162 lines
15 KiB
HTML
162 lines
15 KiB
HTML
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<title>std::allocator<T>::deallocate</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_memory_allocator_deallocate 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::allocator<T>::</span>deallocate</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>‎ | <a href="cpp-memory-allocator.html">allocator</a></span></div>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">void</span> deallocate<span class="br0">(</span> T<span class="sy2">*</span> p, <a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> n <span class="br0">)</span><span class="sy4">;</span></span></div>
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<td><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="kw4">constexpr</span> <span class="kw4">void</span> deallocate<span class="br0">(</span> T<span class="sy2">*</span> p, <a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> n <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>从指针 <code>p</code> 所引用的存储解分配，指针必须是通过先前调用 <span class="t-lc"><a href="cpp-memory-allocator-allocate.html">allocate()</a></span> <span class="t-rev-inl t-since-cxx23"><span>或 <a href="cpp-memory-allocator-allocate_at_least.html"><code>allocate_at_least()</code></a></span> <span><span class="t-mark-rev t-since-cxx23">(C++23 起)</span></span></span> 获得的指针。</p>
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<p>参数 <code>n</code> 必须等于对原先生成 <code>p</code> 的 <span class="t-lc"><a href="cpp-memory-allocator-allocate.html">allocate()</a></span> 调用的首参数<span class="t-rev-inl t-since-cxx23"><span>，或若 <code>p</code> 由返回 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">{</span>p, count<span class="br0">}</span></span></span> 的调用 <span class="t-c"><span class="mw-geshi cpp source-cpp">allocate_at_least<span class="br0">(</span>m<span class="br0">)</span></span></span> 获得，则在范围 <code>[m, count]</code> 中</span> <span><span class="t-mark-rev t-since-cxx23">(C++23 起)</span></span></span>；否则行为未定义。</p>
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<p>调用 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy4">::</span><a href="cpp-memory-new-operator_delete.html"><span class="kw689">operator delete</span></a><span class="br0">(</span><span class="kw4">void</span><span class="sy2">*</span><span class="br0">)</span></span></span> <span class="t-rev-inl t-since-cxx17"><span>或 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy4">::</span><a href="cpp-memory-new-operator_delete.html"><span class="kw689">operator delete</span></a><span class="br0">(</span><span class="kw4">void</span><span class="sy2">*</span>, <a href="cpp-memory-new-align_val_t.html"><span class="kw697">std::<span class="me2">align_val_t</span></span></a><span class="br0">)</span></span></span></span> <span><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></span></span>，但何时及如何调用是未指定的。</p>
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<p>在常量表达式的求值中，此函数必须解分配在同一表达式的求值内分配的存储。</p>
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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=".E5.8F.82.E6.95.B0">参数</span></h3>
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<table class="t-par-begin">
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<td>p</td>
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<td>-</td>
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<td>从 <code>allocate()</code> <span class="t-rev-inl t-since-cxx23"><span>或 <a href="cpp-memory-allocator-allocate_at_least.html"><code>allocate_at_least()</code></a></span> <span><span class="t-mark-rev t-since-cxx23">(C++23 起)</span></span></span> 获得的指针</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>先前传递给 <code>allocate()</code> 的对象数<span class="t-rev-inl t-since-cxx23"><span>，或在经由 <a href="cpp-memory-allocator-allocate_at_least.html"><code>allocate_at_least()</code></a> 请求和实际分配的对象数之间的数（可以等于任一边界）</span> <span><span class="t-mark-rev t-since-cxx23">(C++23 起)</span></span></span></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=".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 <algorithm></span>
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<span class="co2">#include <cstddef></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="kw1">class</span> S <span class="br0">{</span>
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<span class="kw1">inline</span> <span class="kw4">static</span> <span class="kw4">int</span> n<span class="br0">{</span><span class="nu0">1</span><span class="br0">}</span><span class="sy4">;</span>
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<span class="kw4">int</span> m<span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>
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<span class="kw4">void</span> pre<span class="br0">(</span><span class="br0">)</span> <span class="kw4">const</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">"#"</span> <span class="sy1"><<</span> m <span class="sy1"><<</span> <a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a><span class="br0">(</span>m, <span class="st0">' '</span><span class="br0">)</span><span class="sy4">;</span> <span class="br0">}</span>
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<span class="kw1">public</span><span class="sy4">:</span>
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S<span class="br0">(</span><span class="kw4">int</span> x<span class="br0">)</span> <span class="sy4">:</span> m<span class="br0">{</span>n<span class="sy2">++</span><span class="br0">}</span> <span class="br0">{</span> pre<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</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">"S::S("</span> <span class="sy1"><<</span> x <span class="sy1"><<</span> <span class="st0">");<span class="es1">\n</span>"</span><span class="sy4">;</span> <span class="br0">}</span>
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~S<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> pre<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</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">"S::~S();<span class="es1">\n</span>"</span><span class="sy4">;</span> <span class="br0">}</span>
