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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::ranges::</span>uninitialized_default_construct_n</h1>
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<div id="contentSub"><span class="subpages">&lt; <a href="cpp.html">cpp</a>&lrm; | <a href="cpp-memory.html">memory</a></span></div>
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<div>&#23450;&#20041;&#20110;&#22836;&#25991;&#20214; <code><a href="cpp-header-memory.html">&lt;memory&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>no<span class="sy2">-</span>throw<span class="sy2">-</span>forward<span class="sy2">-</span>iterator I<span class="sy1">&gt;</span><br /></span>
<p>requires std<span class="sy4">::</span><span class="me2">default_initializable</span><span class="sy1">&lt;</span>std<span class="sy4">::</span><span class="me2">iter_value_t</span><span class="sy1">&lt;</span>I<span class="sy1">&gt;&gt;</span><br /></p>
I uninitialized_default_construct_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 <span class="br0">)</span><span class="sy4">;</span></div>
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<td class="t-dcl-nopad"></td>
<td><span class="t-mark-rev t-since-cxx20">(C++20 &#36215;)</span></td>
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<p>&#20197;<a href="cpp-language-default_initialization.html">&#40664;&#35748;&#21021;&#22987;&#21270;</a>&#22312;&#22987;&#20110; <code>first</code> &#30340;&#26410;&#21021;&#22987;&#21270;&#23384;&#20648;&#20013;&#26500;&#36896; <code>n</code> &#20010; <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">iter_value_t</span><span class="sy1">&lt;</span>I<span class="sy1">&gt;</span></span></span> &#31867;&#22411;&#23545;&#35937;&#65292;&#22914;&#21516;&#29992;</p>
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<div class="cpp source-cpp">
<pre class="de1"><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="sy4">::</span><span class="me2">new</span> <span class="br0">(</span><span class="kw1">const_cast</span><span class="sy1">&lt;</span><span class="kw4">void</span><span class="sy2">*</span><span class="sy1">&gt;</span><span class="br0">(</span><span class="kw1">static_cast</span><span class="sy1">&lt;</span><span class="kw4">const</span> <span class="kw4">volatile</span> <span class="kw4">void</span><span class="sy2">*</span><span class="sy1">&gt;</span><span class="br0">(</span><a href="cpp-memory-addressof.html"><span class="kw757">std::<span class="me2">addressof</span></span></a><span class="br0">(</span><span class="sy2">*</span>first<span class="br0">)</span><span class="br0">)</span><span class="br0">)</span><span class="br0">)</span>
<a href="cpp-types-remove_reference.html"><span class="kw607">std::<span class="me2">remove_reference_t</span></span></a><span class="sy1">&lt;</span>std<span class="sy4">::</span><span class="me2">iter_reference_t</span><span class="sy1">&lt;</span>I<span class="sy1">&gt;&gt;</span><span class="sy4">;</span></pre></div>
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<p>&#33509;&#21021;&#22987;&#21270;&#26399;&#38388;&#25243;&#20986;&#24322;&#24120;&#65292;&#21017;&#25353;&#26410;&#25351;&#23450;&#39034;&#24207;&#38144;&#27585;&#24050;&#26500;&#36896;&#30340;&#23545;&#35937;&#12290;</p>
<p>&#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>
</ul>
<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, last</td>
<td>-</td>
<td>&#35201;&#21021;&#22987;&#21270;&#30340;&#20803;&#32032;&#33539;&#22260;</td>
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<tr class="t-par">
<td>n</td>
<td>-</td>
<td>&#35201;&#26500;&#36896;&#30340;&#20803;&#32032;&#25968;</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;&#33539;&#22260;&#30340;&#26411;&#23614;&#65288;&#21363; <span class="t-c"><span class="mw-geshi cpp source-cpp">ranges<span class="sy4">::</span><span class="me2">next</span><span class="br0">(</span>first, n<span class="br0">)</span></span></span> &#65289;&#12290;</p>
<h3><span class="mw-headline" id=".E5.A4.8D.E6.9D.82.E5.BA.A6">&#22797;&#26434;&#24230;</span></h3>
<p>&#19982; <span class="t-c"><span class="mw-geshi cpp source-cpp">n</span></span> &#25104;&#32447;&#24615;&#12290;</p>
<h3><span class="mw-headline" id=".E5.BC.82.E5.B8.B8">&#24322;&#24120;</span></h3>
