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<ol class="chapter"><li class="chapter-item expanded "><a href="../Introduction.html">Introduction</a></li><li class="chapter-item expanded "><div>Chapter 1. Deducing Types</div></li><li><ol class="section"><li class="chapter-item expanded "><a href="../1.DeducingTypes/item1.html">Item 1: Understanding template type deduction</a></li><li class="chapter-item expanded "><a href="../1.DeducingTypes/item2.html">Item 2: Understand auto type deduction</a></li><li class="chapter-item expanded "><a href="../1.DeducingTypes/item3.html">Item 3: Understand decltype</a></li><li class="chapter-item expanded "><a href="../1.DeducingTypes/item4.html">Item 4: Know how to view deduced types</a></li></ol></li><li class="chapter-item expanded "><div>Chapter 2. auto</div></li><li><ol class="section"><li class="chapter-item expanded "><a href="../2.Auto/item5.html">Item 5: Prefer auto to explicit type declarations</a></li><li class="chapter-item expanded "><a href="../2.Auto/item6.html">Item 6: Use the explicitly typed initializer idiom when auto deduces undesired types</a></li></ol></li><li class="chapter-item expanded "><div>Chapter 3. Moving to Modern C++</div></li><li><ol class="section"><li class="chapter-item expanded "><a href="../3.MovingToModernCpp/item7.html">Item 7: Distinguish between () and {} when creating objects</a></li><li class="chapter-item expanded "><a href="../3.MovingToModernCpp/item8.html">Item 8: Prefer nullptr to 0 or NULL</a></li><li class="chapter-item expanded "><a href="../3.MovingToModernCpp/item9.html">Item 9: Prefer alias declarations to typedefs</a></li><li class="chapter-item expanded "><a href="../3.MovingToModernCpp/item10.html">Item 10: Prefer scoped enums to unscoped enums</a></li><li class="chapter-item expanded "><a href="../3.MovingToModernCpp/item11.html">Item 11: Prefer deleted functions to private undefined ones</a></li><li class="chapter-item expanded "><a href="../3.MovingToModernCpp/item12.html">Item 12: Declare overriding functions override</a></li><li class="chapter-item expanded "><a href="../3.MovingToModernCpp/item13.html">Item 13: Prefer const_iterators to iterators</a></li><li class="chapter-item expanded "><a href="../3.MovingToModernCpp/item14.html">Item 14: Declare functions noexcept if they won't emit exceptions</a></li><li class="chapter-item expanded "><a href="../3.MovingToModernCpp/item15.html">Item 15: Use constexpr whenever possible</a></li><li class="chapter-item expanded "><a href="../3.MovingToModernCpp/item16.html">Item 16: Make const member functions thread safe</a></li><li class="chapter-item expanded "><a href="../3.MovingToModernCpp/item17.html">Item 17: Understand special member funciton generation</a></li></ol></li><li class="chapter-item expanded "><div>Chapter 4. Smart Pointer</div></li><li><ol class="section"><li class="chapter-item expanded "><a href="../4.SmartPointers/item18.html">Item 18: Use std::unique_ptr for exclusive-ownership resource management</a></li><li class="chapter-item expanded "><a href="../4.SmartPointers/item19.html">Item 19: Use std::shared_ptr for shared-ownership resource management</a></li><li class="chapter-item expanded "><a href="../4.SmartPointers/item20.html" class="active">Item 20: Use std::weak_ptr for std::shared_ptr like pointers that can dangle</a></li><li class="chapter-item