147 lines
11 KiB
HTML
147 lines
11 KiB
HTML
<!DOCTYPE html>
|
|
<html lang="zh" dir="ltr" class="client-nojs" xmlns="http://www.w3.org/1999/xhtml" xml:lang="zh">
|
|
<head><meta http-equiv="x-ua-compatible" content="ie=edge">
|
|
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
|
|
<title>std::allocator<T>::allocate</title>
|
|
<link rel="stylesheet" href="ext.css" />
|
|
<link rel="stylesheet" href="site_modules.css" />
|
|
</head>
|
|
<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_memory_allocator_allocate skin-cppreference2 action-view cpp-navbar">
|
|
<div id="cpp-content-base">
|
|
<div id="content"><a id="top"></a>
|
|
<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::allocator<T>::</span>allocate</h1>
|
|
<div id="bodyContent">
|
|
<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>
|
|
<div id="mw-content-text" lang="zh" dir="ltr" class="mw-content-ltr" xml:lang="zh">
|
|
<table class="t-dcl-begin">
|
|
<tbody></tbody>
|
|
<tbody class="t-dcl-rev t-dcl-rev-num">
|
|
<tr class="t-dcl-rev-aux">
|
|
<td></td>
|
|
<td rowspan="142">(1)</td>
|
|
<td></td>
|
|
</tr>
|
|
<tr class="t-dcl t-until-cxx17">
|
|
<td>
|
|
<div><span class="mw-geshi cpp source-cpp">pointer allocate<span class="br0">(</span> size_type n, <span class="kw4">const</span> <span class="kw4">void</span> <span class="sy2">*</span> hint <span class="sy1">=</span> <span class="nu0">0</span> <span class="br0">)</span><span class="sy4">;</span></span></div>
|
|
</td>
|
|
<td class="t-dcl-nopad"></td>
|
|
<td><span class="t-mark-rev t-until-cxx17">(C++17 前)</span></td>
|
|
</tr>
|
|
<tr class="t-dcl t-since-cxx17 t-until-cxx20">
|
|
<td>
|
|
<div><span class="mw-geshi cpp source-cpp">T<span class="sy2">*</span> allocate<span class="br0">(</span> <a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> n, <span class="kw4">const</span> <span class="kw4">void</span> <span class="sy2">*</span> hint<span class="br0">)</span><span class="sy4">;</span></span></div>
|
|
</td>
|
|
<td class="t-dcl-nopad"></td>
|
|
<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span><br />
|
|
<span class="t-mark">(弃用)</span><br />
|
|
<span class="t-mark-rev t-until-cxx20">(C++20 中移除)</span></td>
|
|
</tr>
|
|
</tbody>
|
|
<tbody></tbody>
|
|
<tbody class="t-dcl-rev t-dcl-rev-num">
|
|
<tr class="t-dcl-rev-aux">
|
|
<td></td>
|
|
<td rowspan="142">(2)</td>
|
|
<td></td>
|
|
</tr>
|
|
<tr class="t-dcl t-since-cxx17 t-until-cxx20">
|
|
<td>
|
|
<div><span class="mw-geshi cpp source-cpp">T<span class="sy2">*</span> allocate<span class="br0">(</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="sy4">;</span></span></div>
|
|
</td>
|
|
<td class="t-dcl-nopad"></td>
|
|
<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span><br />
|
|
<span class="t-mark-rev t-until-cxx20">(C++20 前)</span></td>
|
|
</tr>
|
|
<tr class="t-dcl t-since-cxx20">
|
|
<td>
|
|
<div><span class="mw-geshi cpp source-cpp"><span class="br0">[</span><span class="br0">[</span>nodiscard<span class="br0">]</span><span class="br0">]</span> <span class="kw4">constexpr</span> T<span class="sy2">*</span> allocate<span class="br0">(</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="sy4">;</span></span></div>
|
|
</td>
|
|
<td class="t-dcl-nopad"></td>
|
|
<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
|
|
</tr>
|
|
</tbody>
|
|
<tbody>
|
|
<tr class="t-dcl-sep">
|
|
<td></td>
|
|
<td></td>
|
|
<td></td>
|
|
</tr>
|
|
</tbody>
|
|
</table>
|
|
<p>调用 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy4">::</span><a href="cpp-memory-new-operator_new.html"><span class="kw687">operator new</span></a><span class="br0">(</span><a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a><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_new.html"><span class="kw687">operator new</span></a><span class="br0">(</span><a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a>, <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>分配 <span class="t-c"><span class="mw-geshi cpp source-cpp">n <span class="sy2">*</span> sizeof<span class="br0">(</span>T<span class="br0">)</span></span></span> 字节的未初始化存储，但何时及如何调用此函数是未指定的。指针 <code>hint</code> 可用于提供引用的局部性：若实现支持，则 allocator 会试图分配尽可能接近 <code>hint</code> 的新内存块。</p>