<p>&#26500;&#36896;&#30446;&#26631;&#33539;&#22260;&#20013;&#30340;&#20803;&#32032;&#26102;&#25243;&#20986;&#30340;&#24322;&#24120;&#65292;&#33509;&#23384;&#22312;&#12290;</p>
<h3><span class="mw-headline" id=".E6.B3.A8.E8.A7.A3">&#27880;&#35299;</span></h3>
<p>&#33509;&#40664;&#35748;&#21021;&#22987;&#21270; <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">iter_value_t</span><span class="sy1">&lt;</span>I<span class="sy1">&gt;</span></span></span> &#23545;&#35937;&#26102;&#19981;&#35843;&#29992;&#38750;&#24179;&#20961;&#30340;&#40664;&#35748;&#26500;&#36896;&#20989;&#25968;&#65292;&#21017;&#23454;&#29616;&#21487;&#20197;&#36339;&#36807;&#23545;&#35937;&#26500;&#36896;&#65288;&#32780;&#19981;&#26356;&#25913;&#21487;&#35266;&#23519;&#20316;&#29992;&#65289;&#65292;&#36825;&#33021;&#30001; <span class="t-lc"><a href="cpp-types-is_default_constructible.html">std::is_trivially_default_constructible_v</a></span> &#26816;&#27979;&#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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<pre class="de1"><span class="kw1">struct</span> uninitialized_default_construct_n_fn <span class="br0">{</span>
<span class="kw1">template</span> <span class="sy1">&lt;</span>no<span class="sy2">-</span>throw<span class="sy2">-</span>forward<span class="sy2">-</span>iterator I<span class="sy1">&gt;</span>
requires std<span class="sy4">::</span><span class="me2">default_initializable</span><span class="sy1">&lt;</span>std<span class="sy4">::</span><span class="me2">iter_value_t</span><span class="sy1">&lt;</span>I<span class="sy1">&gt;&gt;</span>
I 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 <span class="br0">)</span> <span class="kw4">const</span> <span class="br0">{</span>
<span class="kw1">using</span> ValueType <span class="sy1">=</span> <a href="cpp-types-remove_reference.html"><span class="kw607">std::<span class="me2">remove_reference_t</span></span></a><span class="sy1">&lt;</span>std<span class="sy4">::</span><span class="me2">iter_reference_t</span><span class="sy1">&lt;</span>I<span class="sy1">&gt;&gt;</span><span class="sy4">;</span>
<span class="kw1">if</span> <span class="kw4">constexpr</span> <span class="br0">(</span><a href="cpp-types-is_default_constructible.html"><span class="kw529">std::<span class="me2">is_trivially_default_constructible_v</span></span></a><span class="sy1">&lt;</span>ValueType<span class="sy1">&gt;</span><span class="br0">)</span>
<span class="kw1">return</span> ranges<span class="sy4">::</span><span class="me2">next</span><span class="br0">(</span>first, n<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// &#36339;&#36807;&#21021;&#22987;&#21270;</span>
I rollback <span class="br0">{</span>first<span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">try</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="sy4">::</span><span class="me2">new</span> <span class="br0">(</span><span class="kw1">const_cast</span><span class="sy1">&lt;</span><span class="kw4">void</span><span class="sy2">*</span><span class="sy1">&gt;</span><span class="br0">(</span><span class="kw1">static_cast</span><span class="sy1">&lt;</span><span class="kw4">const</span> <span class="kw4">volatile</span> <span class="kw4">void</span><span class="sy2">*</span><span class="sy1">&gt;</span>
<span class="br0">(</span><a href="cpp-memory-addressof.html"><span class="kw757">std::<span class="me2">addressof</span></span></a><span class="br0">(</span><span class="sy2">*</span>first<span class="br0">)</span><span class="br0">)</span><span class="br0">)</span><span class="br0">)</span> ValueType<span class="sy4">;</span>
<span class="kw1">return</span> first<span class="sy4">;</span>
<span class="br0">}</span> <span class="kw1">catch</span> <span class="br0">(</span>...<span class="br0">)</span> <span class="br0">{</span> <span class="co1">// &#22238;&#28378;&#65306;&#38144;&#27585;&#24050;&#26500;&#36896;&#30340;&#20803;&#32032;</span>
<span class="kw1">for</span> <span class="br0">(</span><span class="sy4">;</span> rollback <span class="sy3">!</span><span class="sy1">=</span> first<span class="sy4">;</span> <span class="sy2">++</span>rollback<span class="br0">)</span>
ranges<span class="sy4">::</span><span class="me2">destroy_at</span><span class="br0">(</span><a href="cpp-memory-addressof.html"><span class="kw757">std::<span class="me2">addressof</span></span></a><span class="br0">(</span><span class="sy2">*</span>rollback<span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
