187 lines
25 KiB
HTML
187 lines
25 KiB
HTML
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<title>std::allocate_shared, std::allocate_shared_for_overwrite</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_memory_shared_ptr_allocate_shared 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::</span>allocate_shared, <span style="font-size:0.7em; line-height:130%">std::</span>allocate_shared_for_overwrite</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-shared_ptr.html">shared ptr</a></span></div>
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<table class="t-dcl-begin">
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<div>定义于头文件 <code><a href="cpp-header-memory.html"><memory></a></code></div>
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<td></td>
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<td></td>
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<tr class="t-dcl t-since-cxx11">
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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> T, <span class="kw1">class</span> Alloc, <span class="kw1">class</span>... <span class="me1">Args</span> <span class="sy1">></span><br />
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shared_ptr<span class="sy1"><</span>T<span class="sy1">></span> allocate_shared<span class="br0">(</span> <span class="kw4">const</span> Alloc<span class="sy3">&</span> alloc, 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>
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<td>(1)</td>
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<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span><br />
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<span class="t-mark">(T 非数组)</span></td>
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</tr>
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<tr class="t-dcl t-since-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> T, <span class="kw1">class</span> Alloc <span class="sy1">></span><br />
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shared_ptr<span class="sy1"><</span>T<span class="sy1">></span> allocate_shared<span class="br0">(</span> <span class="kw4">const</span> Alloc<span class="sy3">&</span> alloc, <a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> N <span class="br0">)</span><span class="sy4">;</span></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-cxx20">(C++20 起)</span><br />
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<span class="t-mark">(T 为 U[])</span></td>
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</tr>
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<tr class="t-dcl t-since-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> T, <span class="kw1">class</span> Alloc <span class="sy1">></span><br />
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shared_ptr<span class="sy1"><</span>T<span class="sy1">></span> allocate_shared<span class="br0">(</span> <span class="kw4">const</span> Alloc<span class="sy3">&</span> alloc <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-cxx20">(C++20 起)</span><br />
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<span class="t-mark">(T 为 U[N])</span></td>
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</tr>
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<tr class="t-dcl t-since-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> T, <span class="kw1">class</span> Alloc <span class="sy1">></span><br /></span>
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<p>shared_ptr<span class="sy1"><</span>T<span class="sy1">></span> allocate_shared<span class="br0">(</span> <span class="kw4">const</span> Alloc<span class="sy3">&</span> alloc, <a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> N,<br /></p>
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<span class="kw4">const</span> <a href="cpp-types-remove_extent.html"><span class="kw621">std::<span class="me2">remove_extent_t</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="sy3">&</span> u <span class="br0">)</span><span class="sy4">;</span></div>
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<td>(4)</td>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span><br />
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<span class="t-mark">(T 为 U[])</span></td>
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</tr>
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<tr class="t-dcl t-since-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> T, <span class="kw1">class</span> Alloc <span class="sy1">></span><br /></span>
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<p>shared_ptr<span class="sy1"><</span>T<span class="sy1">></span> allocate_shared<span class="br0">(</span> <span class="kw4">const</span> Alloc<span class="sy3">&</span> alloc,<br /></p>
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<span class="kw4">const</span> <a href="cpp-types-remove_extent.html"><span class="kw621">std::<span class="me2">remove_extent_t</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="sy3">&</span> u <span class="br0">)</span><span class="sy4">;</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-cxx20">(C++20 起)</span><br />
