248 lines
25 KiB
HTML
248 lines
25 KiB
HTML
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<title>std::ranges::uninitialized_default_construct</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_memory_ranges_uninitialized_default_construct 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_default_construct</h1>
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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>定义于头文件 <code><a href="cpp-header-memory.html"><memory></a></code></div>
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<div>调用签名</div>
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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, no<span class="sy2">-</span>throw<span class="sy2">-</span>sentinel<span class="sy2">-</span><span class="kw1">for</span><span class="sy1"><</span>I<span class="sy1">></span> S<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_default_construct<span class="br0">(</span> I first, S last <span class="br0">)</span><span class="sy4">;</span></div>
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<td>(1)</td>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></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 R<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>ranges<span class="sy4">::</span><span class="me2">range_value_t</span><span class="sy1"><</span>R<span class="sy1">>></span><br />
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ranges<span class="sy4">::</span><span class="me2">borrowed_iterator_t</span><span class="sy1"><</span>R<span class="sy1">></span><br /></p>
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uninitialized_default_construct<span class="br0">(</span> R<span class="sy3">&&</span> r <span class="br0">)</span><span class="sy4">;</span></div>
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<td>(2)</td>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
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<div class="t-li1"><span class="t-li">1)</span> 在 <code>[first, last)</code> 所指代的未初始化范围中构造以<a href="cpp-language-default_initialization.html">默认初始化</a>构造 <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
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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> first <span class="sy3">!</span><span class="sy1">=</span> last<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>
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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="sy4">;</span></pre></div>
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<div class="t-li1"><span class="t-li"></span> 若在初始化期间抛出异常，则以未指定顺序销毁已构造的对象。</div>
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<div class="t-li1"><span class="t-li">2)</span> 同 <span class="t-v">(1)</span> ，但以 <code>r</code> 为范围，如同以 <span class="t-c"><span class="mw-geshi cpp source-cpp">ranges<span class="sy4">::</span><span class="me2">begin</span><span class="br0">(</span>r<span class="br0">)</span></span></span> 为 <code>first</code> 并以 <span class="t-c"><span class="mw-geshi cpp source-cpp">ranges<span class="sy4">::</span><span class="me2">end</span><span class="br0">(</span>r<span class="br0">)</span></span></span> 为 <code>last</code> 。</div>
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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 class="t-par">
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<td>r</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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</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>等于 <code>last</code> 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 <code>first</code> 和 <code>last</code> 间的距离成线性。</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>若默认初始化 <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> 对象时不调用非平凡的默认构造函数，则实现可以跳过对象构造（而不更改可观察作用），这能由 <span class="t-lc"><a href="cpp-types-is_default_constructible.html">std::is_trivially_default_constructible_v</a></span> 检测。</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 class="cpp source-cpp">
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<pre class="de1"><span class="kw1">struct</span> uninitialized_default_construct_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, no<span class="sy2">-</span>throw<span class="sy2">-</span>sentinel<span class="sy2">-</span><span class="kw1">for</span><span class="sy1"><</span>I<span class="sy1">></span> S<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, S last <span class="br0">)</span> <span class="kw4">const</span> <span class="br0">{</span>
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<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"><</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_default_constructible.html"><span class="kw529">std::<span class="me2">is_trivially_default_constructible_v</span></span></a><span class="sy1"><</span>ValueType<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">next</span><span class="br0">(</span>first, last<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 跳过初始化</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> <span class="sy3">!</span><span class="br0">(</span>first <span class="sy1">==</span> last<span class="br0">)</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> ValueType<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="kw1">template</span> <span class="sy1"><</span>no<span class="sy2">-</span>throw<span class="sy2">-</span>forward<span class="sy2">-</span>range R<span class="sy1">></span>
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requires std<span class="sy4">::</span><span class="me2">default_initializable</span><span class="sy1"><</span>ranges<span class="sy4">::</span><span class="me2">range_value_t</span><span class="sy1"><</span>R<span class="sy1">>></span>
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ranges<span class="sy4">::</span><span class="me2">borrowed_iterator_t</span><span class="sy1"><</span>R<span class="sy1">></span>
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operator<span class="br0">(</span><span class="br0">)</span><span class="br0">(</span> R<span class="sy3">&&</span> r <span class="br0">)</span> <span class="kw4">const</span> <span class="br0">{</span>
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<span class="kw1">return</span> <span class="br0">(</span><span class="sy2">*</span>this<span class="br0">)</span><span class="br0">(</span>ranges<span class="sy4">::</span><span class="me2">begin</span><span class="br0">(</span>r<span class="br0">)</span>, ranges<span class="sy4">::</span><span class="me2">end</span><span class="br0">(</span>r<span class="br0">)</span><span class="br0">)</span><span class="sy4">;</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_default_construct_fn uninitialized_default_construct<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 <cstring></span>
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<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="br0">{</span>first <span class="sy2">+</span> n<span class="br0">}</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_default_construct</span><span class="br0">(</span>first, last<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> <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> <span class="st0">"Exception!<span class="es1">\n</span>"</span><span class="sy4">;</span> <span class="br0">}</span>
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<span class="co1">// 注意对于“平凡类型” uninitialized_default_construct 通常不以零填充给定的未初始化内存区域。</span>
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<span class="kw4">constexpr</span> <span class="kw4">char</span> etalon<span class="br0">[</span><span class="br0">]</span> <span class="br0">{</span> <span class="st0">'A'</span>, <span class="st0">'B'</span>, <span class="st0">'C'</span>, <span class="st0">'D'</span>, <span class="st0">'<span class="es1">\n</span>'</span> <span class="br0">}</span><span class="sy4">;</span>
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<span class="kw4">char</span> v<span class="br0">[</span><span class="br0">]</span> <span class="br0">{</span> <span class="st0">'A'</span>, <span class="st0">'B'</span>, <span class="st0">'C'</span>, <span class="st0">'D'</span>, <span class="st0">'<span class="es1">\n</span>'</span> <span class="br0">}</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_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>
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<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>
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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="co1">// 可能为未定义行为，待定的 CWG 1997 ：</span>
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<span class="co1">// for (const char c : v) { std::cout << c << ' '; }</span>
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<span class="kw1">for</span> <span class="br0">(</span><span class="kw4">const</span> <span class="kw4">char</span> c <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"><<</span> c <span class="sy1"><<</span> <span class="st0">' '</span><span class="sy4">;</span> <span class="br0">}</span>
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<span class="br0">}</span> <span class="kw1">else</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">"Unspecified<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>可能的输出：</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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A B C D</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_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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</div>
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</td>
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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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</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-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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</div>
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</td>
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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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</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-ranges-uninitialized_value_construct_n.html"><span class="t-lines"><span>ranges::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-cxx20">(C++20)</span></span></span></div>
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</div>
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</td>
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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-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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</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_default_construct.html"><span class="t-lines"><span>uninitialized_default_construct</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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</div>
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</td>
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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">(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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</div>
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</div>
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</body>
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</html>
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