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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::</span>uninitialized_default_construct</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><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1">&lt;</span> <span class="kw1">class</span> ForwardIt <span class="sy1">&gt;</span><br />
<span class="kw4">void</span> uninitialized_default_construct<span class="br0">(</span> ForwardIt first, ForwardIt last<span class="br0">)</span><span class="sy4">;</span></span></div>
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<td>(1)</td>
<td><span class="t-mark-rev t-since-cxx17">(C++17 &#36215;)</span></td>
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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> ExecutionPolicy, <span class="kw1">class</span> ForwardIt <span class="sy1">&gt;</span><br />
<span class="kw4">void</span> uninitialized_default_construct<span class="br0">(</span> ExecutionPolicy<span class="sy3">&amp;&amp;</span> policy, ForwardIt first, ForwardIt last <span class="br0">)</span><span class="sy4">;</span></span></div>
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<td>(2)</td>
<td><span class="t-mark-rev t-since-cxx17">(C++17 &#36215;)</span></td>
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<div class="t-li1"><span class="t-li">1)</span> &#20197;<a href="cpp-language-default_initialization.html">&#40664;&#35748;&#21021;&#22987;&#21270;</a>&#22312;&#33539;&#22260; <code>[first, last)</code> &#25152;&#25351;&#20195;&#30340;&#26410;&#21021;&#22987;&#21270;&#23384;&#20648;&#19978;&#26500;&#36896; <code>typename iterator_traits&lt;ForwardIt&gt;::value_type</code> &#31867;&#22411;&#23545;&#35937;&#65292;&#22914;&#21516;&#29992;
<div dir="ltr" class="mw-geshi" style="text-align: left;">
<div class="cpp source-cpp">
<pre class="de1"><span class="kw1">for</span> <span class="br0">(</span><span class="sy4">;</span> first <span class="sy3">!</span><span class="sy1">=</span> last<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="coMULTI">/*VOIDIFY*/</span><span class="br0">(</span><span class="sy2">*</span>first<span class="br0">)</span><span class="br0">)</span>
<span class="kw1">typename</span> <a href="cpp-iterator-iterator_traits.html"><span class="kw651">std::<span class="me2">iterator_traits</span></span></a><span class="sy1">&lt;</span>ForwardIt<span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">value_type</span><span class="sy4">;</span></pre></div>
</div>
<p>&#20854;&#20013; <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="coMULTI">/*VOIDIFY*/</span><span class="br0">(</span>e<span class="br0">)</span></span></span> &#20026;&#65306;</p>
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<dl>
<dd><span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">static_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><a href="cpp-memory-addressof.html"><span class="kw756">std::<span class="me2">addressof</span></span></a><span class="br0">(</span>e<span class="br0">)</span><span class="br0">)</span></span></span></dd>
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<td><span class="t-mark-rev t-until-cxx20">(C++20 &#21069;)</span></td>
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<dl>
<dd><span class="t-c"><span class="mw-geshi cpp source-cpp"><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="kw756">std::<span class="me2">addressof</span></span></a><span class="br0">(</span>e<span class="br0">)</span><span class="br0">)</span><span class="br0">)</span></span></span></dd>
</dl>
</td>
<td><span class="t-mark-rev t-since-cxx20">(C++20 &#36215;)</span></td>
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</div>
<div class="t-li1"><span class="t-li"></span> &#33509;&#21021;&#22987;&#21270;&#26399;&#38388;&#25243;&#20986;&#24322;&#24120;&#65292;&#21017;&#20197;&#26410;&#25351;&#23450;&#39034;&#24207;&#38144;&#27585;&#24050;&#26500;&#36896;&#30340;&#23545;&#35937;&#12290;</div>