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<span class="t-mark">(T 为 U[N])</span></td>
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</tr>
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<tr class="t-dcl t-since-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> T, <span class="kw1">class</span> Alloc <span class="sy1">></span><br />
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shared_ptr<span class="sy1"><</span>T<span class="sy1">></span> allocate_shared_for_overwrite<span class="br0">(</span> <span class="kw4">const</span> Alloc<span class="sy3">&</span> alloc <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-cxx20">(C++20 起)</span><br />
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<span class="t-mark">(T 非 U[])</span></td>
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</tr>
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<tr class="t-dcl t-since-cxx20">
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<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> T, <span class="kw1">class</span> Alloc <span class="sy1">></span><br />
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shared_ptr<span class="sy1"><</span>T<span class="sy1">></span> allocate_shared_for_overwrite<span class="br0">(</span> <span class="kw4">const</span> Alloc<span class="sy3">&</span> alloc, <a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> N <span class="br0">)</span><span class="sy4">;</span></span></div>
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<td>(7)</td>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span><br />
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<span class="t-mark">(T 为 U[])</span></td>
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</tr>
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<tr class="t-dcl-sep">
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<td></td>
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<td></td>
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<div class="t-li1"><span class="t-li">1)</span> 以 <code>args</code> 为 <code>T</code> 的构造函数参数列表，构造 <code>T</code> 类型对象并将它包装于 <span class="t-lc"><a href="cpp-memory-shared_ptr.html">std::shared_ptr</a></span> 。对象如同以表达式 <span class="t-rev-inl t-until-cxx20"><span><span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy4">::</span><span class="me2">new</span> <span class="br0">(</span>pv<span class="br0">)</span> T<span class="br0">(</span>v<span class="br0">)</span></span></span></span> <span><span class="t-mark-rev t-until-cxx20">(C++20 前)</span></span></span><span class="t-rev-inl t-since-cxx20"><span><span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-memory-allocator_traits.html"><span class="kw701">std::<span class="me2">allocator_traits</span></span></a><span class="sy1"><</span>A2<span class="sy1">></span><span class="sy4">::</span><span class="me2">construct</span><span class="br0">(</span>a, pv, v<span class="br0">)</span></span></span></span> <span><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></span></span> 构造，其中 <code>pv</code> 是内部指向适合保有 <code>T</code> 类型对象的存储的 <code>void*</code> 指针，而 <code>a</code> 是重绑定到 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-remove_cv.html"><span class="kw597">std::<span class="me2">remove_cv_t</span></span></a><span class="sy1"><</span>T<span class="sy1">></span></span></span> 的分配器副本。存储通常大于 <code>sizeof(T)</code> ，以对共享指针的控制块和 <code>T</code> 对象使用一次分配。此函数所调用的 <code>std::shared_ptr</code> 构造函数以指向新构造的 <code>T</code> 类型对象指针启用 <code>shared_from_this</code> 。所有内存分配用 <span class="t-c"><span class="mw-geshi cpp source-cpp">alloc</span></span> 的副本执行，而它必须满足<a href="cpp-named_req-Allocator.html"><span style="font-style:italic">分配器</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(Allocator)</span></a> 要求。此重载仅若 T 不是数组类型才参与重载决议。</div>
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<div class="t-li1"><span class="t-li">2,3)</span> 同 <span class="t-v">(1)</span> ，但构造的元素是可能多维的数组，其每个非数组元素如同以表达式 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-memory-allocator_traits.html"><span class="kw701">std::<span class="me2">allocator_traits</span></span></a><span class="sy1"><</span>A2<span class="sy1">></span><span class="sy4">::</span><span class="me2">construct</span><span class="br0">(</span>a2, pv<span class="br0">)</span></span></span> 初始化，其中 <code>A2</code> 类型的 <code>a2</code> 是重绑定为管理 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-remove_cv.html"><span class="kw597">std::<span class="me2">remove_cv_t</span></span></a><span class="sy1"><</span><a href="cpp-types-remove_all_extents.html"><span class="kw623">std::<span class="me2">remove_all_extents_t</span></span></a><span class="sy1"><</span>T<span class="sy1">>></span></span></span> 类型对象的分配器副本。重载 <span class="t-v">(2)</span> 创建第一维上大小为 <code>N</code> 的数组。数组元素以其地址递增顺序初始化，而当其生存期结束时，以原本构造顺序的逆序销毁。</div>
