192 lines
16 KiB
HTML
192 lines
16 KiB
HTML
<!DOCTYPE html>
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<html lang="zh" dir="ltr" class="client-nojs" xmlns="http://www.w3.org/1999/xhtml" xml:lang="zh">
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<title>std::ranges::uninitialized_fill_n</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_memory_ranges_uninitialized_fill_n skin-cppreference2 action-view cpp-navbar">
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<div id="cpp-content-base">
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<div id="content"><a id="top"></a>
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::ranges::</span>uninitialized_fill_n</h1>
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<div id="bodyContent">
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<div id="contentSub"><span class="subpages">< <a href="cpp.html">cpp</a>‎ | <a href="cpp-memory.html">memory</a></span></div>
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<div id="mw-content-text" lang="zh" dir="ltr" class="mw-content-ltr" xml:lang="zh">
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<table class="t-dcl-begin">
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<td>
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<div>定义于头文件 <code><a href="cpp-header-memory.html"><memory></a></code></div>
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<td></td>
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<div>调用签名</div>
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<td></td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span> <span class="sy1"><</span>no<span class="sy2">-</span>throw<span class="sy2">-</span>forward<span class="sy2">-</span>range I, <span class="kw1">class</span> T<span class="sy1">></span><br /></span>
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<p>requires std<span class="sy4">::</span><span class="me2">constructible_from</span><span class="sy1"><</span>std<span class="sy4">::</span><span class="me2">iter_value_t</span><span class="sy1"><</span>I<span class="sy1">></span>, <span class="kw4">const</span> T<span class="sy3">&</span><span class="sy1">></span><br /></p>
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I uninitialized_fill_n<span class="br0">(</span> I first, std<span class="sy4">::</span><span class="me2">iter_difference_t</span><span class="sy1"><</span>I<span class="sy1">></span> n, <span class="kw4">const</span> T<span class="sy3">&</span> x <span class="br0">)</span><span class="sy4">;</span></div>
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</td>
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<td class="t-dcl-nopad"></td>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
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<p>在范围 <code>[first, first + n)</code> 所指代的未初始化内存区域构造给定值 <code>x</code> 的 <code>n</code> 个副本，如同用</p>
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<div dir="ltr" class="mw-geshi" style="text-align: left;">
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<div class="cpp source-cpp">
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<pre class="de1"><span class="kw1">for</span> <span class="br0">(</span><span class="sy4">;</span> n<span class="sy2">--</span><span class="sy4">;</span> <span class="sy2">++</span>first<span class="br0">)</span> <span class="br0">{</span>
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<span class="sy4">::</span><span class="me2">new</span> <span class="br0">(</span>
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<span class="kw1">const_cast</span><span class="sy1"><</span><span class="kw4">void</span><span class="sy2">*</span><span class="sy1">></span><span class="br0">(</span><span class="kw1">static_cast</span><span class="sy1"><</span><span class="kw4">const</span> <span class="kw4">volatile</span> <span class="kw4">void</span><span class="sy2">*</span><span class="sy1">></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>
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<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"><</span>std<span class="sy4">::</span><span class="me2">iter_reference_t</span><span class="sy1"><</span>I<span class="sy1">>></span><span class="br0">(</span>x<span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">}</span></pre></div>
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</div>
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<p>若初始化期间抛出异常，则以未指定顺序销毁已构造的对象。</p>
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<p>此页面上描述的仿函数实体是 <i>niebloid</i> ，即：</p>
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<ul>
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<li>调用它们中的任何一者时不可指定显式模板实参。</li>
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<li>它们无一对<a href="cpp-language-adl.html">实参依赖查找</a>可见。</li>
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<li>在<a href="cpp-language-unqualified_lookup.html">通常无限定查找</a>寻找函数调用运算符左侧的名称找到它们之一时，它禁止<a href="cpp-language-adl.html">实参依赖查找</a>。</li>
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</ul>
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<p>实际上，它们能以函数对象，或以某些特殊编译器扩展实现。</p>
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<h3><span class="mw-headline" id=".E5.8F.82.E6.95.B0">参数</span></h3>
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<table class="t-par-begin">
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<tr class="t-par">
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<td>first</td>
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<td>-</td>
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<td>要初始化的元素范围的起始</td>
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<tr class="t-par">
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<td>n</td>
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<td>-</td>
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<td>构造的元素数</td>
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<tr class="t-par">
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<td>x</td>
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<td>-</td>
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<td>用以构造元素的值</td>
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<h3><span class="mw-headline" id=".E8.BF.94.E5.9B.9E.E5.80.BC">返回值</span></h3>
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<p>等于 <span class="t-c"><span class="mw-geshi cpp source-cpp">first <span class="sy2">+</span> n</span></span> 的迭代器。</p>
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<h3><span class="mw-headline" id=".E5.A4.8D.E6.9D.82.E5.BA.A6">复杂度</span></h3>
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<p>与 <span class="t-c"><span class="mw-geshi cpp source-cpp">n</span></span> 成线性。</p>
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<h3><span class="mw-headline" id=".E5.BC.82.E5.B8.B8">异常</span></h3>
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<p>构造目标范围中的元素时抛出的异常，若存在。</p>
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<h3><span class="mw-headline" id=".E6.B3.A8.E8.A7.A3">注解</span></h3>
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<p>若输出范围的值类型为<a href="cpp-named_req-TrivialType.html"><span style="font-style:italic">平凡类型</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(TrivialType)</span></a> ，则实现可能提升 <code>ranges::uninitialized_fill_n</code> 的效率，例如用 <span class="t-c"><span class="mw-geshi cpp source-cpp">ranges<span class="sy4">::</span><span class="me2">fill_n</span></span></span> 。</p>
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<h3><span class="mw-headline" id=".E5.8F.AF.E8.83.BD.E7.9A.84.E5.AE.9E.E7.8E.B0">可能的实现</span></h3>
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<table class="eq-fun-cpp-table">
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<div dir="ltr" class="mw-geshi" style="text-align: left;">
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<div class="cpp source-cpp">
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<pre class="de1"><span class="kw1">struct</span> uninitialized_fill_n_fn <span class="br0">{</span>
