77 lines
9.6 KiB
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77 lines
9.6 KiB
HTML
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<title>consteval 说明符 (C++20 起)</title>
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<h1 id="firstHeading" class="firstHeading">consteval 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>consteval</b></code> - 指定函数是<i>立即函数（immediate function）</i>，即每次调用该函数必须产生编译时常量。</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>consteval</code> 说明符声明函数或函数模板为<i>立即函数</i>，即该函数的每次潜在求值的调用（即<a href="cpp-language-expressions.html#.E4.B8.8D.E6.B1.82.E5.80.BC.E8.A1.A8.E8.BE.BE.E5.BC.8F">不求值语境</a>外的调用）必须（直接或间接）产生编译时<a href="cpp-language-constant_expression.html">常量表达式</a>。它不可应用于析构函数、分配函数或解分配函数。<code>consteval</code> 说明符蕴含 <code>inline</code> 。同一声明说明符序列中允许出现至多一个 <code>constexpr</code> 、 <code>consteval</code> 及 <code>constinit</code> 说明符。若某个函数或函数模板的任何声明含有 <code>consteval</code> 说明符，则该函数或函数模板的所有声明必须均含该说明符。</p>
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<p>立即函数是 constexpr 函数，而且只要情况符合，必须满足适用于 <a href="cpp-language-constexpr.html">constexpr 函数或 constexpr 构造函数</a>的要求。</p>
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<p>当最内层的非块作用域不是某个立即函数的<a href="cpp-language-scope.html#.E5.87.BD.E6.95.B0.E5.BD.A2.E5.8F.82.E4.BD.9C.E7.94.A8.E5.9F.9F">函数形参作用域</a><span class="t-rev-inl t-since-cxx23"><span>或 <a href="cpp-language-if.html#consteval_if">consteval if 语句</a>的 true 分支</span> <span><span class="t-mark-rev t-since-cxx23">(C++23 起)</span></span></span>时，对立即函数的潜在求值的调用必须产生常量表达式；这种调用称为<i>立即调用（immediate invocation）</i>。</p>
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<pre class="de1">consteval <span class="kw4">int</span> sqr<span class="br0">(</span><span class="kw4">int</span> n<span class="br0">)</span> <span class="br0">{</span>
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<span class="kw1">return</span> n<span class="sy2">*</span>n<span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="kw4">constexpr</span> <span class="kw4">int</span> r <span class="sy1">=</span> sqr<span class="br0">(</span><span class="nu0">100</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK</span>
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<span class="kw4">int</span> x <span class="sy1">=</span> <span class="nu0">100</span><span class="sy4">;</span>
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<span class="kw4">int</span> r2 <span class="sy1">=</span> sqr<span class="br0">(</span>x<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 错误：调用不产生常量</span>
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consteval <span class="kw4">int</span> sqrsqr<span class="br0">(</span><span class="kw4">int</span> n<span class="br0">)</span> <span class="br0">{</span>
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<span class="kw1">return</span> sqr<span class="br0">(</span>sqr<span class="br0">(</span>n<span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 在此点非常量表达式，但是 OK</span>
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<span class="br0">}</span>
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<span class="kw4">constexpr</span> <span class="kw4">int</span> dblsqr<span class="br0">(</span><span class="kw4">int</span> n<span class="br0">)</span> <span class="br0">{</span>
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<span class="kw1">return</span> <span class="nu0">2</span><span class="sy2">*</span>sqr<span class="br0">(</span>n<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 错误：外围函数并非 consteval 且 sqr(n) 不是常量</span>
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<span class="br0">}</span></pre></div>
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<p>指代某个立即函数的<a href="cpp-language-identifiers.html#.E5.9C.A8.E8.A1.A8.E8.BE.BE.E5.BC.8F.E4.B8.AD">标识表达式</a>，仅可出现在立即调用的子表达式内，或某个<i>立即函数语境</i>（即前述的语境，在其中对立即函数的调用不需要是常量表达式）内。可以取得立即函数的指针或引用，但它们不可逃出常量表达式求值：</p>
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<pre class="de1">consteval <span class="kw4">int</span> f<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> <span class="nu0">42</span><span class="sy4">;</span> <span class="br0">}</span>
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consteval <span class="kw4">auto</span> g<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> <span class="sy3">&</span>f<span class="sy4">;</span> <span class="br0">}</span>
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consteval <span class="kw4">int</span> h<span class="br0">(</span><span class="kw4">int</span> <span class="br0">(</span><span class="sy2">*</span>p<span class="br0">)</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1">=</span> g<span class="br0">(</span><span class="br0">)</span><span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> p<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="br0">}</span>
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<span class="kw4">constexpr</span> <span class="kw4">int</span> r <span class="sy1">=</span> h<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK</span>
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<span class="kw4">constexpr</span> <span class="kw4">auto</span> e <span class="sy1">=</span> g<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 非良构：指向立即函数的指针不是常量表达式的容许结果</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-consteval.html"><tt>consteval</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-constexpr.html"><code>constexpr</code></a></li>
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<li><a href="cpp-language-constinit.html"><code>constinit</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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