<div class="t-li1"><span class="t-li">2)</span> &#21516; <span class="t-v">(1)</span> &#65292;&#20294;&#25353;&#29031; <code>policy</code> &#25191;&#34892;&#12290;&#27492;&#37325;&#36733;&#20165;&#33509; <span class="t-rev-inl t-until-cxx20"><span><span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-algorithm-is_execution_policy.html"><span class="kw1563">std::<span class="me2">is_execution_policy_v</span></span></a><span class="sy1">&lt;</span><a href="cpp-types-decay.html"><span class="kw629">std::<span class="me2">decay_t</span></span></a><span class="sy1">&lt;</span>ExecutionPolicy<span class="sy1">&gt;&gt;</span></span></span></span> <span><span class="t-mark-rev t-until-cxx20">(C++20 &#21069;)</span></span></span><span class="t-rev-inl t-since-cxx20"><span><span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-algorithm-is_execution_policy.html"><span class="kw1563">std::<span class="me2">is_execution_policy_v</span></span></a><span class="sy1">&lt;</span>std<span class="sy4">::</span><span class="me2">remove_cvref_t</span><span class="sy1">&lt;</span>ExecutionPolicy<span class="sy1">&gt;&gt;</span></span></span></span> <span><span class="t-mark-rev t-since-cxx20">(C++20 &#36215;)</span></span></span> &#20026; true &#25165;&#21442;&#19982;&#37325;&#36733;&#20915;&#35758;&#12290;</div>
<h3><span class="mw-headline" id=".E5.8F.82.E6.95.B0">&#21442;&#25968;</span></h3>
<table class="t-par-begin">
<tr class="t-par">
<td>first, last</td>
<td>-</td>
<td>&#35201;&#21021;&#22987;&#21270;&#30340;&#20803;&#32032;&#33539;&#22260;</td>
</tr>
<tr class="t-par">
<td>policy</td>
<td>-</td>
<td>&#25152;&#29992;&#30340;&#25191;&#34892;&#31574;&#30053;&#12290;&#32454;&#33410;&#35265;<a href="cpp-algorithm-execution_policy_tag_t.html">&#25191;&#34892;&#31574;&#30053;</a>&#12290;</td>
</tr>
<tr class="t-par-hitem">
<td colspan="3">&#31867;&#22411;&#35201;&#27714;</td>
</tr>
<tr class="t-par-req">
<td colspan="3">-
<div class="spacer"></div>
<code>ForwardIt</code> &#24517;&#39035;&#28385;&#36275;<a href="cpp-named_req-ForwardIterator.html"><span style="font-style:italic">&#32769;&#24335;&#21521;&#21069;&#36845;&#20195;&#22120;</span> <span style="font-family: Georgia, &#39;DejaVu Serif&#39;, serif; font-style:italic">(LegacyForwardIterator)</span></a> &#30340;&#35201;&#27714;&#12290;</td>
</tr>
<tr class="t-par-req">
<td colspan="3">-
<div class="spacer"></div>
&#36890;&#36807; <code>ForwardIt</code> &#21512;&#27861;&#23454;&#20363;&#30340;&#33258;&#22686;&#12289;&#22797;&#21046;&#12289;&#27604;&#36739;&#25110;&#38388;&#25509;&#22343;&#19981;&#25243;&#20986;&#24322;&#24120;&#12290;</td>
</tr>
</table>
<h3><span class="mw-headline" id=".E8.BF.94.E5.9B.9E.E5.80.BC">&#36820;&#22238;&#20540;</span></h3>
<p>&#65288;&#26080;&#65289;</p>
<h3><span class="mw-headline" id=".E5.A4.8D.E6.9D.82.E5.BA.A6">&#22797;&#26434;&#24230;</span></h3>
<p>&#19982; <code>first</code> &#19982; <code>last</code> &#38388;&#30340;&#36317;&#31163;&#25104;&#32447;&#24615;</p>
<h3><span class="mw-headline" id=".E5.BC.82.E5.B8.B8">&#24322;&#24120;</span></h3>
<p>&#25317;&#26377;&#21517;&#20026; <code>ExecutionPolicy</code> &#30340;&#27169;&#26495;&#24418;&#21442;&#30340;&#37325;&#36733;&#25353;&#19979;&#21015;&#26041;&#24335;&#25253;&#21578;&#38169;&#35823;&#65306;</p>
<ul>
<li>&#33509;&#20316;&#20026;&#31639;&#27861;&#19968;&#37096;&#20998;&#35843;&#29992;&#30340;&#20989;&#25968;&#30340;&#25191;&#34892;&#25243;&#20986;&#24322;&#24120;&#65292;&#19988; <code>ExecutionPolicy</code> &#20026;<a href="cpp-algorithm-execution_policy_tag_t.html">&#26631;&#20934;&#31574;&#30053;</a>&#20043;&#19968;&#65292;&#21017;&#35843;&#29992; <span class="t-lc"><a href="cpp-error-terminate.html">std::terminate</a></span> &#12290;&#23545;&#20110;&#20219;&#20309;&#20854;&#20182; <code>ExecutionPolicy</code> &#65292;&#34892;&#20026;&#26159;&#23454;&#29616;&#23450;&#20041;&#30340;&#12290;</li>
<li>&#33509;&#31639;&#27861;&#26080;&#27861;&#20998;&#37197;&#20869;&#23384;&#65292;&#21017;&#25243;&#20986; <span class="t-lc"><a href="cpp-memory-new-bad_alloc.html">std::bad_alloc</a></span> &#12290;</li>
</ul>
<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 dir="ltr" class="mw-geshi" style="text-align: left;">
<div class="cpp source-cpp">
<pre class="de1"><span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> ForwardIt<span class="sy1">&gt;</span>
<span class="kw4">void</span> uninitialized_default_construct<span class="br0">(</span>ForwardIt first, ForwardIt last<span class="br0">)</span>
<span class="br0">{</span>
<span class="kw1">using</span> Value <span class="sy1">=</span> <span class="kw1">typename</span> <a href="cpp-iterator-iterator_traits.html"><span class="kw651">std::<span class="me2">iterator_traits</span></span></a><span class="sy1">&lt;</span>ForwardIt<span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">value_type</span><span class="sy4">;</span>
