376 lines
43 KiB
HTML
376 lines
43 KiB
HTML
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<title>std::shared_ptr<T>::shared_ptr</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_memory_shared_ptr_shared_ptr 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::shared_ptr<T>::</span>shared_ptr</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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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">constexpr</span> shared_ptr<span class="br0">(</span><span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div>
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<td>(1)</td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">constexpr</span> shared_ptr<span class="br0">(</span> <a href="cpp-types-nullptr_t.html"><span class="kw102">std::<span class="me2">nullptr_t</span></span></a> <span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div>
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<td>(2)</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> Y <span class="sy1">></span><br />
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<span class="kw1">explicit</span> shared_ptr<span class="br0">(</span> Y<span class="sy2">*</span> ptr <span class="br0">)</span><span class="sy4">;</span></span></div>
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<td>(3)</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> Y, <span class="kw1">class</span> Deleter <span class="sy1">></span><br />
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shared_ptr<span class="br0">(</span> Y<span class="sy2">*</span> ptr, Deleter d <span class="br0">)</span><span class="sy4">;</span></span></div>
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<td>(4)</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> Deleter <span class="sy1">></span><br />
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shared_ptr<span class="br0">(</span> <a href="cpp-types-nullptr_t.html"><span class="kw102">std::<span class="me2">nullptr_t</span></span></a> ptr, Deleter d <span class="br0">)</span><span class="sy4">;</span></span></div>
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<td>(5)</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> Y, <span class="kw1">class</span> Deleter, <span class="kw1">class</span> Alloc <span class="sy1">></span><br />
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shared_ptr<span class="br0">(</span> Y<span class="sy2">*</span> ptr, Deleter d, Alloc alloc <span class="br0">)</span><span class="sy4">;</span></span></div>
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<td>(6)</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> Deleter, <span class="kw1">class</span> Alloc <span class="sy1">></span><br />
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shared_ptr<span class="br0">(</span> <a href="cpp-types-nullptr_t.html"><span class="kw102">std::<span class="me2">nullptr_t</span></span></a> ptr, Deleter d, Alloc alloc <span class="br0">)</span><span class="sy4">;</span></span></div>
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<td>(7)</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> Y <span class="sy1">></span><br />
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shared_ptr<span class="br0">(</span> <span class="kw4">const</span> shared_ptr<span class="sy1"><</span>Y<span class="sy1">></span><span class="sy3">&</span> r, element_type<span class="sy2">*</span> ptr <span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div>
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<td>(8)</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> Y <span class="sy1">></span><br />
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shared_ptr<span class="br0">(</span> shared_ptr<span class="sy1"><</span>Y<span class="sy1">></span><span class="sy3">&&</span> r, element_type<span class="sy2">*</span> ptr <span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div>
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<td>(8)</td>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