expanded "><a href="../4.SmartPointers/item21.html">Item 21: Prefer std::make_unique and std::make_shared to direct use of new</a></li><li class="chapter-item expanded "><a href="../4.SmartPointers/item22.html">Item 22: When using the Pimpl Idiom, define special member functions in the implementation file</a></li></ol></li><li class="chapter-item expanded "><div>Chapter 5. Rvalue References, Move Semantics, and Perfect Forwarding</div></li><li><ol class="section"><li class="chapter-item expanded "><a href="../5.RRefMovSemPerfForw/item23.html">Item 23: Understand std::move and std::forward</a></li><li class="chapter-item expanded "><a href="../5.RRefMovSemPerfForw/item24.html">Item 24: Distinguish universal references from rvalue references</a></li><li class="chapter-item expanded "><a href="../5.RRefMovSemPerfForw/item25.html">Item 25: Use std::move on rvalue references, std::forward on universal references</a></li><li class="chapter-item expanded "><a href="../5.RRefMovSemPerfForw/item26.html">Item 26: Avoid overloading on universal references</a></li><li class="chapter-item expanded "><a href="../5.RRefMovSemPerfForw/item27.html">Item 27: Familiarize yourself with alternatives to overaloading on univeral references</a></li><li class="chapter-item expanded "><a href="../5.RRefMovSemPerfForw/item28.html">Item 28: Understand reference collapsing</a></li><li class="chapter-item expanded "><a href="../5.RRefMovSemPerfForw/item29.html">Item 29: Assume that move operations are not present not cheap, and not used</a></li><li class="chapter-item expanded "><a href="../5.RRefMovSemPerfForw/item30.html">Item 30: Familiarize yourself with perfect forwarding failure cases</a></li></ol></li><li class="chapter-item expanded "><div>Chapter 6. Lambda Expressions</div></li><li><ol class="section"><li class="chapter-item expanded "><a href="../6.LambdaExpressions/item31.html">Item 31: Avoid default capture modes</a></li><li class="chapter-item expanded "><a href="../6.LambdaExpressions/item32.html">Item 32: Use init capture to move objects into closures</a></li><li class="chapter-item expanded "><a href="../6.LambdaExpressions/item33.html">Item 33: Use decltype on auto&¶meters to std::forward them</a></li><li class="chapter-item expanded "><a href="../6.LambdaExpressions/item34.html">Item 34: Prefer lambdas to std::bind</a></li></ol></li><li class="chapter-item expanded "><div>Chapter 7. The Concurrency API</div></li><li><ol class="section"><li class="chapter-item expanded "><a href="../7.TheConcurrencyAPI/Item35.html">Item 35: Prefer task-based programming to thread-based</a></li><li class="chapter-item expanded "><a href="../7.TheConcurrencyAPI/item36.html">Item 36: Specify std::launch::async if asynchronicity is essential</a></li><li class="chapter-item expanded "><a href="../7.TheConcurrencyAPI/item37.html">Item 37: Make std::threads unjionable on all paths</a></li><li class="chapter-item expanded "><a href="../7.TheConcurrencyAPI/item38.html">Item 38: Be aware of varying thread handle destructor behavior</a></li><li class="chapter-item expanded "><a href="../7.TheConcurrencyAPI/item39.html">Item 39: Consider void futures for one-shot event communication</a></li><li class="chapter-item expanded "><a href="../7.TheConcurrencyAPI/item40.html">Item 40: Use std::atomic for concurrency, volatile for special memory</a></li></ol></li><li class="chapter-item expanded "><div>Chapter 8. Tweaks</div></li><li><ol class="section"><li class="chapter-item expanded "><a href="../8.Tweaks/item41.html">Item 41: Consider pass by value for copyable parameters that are cheap to move and always copied</a></li><li class="chapter-item expanded "><a href="../8.Tweaks/item42.html">Item 42: Consider emplacement instead of insertion</a></li></ol></li></ol>
