95 lines
6.4 KiB
HTML
95 lines
6.4 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::assume_aligned</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_memory_assume_aligned 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>assume_aligned</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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<tbody>
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<tr class="t-dsc-header">
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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 t-since-cxx20">
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1"><</span> <a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> N, <span class="kw1">class</span> T <span class="sy1">></span><br />
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<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> assume_aligned<span class="br0">(</span>T<span class="sy2">*</span> ptr<span class="br0">)</span><span class="sy4">;</span></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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<p>告知编译器 <code>ptr</code> 所指向的对象至少对齐到 <code>N</code> 。实现可用此信息生成更高效的代码，但仅会对通过 <code>assume_aligned</code> 的返回值进行的访问做此假设。</p>
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<p>若 <code>N</code> 不是 2 的幂则程序非良构。若 <code>ptr</code> 不指向 <code>T</code> 类型对象（忽略每层的 cv 限定）或对象的对齐不至少为 <code>N</code> ，则行为未定义。</p>
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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><code>ptr</code> 。</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>不抛出。</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>为确保程序能从以 <code>assume_aligned</code> 启用的优化受益，最好通过其返回值访问对象：</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="kw4">void</span> f<span class="br0">(</span><span class="kw4">int</span><span class="sy2">*</span> p<span class="br0">)</span> <span class="br0">{</span>
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<span class="kw4">int</span><span class="sy2">*</span> p1 <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">assume_aligned</span><span class="sy1"><</span><span class="nu0">256</span><span class="sy1">></span><span class="br0">(</span>p<span class="br0">)</span><span class="sy4">;</span>
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<span class="co1">// 用 p1 而非 p ，以确保从对齐假设受益。</span>
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<span class="co1">// 然而，若 p 未对齐则程序有未定义行为，无关乎是否使用 p1 。</span>
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<span class="br0">}</span></pre></div>
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</div>
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<p>交给程序员确保对齐假设确实成立。调用 <code>assume_aligned</code> 不导致编译器检查或强制这点。</p>
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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><a href="cpp-language-alignof.html"><code>alignof</code> 运算符</a><span class="t-mark-rev t-since-cxx11">(C++11)</span></td>
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<td>查询类型的对齐要求</td>
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</tr>
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<tr class="t-dsc">
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<td><a href="cpp-language-alignas.html"><code>alignas</code> 说明符</a><span class="t-mark-rev t-since-cxx11">(C++11)</span></td>
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<td>指定该变量的存储应该按指定量对齐</td>
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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-types-aligned_storage.html"><span class="t-lines"><span>aligned_storage</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div>
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</div>
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</td>
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<td>定义适于用作给定大小的类型的未初始化存储的类型<br />
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<span class="t-mark">(类模板)</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-align.html"><span class="t-lines"><span>align</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div>
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</div>
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</td>
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<td>对齐缓冲区中的指针<br />
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<span class="t-mark">(函数)</span></td>
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</tr>
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