|
|
<p>然后，此函数在该存储中创建一个 <span class="t-c"><span class="mw-geshi cpp source-cpp">T<span class="br0">[</span>n<span class="br0">]</span></span></span> 数组并开始其生存期，但不开始其任何元素的生存期。</p>
|
|
<p>若 <code>T</code> 为<a href="cpp-language-type.html#.E4.B8.8D.E5.AE.8C.E6.95.B4.E7.B1.BB.E5.9E.8B">不完整类型</a>则此函数的使用为非良构。</p>
|
|
<table class="t-rev-begin">
|
|
<tr class="t-rev t-since-cxx20">
|
|
<td>
|
|
<p>为了在常量表达式中使用此函数，必须在同一表达式的求值内解分配其所分配的存储。</p>
|
|
</td>
|
|
<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
|
|
</tr>
|
|
</table>
|
|
<h3><span class="mw-headline" id=".E5.8F.82.E6.95.B0">参数</span></h3>
|
|
<table class="t-par-begin">
|
|
<tr class="t-par">
|
|
<td>n</td>
|
|
<td>-</td>
|
|
<td>要分配存储的对象数</td>
|
|
</tr>
|
|
<tr class="t-par">
|
|
<td>hint</td>
|
|
<td>-</td>
|
|
<td>指向临近内存位置的指针</td>
|
|
</tr>
|
|
</table>
|
|
<h3><span class="mw-headline" id=".E8.BF.94.E5.9B.9E.E5.80.BC">返回值</span></h3>
|
|
<p>指向 <code>T</code> 类型的 <code>n</code> 个对象的数组首元素的指针，数组元素尚未构造。</p>
|
|
<h3><span class="mw-headline" id=".E5.BC.82.E5.B8.B8">异常</span></h3>
|
|
<table class="t-rev-begin">
|
|
<tr class="t-rev t-since-cxx11">
|
|
<td>
|
|
<p>若 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-numeric_limits.html"><span class="kw344">std::<span class="me2">numeric_limits</span></span></a><span class="sy1"><</span><a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a><span class="sy1">></span><span class="sy4">::</span><span class="me2">max</span><span class="br0">(</span><span class="br0">)</span> <span class="sy2">/</span> sizeof<span class="br0">(</span>T<span class="br0">)</span> <span class="sy1"><</span> n</span></span> 则抛出 <span class="t-lc"><a href="cpp-memory-new-bad_array_new_length.html">std::bad_array_new_length</a></span> 。</p>
|
|
</td>
|
|
<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
|
|
</tr>
|
|
</table>
|
|
<p>若分配失败则抛出 <span class="t-lc"><a href="cpp-memory-new-bad_alloc.html">std::bad_alloc</a></span> 。</p>
|
|
<h3><span class="mw-headline" id=".E6.B3.A8.E8.A7.A3">注解</span></h3>
|
|
<p>遣词“未指定何时及如何”令标准库容器可以<a href="cpp-language-new.html#.E5.88.86.E9.85.8D">组合或优化掉堆分配</a>，即使对直接调用 <code>::operator new</code> 禁止这种优化。例如 libc++ 实现了它（ <a rel="nofollow" class="external autonumber" href="https://github.com/llvm-mirror/libcxx/blob/master@%7B2017-02-09%7D/include/memory#L1766-L1772">[1]</a> 与 <a rel="nofollow" class="external autonumber" href="https://github.com/llvm-mirror/libcxx/blob/master@%7B2017-02-09%7D/include/new#L211-L217">[2]</a> ）</p>
|
|
<p>在调用 <code>allocate</code> 后、构造元素前， <span class="t-c"><span class="mw-geshi cpp source-cpp">T<span class="sy2">*</span></span></span> 的指针算术在分配的数组内是良定义的，但若访问元素则行为未定义。</p>
|
|
<h3><span class="mw-headline" id=".E7.BC.BA.E9.99.B7.E6.8A.A5.E5.91.8A">缺陷报告</span></h3>
|
|
<p>下列更改行为的缺陷报告追溯地应用于以前出版的 C++ 标准。</p>
|
|
<table class="dsctable" style="font-size:0.8em">
|
|
<tr>
|
|
<th>DR</th>
|
|
<th>应用于</th>
|
|
<th>出版时的行为</th>
|
|
<th>正确行为</th>
|
|
</tr>
|
|
<tr>
|
|
<td><a rel="nofollow" class="external text" href="https://cplusplus.github.io/LWG/issue3190">LWG 3190</a></td>
|
|
<td>C++11</td>
|
|
<td><code>allocate</code> 可能分配大小错误的存储</td>
|
|
<td>替而抛出 <code>bad_array_new_length</code></td>
|
|
</tr>
|
|
</table>
|
|
<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">参阅</span></h3>
|
|
<table class="t-dsc-begin">
|
|
<tr class="t-dsc">
|
|
<td>
|
|
<div class="t-dsc-member-div">
|
|
<div><a href="cpp-memory-allocator_traits-allocate.html"><span class="t-lines"><span>allocate</span></span></a></div>
|
|
<div><span class="t-lines"><span><span class="t-cmark">[静态]</span></span></span></div>
|
|
</div>
|
|
</td>
|
|
<td>用分配器分配未初始化的存储<br />
|
|
<span class="t-mark">(<code>std::allocator_traits<Alloc></code> 的公开静态成员函数)</span></td>
|
|
</tr>
|
|
</table>
|
|
</div>
|
|
<div class="visualClear"></div>
|
|
</div>
|
|
</div>
|
|
</div>
|
|
</body>
|
|
</html>
|