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<div><span class="mw-geshi cpp source-cpp">shared_ptr<span class="br0">(</span> <span class="kw4">const</span> shared_ptr<span class="sy3">&</span> r <span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div>
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<td>(9)</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> Y <span class="sy1">></span><br />
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shared_ptr<span class="br0">(</span> <span class="kw4">const</span> shared_ptr<span class="sy1"><</span>Y<span class="sy1">></span><span class="sy3">&</span> r <span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div>
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<td>(9)</td>
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<div><span class="mw-geshi cpp source-cpp">shared_ptr<span class="br0">(</span> shared_ptr<span class="sy3">&&</span> r <span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div>
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<td>(10)</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> Y <span class="sy1">></span><br />
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shared_ptr<span class="br0">(</span> shared_ptr<span class="sy1"><</span>Y<span class="sy1">></span><span class="sy3">&&</span> r <span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></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> Y <span class="sy1">></span><br />
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<span class="kw1">explicit</span> shared_ptr<span class="br0">(</span> <span class="kw4">const</span> <a href="cpp-memory-weak_ptr.html"><span class="kw741">std::<span class="me2">weak_ptr</span></span></a><span class="sy1"><</span>Y<span class="sy1">></span><span class="sy3">&</span> r <span class="br0">)</span><span class="sy4">;</span></span></div>
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<td>(11)</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> Y <span class="sy1">></span><br />
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shared_ptr<span class="br0">(</span> <a href="cpp-memory-auto_ptr.html"><span class="kw742">std::<span class="me2">auto_ptr</span></span></a><span class="sy1"><</span>Y<span class="sy1">></span><span class="sy3">&&</span> r <span class="br0">)</span><span class="sy4">;</span></span></div>
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<td>(12)</td>
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<td><span class="t-mark-rev t-until-cxx17">(C++17 中移除)</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> Y, <span class="kw1">class</span> Deleter <span class="sy1">></span><br />
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shared_ptr<span class="br0">(</span> <a href="cpp-memory-unique_ptr.html"><span class="kw733">std::<span class="me2">unique_ptr</span></span></a><span class="sy1"><</span>Y,Deleter<span class="sy1">></span><span class="sy3">&&</span> r <span class="br0">)</span><span class="sy4">;</span></span></div>
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<td>(13)</td>
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<td></td>
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<p>从指代要管理的对象的各种指针类型构造新的 <code>shared_ptr</code> 。</p>
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<p>为以下描述的目的，若指针类型 <code>Y*</code> 可转换为指针类型 <code>T*</code> ，或 <code>Y</code> 是 <code>U[N]</code> 类型数组而 <code>T</code> 是 <code>U cv []</code> （其中 cv 是某个 cv 限定符集合），则说 <code>Y*</code> <i>兼容</i> <code>T*</code> 。</p>
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<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></td>
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<div class="t-li1"><span class="t-li">1-2)</span> 构造无被管理对象的 <code>shared_ptr</code> ，即空 <code>shared_ptr</code> 。</div>
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<div class="t-li1"><span class="t-li">3-7)</span> 构造 <code>shared_ptr</code> ，管理 <code>ptr</code> 所指向的对象。