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<h2 id="条款二十当stdshard_ptr可能悬空时使用stdweak_ptr"><a class="header" href="#条款二十当stdshard_ptr可能悬空时使用stdweak_ptr">条款二十：当<code>std::shard_ptr</code>可能悬空时使用<code>std::weak_ptr</code></a></h2>
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<p><strong>Item 20: Use <code>std::weak_ptr</code> for <code>std::shared_ptr</code>-like pointers that can dangle</strong></p>
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<p>自相矛盾的是，如果有一个像<code>std::shared_ptr</code>（见<a href="https://github.com/kelthuzadx/EffectiveModernCppChinese/blob/master/4.SmartPointers/item19.md">Item19</a>）的但是不参与资源所有权共享的指针是很方便的。换句话说，是一个类似<code>std::shared_ptr</code>但不影响对象引用计数的指针。这种类型的智能指针必须要解决一个<code>std::shared_ptr</code>不存在的问题：可能指向已经销毁的对象。一个真正的智能指针应该跟踪所指对象，在悬空时知晓，悬空（<em>dangle</em>）就是指针指向的对象不再存在。这就是对<code>std::weak_ptr</code>最精确的描述。</p>
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<p>你可能想知道什么时候该用<code>std::weak_ptr</code>。你可能想知道关于<code>std::weak_ptr</code> API的更多。它什么都好除了不太智能。<code>std::weak_ptr</code>不能解引用，也不能测试是否为空值。因为<code>std::weak_ptr</code>不是一个独立的智能指针。它是<code>std::shared_ptr</code>的增强。</p>
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<p>这种关系在它创建之时就建立了。<code>std::weak_ptr</code>通常从<code>std::shared_ptr</code>上创建。当从<code>std::shared_ptr</code>上创建<code>std::weak_ptr</code>时两者指向相同的对象，但是<code>std::weak_ptr</code>不会影响所指对象的引用计数：</p>
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<pre><code class="language-cpp">auto spw = //spw创建之后，指向的Widget的
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std::make_shared<Widget>(); //引用计数（ref count，RC）为1。
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//std::make_shared的信息参见条款21
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…
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std::weak_ptr<Widget> wpw(spw); //wpw指向与spw所指相同的Widget。RC仍为1
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…
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spw = nullptr; //RC变为0，Widget被销毁。
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//wpw现在悬空
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</code></pre>
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<p>悬空的<code>std::weak_ptr</code>被称作已经<strong>expired</strong>（过期）。你可以用它直接做测试：</p>
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<pre><code class="language-CPP">if (wpw.expired()) … //如果wpw没有指向对象…
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</code></pre>
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<p>但是通常你期望的是检查<code>std::weak_ptr</code>是否已经过期，如果没有过期则访问其指向的对象。这做起来可不是想着那么简单。因为缺少解引用操作，没有办法写这样的代码。即使有，将检查和解引用分开会引入竞态条件：在调用<code>expired</code>和解引用操作之间，另一个线程可能对指向这对象的<code>std::shared_ptr</code>重新赋值或者析构，并由此造成对象已析构。这种情况下，你的解引用将会产生未定义行为。</p>
