225 lines
22 KiB
HTML
225 lines
22 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_value_construct_n</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_memory_ranges_uninitialized_value_construct_n skin-cppreference2 action-view cpp-navbar">
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::ranges::</span>uninitialized_value_construct_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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<td></td>
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<tr class="t-dcl-h">
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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>iterator I<span class="sy1">></span><br /></span>
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<p>requires std<span class="sy4">::</span><span class="me2">default_initializable</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><br /></p>
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I uninitialized_value_construct_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="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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</tr>
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<tr class="t-dcl-sep">
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<td></td>
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<td></td>
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</tbody>
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<p>以<a href="cpp-language-value_initialization.html">值初始化</a>在始于 <code>first</code> 的未初始化存储中构造 <code>n</code> 个 <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"><</span>I<span class="sy1">></span></span></span> 类型对象，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="sy1">></span> <span class="nu0">0</span><span class="sy4">;</span> <span class="sy2">++</span>first<span class="br0">)</span>
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<span class="sy4">::</span><span class="me2">new</span> <span class="br0">(</span><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><span class="br0">)</span>
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<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><span class="br0">)</span><span class="sy4">;</span></pre></div>
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</div>
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<p>若初始化期间抛出异常，则按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, last</td>
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<td>-</td>
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<td>要初始化的元素范围</td>
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</tr>
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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>要构造的元素数</td>
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</tr>
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</table>
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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>对象范围的末尾（即 <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> ）。</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> <i>及</i><a href="cpp-named_req-CopyAssignable.html"><span style="font-style:italic">可复制赋值</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(CopyAssignable)</span></a> ，则实现可以提升 <code>ranges::uninitialized_value_construct_n</code> 的效率，例如用 <span class="t-lc">ranges::fill</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_value_construct_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>iterator I<span class="sy1">></span>
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requires std<span class="sy4">::</span><span class="me2">default_initializable</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>
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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="br0">)</span> <span class="kw4">const</span> <span class="br0">{</span>
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<span class="kw1">using</span> T <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"><</span>std<span class="sy4">::</span><span class="me2">iter_reference_t</span><span class="sy1"><</span>I<span class="sy1">>></span><span class="sy4">;</span>
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<span class="kw1">if</span> <span class="kw4">constexpr</span> <span class="br0">(</span><a href="cpp-types-is_trivial.html"><span class="kw499">std::<span class="me2">is_trivial_v</span></span></a><span class="sy1"><</span>T<span class="sy1">></span> <span class="sy3">&&</span> <a href="cpp-types-is_copy_assignable.html"><span class="kw551">std::<span class="me2">is_copy_assignable_v</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="br0">)</span>
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<span class="kw1">return</span> ranges<span class="sy4">::</span><span class="me2">fill_n</span><span class="br0">(</span>first, n, T<span class="br0">(</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</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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<span class="sy4">::</span><span class="me2">new</span> <span class="br0">(</span><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>
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<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> T<span class="br0">(</span><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_value_construct_n_fn uninitialized_value_construct_n<span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span></pre></div>
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</td>
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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="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">"█▓▒░ █▓▒░ █▓▒░ "</span> <span class="br0">}</span><span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span>
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<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>
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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>
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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>S<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_value_construct_n</span><span class="br0">(</span>first, n<span class="br0">)</span><span class="sy4">;</span>
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<span class="kw4">auto</span> count <span class="br0">{</span><span class="nu0">1</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> count<span class="sy2">++</span> <span class="sy1"><<</span> <span class="st0">' '</span> <span class="sy1"><<</span> it<span class="sy2">-</span><span class="sy1">></span>m <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="co1">// 注意对于“平凡类型” the uninitialized_value_construct_n 零初始化给定的未初始化内存区域。</span>
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<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="nu0">7</span>, <span class="nu0">8</span> <span class="br0">}</span><span class="sy4">;</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">' '</span><span class="sy4">;</span>
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<span class="kw1">for</span> <span class="br0">(</span><span class="kw4">const</span> <span class="kw4">int</span> i <span class="sy4">:</span> v<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"><<</span> i <span class="sy1"><<</span> <span class="st0">' '</span><span class="sy4">;</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="st0">"<span class="es1">\n</span> "</span><span class="sy4">;</span>
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std<span class="sy4">::</span><span class="me2">ranges</span><span class="sy4">::</span><span class="me2">uninitialized_value_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>
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<span class="kw1">for</span> <span class="br0">(</span><span class="kw4">const</span> <span class="kw4">int</span> i <span class="sy4">:</span> v<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"><<</span> i <span class="sy1"><<</span> <span class="st0">' '</span><span class="sy4">;</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="st0">'<span class="es1">\n</span>'</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>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">1 █▓▒░ █▓▒░ █▓▒░
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2 █▓▒░ █▓▒░ █▓▒░
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3 █▓▒░ █▓▒░ █▓▒░
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4 █▓▒░ █▓▒░ █▓▒░
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1 2 3 4 5 6 7 8
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0 0 0 0 0 0 0 0</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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<div><a href="cpp-memory-ranges-uninitialized_value_construct.html"><span class="t-lines"><span>ranges::uninitialized_value_construct</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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<td>在范围所定义的未初始化的内存区域以<a href="cpp-language-value_initialization.html">值初始化</a>构造对象<br />
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<span class="t-mark">(niebloid)</span></td>
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<div><a href="cpp-memory-ranges-uninitialized_default_construct.html"><span class="t-lines"><span>ranges::uninitialized_default_construct</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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<td>在范围所定义的未初始化的内存区域以<a href="cpp-language-default_initialization.html">默认初始化</a>构造对象<br />
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<span class="t-mark">(niebloid)</span></td>
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<div><a href="cpp-memory-ranges-uninitialized_default_construct_n.html"><span class="t-lines"><span>ranges::uninitialized_default_construct_n</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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<td>在起始与计数所定义的未初始化的内存区域以<a href="cpp-language-default_initialization.html">默认初始化</a>构造对象<br />
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<span class="t-mark">(niebloid)</span></td>
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<div><a href="cpp-memory-uninitialized_value_construct_n.html"><span class="t-lines"><span>uninitialized_value_construct_n</span></span></a></div>
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<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>
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<td>在起始和计数所定义的未初始化内存区域以<a href="cpp-language-value_initialization.html">值初始化</a>构造对象<br />
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<span class="t-mark">(函数模板)</span></td>
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