203 lines
23 KiB
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203 lines
23 KiB
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::pmr::polymorphic_allocator<T>::</span>construct</h1>
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<div id="contentSub"><span class="subpages">< <a href="cpp.html">cpp</a>‎ | <a href="cpp-memory.html">memory</a>‎ | <a href="cpp-memory-polymorphic_allocator.html">polymorphic allocator</a></span></div>
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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="kw1">class</span>... <span class="me1">Args</span> <span class="sy1">></span><br />
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<span class="kw4">void</span> construct<span class="br0">(</span> U<span class="sy2">*</span> p, Args<span class="sy3">&&</span>... <span class="me1">args</span> <span class="br0">)</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-cxx17">(C++17 起)</span></td>
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<tr class="t-dcl t-since-cxx17 t-until-cxx20">
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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> T1, <span class="kw1">class</span> T2, <span class="kw1">class</span>... <span class="me1">Args1</span>, <span class="kw1">class</span>... <span class="me1">Args2</span> <span class="sy1">></span><br /></span>
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<p><span class="kw4">void</span> construct<span class="br0">(</span> <a href="cpp-utility-pair.html"><span class="kw1096">std::<span class="me2">pair</span></span></a><span class="sy1"><</span>T1, T2<span class="sy1">></span><span class="sy2">*</span> p,<br />
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<a href="cpp-utility-piecewise_construct_t.html"><span class="kw1105">std::<span class="me2">piecewise_construct_t</span></span></a>,<br />
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<a href="cpp-utility-tuple.html"><span class="kw1098">std::<span class="me2">tuple</span></span></a><span class="sy1"><</span>Args1...<span class="sy1">></span> x,<br /></p>
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<a href="cpp-utility-tuple.html"><span class="kw1098">std::<span class="me2">tuple</span></span></a><span class="sy1"><</span>Args2...<span class="sy1">></span> y <span class="br0">)</span><span class="sy4">;</span></div>
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</td>
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<td>(2)</td>
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<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span><br />
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<span class="t-mark-rev t-until-cxx20">(C++20 前)</span></td>
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</tr>
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<tr class="t-dcl t-since-cxx17 t-until-cxx20">
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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> T1, <span class="kw1">class</span> T2 <span class="sy1">></span><br />
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<span class="kw4">void</span> construct<span class="br0">(</span> <a href="cpp-utility-pair.html"><span class="kw1096">std::<span class="me2">pair</span></span></a><span class="sy1"><</span>T1, T2<span class="sy1">></span><span class="sy2">*</span> p <span class="br0">)</span><span class="sy4">;</span></span></div>
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</td>
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<td>(3)</td>
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<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span><br />
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<span class="t-mark-rev t-until-cxx20">(C++20 前)</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> T1, <span class="kw1">class</span> T2, <span class="kw1">class</span> U, <span class="kw1">class</span> V <span class="sy1">></span><br />
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<span class="kw4">void</span> construct<span class="br0">(</span> <a href="cpp-utility-pair.html"><span class="kw1096">std::<span class="me2">pair</span></span></a><span class="sy1"><</span>T1, T2<span class="sy1">></span><span class="sy2">*</span> p, U<span class="sy3">&&</span> x, V<span class="sy3">&&</span> y <span class="br0">)</span><span class="sy4">;</span></span></div>
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</td>
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<td>(4)</td>
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<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span><br />
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<span class="t-mark-rev t-until-cxx20">(C++20 前)</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> T1, <span class="kw1">class</span> T2, <span class="kw1">class</span> U, <span class="kw1">class</span> V <span class="sy1">></span><br />
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<span class="kw4">void</span> construct<span class="br0">(</span> <a href="cpp-utility-pair.html"><span class="kw1096">std::<span class="me2">pair</span></span></a><span class="sy1"><</span>T1, T2<span class="sy1">></span><span class="sy2">*</span> p, <span class="kw4">const</span> <a href="cpp-utility-pair.html"><span class="kw1096">std::<span class="me2">pair</span></span></a><span class="sy1"><</span>U, V<span class="sy1">></span><span class="sy3">&</span> xy <span class="br0">)</span><span class="sy4">;</span></span></div>
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</td>
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<td>(5)</td>
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<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span><br />
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<span class="t-mark-rev t-until-cxx20">(C++20 前)</span></td>