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<p>你需要的是一个原子操作检查<code>std::weak_ptr</code>是否已经过期，如果没有过期就访问所指对象。这可以通过从<code>std::weak_ptr</code>创建<code>std::shared_ptr</code>来实现，具体有两种形式可以从<code>std::weak_ptr</code>上创建<code>std::shared_ptr</code>，具体用哪种取决于<code>std::weak_ptr</code>过期时你希望<code>std::shared_ptr</code>表现出什么行为。一种形式是<code>std::weak_ptr::lock</code>，它返回一个<code>std::shared_ptr</code>，如果<code>std::weak_ptr</code>过期这个<code>std::shared_ptr</code>为空：</p>
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<pre><code class="language-cpp">std::shared_ptr<Widget> spw1 = wpw.lock(); //如果wpw过期，spw1就为空
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auto spw2 = wpw.lock(); //同上，但是使用auto
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</code></pre>
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<p>另一种形式是以<code>std::weak_ptr</code>为实参构造<code>std::shared_ptr</code>。这种情况中，如果<code>std::weak_ptr</code>过期，会抛出一个异常：</p>
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<pre><code class="language-cpp">std::shared_ptr<Widget> spw3(wpw); //如果wpw过期，抛出std::bad_weak_ptr异常
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</code></pre>
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<p>但是你可能还想知道为什么<code>std::weak_ptr</code>就有用了。考虑一个工厂函数，它基于一个唯一ID从只读对象上产出智能指针。根据<a href="https://github.com/kelthuzadx/EffectiveModernCppChinese/blob/master/4.SmartPointers/item19.md">Item18</a>的描述，工厂函数会返回一个该对象类型的<code>std::unique_ptr</code>：</p>
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<pre><code class="language-cpp">std::unique_ptr<const Widget> loadWidget(WidgetID id);
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</code></pre>
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<p>如果调用<code>loadWidget</code>是一个昂贵的操作（比如它操作文件或者数据库I/O）并且重复使用ID很常见，一个合理的优化是再写一个函数除了完成<code>loadWidget</code>做的事情之外再缓存它的结果。当每个请求获取的<code>Widget</code>阻塞了缓存也会导致本身性能问题，所以另一个合理的优化可以是当<code>Widget</code>不再使用的时候销毁它的缓存。</p>
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<p>对于可缓存的工厂函数，返回<code>std::unique_ptr</code>不是好的选择。调用者应该接收缓存对象的智能指针，调用者也应该确定这些对象的生命周期，但是缓存本身也需要一个指针指向它所缓存的对象。缓存对象的指针需要知道它是否已经悬空，因为当工厂客户端使用完工厂产生的对象后，对象将被销毁，关联的缓存条目会悬空。所以缓存应该使用<code>std::weak_ptr</code>，这可以知道是否已经悬空。这意味着工厂函数返回值类型应该是<code>std::shared_ptr</code>，因为只有当对象的生命周期由<code>std::shared_ptr</code>管理时，<code>std::weak_ptr</code>才能检测到悬空。</p>
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<p>下面是一个临时凑合的<code>loadWidget</code>的缓存版本的实现：</p>
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<pre><code class="language-cpp">std::shared_ptr<const Widget> fastLoadWidget(WidgetID id)
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{
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static std::unordered_map<WidgetID,
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std::weak_ptr<const Widget>> cache;
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//译者注：这里std::weak_ptr<const Widget>是高亮
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auto objPtr = cache[id].lock(); //objPtr是去缓存对象的
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//std::shared_ptr（或
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//当对象不在缓存中时为null）
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if (!objPtr) { //如果不在缓存中
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objPtr = loadWidget(id); //加载它
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cache[id] = objPtr; //缓存它
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}
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return objPtr;
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}