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<div class="t-li1"><span class="t-li">4,5)</span> 同 <span class="t-v">(2,3)</span> ，但每个元素从默认值 <code>u</code> 初始化。若 <code>U</code> 不是数组类型，则这如同以与 <span class="t-v">(1)</span> 中相同的分配器表达式进行，除了把分配器重绑定到 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-remove_cv.html"><span class="kw597">std::<span class="me2">remove_cv_t</span></span></a><span class="sy1"><</span><a href="cpp-types-remove_all_extents.html"><span class="kw623">std::<span class="me2">remove_all_extents_t</span></span></a><span class="sy1"><</span>T<span class="sy1">>></span></span></span> 。否则，这如同以如同 <span class="t-v">(1)</span> 中的分配器表达式，从来自 <code>u</code> 的对应元素初始化（可能多维的）数组的每个非数组元素，除了重绑定分配器到 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-remove_cv.html"><span class="kw597">std::<span class="me2">remove_cv_t</span></span></a><span class="sy1"><</span><a href="cpp-types-remove_all_extents.html"><span class="kw623">std::<span class="me2">remove_all_extents_t</span></span></a><span class="sy1"><</span>T<span class="sy1">>></span></span></span> 类型。重载 <span class="t-v">(4)</span>创建第一维上大小为 <code>N</code> 的数组。数组元素以其地址递增顺序初始化，而当其生存期结束时，以原本构造顺序的逆序销毁。</div>
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<div class="t-li1"><span class="t-li">6)</span> 若 <code>T</code> 非数组类型则同 <span class="t-v">(1)</span> 而若 <code>T</code> 为 <code>U[N]</code> 则同 <span class="t-v">(3)</span> ，除了<a href="cpp-language-default_initialization.html">默认初始化</a>创建的元素。</div>
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<div class="t-li1"><span class="t-li">7)</span> 同 <span class="t-v">(2)</span> ，除了<a href="cpp-language-default_initialization.html">默认初始化</a>单独的数组元素。</div>
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<p>对于 <code>allocate_shared</code> ，通过表达式 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-memory-allocator_traits.html"><span class="kw701">std::<span class="me2">allocator_traits</span></span></a><span class="sy1"><</span>A2<span class="sy1">></span><span class="sy4">::</span><span class="me2">destroy</span><span class="br0">(</span>a, p<span class="br0">)</span></span></span> 销毁对象{{rev inl|since=c++20|（或对于 <span class="t-v">(2-5)</span> 为单独的数组元素），其中 <code>p</code> 是指向对象的指针而 <code>a</code> 是传递给 <span class="t-c"><span class="mw-geshi cpp source-cpp">allocate_shared</span></span> 的分配器副本，重绑定到正在销毁的对象类型。</p>
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<table class="t-rev-begin">
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<p>对于 <code>allocate_shared_for_overwrite</code> ，对象（或若 <code>T</code> 为数组类型则为单独的数组元素）将会被 <span class="t-c"><span class="mw-geshi cpp source-cpp">p<span class="sy2">-</span><span class="sy1">></span>~X<span class="br0">(</span><span class="br0">)</span></span></span> 销毁，其中 <code>p</code> 是指向对象的指针而 <code>X</code> 是对象的类型。</p>
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</td>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</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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<tr class="t-par">
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<td>alloc</td>
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<td>-</td>
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<td>使用的<a href="cpp-named_req-Allocator.html"><span style="font-style:italic">分配器</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(Allocator)</span></a> 。</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>构造 <code>T</code> 示例将要用到的参数列表。</td>
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</tr>
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<tr class="t-par">
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<td>N</td>
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<td>-</td>
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<td>所用的数组大小</td>
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</tr>
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<tr class="t-par">
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<td>u</td>
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<td>-</td>
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<td>初始化数组每个元素的初值</td>
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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><code>T</code> 类型实例的的 <span class="t-lc"><a href="cpp-memory-shared_ptr.html">std::shared_ptr</a></span> 。</p>
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<h3><span class="mw-headline" id=".E5.BC.82.E5.B8.B8">异常</span></h3>
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<p>能抛出从 <span class="t-c"><span class="mw-geshi cpp source-cpp">Alloc<span class="sy4">::</span><span class="me2">allocate</span><span class="br0">(</span><span class="br0">)</span></span></span> 抛出的异常，或从 <code>T</code> 构造函数抛出的异常。若抛出异常，则 <span class="t-v">(1)</span> 无效果。<span class="t-rev-inl t-since-cxx20"><span>若异常在数组的构造中抛出，则已初始化元素以逆序销毁。</span> <span><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></span></span></p>
