97 lines
9.6 KiB
HTML
97 lines
9.6 KiB
HTML
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<title>std::experimental::propagate_const<T>::propagate_const</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_experimental_propagate_const_propagate_const 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::experimental::propagate_const<T>::</span>propagate_const</h1>
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<div id="contentSub"><span class="subpages">< <a href="cpp.html">cpp</a>‎ | <a href="cpp-experimental.html">experimental</a>‎ | <a href="cpp-experimental-propagate_const.html">propagate const</a></span></div>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">constexpr</span> propagate_const<span class="br0">(</span><span class="br0">)</span> <span class="sy1">=</span> <span class="kw1">default</span><span class="sy4">;</span></span></div>
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<td>(1)</td>
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<td><span class="t-mark-rev t-since-libfund-ts-2 t-mark-ts">(库基础 TS v2)</span></td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">constexpr</span> propagate_const<span class="br0">(</span> propagate_const<span class="sy3">&&</span> p <span class="br0">)</span> <span class="sy1">=</span> <span class="kw1">default</span><span class="sy4">;</span></span></div>
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<td>(2)</td>
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<td><span class="t-mark-rev t-since-libfund-ts-2 t-mark-ts">(库基础 TS v2)</span></td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> U<span class="sy1">></span><br />
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<span class="coMULTI">/* see below */</span> <span class="kw4">constexpr</span> propagate_const<span class="br0">(</span> propagate_const<span class="sy1"><</span>U<span class="sy1">></span><span class="sy3">&&</span> pu <span class="br0">)</span><span class="sy4">;</span></span></div>
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<td>(3)</td>
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<td><span class="t-mark-rev t-since-libfund-ts-2 t-mark-ts">(库基础 TS v2)</span></td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> U<span class="sy1">></span><br />
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<span class="coMULTI">/* see below */</span> <span class="kw4">constexpr</span> propagate_const<span class="br0">(</span> U<span class="sy3">&&</span> u <span class="br0">)</span><span class="sy4">;</span></span></div>
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<td>(4)</td>
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<td><span class="t-mark-rev t-since-libfund-ts-2 t-mark-ts">(库基础 TS v2)</span></td>
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<div><span class="mw-geshi cpp source-cpp">propagate_const<span class="br0">(</span> <span class="kw4">const</span> propagate_const<span class="sy3">&</span> <span class="br0">)</span> <span class="sy1">=</span> delete<span class="sy4">;</span></span></div>
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<td>(5)</td>
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<td><span class="t-mark-rev t-since-libfund-ts-2 t-mark-ts">(库基础 TS v2)</span></td>
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<p>令 <code>t_</code> 指代私有数据成员，即被包装的仿指针对象。</p>
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<div class="t-li1"><span class="t-li">1)</span> 构造 <code>propagate_const</code> ，默认初始化 <span class="t-c"><span class="mw-geshi cpp source-cpp">this<span class="sy2">-</span><span class="sy1">></span>t_</span></span> 。</div>
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<div class="t-li1"><span class="t-li">2)</span> 显式默认化的移动构造函数，从 <span class="t-c"><span class="mw-geshi cpp source-cpp">p.<span class="me1">t_</span></span></span> 移动构造 <span class="t-c"><span class="mw-geshi cpp source-cpp">this<span class="sy2">-</span><span class="sy1">></span>t_</span></span> 。</div>
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<div class="t-li1"><span class="t-li">3)</span> 如同以直接非列表初始化，从表达式 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">move</span><span class="br0">(</span>pu.<span class="me1">t_</span><span class="br0">)</span></span></span> 初始化 <span class="t-c"><span class="mw-geshi cpp source-cpp">this<span class="sy2">-</span><span class="sy1">></span>t_</span></span> 。<br />
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此构造函数仅若 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_constructible.html"><span class="kw520">std::<span class="me2">is_constructible</span></span></a><span class="sy1"><</span>T, U<span class="sy1">></span><span class="sy4">::</span><span class="me2">value</span></span></span> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> 才参与重载决议，若且唯若 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_convertible.html"><span class="kw592">std::<span class="me2">is_convertible</span></span></a><span class="sy1"><</span>U, T<span class="sy1">></span><span class="sy4">::</span><span class="me2">value</span></span></span> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">false</span></span></span> 才为 <code>explicit</code> 。</div>
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<div class="t-li1"><span class="t-li">4)</span> 如同以直接非列表初始化，用表达式 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-utility-forward.html"><span class="kw1117">std::<span class="me2">forward</span></span></a><span class="sy1"><</span>U<span class="sy1">></span><span class="br0">(</span>u<span class="br0">)</span></span></span> 初始化 <span class="t-c"><span class="mw-geshi cpp source-cpp">this<span class="sy2">-</span><span class="sy1">></span>t_</span></span> 。<br />
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此构造函数仅若 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_constructible.html"><span class="kw520">std::<span class="me2">is_constructible</span></span></a><span class="sy1"><</span>T, U<span class="sy1">></span><span class="sy4">::</span><span class="me2">value</span></span></span> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> 而 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-decay.html"><span class="kw629">std::<span class="me2">decay_t</span></span></a><span class="sy1"><</span>U<span class="sy1">></span></span></span> 不是 <code>propagate_const</code> 的特化才参与重载决议。此构造函数若且唯若 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_convertible.html"><span class="kw592">std::<span class="me2">is_convertible</span></span></a><span class="sy1"><</span>U, T<span class="sy1">></span><span class="sy4">::</span><span class="me2">value</span></span></span> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">false</span></span></span> 才为 <code>explicit</code> 。</div>
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<div class="t-li1"><span class="t-li">5)</span> 复制构造函数被显式删除。 <code>propagate_const</code> 不可复制。</div>
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<h3><span class="mw-headline" id=".E5.8F.82.E6.95.B0">参数</span></h3>
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<td>p</td>
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<td>-</td>
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<td>要移动的另一 <code>propagate_const</code> 对象</td>
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<td>pu</td>
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<td>-</td>
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<td>要移动的另一不同特化的 <code>propagate_const</code> 对象</td>
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<td>u</td>
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<td>-</td>
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<td>用来初始化所含指针的另一对象</td>
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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>在库基础 TS v2 （基于 C++14 ）中， <span class="t-v">(4)</span> 与 <span class="t-v">(5)</span> 常以 <code>explicit</code>/非 <code>explicit</code> 重载对实现。从库基础 TS v3 （基于 C++20 ）起，它们能以条件性 <code>explicit</code> 说明符实现。</p>
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