<span class="kw1">throw</span><span class="sy4">;</span>
<span class="br0">}</span>
<span class="br0">}</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">inline</span> <span class="kw4">constexpr</span> uninitialized_default_construct_n_fn uninitialized_default_construct_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">&#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;cstring&gt;</span>
<span class="co2">#include &lt;iostream&gt;</span>
<span class="co2">#include &lt;memory&gt;</span>
<span class="co2">#include &lt;string&gt;</span>
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
<span class="kw1">struct</span> S <span class="br0">{</span> <a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a> m<span class="br0">{</span> <span class="st0">&quot;&#9608;&#9619;&#9618;&#9617; &#9608;&#9619;&#9618;&#9617; &quot;</span> <span class="br0">}</span><span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span>
<span class="kw4">constexpr</span> <span class="kw4">int</span> n <span class="br0">{</span><span class="nu0">4</span><span class="br0">}</span><span class="sy4">;</span>
alignas<span class="br0">(</span>alignof<span class="br0">(</span>S<span class="br0">)</span><span class="br0">)</span> <span class="kw4">char</span> out<span class="br0">[</span>n <span class="sy2">*</span> sizeof<span class="br0">(</span>S<span class="br0">)</span><span class="br0">]</span><span class="sy4">;</span>
<span class="kw1">try</span>
<span class="br0">{</span>
<span class="kw4">auto</span> first <span class="br0">{</span><span class="kw1">reinterpret_cast</span><span class="sy1">&lt;</span>S<span class="sy2">*</span><span class="sy1">&gt;</span><span class="br0">(</span>out<span class="br0">)</span><span class="br0">}</span><span class="sy4">;</span>
<span class="kw4">auto</span> last <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">ranges</span><span class="sy4">::</span><span class="me2">uninitialized_default_construct_n</span><span class="br0">(</span>first, n<span class="br0">)</span><span class="sy4">;</span>
<span class="kw4">auto</span> count <span class="br0">{</span><span class="nu0">1</span><span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">for</span> <span class="br0">(</span><span class="kw4">auto</span> it <span class="br0">{</span>first<span class="br0">}</span><span class="sy4">;</span> it <span class="sy3">!</span><span class="sy1">=</span> last<span class="sy4">;</span> <span class="sy2">++</span>it<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> count<span class="sy2">++</span> <span class="sy1">&lt;&lt;</span> <span class="st0">&#39; &#39;</span> <span class="sy1">&lt;&lt;</span> it<span class="sy2">-</span><span class="sy1">&gt;</span>m <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>
std<span class="sy4">::</span><span class="me2">ranges</span><span class="sy4">::</span><span class="me2">destroy</span><span class="br0">(</span>first, last<span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span>
<span class="kw1">catch</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> <span class="st0">&quot;Exception!<span class="es1">\n</span>&quot;</span><span class="sy4">;</span> <span class="br0">}</span>
<span class="co1">// &#27880;&#24847; uninitialized_default_construct_n &#23545;&ldquo;&#24179;&#20961;&#31867;&#22411;&rdquo;</span>
<span class="co1">// &#36890;&#24120;&#19981;&#20250;&#20197;&#38646;&#22635;&#20805;&#32473;&#23450;&#30340;&#26410;&#21021;&#22987;&#21270;&#20869;&#23384;&#21306;&#22495;&#12290;</span>
<span class="kw4">constexpr</span> <span class="kw4">int</span> etalon<span class="br0">[</span><span class="br0">]</span> <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="nu0">6</span> <span class="br0">}</span><span class="sy4">;</span>
<span class="kw4">int</span> v<span class="br0">[</span><span class="br0">]</span> <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="nu0">6</span> <span class="br0">}</span><span class="sy4">;</span>
std<span class="sy4">::</span><span class="me2">ranges</span><span class="sy4">::</span><span class="me2">uninitialized_default_construct_n</span><span class="br0">(</span><a href="cpp-iterator-begin.html"><span class="kw676">std::<span class="me2">begin</span></span></a><span class="br0">(</span>v<span class="br0">)</span>, <a href="cpp-iterator-size.html"><span class="kw684">std::<span class="me2">size</span></span></a><span class="br0">(</span>v<span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
