br0">}</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="co1">// &#33539;&#22260;&#25171;&#21360;&#22120;&#33021;&#25171;&#21360;&#25237;&#24433;&#65288;&#20462;&#25913;&#65289;&#21518;&#30340;&#33539;&#22260;&#20803;&#32032;&#12290;</span>
<span class="kw1">template</span> <span class="sy1">&lt;</span>std<span class="sy4">::</span><span class="me2">ranges</span><span class="sy4">::</span><span class="me2">input_range</span> R,
<span class="kw1">typename</span> Projection <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">identity</span><span class="sy1">&gt;</span> <span class="co1">//&lt;- &#27880;&#24847;&#40664;&#35748;&#25237;&#24433;</span>
<span class="kw4">void</span> print<span class="br0">(</span><a href="cpp-string-basic_string_view.html"><span class="kw1248">std::<span class="me2">string_view</span></span></a> <span class="kw4">const</span> rem, R<span class="sy3">&amp;&amp;</span> r, Projection proj <span class="sy1">=</span> <span class="br0">{</span><span class="br0">}</span><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> rem <span class="sy1">&lt;&lt;</span> <span class="st0">&quot;{ &quot;</span><span class="sy4">;</span>
std<span class="sy4">::</span><span class="me2">ranges</span><span class="sy4">::</span><span class="me2">for_each</span><span class="br0">(</span>r, <span class="br0">[</span><span class="br0">]</span><span class="br0">(</span><span class="kw4">const</span> <span class="kw4">auto</span><span class="sy3">&amp;</span> o<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> o <span class="sy1">&lt;&lt;</span> <span class="st0">&#39; &#39;</span><span class="sy4">;</span> <span class="br0">}</span>, proj<span class="br0">)</span><span class="sy4">;</span>
<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">&quot;}<span class="es1">\n</span>&quot;</span><span class="sy4">;</span>
<span class="br0">}</span>
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
<span class="kw4">const</span> <a href="cpp-container-vector.html"><span class="kw1269">std::<span class="me2">vector</span></span></a><span class="sy1">&lt;</span>Pair<span class="sy1">&gt;</span> v<span class="br0">{</span> <span class="br0">{</span><span class="nu0">1</span>, <span class="st0">&quot;one&quot;</span><span class="br0">}</span>, <span class="br0">{</span><span class="nu0">2</span>, <span class="st0">&quot;two&quot;</span><span class="br0">}</span>, <span class="br0">{</span><span class="nu0">3</span>, <span class="st0">&quot;three&quot;</span><span class="br0">}</span> <span class="br0">}</span><span class="sy4">;</span>
print<span class="br0">(</span><span class="st0">&quot;Print using std::identity as a projection: &quot;</span>, v<span class="br0">)</span><span class="sy4">;</span>
print<span class="br0">(</span><span class="st0">&quot;Project the Pair::n: &quot;</span>, v, <span class="sy3">&amp;</span>Pair<span class="sy4">::</span><span class="me2">n</span><span class="br0">)</span><span class="sy4">;</span>
print<span class="br0">(</span><span class="st0">&quot;Project the Pair::s: &quot;</span>, v, <span class="sy3">&amp;</span>Pair<span class="sy4">::</span><span class="me2">s</span><span class="br0">)</span><span class="sy4">;</span>
print<span class="br0">(</span><span class="st0">&quot;Print using custom closure as a projection: &quot;</span>, v,
<span class="br0">[</span><span class="br0">]</span><span class="br0">(</span>Pair <span class="kw4">const</span><span class="sy3">&amp;</span> p<span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> <a href="cpp-string-basic_string-to_string.html"><span class="kw1265">std::<span class="me2">to_string</span></span></a><span class="br0">(</span>p.<span class="me1">n</span><span class="br0">)</span> <span class="sy2">+</span> <span class="st0">&#39;:&#39;</span> <span class="sy2">+</span> p.<span class="me1">s</span><span class="sy4">;</span> <span class="br0">}</span><span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span></pre></div>
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<p>&#36755;&#20986;&#65306;</p>
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<pre class="de1">Print using std::identity as a projection: { { 1, one } { 2, two } { 3, three } }
Project the Pair::n: { 1 2 3 }
Project the Pair::s: { one two three }
Print using custom closure as a projection: { 1:one 2:two 3:three }</pre></div>
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<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">&#21442;&#38405;</span></h3>
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<div><a href="cpp-types-type_identity.html"><span class="t-lines"><span>type_identity</span></span></a></div>
<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span></span></div>
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<td>&#36820;&#22238;&#19981;&#26356;&#25913;&#30340;&#31867;&#22411;&#23454;&#21442;<br />
<span class="t-mark">(&#31867;&#27169;&#26495;)</span></td>
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