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<p>对于 <span class="t-v">(3-4,6)</span> ， <code>Y*</code> 必须可转换为 <code>T*</code> 。</p>
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<td><span class="t-mark-rev t-until-cxx17">(C++17 前)</span></td>
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<p>若 <code>T</code> 是数组类型 <code>U[N]</code> ，则若 <code>Y(*)[N]</code> 不可转换为 <code>T*</code> 则 <span class="t-v">(3-4,6)</span> 不参与重载决议。若 <code>T</code> 是数组类型 <code>U[]</code> ，则若 <code>Y(*)[]</code> 不可转换为 <code>T*</code> 则 <span class="t-v">(3-4,6)</span> 不参与重载决议。否则，若 <code>Y*</code> 不可转换为 <code>T*</code> 则 <span class="t-v">(3-4,6)</span> 不参与重载决议。</p>
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<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></td>
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另外：</div>
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<div class="t-li2"><span class="t-li">3)</span> 以 <a href="cpp-language-delete.html">delete 表达式</a> <span class="t-c"><span class="mw-geshi cpp source-cpp">delete ptr</span></span> 为删除器<span class="t-rev-inl t-since-cxx17"><span>，若 <code>T</code> 不是数组类型；以 <span class="t-c"><span class="mw-geshi cpp source-cpp">delete<span class="br0">[</span><span class="br0">]</span> ptr</span></span> 为删除器，若 <code>T</code> 是数组类型</span> <span><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></span></span>。 <code>Y</code> 必须是完整类型。 delete 表达式必须为良构，拥有良好定义行为且不抛异常。<span class="t-rev-inl t-since-cxx17"><span>若 delete 表达式不为良构，则此构造函数不参与重载决议。</span> <span><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></span></span></div>
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<div class="t-li2"><span class="t-li">4-5)</span> 以指定的删除器 <code>d</code> 为删除器。表达式 <span class="t-c"><span class="mw-geshi cpp source-cpp">d<span class="br0">(</span>ptr<span class="br0">)</span></span></span> 表达式必须为良构，拥有良好定义行为且不抛异常。 <code>d</code> 的构造和从 <code>d</code> 构造存储的删除器必须不抛异常。
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<p><code>Deleter</code> 必须<a href="cpp-named_req-CopyConstructible.html"><span style="font-style:italic">可复制构造</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(CopyConstructible)</span></a> 。</p>
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<td><span class="t-mark-rev t-until-cxx17">(C++17 前)</span></td>
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<p>若表达式 <span class="t-c"><span class="mw-geshi cpp source-cpp">d<span class="br0">(</span>ptr<span class="br0">)</span></span></span> 非良构，或若 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_move_constructible.html"><span class="kw538">std::<span class="me2">is_move_constructible</span></span></a><span class="sy1"><</span>D<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> ，则这些构造函数不额外参与重载决议。</p>
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<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></td>
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<div class="t-li2"><span class="t-li">6-7)</span> 同 <span class="t-v">(4-5)</span> ，但额外地用 <code>alloc</code> 的副本分配内部使用的数据。 <code>Alloc</code> 必须是<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> 。</div>
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<div class="t-li1"><span class="t-li">8)</span> <i>别名使用构造函数</i>：构造 <code>shared_ptr</code> ，与 <code>r</code> 的初始值共享所有权信息，但保有无关且不管理的指针 <code>ptr</code> 。若此 <code>shared_ptr</code> 是离开作用域的组中的最后者，则它将调用最初 <code>r</code> 所管理对象的析构函数。然而，在此 <code>shared_ptr</code> 上调用 <code>get()</code> 将始终返回 <code>ptr</code> 的副本。程序员负责确保只要此 <code>shared_ptr</code> 存在，此 <code>ptr</code> 就保持合法，例如在典型使用情况中，其中 <code>ptr</code> 是 <code>r</code> 所管理对象的成员，或是 <code>r.get()</code> 的别名（例如向下转型）。<span class="t-rev-inl t-since-cxx20"><span>对于接收右值的第二重载，调用后 <code>r</code> 为空且 <span class="t-c"><span class="mw-geshi cpp source-cpp">r.<span class="me1">get</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1">==</span> nullptr</span></span> 。</span> <span><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></span></span></div>