ForwardIt current <span class="sy1">=</span> first<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> current <span class="sy3">!</span><span class="sy1">=</span> last<span class="sy4">;</span> <span class="sy2">++</span>current<span class="br0">)</span> <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="kw756">std::<span class="me2">addressof</span></span></a><span class="br0">(</span><span class="sy2">*</span>current<span class="br0">)</span><span class="br0">)</span><span class="br0">)</span><span class="br0">)</span> Value<span class="sy4">;</span>
<span class="br0">}</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-memory-destroy.html"><span class="kw726">std::<span class="me2">destroy</span></span></a><span class="br0">(</span>first, current<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></pre></div>
</div>
</td>
</tr>
</table>
<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;iostream&gt;</span>
<span class="co2">#include &lt;memory&gt;</span>
<span class="co2">#include &lt;string&gt;</span>
<span class="co2">#include &lt;cstring&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="kw1229">std::<span class="me2">string</span></span></a> m<span class="br0">{</span> <span class="st0">&quot;Default value&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">3</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">unsigned</span> <span class="kw4">char</span> mem<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>mem<span class="br0">)</span><span class="br0">}</span><span class="sy4">;</span>
<span class="kw4">auto</span> last <span class="br0">{</span>first <span class="sy2">+</span> n<span class="br0">}</span><span class="sy4">;</span>
std<span class="sy4">::</span><span class="me2">uninitialized_default_construct</span><span class="br0">(</span>first, last<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="kw1757">std::<span class="me2">cout</span></span></a> <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>
<a href="cpp-memory-destroy.html"><span class="kw726">std::<span class="me2">destroy</span></span></a><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="kw1757">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;&#23545;&#20110;&ldquo;&#24179;&#20961;&#31867;&#22411;&rdquo; uninitialized_default_construct &#36890;&#24120;&#19981;&#20250;&#20197;&#38646;&#22635;&#20805;&#26410;&#21021;&#22987;&#21270;&#30340;&#20869;&#23384;&#21306;&#22495;&#12290;</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="br0">}</span><span class="sy4">;</span>
<span class="kw4">const</span> <span class="kw4">int</span> original<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="br0">}</span><span class="sy4">;</span>
std<span class="sy4">::</span><span class="me2">uninitialized_default_construct</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-end.html"><span class="kw680">std::<span class="me2">end</span></span></a><span class="br0">(</span>v<span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
<span class="co1">// for (const int i : v) { std::cout &lt;&lt; i &lt;&lt; &#39; &#39;; }</span>
<span class="co1">// &#21487;&#33021;&#20026;&#26410;&#23450;&#20041;&#34892;&#20026;&#65292;&#24453;&#23450;&#30340; CWG 1997 &#12290;</span>
<a href="cpp-io-cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span>
<span class="br0">(</span><a href="cpp-string-byte-memcmp.html"><span class="kw1156">std::<span class="me2">memcmp</span></span></a><span class="br0">(</span>v, original, 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="sy4">?</span> <span class="st0">&quot;Unmodified<span class="es1">\n</span>&quot;</span> <span class="sy4">:</span> <span class="st0">&quot;Modified<span class="es1">\n</span>&quot;</span><span class="br0">)</span><span class="sy4">;</span>
<span class="co1">// &#32467;&#26524;&#26410;&#25351;&#23450;&#12290;</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">Default value
Default value
Default value
Unmodified</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>
<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>
<tr class="t-dsc">
<td>
<div class="t-dsc-member-div">
<div><a href="cpp-memory-uninitialized_value_construct.html"><span class="t-lines"><span>uninitialized_value_construct</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;&#33539;&#22260;&#25152;&#23450;&#20041;&#30340;&#26410;&#21021;&#22987;&#21270;&#20869;&#23384;&#20013;&#29992;<a href="cpp-language-value_initialization.html">&#20540;&#21021;&#22987;&#21270;</a>&#26500;&#36896;&#23545;&#35937;<br />
<span class="t-mark">(&#20989;&#25968;&#27169;&#26495;)</span></td>
</tr>
<tr class="t-dsc">
<td>
<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>
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<div class="visualClear"></div>
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