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<span class="kw1">template</span> <span class="sy1"><</span>no<span class="sy2">-</span>throw<span class="sy2">-</span>forward<span class="sy2">-</span>range I, <span class="kw1">class</span> T<span class="sy1">></span>
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requires std<span class="sy4">::</span><span class="me2">constructible_from</span><span class="sy1"><</span>std<span class="sy4">::</span><span class="me2">iter_value_t</span><span class="sy1"><</span>I<span class="sy1">></span>, <span class="kw4">const</span> T<span class="sy3">&</span><span class="sy1">></span>
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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"><</span>I<span class="sy1">></span> n, <span class="kw4">const</span> T<span class="sy3">&</span> x <span class="br0">)</span> <span class="kw4">const</span> <span class="br0">{</span>
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I rollback <span class="br0">{</span>first<span class="br0">}</span><span class="sy4">;</span>
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<span class="kw1">try</span> <span class="br0">{</span>
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<span class="kw1">for</span> <span class="br0">(</span><span class="sy4">;</span> n<span class="sy2">--</span> <span class="sy1">></span> <span class="nu0">0</span><span class="sy4">;</span> <span class="sy2">++</span>first<span class="br0">)</span>
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ranges<span class="sy4">::</span><span class="me2">construct_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>first<span class="br0">)</span>, x<span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">return</span> first<span class="sy4">;</span>
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<span class="br0">}</span> <span class="kw1">catch</span> <span class="br0">(</span>...<span class="br0">)</span> <span class="br0">{</span> <span class="co1">// 回滚：销毁已构造的元素</span>
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<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>
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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>
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<span class="kw1">throw</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="br0">}</span>
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<span class="br0">}</span><span class="sy4">;</span>
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<span class="kw1">inline</span> <span class="kw4">constexpr</span> uninitialized_fill_n_fn uninitialized_fill_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">示例</span></h3>
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<div class="t-example">
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<div class="t-example-live-link"></div>
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<div dir="ltr" class="mw-geshi" style="text-align: left;">
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<div class="cpp source-cpp">
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<pre class="de1"><span class="co2">#include <iostream></span>
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<span class="co2">#include <memory></span>
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<span class="co2">#include <string></span>
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<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
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<span class="br0">{</span>
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<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>
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alignas<span class="br0">(</span>alignof<span class="br0">(</span><a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a><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><a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a><span class="br0">)</span><span class="br0">]</span><span class="sy4">;</span>
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<span class="kw1">try</span>
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<span class="br0">{</span>
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<span class="kw4">auto</span> first <span class="br0">{</span><span class="kw1">reinterpret_cast</span><span class="sy1"><</span><a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a><span class="sy2">*</span><span class="sy1">></span><span class="br0">(</span>out<span class="br0">)</span><span class="br0">}</span><span class="sy4">;</span>
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<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_fill_n</span><span class="br0">(</span>first, n, <span class="st0">"cppreference"</span><span class="br0">)</span><span class="sy4">;</span>
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<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>
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<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="sy2">*</span>it <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
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<span class="br0">}</span>
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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>
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<span class="br0">}</span>
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<span class="kw1">catch</span><span class="br0">(</span>...<span class="br0">)</span>
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<span class="br0">{</span>
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<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"Exception!<span class="es1">\n</span>"</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="br0">}</span></pre></div>
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</div>
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<p>输出：</p>
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<div dir="ltr" class="mw-geshi" style="text-align: left;">
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<div class="text source-text">
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<pre class="de1">cppreference
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cppreference
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cppreference</pre></div>
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</div>
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</div>
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<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">参阅</span></h3>
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<table class="t-dsc-begin">
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<tr class="t-dsc">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-memory-ranges-uninitialized_fill.html"><span class="t-lines"><span>ranges::uninitialized_fill</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span></span></div>
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</div>
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</td>
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<td>复制一个对象到范围所定义的未初始化的内存区域<br />
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<span class="t-mark">(niebloid)</span></td>
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</tr>
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<tr class="t-dsc">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-memory-uninitialized_fill_n.html"><span class="t-lines"><span>uninitialized_fill_n</span></span></a></div>
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</div>
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</td>
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<td>复制一个对象到以起点和计数定义的未初始化内存区域<br />
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<span class="t-mark">(函数模板)</span></td>
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</tr>
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</table>
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</div>
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<div class="visualClear"></div>
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</div>
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