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<div class="t-li1"><span class="t-li">9)</span> 构造 <code>shared_ptr</code> ，共享 <code>r</code> 所管理对象的所有权。若 <code>r</code> 不管理对象，则 <code>*this</code> 亦不管理对象。若 <code>Y*</code> 不<span class="t-rev-inl t-until-cxx17"><span>可隐式转换为</span> <span><span class="t-mark-rev t-until-cxx17">(C++17 前)</span></span></span><span class="t-rev-inl t-since-cxx17"><span><i>兼容</i></span> <span><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></span></span> <code>T*</code> ，则模板重载不参与重载决议。</div>
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<div class="t-li1"><span class="t-li">10)</span> 从 <code>r</code> 移动构造 <code>shared_ptr</code> 。构造后， <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>this</span></span> 含 <code>r</code> 先前状态的副本，而 <code>r</code> 为空且其存储的指针为空。若 <code>Y*</code> 不<span class="t-rev-inl t-until-cxx17"><span>可隐式转换为</span> <span><span class="t-mark-rev t-until-cxx17">(C++17 前)</span></span></span><span class="t-rev-inl t-since-cxx17"><span><i>兼容</i></span> <span><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></span></span> <code>T*</code> ，则模板重载不参与重载决议。</div>
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<div class="t-li1"><span class="t-li">11)</span> 构造 <code>shared_ptr</code> ，共享 <code>r</code> 所管理对象的所有权。 <span class="t-rev-inl t-until-cxx17"><span><code>Y*</code> 必须可隐式转换为 <code>T*</code> 。</span> <span><span class="t-mark-rev t-until-cxx17">(C++17 前)</span></span></span><span class="t-rev-inl t-since-cxx17"><span>此重载仅若 <code>Y*</code> 兼容 <code>T*</code> 才参与重载决议。</span> <span><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></span></span>注意可为相同目的用 <span class="t-c"><span class="mw-geshi cpp source-cpp">r.<span class="me1">lock</span><span class="br0">(</span><span class="br0">)</span></span></span> ：区别是若参数为空则此构造函数抛异常，而 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-memory-weak_ptr.html"><span class="kw741">std::<span class="me2">weak_ptr</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="sy4">::</span><span class="me2">lock</span><span class="br0">(</span><span class="br0">)</span></span></span> 在该情况下构造空的 <code>std::shared_ptr</code> 。</div>
|
|
<div class="t-li1"><span class="t-li">12)</span> 构造 <code>shared_ptr</code> ，存储并占有 <code>r</code> 先前占有的对象。 <code>Y*</code> 必须可转换为 <code>T*</code> 。构造后， <code>r</code> 为空。</div>
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|
<div class="t-li1"><span class="t-li">13)</span> 构造 <code>shared_ptr</code> ，管理当前为 <code>r</code> 所管理的对象。存储与 <code>r</code> 关联的删除器以在未来删除被管理对象。调用后 <code>r</code> 不管理对象。
|
|
<table class="t-rev-begin">
|
|
<tr class="t-rev t-since-cxx17">
|
|
<td>若 <code>std::unique_ptr<Y, Deleter>::pointer</code> 不<i>兼容</i> <code>T*</code> 则此重载不参与重载决议。若 <span class="t-c"><span class="mw-geshi cpp source-cpp">r.<span class="me1">get</span><span class="br0">(</span><span class="br0">)</span></span></span> 是空指针，则此重载等价于默认构造函数 <span class="t-v">(1)</span> 。</td>
|
|
<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></td>
|
|
</tr>
|
|
</table>
|
|
若 <code>Deleter</code> 是引用类型，则等价于 <span class="t-c"><span class="mw-geshi cpp source-cpp">shared_ptr<span class="br0">(</span>r.<span class="me1">release</span><span class="br0">(</span><span class="br0">)</span>, <a href="cpp-utility-functional-ref.html"><span class="kw1069">std::<span class="me2">ref</span></span></a><span class="br0">(</span>r.<span class="me1">get_deleter</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span></span></span> 。否则，等价于 <span class="t-c"><span class="mw-geshi cpp source-cpp">shared_ptr<span class="br0">(</span>r.<span class="me1">release</span><span class="br0">(</span><span class="br0">)</span>, std<span class="sy4">::</span><span class="me2">move</span><span class="br0">(</span>r.<span class="me1">get_deleter</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span><span class="br0">)</span></span></span> 。</div>
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|