<span class="kw1">if</span> <span class="br0">(</span><a href="cpp-string-byte-memcmp.html"><span class="kw1157">std::<span class="me2">memcmp</span></span></a><span class="br0">(</span>v, etalon, sizeof<span class="br0">(</span>v<span class="br0">)</span><span class="br0">)</span> <span class="sy1">==</span> <span class="nu0">0</span><span class="br0">)</span> <span class="br0">{</span>
<span class="co1">// &#21487;&#33021;&#20026;&#26410;&#23450;&#20041;&#34892;&#20026;&#65292;&#24453;&#20915;&#30340; CWG 1997 &#65306;</span>
<span class="co1">// for (const int i : v) { std::cout &lt;&lt; i &lt;&lt; &#39; &#39;; }</span>
<span class="kw1">for</span> <span class="br0">(</span><span class="kw4">const</span> <span class="kw4">int</span> i <span class="sy4">:</span> etalon<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> i <span class="sy1">&lt;&lt;</span> <span class="st0">&#39; &#39;</span><span class="sy4">;</span> <span class="br0">}</span>
<span class="br0">}</span> <span class="kw1">else</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;Unspecified!&quot;</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">&#39;<span class="es1">\n</span>&#39;</span><span class="sy4">;</span>
<span class="br0">}</span></pre></div>
</div>
<p>&#21487;&#33021;&#30340;&#36755;&#20986;&#65306;</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;">
<div class="text source-text">
<pre class="de1">1 &#9608;&#9619;&#9618;&#9617; &#9608;&#9619;&#9618;&#9617;
2 &#9608;&#9619;&#9618;&#9617; &#9608;&#9619;&#9618;&#9617;
3 &#9608;&#9619;&#9618;&#9617; &#9608;&#9619;&#9618;&#9617;
4 &#9608;&#9619;&#9618;&#9617; &#9608;&#9619;&#9618;&#9617;
1 2 3 4 5 6</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">
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<div class="t-dsc-member-div">
<div><a href="cpp-memory-ranges-uninitialized_default_construct.html"><span class="t-lines"><span>ranges::uninitialized_default_construct</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>&#22312;&#33539;&#22260;&#25152;&#23450;&#20041;&#30340;&#26410;&#21021;&#22987;&#21270;&#30340;&#20869;&#23384;&#21306;&#22495;&#20197;<a href="cpp-language-default_initialization.html">&#40664;&#35748;&#21021;&#22987;&#21270;</a>&#26500;&#36896;&#23545;&#35937;<br />
<span class="t-mark">(niebloid)</span></td>
</tr>
<tr class="t-dsc">
<td>
<div class="t-dsc-member-div">
<div><a href="cpp-memory-ranges-uninitialized_value_construct.html"><span class="t-lines"><span>ranges::uninitialized_value_construct</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>&#22312;&#33539;&#22260;&#25152;&#23450;&#20041;&#30340;&#26410;&#21021;&#22987;&#21270;&#30340;&#20869;&#23384;&#21306;&#22495;&#20197;<a href="cpp-language-value_initialization.html">&#20540;&#21021;&#22987;&#21270;</a>&#26500;&#36896;&#23545;&#35937;<br />
<span class="t-mark">(niebloid)</span></td>
</tr>
<tr class="t-dsc">
<td>
<div class="t-dsc-member-div">
<div><a href="cpp-memory-ranges-uninitialized_value_construct_n.html"><span class="t-lines"><span>ranges::uninitialized_value_construct_n</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>&#22312;&#36215;&#22987;&#19982;&#35745;&#25968;&#25152;&#23450;&#20041;&#30340;&#26410;&#21021;&#22987;&#21270;&#30340;&#20869;&#23384;&#21306;&#22495;&#20197;<a href="cpp-language-value_initialization.html">&#20540;&#21021;&#22987;&#21270;</a>&#26500;&#36896;&#23545;&#35937;<br />
<span class="t-mark">(niebloid)</span></td>
</tr>
<tr class="t-dsc">
<td>
<div class="t-dsc-member-div">
<div><a href="cpp-memory-uninitialized_default_construct_n.html"><span class="t-lines"><span>uninitialized_default_construct_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>&#22312;&#36215;&#22987;&#21644;&#35745;&#25968;&#25152;&#23450;&#20041;&#30340;&#26410;&#21021;&#22987;&#21270;&#20869;&#23384;&#21306;&#22495;&#29992;<a href="cpp-language-default_initialization.html">&#40664;&#35748;&#21021;&#22987;&#21270;</a>&#26500;&#36896;&#23545;&#35937;<br />
<span class="t-mark">(&#20989;&#25968;&#27169;&#26495;)</span></td>
</tr>
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