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<tr class="t-dcl t-since-cxx17 t-until-cxx20">
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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> T1, <span class="kw1">class</span> T2, <span class="kw1">class</span> U, <span class="kw1">class</span> V <span class="sy1">></span><br />
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<span class="kw4">void</span> construct<span class="br0">(</span> <a href="cpp-utility-pair.html"><span class="kw1096">std::<span class="me2">pair</span></span></a><span class="sy1"><</span>T1, T2<span class="sy1">></span><span class="sy2">*</span> p, <a href="cpp-utility-pair.html"><span class="kw1096">std::<span class="me2">pair</span></span></a><span class="sy1"><</span>U, V<span class="sy1">></span><span class="sy3">&&</span> xy <span class="br0">)</span><span class="sy4">;</span></span></div>
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<td>(6)</td>
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<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span><br />
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<span class="t-mark-rev t-until-cxx20">(C++20 前)</span></td>
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<p>在 <code>p</code> 所指的，分配但未初始化的存储上，以提供的构造函数参数构造一个对象。若对象自身拥有使用分配器的类型，或它是 std::pair ，则传递 <code>this->resource()</code> 给被构造的对象。</p>
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<div class="t-li1"><span class="t-li">1)</span> 以<a href="cpp-memory-uses_allocator.html#.E4.BD.BF.E7.94.A8.E5.88.86.E9.85.8D.E5.99.A8.E6.9E.84.E9.80.A0">使用分配器构造</a>的手段在 <span class="t-spar">p</span> 所指示的未初始化内存位置，以 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>this</span></span> 为分配器创建给定类型 <code>U</code> 的对象。<span class="t-rev-inl t-until-cxx20"><span>此重载仅若 <code>U</code> 不是 <span class="t-lc"><a href="cpp-utility-pair.html">std::pair</a></span> 的特化才参与重载决议。</span> <span><span class="t-mark-rev t-until-cxx20">(C++20 前)</span></span></span></div>
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<div class="t-li1"><span class="t-li">2)</span> 首先，若 <code>T1</code> 或 <code>T2</code> 之一具分配器，则修改 <code>x</code> 与 <code>y</code> 的 tuple 以包含 <code>this->resource()</code> ，产生二个新 tuple <code>xprime</code> 与 <code>yprime</code> ，按照下列规则：</div>
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<div class="t-li2"><span class="t-li">2a)</span> 若 <code>T1</code> 不具分配器（ <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-memory-uses_allocator.html"><span class="kw704">std::<span class="me2">uses_allocator</span></span></a><span class="sy1"><</span>T1, polymorphic_allocator<span class="sy1">></span><span class="sy4">::</span><span class="me2">value</span><span class="sy1">==</span><span class="kw2">false</span></span></span> ）且 <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>T1, Args1...<span class="sy1">></span><span class="sy4">::</span><span class="me2">value</span><span class="sy1">==</span><span class="kw2">true</span></span></span> ，则 <code>xprime</code> 是未修改的 <code>x</code> 。</div>
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<div class="t-li2"><span class="t-li">2b)</span> 若 <code>T1</code> 具分配器（ <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-memory-uses_allocator.html"><span class="kw704">std::<span class="me2">uses_allocator</span></span></a><span class="sy1"><</span>T1, polymorphic_allocator<span class="sy1">></span><span class="sy4">::</span><span class="me2">value</span><span class="sy1">==</span><span class="kw2">true</span></span></span> ），且其构造函数使用分配器标签（ <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>T1, <a href="cpp-memory-allocator_arg_t.html"><span class="kw702">std::<span class="me2">allocator_arg_t</span></span></a>, polymorphic_allocator, Args1...<span class="sy1">></span><span class="sy4">::</span><span class="me2">value</span><span class="sy1">==</span><span class="kw2">true</span></span></span> ），则 <code>xprime</code> 是 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-utility-tuple-tuple_cat.html"><span class="kw1115">std::<span class="me2">tuple_cat</span></span></a><span class="br0">(</span><a href="cpp-utility-tuple-make_tuple.html"><span class="kw1112">std::<span class="me2">make_tuple</span></span></a><span class="br0">(</span><a href="cpp-memory-allocator_arg.html"><span class="kw703">std::<span class="me2">allocator_arg</span></span></a>, <span class="sy2">*</span>this<span class="br0">)</span>, std<span class="sy4">::</span><span class="me2">move</span><span class="br0">(</span>x<span class="br0">)</span><span class="br0">)</span></span></span></div>
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<div class="t-li2"><span class="t-li">2c)</span> 若 <code>T1</code> 具分配器（ <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-memory-uses_allocator.html"><span class="kw704">std::<span class="me2">uses_allocator</span></span></a><span class="sy1"><</span>T1, polymorphic_allocator<span class="sy1">></span><span class="sy4">::</span><span class="me2">value</span><span class="sy1">==</span><span class="kw2">true</span></span></span> ），且其构造函数接受分配器为最后参数（ <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>T1, Args1..., polymorphic_allocator<span class="sy1">></span><span class="sy4">::</span><span class="me2">value</span><span class="sy1">==</span><span class="kw2">true</span></span></span> ），则 <code>xprime</code> 是 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-utility-tuple-tuple_cat.html"><span class="kw1115">std::<span class="me2">tuple_cat</span></span></a><span class="br0">(</span>std<span class="sy4">::</span><span class="me2">move</span><span class="br0">(</span>x<span class="br0">)</span>, <a href="cpp-utility-tuple-make_tuple.html"><span class="kw1112">std::<span class="me2">make_tuple</span></span></a><span class="br0">(</span><span class="sy2">*</span>this<span class="br0">)</span><span class="br0">)</span></span></span> 。</div>
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<div class="t-li2"><span class="t-li">2d)</span> 否则程序为病式。</div>