<p><code>T</code> 不是数组类型时，重载 <span class="t-v">(3)</span> 、 <span class="t-v">(4)</span> 及 <span class="t-v">(6)</span> 以 <code>ptr</code> 启用 <code>shared_from_this</code> ，而重载 <span class="t-v">(13)</span> 以 <span class="t-c"><span class="mw-geshi cpp source-cpp">r.<span class="me1">release</span><span class="br0">(</span><span class="br0">)</span></span></span> 所返回的指针启用 <code>shared_from_this</code> 。</p>
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|
<h3><span class="mw-headline" id=".E6.B3.A8.E8.A7.A3">注解</span></h3>
|
|
<p><code>T</code> 不是数组类型是，重载 <span class="t-v">(3)</span> 、 <span class="t-v">(4)</span> 及 <span class="t-v">(6)</span> 以 <code>ptr</code> 启用 shared_from_this ，而重载 <span class="t-v">(13)</span> 以 <span class="t-c"><span class="mw-geshi cpp source-cpp">r.<span class="me1">release</span><span class="br0">(</span><span class="br0">)</span></span></span> 所返回的指针启用 shared_from_this 。</p>
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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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<p>裸指针重载认定被指向对象的所有权。从而，用裸指针重载为已经为 <code>shared_ptr</code> 所管理对象构造 <code>shared_ptr</code> ，例如以 <span class="t-c"><span class="mw-geshi cpp source-cpp">shared_ptr<span class="br0">(</span>ptr.<span class="me1">get</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span></span></span> 很可能导致未定义行为，即使对象有导出自 <span class="t-lc"><a href="cpp-memory-enable_shared_from_this.html">std::enable_shared_from_this</a></span> 的类型。</p>
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<p>因为默认构造函数是 <code>constexpr</code> ，静态 shared_ptrs 作为<a href="cpp-language-initialization.html#.E9.9D.9E.E5.B1.80.E9.83.A8.E5.8F.98.E9.87.8F">静态非局部初始化</a>的一部分初始化，在任何动态初始化开始前。这使得在任何静态对象的构造函数中使用 shared_ptr 是安全的。</p>
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<p>C++11 和 C++14 中，从 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-memory-unique_ptr.html"><span class="kw733">std::<span class="me2">unique_ptr</span></span></a><span class="sy1"><</span>T<span class="br0">[</span><span class="br0">]</span><span class="sy1">></span></span></span> 构造 <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> 是合法的：</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"><a href="cpp-memory-unique_ptr.html"><span class="kw733">std::<span class="me2">unique_ptr</span></span></a><span class="sy1"><</span><span class="kw4">int</span><span class="br0">[</span><span class="br0">]</span><span class="sy1">></span> arr<span class="br0">(</span>new <span class="kw4">int</span><span class="br0">[</span><span class="nu0">1</span><span class="br0">]</span><span class="br0">)</span><span class="sy4">;</span>
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<a href="cpp-memory-shared_ptr.html"><span class="kw734">std::<span class="me2">shared_ptr</span></span></a><span class="sy1"><</span><span class="kw4">int</span><span class="sy1">></span> ptr<span class="br0">(</span>std<span class="sy4">::</span><span class="me2">move</span><span class="br0">(</span>arr<span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span></pre></div>
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</div>
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|
<p>因为 <code>shared_ptr</code> 从 <code>unique_ptr</code> 获得其删除器（ <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-memory-default_delete.html"><span class="kw746">std::<span class="me2">default_delete</span></span></a><span class="sy1"><</span>T<span class="br0">[</span><span class="br0">]</span><span class="sy1">></span></span></span> 对象），故能正确解分配数组。</p>
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|
<p>C++17 中这不再受允许。应当使用替代的数组形式 <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="br0">[</span><span class="br0">]</span><span class="sy1">></span></span></span> 。</p>
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|
<h3><span class="mw-headline" id=".E5.8F.82.E6.95.B0">参数</span></h3>
|
|
<table class="t-par-begin">
|
|
<tr class="t-par">
|
|
<td>ptr</td>
|
|
<td>-</td>
|
|
<td>指向要管理的对象的指针</td>
|
|
</tr>
|
|
<tr class="t-par">
|
|
<td>d</td>
|
|
<td>-</td>
|
|
<td>用于销毁对象的删除器</td>
|
|
</tr>
|
|
<tr class="t-par">
|
|
<td>alloc</td>
|
|
<td>-</td>
|
|
<td>用于分配内部使用的数据的分配器</td>
|
|
</tr>
|
|
<tr class="t-par">