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<span class="kw4">void</span> id<span class="br0">(</span><span class="br0">)</span> <span class="kw4">const</span> <span class="br0">{</span> pre<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</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">"S::id();<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> <span class="br0">{</span>
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<span class="kw4">constexpr</span> <a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> n<span class="br0">{</span><span class="nu0">4</span><span class="br0">}</span><span class="sy4">;</span>
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<a href="cpp-memory-allocator.html"><span class="kw700">std::<span class="me2">allocator</span></span></a><span class="sy1"><</span>S<span class="sy1">></span> allocator<span class="sy4">;</span>
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<span class="kw1">try</span> <span class="br0">{</span>
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S<span class="sy2">*</span> s <span class="sy1">=</span> allocator.<span class="me1">allocate</span><span class="br0">(</span>n<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 可能抛出</span>
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<span class="kw1">for</span> <span class="br0">(</span><a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> i<span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span> i <span class="sy3">!</span><span class="sy1">=</span> n<span class="sy4">;</span> <span class="sy2">++</span>i<span class="br0">)</span> <span class="br0">{</span>
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<span class="co1">// allocator.construct(&s[i], i+42); // C++20 中移除</span>
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std<span class="sy4">::</span><span class="me2">construct_at</span><span class="br0">(</span><span class="sy3">&</span>s<span class="br0">[</span>i<span class="br0">]</span>, i<span class="sy2">+</span><span class="nu0">42</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// C++20 起</span>
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<span class="br0">}</span>
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<a href="cpp-algorithm-for_each_n.html"><span class="kw1569">std::<span class="me2">for_each_n</span></span></a><span class="br0">(</span>s, n, <span class="br0">[</span><span class="br0">]</span><span class="br0">(</span><span class="kw4">const</span> <span class="kw4">auto</span><span class="sy3">&</span> e<span class="br0">)</span> <span class="br0">{</span> e.<span class="me1">id</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="br0">}</span><span class="br0">)</span><span class="sy4">;</span>
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<a href="cpp-memory-destroy_n.html"><span class="kw729">std::<span class="me2">destroy_n</span></span></a><span class="br0">(</span>s, n<span class="br0">)</span><span class="sy4">;</span>
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allocator.<span class="me1">deallocate</span><span class="br0">(</span>s, n<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><a href="cpp-memory-new-bad_array_new_length.html"><span class="kw694">std::<span class="me2">bad_array_new_length</span></span></a> <span class="kw4">const</span><span class="sy3">&</span> ex<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> ex.<span class="me1">what</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span> <span class="br0">}</span>
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<span class="kw1">catch</span><span class="br0">(</span><a href="cpp-memory-new-bad_alloc.html"><span class="kw693">std::<span class="me2">bad_alloc</span></span></a> <span class="kw4">const</span><span class="sy3">&</span> ex<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> ex.<span class="me1">what</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span> <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">#1 S::S(42);
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#2 S::S(43);
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#3 S::S(44);
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#4 S::S(45);
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#1 S::id();
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#2 S::id();
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#3 S::id();
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#4 S::id();
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#1 S::~S();
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#2 S::~S();
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#3 S::~S();
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#4 S::~S();</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-allocator-allocate.html"><span class="t-lines"><span>allocate</span></span></a></div>
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</div>
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</td>
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<td>分配未初始化的存储<br />
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<span class="t-mark">(公开成员函数)</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-allocator-allocate_at_least.html"><span class="t-lines"><span>allocate_at_least</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx23">(C++23)</span></span></span></div>
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</div>
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</td>
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<td>分配与请求的大小至少一样大的未初始化存储<br />
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<span class="t-mark">(公开成员函数)</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-allocator_traits-deallocate.html"><span class="t-lines"><span>deallocate</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-cmark">[静态]</span></span></span></div>
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</div>
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</td>
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<td>用分配器解分配存储<br />
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<span class="t-mark">(<code>std::allocator_traits<Alloc></code> 的公开静态成员函数)</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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