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</code></pre>
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<p>这个实现使用了C++11的hash表容器<code>std::unordered_map</code>，但是需要的<code>WidgetID</code>哈希和相等性比较函数在这里没有展示。</p>
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<p><code>fastLoadWidget</code>的实现忽略了以下事实：缓存可能会累积过期的<code>std::weak_ptr</code>，这些指针对应了不再使用的<code>Widget</code>（也已经被销毁了）。其实可以改进实现方式，但是花时间在这个问题上不会让我们对<code>std::weak_ptr</code>有更深入的理解，让我们考虑第二个用例：观察者设计模式（Observer design pattern）。此模式的主要组件是subjects（状态可能会更改的对象）和observers（状态发生更改时要通知的对象）。在大多数实现中，每个subject都包含一个数据成员，该成员持有指向其observers的指针。这使subjects很容易发布状态更改通知。subjects对控制observers的生命周期（即它们什么时候被销毁）没有兴趣，但是subjects对确保另一件事具有极大的兴趣，那事就是一个observer被销毁时，不再尝试访问它。一个合理的设计是每个subject持有一个<code>std::weak_ptr</code>s容器指向observers，因此可以在使用前检查是否已经悬空。</p>
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<p>作为最后一个使用<code>std::weak_ptr</code>的例子，考虑一个持有三个对象<code>A</code>、<code>B</code>、<code>C</code>的数据结构，<code>A</code>和<code>C</code>共享<code>B</code>的所有权，因此持有<code>std::shared_ptr</code>：</p>
|
||
<p><img src="media/item20_fig1.png" alt="item20_fig1" /></p>
|
||
<p>假定从B指向A的指针也很有用。应该使用哪种指针？</p>
|
||
<p><img src="media/item20_fig2.png" alt="item20_fig2" /></p>
|
||
<p>有三种选择：</p>
|
||
<ul>
|
||
<li><strong>原始指针</strong>。使用这种方法，如果<code>A</code>被销毁，但是<code>C</code>继续指向<code>B</code>，<code>B</code>就会有一个指向<code>A</code>的悬空指针。而且<code>B</code>不知道指针已经悬空，所以<code>B</code>可能会继续访问，就会导致未定义行为。</li>
|
||
<li><strong><code>std::shared_ptr</code></strong>。这种设计，<code>A</code>和<code>B</code>都互相持有对方的<code>std::shared_ptr</code>，导致的<code>std::shared_ptr</code>环状结构（<code>A</code>指向<code>B</code>，<code>B</code>指向<code>A</code>）阻止<code>A</code>和<code>B</code>的销毁。甚至<code>A</code>和<code>B</code>无法从其他数据结构访问了（比如，<code>C</code>不再指向<code>B</code>），每个的引用计数都还是1。如果发生了这种情况，<code>A</code>和<code>B</code>都被泄漏：程序无法访问它们，但是资源并没有被回收。</li>
|
||
<li><strong><code>std::weak_ptr</code></strong>。这避免了上述两个问题。如果<code>A</code>被销毁，<code>B</code>指向它的指针悬空，但是<code>B</code>可以检测到这件事。尤其是，尽管<code>A</code>和<code>B</code>互相指向对方，<code>B</code>的指针不会影响<code>A</code>的引用计数，因此在没有<code>std::shared_ptr</code>指向<code>A</code>时不会导致<code>A</code>无法被销毁。</li>
|
||
</ul>
|
||
<p>使用<code>std::weak_ptr</code>显然是这些选择中最好的。但是，需要注意使用<code>std::weak_ptr</code>打破<code>std::shared_ptr</code>循环并不常见。在严格分层的数据结构比如树中，子节点只被父节点持有。当父节点被销毁时，子节点就被销毁。从父到子的链接关系可以使用<code>std::unique_ptr</code>很好的表征。从子到父的反向连接可以使用原始指针安全实现，因为子节点的生命周期肯定短于父节点。因此没有子节点解引用一个悬垂的父节点指针这样的风险。</p>
|
||
<p>当然，不是所有的使用指针的数据结构都是严格分层的，所以当发生这种情况时，比如上面所述缓存和观察者列表的实现之类的，知道<code>std::weak_ptr</code>随时待命也是不错的。</p>
|
||
<p>从效率角度来看，<code>std::weak_ptr</code>与<code>std::shared_ptr</code>基本相同。两者的大小是相同的，使用相同的控制块（参见<a href="https://github.com/kelthuzadx/EffectiveModernCppChinese/blob/master/4.SmartPointers/item19.md">Item19</a>），构造、析构、赋值操作涉及引用计数的原子操作。这可能让你感到惊讶，因为本条款开篇就提到<code>std::weak_ptr</code>不影响引用计数。我写的是<code>std::weak_ptr</code>不参与对象的<strong>共享所有权</strong>，因此不影响<strong>指向对象的引用计数</strong>。实际上在控制块中还是有第二个引用计数，<code>std::weak_ptr</code>操作的是第二个引用计数。想了解细节的话，继续看<a href="https://github.com/kelthuzadx/EffectiveModernCppChinese/blob/master/4.SmartPointers/item21.md">Item21</a>吧。</p>
|
||
<p><strong>请记住：</strong></p>
|
||
<ul>
|
||
<li>用<code>std::weak_ptr</code>替代可能会悬空的<code>std::shared_ptr</code>。</li>
|
||
<li><code>std::weak_ptr</code>的潜在使用场景包括：缓存、观察者列表、打破<code>std::shared_ptr</code>环状结构。</li>
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