187 lines
12 KiB
HTML
187 lines
12 KiB
HTML
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<title>std::destroy_at</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_memory_destroy_at 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::</span>destroy_at</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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<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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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1"><</span> <span class="kw1">class</span> T <span class="sy1">></span><br />
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<span class="kw4">void</span> destroy_at<span class="br0">(</span> T<span class="sy2">*</span> p <span class="br0">)</span><span class="sy4">;</span></span></div>
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<td class="t-dcl-nopad"></td>
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<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span><br />
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<span class="t-mark-rev t-until-cxx20">(C++20 前)</span></td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1"><</span> <span class="kw1">class</span> T <span class="sy1">></span><br />
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<span class="kw4">constexpr</span> <span class="kw4">void</span> destroy_at<span class="br0">(</span> T<span class="sy2">*</span> p <span class="br0">)</span><span class="sy4">;</span></span></div>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
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<p>若 <code>T</code> 不是数组类型，则调用 <code>p</code> 所指向对象的析构函数，如同用 <code>p->~T()</code> 。</p>
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<p>若 <code>T</code> 是数组类型，则<span class="t-rev-inl t-until-cxx20"><span>程序非良构</span> <span><span class="t-mark-rev t-until-cxx20">(C++20 前)</span></span></span><span class="t-rev-inl t-since-cxx20"><span>按顺序递归地销毁 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>p</span></span> 的元素，如同通过调用 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-memory-destroy.html"><span class="kw727">std::<span class="me2">destroy</span></span></a><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><span class="sy2">*</span>p<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><span class="sy2">*</span>p<span class="br0">)</span><span class="br0">)</span></span></span></span> <span><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></span></span>。</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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<td>p</td>
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<td>-</td>
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<td>指向要被销毁的对象的指针</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>（无）</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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<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">template</span><span class="sy1"><</span><span class="kw1">class</span> T<span class="sy1">></span>
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<span class="kw4">constexpr</span> <span class="kw4">void</span> destroy_at<span class="br0">(</span>T<span class="sy2">*</span> p<span class="br0">)</span>
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<span class="br0">{</span>
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<span class="kw1">if</span> <span class="kw4">constexpr</span> <span class="br0">(</span><a href="cpp-types-is_array.html"><span class="kw461">std::<span class="me2">is_array_v</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="br0">)</span>
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<span class="kw1">for</span> <span class="br0">(</span><span class="kw4">auto</span> <span class="sy3">&</span>elem <span class="sy4">:</span> <span class="sy2">*</span>p<span class="br0">)</span>
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<span class="br0">(</span>destroy_at<span class="br0">)</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>elem<span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">else</span>
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p<span class="sy2">-</span><span class="sy1">></span>~T<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="co1">// C++17 版本：</span>
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<span class="co1">// template<class T> void destroy_at(T* p) { p->~T(); }</span></pre></div>
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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><code>destroy_at</code> 推导要销毁的对象类型，从而避免在析构函数调用中显式写出它。</p>
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<p>在某<a href="cpp-language-constant_expression.html">常量表达式</a> <span class="t-c"><span class="mw-geshi cpp source-cpp">e</span></span> 的求值中调用 <code>destroy_at</code> 时，参数 <code>p</code> 必须指向生存期始于 <span class="t-c"><span class="mw-geshi cpp source-cpp">e</span></span> 的求值内的对象。</p>
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</td>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></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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<p>下列示例演示如何用 <code>destroy_at</code> 销毁元素的相接序列。</p>
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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 <memory></span>
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<span class="co2">#include <new></span>
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<span class="co2">#include <iostream></span>
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<span class="kw1">struct</span> Tracer <span class="br0">{</span>
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<span class="kw4">int</span> value<span class="sy4">;</span>
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~Tracer<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"><<</span> value <span class="sy1"><<</span> <span class="st0">" destructed<span class="es1">\n</span>"</span><span class="sy4">;</span> <span class="br0">}</span>
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<span class="br0">}</span><span class="sy4">;</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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alignas<span class="br0">(</span>Tracer<span class="br0">)</span> <span class="kw4">unsigned</span> <span class="kw4">char</span> buffer<span class="br0">[</span>sizeof<span class="br0">(</span>Tracer<span class="br0">)</span> <span class="sy2">*</span> <span class="nu0">8</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">int</span> i <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span> i <span class="sy1"><</span> <span class="nu0">8</span><span class="sy4">;</span> <span class="sy2">++</span>i<span class="br0">)</span>
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new<span class="br0">(</span>buffer <span class="sy2">+</span> sizeof<span class="br0">(</span>Tracer<span class="br0">)</span> <span class="sy2">*</span> i<span class="br0">)</span> Tracer<span class="br0">{</span>i<span class="br0">}</span><span class="sy4">;</span> <span class="co1">// 手工构造对象</span>
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<span class="kw4">auto</span> ptr <span class="sy1">=</span> <a href="cpp-utility-launder.html"><span class="kw1121">std::<span class="me2">launder</span></span></a><span class="br0">(</span><span class="kw1">reinterpret_cast</span><span class="sy1"><</span>Tracer<span class="sy2">*</span><span class="sy1">></span><span class="br0">(</span>buffer<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">int</span> i <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span> i <span class="sy1"><</span> <span class="nu0">8</span><span class="sy4">;</span> <span class="sy2">++</span>i<span class="br0">)</span>
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std<span class="sy4">::</span><span class="me2">destroy_at</span><span class="br0">(</span>ptr <span class="sy2">+</span> i<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>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">0 destructed
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1 destructed
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2 destructed
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3 destructed
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4 destructed
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5 destructed
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6 destructed
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7 destructed</pre></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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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-memory-destroy.html"><span class="t-lines"><span>destroy</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>销毁一个范围中的对象<br />
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<span class="t-mark">(函数模板)</span></td>
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</tr>
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<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-destroy_n.html"><span class="t-lines"><span>destroy_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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</div>
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</td>
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<td>销毁范围中一定数量的对象<br />
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<span class="t-mark">(函数模板)</span></td>
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</tr>
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<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-construct_at.html"><span class="t-lines"><span>construct_at</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>在给定地址创建对象<br />
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<span class="t-mark">(函数模板)</span></td>
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</tr>
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<tr class="t-dsc">
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<div class="t-dsc-member-div">
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<div><a href="cpp-memory-ranges-destroy_at.html"><span class="t-lines"><span>ranges::destroy_at</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>销毁位于给定地址的元素<br />
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<span class="t-mark">(niebloid)</span></td>
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</tr>
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</table>
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