|
|
<td>r</td>
|
|
<td>-</td>
|
|
<td>要共享所有权或从它获得所有权的另一智能指针</td>
|
|
</tr>
|
|
</table>
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|
<h3><span class="mw-headline" id=".E5.BC.82.E5.B8.B8">异常</span></h3>
|
|
<div class="t-li1"><span class="t-li">3)</span> 若无法获得要求的附加内存则为 <span class="t-lc"><a href="cpp-memory-new-bad_alloc.html">std::bad_alloc</a></span> 。可能因其他错误抛出实现定义的异常。若异常出现，则调用 <span class="t-c"><span class="mw-geshi cpp source-cpp">delete ptr</span></span><span class="t-rev-inl t-since-cxx17"><span>，若 <code>T</code> 非数组类型，否则调用 <span class="t-c"><span class="mw-geshi cpp source-cpp">delete<span class="br0">[</span><span class="br0">]</span> ptr</span></span></span> <span><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></span></span> 。</div>
|
|
<div class="t-li1"><span class="t-li">4-7)</span> 若无法获得要求的附加内存则为 <span class="t-lc"><a href="cpp-memory-new-bad_alloc.html">std::bad_alloc</a></span> 。可能因其他错误抛出实现定义的异常。若异常出现，则调用 <span class="t-c"><span class="mw-geshi cpp source-cpp">d<span class="br0">(</span>ptr<span class="br0">)</span></span></span> 。</div>
|
|
<div class="t-li1"><span class="t-li">11)</span> 若 <span class="t-c"><span class="mw-geshi cpp source-cpp">r.<span class="me1">expired</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1">==</span> <span class="kw2">true</span></span></span> 则为 <span class="t-lc"><a href="cpp-memory-bad_weak_ptr.html">std::bad_weak_ptr</a></span> 。若异常出现，则此构造函数无效果。</div>
|
|
<div class="t-li1"><span class="t-li">12)</span> 若无法获得要求的附加内存则为 <span class="t-lc"><a href="cpp-memory-new-bad_alloc.html">std::bad_alloc</a></span> 。可能因其他错误抛出实现定义的异常。若异常出现，则此构造函数无效果。</div>
|
|
<div class="t-li1"><span class="t-li">13)</span> 若抛异常，则构造函数无效果。</div>
|
|
<h3><span class="mw-headline" id=".E7.A4.BA.E4.BE.8B">示例</span></h3>
|
|
<div class="t-example">
|
|
<div class="t-example-live-link"></div>
|
|
<div dir="ltr" class="mw-geshi" style="text-align: left;">
|
|
<div class="cpp source-cpp">
|
|
<pre class="de1"><span class="co2">#include <memory></span>
|
|
<span class="co2">#include <iostream></span>
|
|
|
|
<span class="kw1">struct</span> Foo <span class="br0">{</span>
|
|
Foo<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"Foo...<span class="es1">\n</span>"</span><span class="sy4">;</span> <span class="br0">}</span>
|
|
~Foo<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"~Foo...<span class="es1">\n</span>"</span><span class="sy4">;</span> <span class="br0">}</span>
|
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<span class="br0">}</span><span class="sy4">;</span>
|
|
|
|
<span class="kw1">struct</span> D <span class="br0">{</span>
|
|
<span class="kw4">void</span> operator<span class="br0">(</span><span class="br0">)</span><span class="br0">(</span>Foo<span class="sy2">*</span> p<span class="br0">)</span> <span class="kw4">const</span> <span class="br0">{</span>
|
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<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"Call delete from function object...<span class="es1">\n</span>"</span><span class="sy4">;</span>
|
|
delete p<span class="sy4">;</span>
|
|
<span class="br0">}</span>
|
|
<span class="br0">}</span><span class="sy4">;</span>
|
|
|
|
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
|
|
<span class="br0">{</span>
|
|
<span class="br0">{</span>
|
|
<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"constructor with no managed object<span class="es1">\n</span>"</span><span class="sy4">;</span>
|
|
<a href="cpp-memory-shared_ptr.html"><span class="kw734">std::<span class="me2">shared_ptr</span></span></a><span class="sy1"><</span>Foo<span class="sy1">></span> sh1<span class="sy4">;</span>
|
|
<span class="br0">}</span>
|
|
|
|
<span class="br0">{</span>
|
|
<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"constructor with object<span class="es1">\n</span>"</span><span class="sy4">;</span>
|
|
<a href="cpp-memory-shared_ptr.html"><span class="kw734">std::<span class="me2">shared_ptr</span></span></a><span class="sy1"><</span>Foo<span class="sy1">></span> sh2<span class="br0">(</span>new Foo<span class="br0">)</span><span class="sy4">;</span>
