65 lines
7.1 KiB
HTML
65 lines
7.1 KiB
HTML
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<title>constinit 说明符 (C++20 起)</title>
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<h1 id="firstHeading" class="firstHeading">constinit 说明符 <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></span></div>
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<li><code><b>constinit</b></code> - 断言变量拥有静态初始化，即<a href="cpp-language-zero_initialization.html">零初始化</a>与<a href="cpp-language-constant_initialization.html">常量初始化</a>，否则程序非良构。</li>
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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><code>constinit</code> 说明符声明拥有静态或线程<a href="cpp-language-storage_duration.html">存储期</a>的变量。若变量以 <code>constinit</code> 声明，则其<a href="cpp-language-initialization.html">初始化声明</a>必须应用 <code>constinit</code> 。若以 <code>constinit</code> 声明的变量拥有动态初始化，则程序非良构。若在初始化声明点无可达的 <code>constinit</code> 声明，则程序非良构，不需要诊断。</p>
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<p><code>constinit</code> 不能和 <code>constexpr</code> 或 <code>consteval</code> 一同使用。声明的变量为引用时， <code>constinit</code> 等价于 <code>constexpr</code> 。声明的变量为对象时， <code>constexpr</code> 强制对象必须拥有静态初始化和常量析构，并使对象有 const 限定，然而 <code>constinit</code> 不强制常量析构和 const 限定。结果是拥有 constexpr 构造函数且无 constexpr 析构函数的类型（例如 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-memory-shared_ptr.html"><span class="kw734">std::<span class="me2">shared_ptr</span></span></a><span class="sy1"><</span>T<span class="sy1">></span></span></span> ）的对象可能可以用 <code>constinit</code> ，但不能用 <code>constexpr</code> 声明。</p>
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<pre class="de1"><span class="kw4">const</span> <span class="kw4">char</span> <span class="sy2">*</span>g<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> <span class="st0">"dynamic initialization"</span><span class="sy4">;</span> <span class="br0">}</span>
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<span class="kw4">constexpr</span> <span class="kw4">const</span> <span class="kw4">char</span> <span class="sy2">*</span>f<span class="br0">(</span><span class="kw4">bool</span> p<span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> p <span class="sy4">?</span> <span class="st0">"constant initializer"</span> <span class="sy4">:</span> g<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="br0">}</span>
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constinit <span class="kw4">const</span> <span class="kw4">char</span> <span class="sy2">*</span>c <span class="sy1">=</span> f<span class="br0">(</span><span class="kw2">true</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK</span>
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<span class="co1">// constinit const char *d = f(false); // 错误</span></pre></div>
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<p><code>constinit</code> 亦能用于非初始化声明，以告知编译器 <code>thread_local</code> 变量已被初始化，以减少隐藏的防卫变量所致的<a href="cpp-language-storage_duration.html#.E9.9D.99.E6.80.81.E5.B1.80.E9.83.A8.E5.8F.98.E9.87.8F">开销</a>。</p>
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<pre class="de1"><span class="kw4">extern</span> thread_local constinit <span class="kw4">int</span> x<span class="sy4">;</span>
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<span class="kw4">int</span> f<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> x<span class="sy4">;</span> <span class="br0">}</span> <span class="co1">// 无需检查防卫变量</span></pre></div>
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<h3><span class="mw-headline" id=".E5.85.B3.E9.94.AE.E8.AF.8D">关键词</span></h3>
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<p><a href="cpp-keyword-constinit.html"><tt>constinit</tt></a></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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<td class="mbox-text" style="">本节未完成<br />
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原因：暂无示例</td>
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<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">参阅</span></h3>
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<ul>
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<li><a href="cpp-language-consteval.html"><code>consteval</code></a></li>
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<li><a href="cpp-language-constexpr.html"><code>constexpr</code></a></li>
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<li><a href="cpp-language-constant_expression.html">常量表达式</a></li>
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<li><a href="cpp-language-constant_initialization.html">常量初始化</a></li>
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<li><a href="cpp-language-zero_initialization.html">零初始化</a></li>
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