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<div class="t-li1"><span class="t-li"></span> 同样的规则应用于 <code>T2</code> 及以 <code>yprime</code> 替换 <code>y</code> 的情况。</div>
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<div class="t-li1"><span class="t-li"></span> 一旦构造了 <code>xprime</code> 和 <code>yprime</code> ，就如同以用 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy4">::</span><span class="me2">new</span><span class="br0">(</span><span class="br0">(</span><span class="kw4">void</span> <span class="sy2">*</span><span class="br0">)</span> p<span class="br0">)</span> pair<span class="sy1"><</span>T1, T2<span class="sy1">></span><span class="br0">(</span><a href="cpp-utility-piecewise_construct.html"><span class="kw1106">std::<span class="me2">piecewise_construct</span></span></a>, std<span class="sy4">::</span><span class="me2">move</span><span class="br0">(</span>xprime<span class="br0">)</span>, std<span class="sy4">::</span><span class="me2">move</span><span class="br0">(</span>yprime<span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span></span></span> 于分配的存储构造 pair <code>p</code> 。</div>
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<div class="t-li1"><span class="t-li">3)</span> 等价于 <span class="t-c"><span class="mw-geshi cpp source-cpp">construct<span class="br0">(</span>p, <a href="cpp-utility-piecewise_construct.html"><span class="kw1106">std::<span class="me2">piecewise_construct</span></span></a>, <a href="cpp-utility-tuple.html"><span class="kw1098">std::<span class="me2">tuple</span></span></a><span class="sy1"><></span><span class="br0">(</span><span class="br0">)</span>, <a href="cpp-utility-tuple.html"><span class="kw1098">std::<span class="me2">tuple</span></span></a><span class="sy1"><></span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span></span></span> ，即传递memory_resouce 到 pair 的成员类型上，若它接受它们。</div>
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<div class="t-li1"><span class="t-li">4)</span> 等价于
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<div class="cpp source-cpp">
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<pre class="de1"> construct<span class="br0">(</span>p, <a href="cpp-utility-piecewise_construct.html"><span class="kw1106">std::<span class="me2">piecewise_construct</span></span></a>, <a href="cpp-utility-tuple-forward_as_tuple.html"><span class="kw1114">std::<span class="me2">forward_as_tuple</span></span></a><span class="br0">(</span><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>x<span class="br0">)</span><span class="br0">)</span>,
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<a href="cpp-utility-tuple-forward_as_tuple.html"><span class="kw1114">std::<span class="me2">forward_as_tuple</span></span></a><span class="br0">(</span><a href="cpp-utility-forward.html"><span class="kw1117">std::<span class="me2">forward</span></span></a><span class="sy1"><</span>V<span class="sy1">></span><span class="br0">(</span>y<span class="br0">)</span><span class="br0">)</span><span class="br0">)</span></pre></div>
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<div class="t-li1"><span class="t-li">5)</span> 等价于
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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"> construct<span class="br0">(</span>p, <a href="cpp-utility-piecewise_construct.html"><span class="kw1106">std::<span class="me2">piecewise_construct</span></span></a>, <a href="cpp-utility-tuple-forward_as_tuple.html"><span class="kw1114">std::<span class="me2">forward_as_tuple</span></span></a><span class="br0">(</span>xy.<span class="me1">first</span><span class="br0">)</span>,
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<a href="cpp-utility-tuple-forward_as_tuple.html"><span class="kw1114">std::<span class="me2">forward_as_tuple</span></span></a><span class="br0">(</span>xy.<span class="me1">second</span><span class="br0">)</span><span class="br0">)</span></pre></div>
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<div class="t-li1"><span class="t-li">6)</span> 等价于
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<div class="cpp source-cpp">
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<pre class="de1"> construct<span class="br0">(</span>p, <a href="cpp-utility-piecewise_construct.html"><span class="kw1106">std::<span class="me2">piecewise_construct</span></span></a>, <a href="cpp-utility-tuple-forward_as_tuple.html"><span class="kw1114">std::<span class="me2">forward_as_tuple</span></span></a><span class="br0">(</span><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>xy.<span class="me1">first</span><span class="br0">)</span><span class="br0">)</span>,
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<a href="cpp-utility-tuple-forward_as_tuple.html"><span class="kw1114">std::<span class="me2">forward_as_tuple</span></span></a><span class="br0">(</span><a href="cpp-utility-forward.html"><span class="kw1117">std::<span class="me2">forward</span></span></a><span class="sy1"><</span>V<span class="sy1">></span><span class="br0">(</span>xy.<span class="me1">second</span><span class="br0">)</span><span class="br0">)</span><span class="br0">)</span></pre></div>
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<td><span class="t-mark-rev t-until-cxx20">(C++20 前)</span></td>
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<h3><span class="mw-headline" id=".E5.8F.82.E6.95.B0">参数</span></h3>
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<table class="t-par-begin">
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<td>p</td>
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<td>-</td>
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<td>指向分配而未初始化存储的指针</td>
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</tr>
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<tr class="t-par">
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<td>args...</td>