|
|
<a href="cpp-memory-shared_ptr.html"><span class="kw734">std::<span class="me2">shared_ptr</span></span></a><span class="sy1"><</span>Foo<span class="sy1">></span> sh3<span class="br0">(</span>sh2<span class="br0">)</span><span class="sy4">;</span>
|
|
<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> sh2.<span class="me1">use_count</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
|
|
<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> sh3.<span class="me1">use_count</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
|
|
<span class="br0">}</span>
|
|
|
|
<span class="br0">{</span>
|
|
<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"constructor with object and deleter<span class="es1">\n</span>"</span><span class="sy4">;</span>
|
|
<a href="cpp-memory-shared_ptr.html"><span class="kw734">std::<span class="me2">shared_ptr</span></span></a><span class="sy1"><</span>Foo<span class="sy1">></span> sh4<span class="br0">(</span>new Foo, D<span class="br0">(</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
|
|
<a href="cpp-memory-shared_ptr.html"><span class="kw734">std::<span class="me2">shared_ptr</span></span></a><span class="sy1"><</span>Foo<span class="sy1">></span> sh5<span class="br0">(</span>new Foo, <span class="br0">[</span><span class="br0">]</span><span class="br0">(</span><span class="kw4">auto</span> p<span class="br0">)</span> <span class="br0">{</span>
|
|
<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"Call delete from lambda...<span class="es1">\n</span>"</span><span class="sy4">;</span>
|
|
delete p<span class="sy4">;</span>
|
|
<span class="br0">}</span><span class="br0">)</span><span class="sy4">;</span>
|
|
<span class="br0">}</span>
|
|
<span class="br0">}</span></pre></div>
|
|
</div>
|
|
<p>输出：</p>
|
|
<div dir="ltr" class="mw-geshi" style="text-align: left;">
|
|
<div class="text source-text">
|
|
<pre class="de1">constructor with no managed object
|
|
constructor with object
|
|
Foo...
|
|
2
|
|
2
|
|
~Foo...
|
|
constructor with object and deleter
|
|
Foo...
|
|
Foo...
|
|
Call delete from lambda...
|
|
~Foo...
|
|
Call delete from function object...
|
|
~Foo..</pre></div>
|
|
</div>
|
|
</div>
|
|
<h3><span class="mw-headline" id=".E7.BC.BA.E9.99.B7.E6.8A.A5.E5.91.8A">缺陷报告</span></h3>
|
|
<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>
|
|
<table class="dsctable" style="font-size:0.8em">
|
|
<tr>
|
|
<th>DR</th>
|
|
<th>应用于</th>
|
|
<th>出版时的行为</th>
|
|
<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/issue3548">LWG 3548</a></td>
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<td>C++11</td>
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<td>源自 <code>unique_ptr</code> 的构造函数复制构造删除器</td>
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<td>改为移动构造</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-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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<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-allocate_shared.html"><span class="t-lines"><span>allocate_shared</span><span>allocate_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>创建管理一个用分配器分配的新对象的共享指针<br />
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<span class="t-mark">(函数模板)</span></td>
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</tr>
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<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-enable_shared_from_this.html"><span class="t-lines"><span>enable_shared_from_this</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div>
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</div>
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</td>
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<td>允许对象创建指代自身的 <code>shared_ptr</code><br />
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<span class="t-mark">(类模板)</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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