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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::ranges::</span>empty</h1>
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<div id="contentSub"><span class="subpages">&lt; <a href="cpp.html">cpp</a>&lrm; | <a href="cpp-ranges.html">ranges</a></span></div>
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<div>&#23450;&#20041;&#20110;&#22836;&#25991;&#20214; <code><a href="cpp-header-ranges.html">&lt;ranges&gt;</a></code></div>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">inline</span> <span class="kw1">namespace</span> <span class="coMULTI">/*unspecified*/</span> <span class="br0">{</span><br /></span>
<p>&nbsp; &nbsp; <span class="kw1">inline</span> <span class="kw4">constexpr</span> <span class="kw4">auto</span> empty <span class="sy1">=</span> <span class="coMULTI">/*unspecified*/</span><span class="sy4">;</span><br /></p>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 &#36215;)</span><br />
<span class="t-mark">(&#23450;&#21046;&#28857;&#23545;&#35937;)</span></td>
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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> <span class="kw1">class</span> T <span class="sy1">&gt;</span><br /></span>
<p>&nbsp; &nbsp; requires <span class="coMULTI">/* see below */</span><br /></p>
<span class="kw4">constexpr</span> <span class="kw4">bool</span> empty<span class="br0">(</span>T<span class="sy3">&amp;&amp;</span> t<span class="br0">)</span><span class="sy4">;</span></div>
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<p>&#30830;&#23450; <code>t</code> &#26159;&#21542;&#25317;&#26377;&#20219;&#20309;&#20803;&#32032;&#12290;</p>
<p>&#20196; <code>t</code> &#20026; <code>T</code> &#31867;&#22411;&#23545;&#35937;&#12290;&#23545; <code>ranges::empty</code> &#30340;&#35843;&#29992;&#34920;&#36798;&#24335;&#31561;&#20215;&#20110;&#65306;</p>
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<li><span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span><span class="br0">(</span><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 class="me1">empty</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span></span></span> &#65292;&#33509;&#35813;&#34920;&#36798;&#24335;&#21512;&#27861;&#12290;</li>
<li>&#21542;&#21017;&#20026; <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">(</span>ranges<span class="sy4">::</span><span class="me2">size</span><span class="br0">(</span><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 class="br0">)</span> <span class="sy1">==</span> <span class="nu0">0</span><span class="br0">)</span></span></span> &#65292;&#33509;&#35813;&#34920;&#36798;&#24335;&#21512;&#27861;&#12290;</li>
<li>&#21542;&#21017;&#20026; <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span><span class="br0">(</span>ranges<span class="sy4">::</span><span class="me2">begin</span><span class="br0">(</span>t<span class="br0">)</span> <span class="sy1">==</span> ranges<span class="sy4">::</span><span class="me2">end</span><span class="br0">(</span>t<span class="br0">)</span><span class="br0">)</span></span></span> &#12290;</li>
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<p>&#25152;&#26377;&#20854;&#20182;&#24773;&#20917;&#19979;&#65292;&#23545; <code>ranges::empty</code> &#30340;&#35843;&#29992;&#38750;&#33391;&#26500;&#65292;&#36825;&#33021;&#22312; <span class="t-c"><span class="mw-geshi cpp source-cpp">ranges<span class="sy4">::</span><span class="me2">empty</span><span class="br0">(</span>t<span class="br0">)</span></span></span> &#20986;&#29616;&#20110;&#27169;&#26495;&#23454;&#20363;&#21270;&#30340;&#31435;&#21363;&#35821;&#22659;&#20013;&#26102;&#23548;&#33268;<a href="cpp-language-sfinae.html">&#26367;&#25442;&#22833;&#36133;</a>&#12290;</p>
<h3><span class="mw-headline" id=".E8.A1.A8.E8.BE.BE.E5.BC.8F.E7.AD.89.E4.BB.B7">&#34920;&#36798;&#24335;&#31561;&#20215;</span></h3>
<p>&#34920;&#36798;&#24335; <span class="t-c"><span class="mw-geshi cpp source-cpp">e</span></span> <i>&#34920;&#36798;&#24335;&#31561;&#20215;</i>&#20110;&#34920;&#36798;&#24335; <span class="t-c"><span class="mw-geshi cpp source-cpp">f</span></span> &#65292;&#33509; <span class="t-c"><span class="mw-geshi cpp source-cpp">e</span></span> &#19982; <span class="t-c"><span class="mw-geshi cpp source-cpp">f</span></span> &#25317;&#26377;&#30456;&#21516;&#25928;&#26524;&#65292;&#22343;&#20026;<a href="cpp-language-noexcept_spec.html">&#28508;&#22312;&#25243;&#20986;</a>&#25110;&#22343;&#38750;&#28508;&#22312;&#25243;&#20986;&#65288;&#21363; <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> &#65289;&#65292;&#19988;&#22343;&#20026;<a href="cpp-language-constant_expression.html">&#24120;&#37327;&#23376;&#34920;&#36798;&#24335;</a>&#25110;&#22343;&#38750;&#24120;&#37327;&#23376;&#34920;&#36798;&#24335;&#12290;</p>