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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>类似 <span class="t-lc"><a href="cpp-memory-shared_ptr-make_shared.html">std::make_shared</a></span> ，此函数通常只进行一次分配，并将 <code>T</code> 对象与控制块置于分配的内存块中（标准推荐但不要求如此，所有已知实现均如此）。 <code>alloc</code> 的副本作为控制块的一部分存储，从而它能用于解分配之，若所有共享及若引用计数抵达零。</p>
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<p>不同于 <code>std::shared_ptr</code> 构造函数， <code>std::allocate_shared</code> 不接受另外的自定义删除器：用提供的分配器析构控制块和 <code>T</code> 对象，及解分配其共享内存块。</p>
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<table class="t-rev-begin">
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<tr class="t-rev t-until-cxx20">
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<p><span class="t-lc"><a href="cpp-memory-shared_ptr.html">std::shared_ptr</a></span> 支持数组类型（从 C++17 起），但 <code>std::allocate_shared</code> 不支持。 <a rel="nofollow" class="external text" href="http://www.boost.org/doc/libs/1_66_0/libs/smart_ptr/doc/html/smart_ptr.html#make_shared">boost::allocate_shared</a> 支持此功能。</p>
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</td>
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<td><span class="t-mark-rev t-until-cxx20">(C++20 前)</span></td>
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<p>构造函数以 <code>U*</code> 类型指针 <code>ptr</code> <i>启用 <code>shared_from_this</code></i> ，表示它确定 <code>U</code> 是否拥有作为 <span class="t-lc"><a href="cpp-memory-enable_shared_from_this.html">std::enable_shared_from_this</a></span> 特化的<span class="t-rev-inl t-since-cxx17"><span>无歧义且可访问</span> <span><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></span></span>基类，而若如此，则构造函数求值该语句：</p>
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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"><span class="kw1">if</span> <span class="br0">(</span>ptr <span class="sy3">!</span><span class="sy1">=</span> nullptr <span class="sy3">&&</span> ptr<span class="sy2">-</span><span class="sy1">></span>weak_this.<span class="me1">expired</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span>
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ptr<span class="sy2">-</span><span class="sy1">></span>weak_this <span class="sy1">=</span> <a href="cpp-memory-shared_ptr.html"><span class="kw734">std::<span class="me2">shared_ptr</span></span></a><span class="sy1"><</span><a href="cpp-types-remove_cv.html"><span class="kw597">std::<span class="me2">remove_cv_t</span></span></a><span class="sy1"><</span>U<span class="sy1">>></span><span class="br0">(</span><span class="sy2">*</span>this,
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<span class="kw1">const_cast</span><span class="sy1"><</span><a href="cpp-types-remove_cv.html"><span class="kw597">std::<span class="me2">remove_cv_t</span></span></a><span class="sy1"><</span>U<span class="sy1">></span><span class="sy2">*</span><span class="sy1">></span><span class="br0">(</span>ptr<span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span></pre></div>
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</div>
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<p>其中 <code>weak_this</code> 是 <span class="t-lc">std::shared_from_this</span> 的隐藏 mutable <code>std::weak_ptr</code> 成员。对 <code>weak_this</code> 成员的赋值不是原子的，且与任何到同一对象的潜在并发访问冲突。这确保将来对 <a href="cpp-memory-enable_shared_from_this-shared_from_this.html"><tt>shared_from_this()</tt></a> 调用，将与此裸指针构造函数所创建的 <code>shared_ptr</code> 共享所有权。</p>
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<p>上述解释代码中，测试 <code>ptr->weak_this.expired()</code> 是为确保若 <code>weak_this</code> 指示已有占有者则重赋值它。从 C++17 起要求此测试。</p>
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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 t-dsc-member-nobold-div">
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<div><a href="cpp-memory-shared_ptr-shared_ptr.html"><span class="t-lines"><span>(构造函数)</span></span></a></div>
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</div>
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</td>
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<td>构造新的 <code>shared_ptr</code><br />
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<span class="t-mark">(公开成员函数)</span></td>
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</tr>
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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-shared_ptr-make_shared.html"><span class="t-lines"><span>make_shared</span><span>make_shared_for_overwrite</span></span></a></div>
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<div><span class="t-lines"><span></span><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span></span></div>
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</div>
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</td>
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<td>创建管理一个新对象的共享指针<br />
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<span class="t-mark">(函数模板)</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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