97 lines
8.3 KiB
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97 lines
8.3 KiB
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<title>C++ 属性： no_unique_address (C++20 起)</title>
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">C++ 属性：</span> no_unique_address <span class="t-mark-rev t-since-cxx20">(C++20 起)</span></h1>
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<div id="contentSub"><span class="subpages">< <a href="cpp.html">cpp</a>‎ | <a href="cpp-language.html">language</a>‎ | <a href="cpp-language-attributes.html">attributes</a></span></div>
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<p>指示此数据成员不需要具有不同于其类的所有其他非静态数据成员的地址。</p>
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<h3><span class="mw-headline" id=".E8.AF.AD.E6.B3.95">语法</span></h3>
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<td class="t-sdsc-nopad"><code><b>[[no_unique_address]]</b></code></td>
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<h3><span class="mw-headline" id=".E8.A7.A3.E9.87.8A">解释</span></h3>
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<p>应用到非位域非静态数据成员的声明中所声明的名字。</p>
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<p>指示此数据成员不需要具有不同于其类的所有其他非静态数据成员的地址。这表示若该成员拥有空类型（例如无状态分配器），则编译器可将它优化为不占空间，正如同假如它是<a href="cpp-language-ebo.html">空基类</a>一样。若该成员非空，则其中的任何尾随填充空间亦可复用于存储其他数据成员。</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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<pre class="de1"><span class="co2">#include <iostream></span>
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<span class="kw1">struct</span> Empty <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span> <span class="co1">// 空类</span>
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<span class="kw1">struct</span> X <span class="br0">{</span>
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<span class="kw4">int</span> i<span class="sy4">;</span>
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Empty e<span class="sy4">;</span>
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<span class="br0">}</span><span class="sy4">;</span>
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<span class="kw1">struct</span> Y <span class="br0">{</span>
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<span class="kw4">int</span> i<span class="sy4">;</span>
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<span class="br0">[</span><span class="br0">[</span>no_unique_address<span class="br0">]</span><span class="br0">]</span> Empty e<span class="sy4">;</span>
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<span class="br0">}</span><span class="sy4">;</span>
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<span class="kw1">struct</span> Z <span class="br0">{</span>
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<span class="kw4">char</span> c<span class="sy4">;</span>
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<span class="br0">[</span><span class="br0">[</span>no_unique_address<span class="br0">]</span><span class="br0">]</span> Empty e1, e2<span class="sy4">;</span>
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<span class="br0">}</span><span class="sy4">;</span>
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<span class="kw1">struct</span> W <span class="br0">{</span>
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<span class="kw4">char</span> c<span class="br0">[</span><span class="nu0">2</span><span class="br0">]</span><span class="sy4">;</span>
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<span class="br0">[</span><span class="br0">[</span>no_unique_address<span class="br0">]</span><span class="br0">]</span> Empty e1, e2<span class="sy4">;</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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<span class="co1">// 任何空类类型对象的大小至少为 1</span>
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static_assert<span class="br0">(</span>sizeof<span class="br0">(</span>Empty<span class="br0">)</span> <span class="sy1">>=</span> <span class="nu0">1</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="co1">// 至少需要多一个字节以给 e 唯一地址</span>
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static_assert<span class="br0">(</span>sizeof<span class="br0">(</span>X<span class="br0">)</span> <span class="sy1">>=</span> sizeof<span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span> <span class="sy2">+</span> <span class="nu0">1</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="co1">// 优化掉空成员</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">"sizeof(Y) == sizeof(int) is "</span> <span class="sy1"><<</span> <a href="cpp-io-manip-boolalpha.html"><span class="kw1764">std::<span class="me2">boolalpha</span></span></a>
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<span class="sy1"><<</span> <span class="br0">(</span>sizeof<span class="br0">(</span>Y<span class="br0">)</span> <span class="sy1">==</span> sizeof<span class="br0">(</span><span class="kw4">int</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>
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<span class="co1">// e1 与 e2 不能共享同一地址，因为它们拥有相同类型，尽管它们标记有 [[no_unique_address]]。</span>
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<span class="co1">// 然而，其中一者可以与 c 共享地址。</span>
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static_assert<span class="br0">(</span>sizeof<span class="br0">(</span>Z<span class="br0">)</span> <span class="sy1">>=</span> <span class="nu0">2</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="co1">// e1 与 e2 不能拥有同一地址，但它们之一能与 c[0] 共享，而另一者与 c[1] 共享</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">"sizeof(W) == 2 is "</span> <span class="sy1"><<</span> <span class="br0">(</span>sizeof<span class="br0">(</span>W<span class="br0">)</span> <span class="sy1">==</span> <span class="nu0">2</span><span class="br0">)</span> <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></pre></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">sizeof(Y) == sizeof(int) is true
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sizeof(W) == 2 is true</pre></div>
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