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<td>-</td>
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<td>传递给 <code>T</code> 构造函数的构造函数参数</td>
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</tr>
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<tr class="t-par">
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<td>x</td>
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<td>-</td>
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<td>传递给 <code>T1</code> 构造函数的构造函数参数</td>
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</tr>
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<tr class="t-par">
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<td>y</td>
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<td>-</td>
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<td>传递给 <code>T2</code> 构造函数的构造函数参数</td>
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</tr>
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<tr class="t-par">
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<td>xy</td>
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<td>-</td>
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<td>成员是 <code>T1</code> 与 <code>T2</code> 构造函数参数的 pair</td>
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</tr>
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</table>
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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>（无）</p>
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<h3><span class="mw-headline" id=".E6.B3.A8.E6.84.8F">注意</span></h3>
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<p>此函数为任何具分配器对象调用（通过 <span class="t-lc"><a href="cpp-memory-allocator_traits.html">std::allocator_traits</a></span> ），例如 <span class="t-lc"><a href="cpp-container-vector.html">std::pmr::vector</a></span> （或另一个给定了 <code>std::std::polymorphic_allocator</code> 作为所用分配器的 <span class="t-lc"><a href="cpp-container-vector.html">std::vector</a></span> ）。</p>
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<h3><span class="mw-headline" id=".E7.BC.BA.E9.99.B7.E6.8A.A5.E5.91.8A">缺陷报告</span></h3>
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<p>下列更改行为的缺陷报告追溯地应用于以前出版的 C++ 标准。</p>
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<table class="dsctable" style="font-size:0.8em">
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<tr>
|
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<th>DR</th>
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<th>应用于</th>
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<th>出版时的行为</th>
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<th>正确行为</th>
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</tr>
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<tr>
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<td><a rel="nofollow" class="external text" href="https://cplusplus.github.io/LWG/issue2969">LWG 2969</a></td>
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<td>C++17</td>
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<td>uses-allocator 构造传递 <code>resource()</code></td>
|
|
<td>传递 <code>*this</code></td>
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</tr>
|
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<tr>
|
|
<td><a rel="nofollow" class="external text" href="https://cplusplus.github.io/LWG/issue2975">LWG 2975</a></td>
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<td>C++17</td>
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|
<td>某些情况下 pair 构造错误地使用第一重载</td>
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<td>制约为不接受 pair</td>
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</tr>
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</table>
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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>
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<div class="t-dsc-member-div">
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<div><a href="cpp-memory-allocator_traits-construct.html"><span class="t-lines"><span>construct</span></span></a></div>
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|
<div><span class="t-lines"><span><span class="t-cmark">[静态]</span></span></span></div>
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</div>
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</td>
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<td>在分配的存储构造对象<br />
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<span class="t-mark">(函数模板)</span></td>
|
|
</tr>
|
|
<tr class="t-dsc">
|
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<td>
|
|
<div class="t-dsc-member-div">
|
|
<div><a href="cpp-memory-allocator-construct.html"><span class="t-lines"><span>construct</span></span></a></div>
|
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<div><span class="t-lines"><span><span class="t-mark-rev t-deprecated-cxx17">(C++17 中弃用)</span></span><span><span class="t-mark-rev t-until-cxx20">(C++20 中移除)</span></span></span></div>
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</div>
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</td>
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<td>在分配的存储构造对象<br />
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<span class="t-mark">(<code>std::allocator<T></code> 的公开成员函数)</span></td>
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</tr>
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</table>
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</div>
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<div class="visualClear"></div>
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</div>
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</div>
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</div>
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</body>
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</html>
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