<h3><span class="mw-headline" id=".E5.AE.9A.E5.88.B6.E7.82.B9.E5.AF.B9.E8.B1.A1">&#23450;&#21046;&#28857;&#23545;&#35937;</span></h3>
<p>&#21517;&#23383; <code>ranges::empty</code> &#20195;&#34920;&#19968;&#20010;<i>&#23450;&#21046;&#28857;&#23545;&#35937;</i>&#65292;&#23427;&#26159;<a href="cpp-named_req-LiteralType.html">&#23383;&#38754;</a> <a href="cpp-concepts-semiregular.html"><code>semiregular</code></a> &#31867;&#31867;&#22411;&#30340; const <a href="cpp-named_req-FunctionObject.html">&#20989;&#25968;&#23545;&#35937;</a>&#12290;&#20026;&#35828;&#26126;&#30446;&#30340;&#65292;&#20197; <code><i>__empty_fn</i></code> &#34920;&#31034;&#20854;&#31867;&#22411;&#30340; cv &#26080;&#38480;&#23450;&#29256;&#26412;&#12290;</p>
<p><code><i>__empty_fn</i></code> &#30340;&#25152;&#26377;&#23454;&#20363;&#22343;&#30456;&#31561;&#12290;&#22312;&#30456;&#21516;&#21442;&#25968;&#19978;&#35843;&#29992;&#31867;&#22411; <code><i>__empty_fn</i></code> &#30340;&#19981;&#21516;&#23454;&#20363;&#30340;&#25928;&#26524;&#26159;&#31561;&#20215;&#30340;&#65292;&#26080;&#20851;&#20046;&#25351;&#20195;&#35813;&#23454;&#20363;&#30340;&#34920;&#36798;&#24335;&#26159;&#24038;&#20540;&#36824;&#26159;&#21491;&#20540;&#65292;&#20197;&#21450;&#26159;&#21542;&#20026; const &#38480;&#23450;&#65288;&#28982;&#32780;&#19981;&#35201;&#27714; volatile &#38480;&#23450;&#30340;&#23454;&#20363;&#21487;&#35843;&#29992;&#65289;&#12290;&#20174;&#32780;&#33021;&#33258;&#30001;&#22320;&#22797;&#21046; <code>ranges::empty</code> &#24182;&#19988;&#33021;&#24444;&#27492;&#26367;&#20195;&#22320;&#20351;&#29992;&#20854;&#21103;&#26412;&#12290;</p>
<p>&#32473;&#23450;&#31867;&#22411;&#38598;&#21512; <code>Args...</code> &#65292;&#33509; <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-utility-declval.html"><span class="kw1120">std::<span class="me2">declval</span></span></a><span class="sy1">&lt;</span>Args<span class="sy1">&gt;</span><span class="br0">(</span><span class="br0">)</span>...</span></span> &#28385;&#36275;&#19978;&#38754;&#23545;&#20110; <code>ranges::empty</code> &#30340;&#21442;&#25968;&#35201;&#27714;&#65292;&#21017; <code><i>__empty_fn</i></code> &#23454;&#29616; <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">invocable</span><span class="sy1">&lt;</span>__empty_fn, Args...<span class="sy1">&gt;</span></span></span>&#12289; <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">invocable</span><span class="sy1">&lt;</span><span class="kw4">const</span> __empty_fn, Args...<span class="sy1">&gt;</span></span></span>&#12289; <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">invocable</span><span class="sy1">&lt;</span>__empty_fn<span class="sy3">&amp;</span>, Args...<span class="sy1">&gt;</span></span></span> &#21450; <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">invocable</span><span class="sy1">&lt;</span><span class="kw4">const</span> __empty_fn<span class="sy3">&amp;</span>, Args...<span class="sy1">&gt;</span></span></span> &#12290;&#21542;&#21017;&#65292; <code><i>__empty_fn</i></code> &#30340;&#20989;&#25968;&#35843;&#29992;&#36816;&#31639;&#31526;&#19981;&#21442;&#19982;&#37325;&#36733;&#20915;&#35758;&#12290;</p>
<h3><span class="mw-headline" id=".E7.A4.BA.E4.BE.8B">&#31034;&#20363;</span></h3>
<div class="t-example">
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<pre class="de1"><span class="co2">#include &lt;iostream&gt;</span>
<span class="co2">#include &lt;ranges&gt;</span>
<span class="co2">#include &lt;vector&gt;</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="sy1">&gt;</span>
<span class="kw4">void</span> print<span class="br0">(</span>R<span class="sy3">&amp;&amp;</span> r<span class="br0">)</span>
<span class="br0">{</span>
<span class="kw1">if</span> <span class="br0">(</span>std<span class="sy4">::</span><span class="me2">ranges</span><span class="sy4">::</span><span class="me2">empty</span><span class="br0">(</span>r<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> <span class="st0">&quot;<span class="es1">\t</span>Empty<span class="es1">\n</span>&quot;</span><span class="sy4">;</span>
<span class="kw1">return</span><span class="sy4">;</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> <span class="st0">&quot;<span class="es1">\t</span>Elements:&quot;</span><span class="sy4">;</span>
<span class="kw1">for</span> <span class="br0">(</span><span class="kw4">const</span> <span class="kw4">auto</span><span class="sy3">&amp;</span> element <span class="sy4">:</span> r<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> <span class="st0">&#39; &#39;</span> <span class="sy1">&lt;&lt;</span> element<span class="sy4">;</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> <span class="st0">&#39;<span class="es1">\n</span>&#39;</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="br0">{</span>
<span class="kw4">auto</span> v <span class="sy1">=</span> <a href="cpp-container-vector.html"><span class="kw1269">std::<span class="me2">vector</span></span></a><span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy1">&gt;</span><span class="br0">{</span><span class="nu0">1</span>, <span class="nu0">2</span>, <span class="nu0">3</span><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;1. calling ranges::empty on std::vector:<span class="es1">\n</span>&quot;</span><span class="sy4">;</span>
print<span class="br0">(</span>v<span class="br0">)</span><span class="sy4">;</span>
v.<span class="me1">clear</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
print<span class="br0">(</span>v<span class="br0">)</span><span class="sy4">;</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> <span class="st0">&quot;2. calling ranges::empty on std::initializer_list:<span class="es1">\n</span>&quot;</span><span class="sy4">;</span>
<span class="kw4">auto</span> il <span class="sy1">=</span> <span class="br0">{</span><span class="nu0">7</span>, <span class="nu0">8</span>, <span class="nu0">9</span><span class="br0">}</span><span class="sy4">;</span>
print<span class="br0">(</span>il<span class="br0">)</span><span class="sy4">;</span>
print<span class="br0">(</span><a href="cpp-utility-initializer_list.html"><span class="kw960">std::<span class="me2">initializer_list</span></span></a><span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy1">&gt;</span><span class="br0">{</span><span class="br0">}</span><span class="br0">)</span><span class="sy4">;</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> <span class="st0">&quot;2. calling ranges::empty on a raw array:<span class="es1">\n</span>&quot;</span><span class="sy4">;</span>
<span class="kw4">int</span> array<span class="br0">[</span><span class="br0">]</span> <span class="sy1">=</span> <span class="br0">{</span><span class="nu0">4</span>, <span class="nu0">5</span>, <span class="nu0">6</span><span class="br0">}</span><span class="sy4">;</span> <span class="co1">// &#25968;&#32452;&#25317;&#26377;&#24050;&#30693;&#36793;&#30028;</span>
print<span class="br0">(</span>array<span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span>
<span class="br0">{</span>
<span class="kw1">struct</span> NoEmptyNorSize <span class="sy4">:</span> <span class="kw1">private</span> <a href="cpp-container-vector.html"><span class="kw1269">std::<span class="me2">vector</span></span></a><span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy1">&gt;</span> <span class="br0">{</span>
<span class="kw4">auto</span> begin<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> <a href="cpp-container-vector.html"><span class="kw1269">std::<span class="me2">vector</span></span></a><span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">begin</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="br0">}</span>
<span class="kw4">auto</span> end<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> <a href="cpp-container-vector.html"><span class="kw1269">std::<span class="me2">vector</span></span></a><span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">end</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="br0">}</span>
<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;3. calling ranges::empty on an object that satisfies only case 3):<span class="es1">\n</span>&quot;</span><span class="sy4">;</span>
print<span class="br0">(</span>NoEmptyNorSize<span class="br0">{</span><span class="br0">}</span><span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span>
<span class="br0">}</span></pre></div>
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<p>&#36755;&#20986;&#65306;</p>
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<div class="text source-text">
<pre class="de1">1. calling ranges::empty on std::vector:
Elements: 1 2 3
Empty
2. calling ranges::empty on std::initializer_list:
Elements: 7 8 9
Empty
2. calling ranges::empty on a raw array:
Elements: 4 5 6
3. calling ranges::empty on an object that satisfies only case 3):
Empty</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 class="t-dsc-member-div">
<div><a href="cpp-iterator-empty.html"><span class="t-lines"><span>empty</span></span></a></div>
<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span></span></div>
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<td>&#26816;&#26597;&#23481;&#22120;&#26159;&#21542;&#20026;&#31354;<br />
<span class="t-mark">(&#20989;&#25968;&#27169;&#26495;)</span></td>
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