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< title > Item 20:当std::shared_ptr可能悬空时使用std::weak_ptr - Effective Modern C++< / title >
2022-06-30 10:23:03 +08:00
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2022-11-18 22:12:20 +08:00
< ol class = "chapter" > < li class = "chapter-item expanded " > < a href = "../Introduction.html" > 简介< / a > < / li > < li class = "chapter-item expanded " > < div > 第一章 类型推导< / div > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../1.DeducingTypes/item1.html" > Item 1:理解模板类型推导< / a > < / li > < li class = "chapter-item expanded " > < a href = "../1.DeducingTypes/item2.html" > Item 2:理解auto类型推导< / a > < / li > < li class = "chapter-item expanded " > < a href = "../1.DeducingTypes/item3.html" > Item 3:理解decltype< / a > < / li > < li class = "chapter-item expanded " > < a href = "../1.DeducingTypes/item4.html" > Item 4:学会查看类型推导结果< / a > < / li > < / ol > < / li > < li class = "chapter-item expanded " > < div > 第二章 auto< / div > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../2.Auto/item5.html" > Item 5:优先考虑auto而非显式类型声明< / a > < / li > < li class = "chapter-item expanded " > < a href = "../2.Auto/item6.html" > Item 6:auto推导若非己愿， < / a > < / li > < / ol > < / li > < li class = "chapter-item expanded " > < div > 第三章 移步现代C++< / div > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../3.MovingToModernCpp/item7.html" > Item 7:区别使用()和{}创建对象< / a > < / li > < li class = "chapter-item expanded " > < a href = "../3.MovingToModernCpp/item8.html" > Item 8:优先考虑nullptr而非0和NULL< / a > < / li > < li class = "chapter-item expanded " > < a href = "../3.MovingToModernCpp/item9.html" > Item 9:优先考虑别名声明而非typedefs< / a > < / li > < li class = "chapter-item expanded " > < a href = "../3.MovingToModernCpp/item10.html" > Item 10:优先考虑限域枚举而非未限域枚举< / a > < / li > < li class = "chapter-item expanded " > < a href = "../3.MovingToModernCpp/item11.html" > Item 11:优先考虑使用deleted函数而非使用未定义的私有声明< / a > < / li > < li class = "chapter-item expanded " > < a href = "../3.MovingToModernCpp/item12.html" > Item 12:使用override声明重载函数< / a > < / li > < li class = "chapter-item expanded " > < a href = "../3.MovingToModernCpp/item13.html" > Item 13:优先考虑const_iterator而非iterator< / a > < / li > < li class = "chapter-item expanded " > < a href = "../3.MovingToModernCpp/item14.html" > Item 14:如果函数不抛出异常请使用noexcept< / a > < / li > < li class = "chapter-item expanded " > < a href = "../3.MovingToModernCpp/item15.html" > Item 15:尽可能的使用constexpr< / a > < / li > < li class = "chapter-item expanded " > < a href = "../3.MovingToModernCpp/item16.html" > Item 16:让const成员函数线程安全< / a > < / li > < li class = "chapter-item expanded " > < a href = "../3.MovingToModernCpp/item17.html" > Item 17:理解特殊成员函数函数的生成< / a > < / li > < / ol > < / li > < li class = "chapter-item expanded " > < div > 第四章 智能指针< / div > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../4.SmartPointers/item18.html" > Item 18:对于独占资源使用std::unique_ptr< / a > < / li > < li class = "chapter-item expanded " > < a href = "../4.SmartPointers/item19.html" > Item 19:对于共享资源使用std::shared_ptr< / a > < / li > < li class = "chapter-item expanded " > < a href = "../4.SmartPointers/item20.html" class = "active" > Item 20:当std::shared_ptr可能悬空时使用std::weak_ptr< / a > < / li > < li class = "chapter-item expanded " > < a href = "../4.SmartPointers/item21.html" > Item 21:优先考虑使用std::make_unique和std::make_shared而非new< / a > < / li > < li class = "chapter-item expanded " > < a href = "../4.SmartPointers/item22.html" > Item 22:当使用Pimpl惯用法， < / a > < / li > < / ol > < / li > < li class = "chapter-item expanded " > < div > 第五章 右值引用，移动语义，完美转发< / div > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../5.RRefMovSemPerfForw/item23.html" > Item 23:理解std::move和std::forward< / a > < / li > < li class = "chapter-item expanded " > < a href = "../5.RRefMovSemPerfForw/item24.html" > Item 24:区别通用引用和右值引用< / a > < / li > < li class = "chapter-item expanded " > < a href = "../5.RRefMovSemPerfForw/item25.html" > Item 25:对于右值引用使用std::move， < / a > < / li > < li class = "chapter-item expanded " > < a href = "../5
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< h2 id = "条款二十当stdshared_ptr可能悬空时使用stdweak_ptr" > < a class = "header" href = "#条款二十当stdshared_ptr可能悬空时使用stdweak_ptr" > 条款二十：当< code > std::shared_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 >
2022-11-25 13:43:31 +08:00
< p > 自相矛盾的是，如果有一个像< code > std::shared_ptr< / code > （见< a href = "../4.SmartPointers/item19.html" > 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 >
< 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 >
< pre > < code class = "language-cpp" > auto spw = //spw创建之后，
std::make_shared< Widget> (); //引用计数（ ， ）
//std::make_shared的信息参见条款21
…
std::weak_ptr< Widget> wpw(spw); //wpw指向与spw所指相同的Widget。RC仍为1
…
spw = nullptr; //RC变为0，
//wpw现在悬空
< / code > < / pre >
< 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 >
< pre > < code class = "language-CPP" > if (wpw.expired()) … //如果wpw没有指向对象…
< / code > < / pre >
< 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 >
< 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 >
< pre > < code class = "language-cpp" > std::shared_ptr< Widget> spw1 = wpw.lock(); //如果wpw过期，
auto spw2 = wpw.lock(); //同上，
< / code > < / pre >
< 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 >
< pre > < code class = "language-cpp" > std::shared_ptr< Widget> spw3(wpw); //如果wpw过期，
< / 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， < a href = "../4.SmartPointers/item19.html" > 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 >
2022-06-30 10:23:03 +08:00
< pre > < code class = "language-cpp" > std::unique_ptr< const Widget> loadWidget(WidgetID id);
< / code > < / pre >
< p > 如果调用< code > loadWidget< / code > 是一个昂贵的操作（ ） ， < 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 >
< 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 >
< p > 下面是一个临时凑合的< code > loadWidget< / code > 的缓存版本的实现：< / p >
< pre > < code class = "language-cpp" > std::shared_ptr< const Widget> fastLoadWidget(WidgetID id)
{
static std::unordered_map< WidgetID,
std::weak_ptr< const Widget> > cache;
//译者注： < const Widget> 是高亮
auto objPtr = cache[id].lock(); //objPtr是去缓存对象的
//std::shared_ptr（
//当对象不在缓存中时为null）
if (!objPtr) { //如果不在缓存中
objPtr = loadWidget(id); //加载它
cache[id] = objPtr; //缓存它
}
return objPtr;
}
< / code > < / pre >
< 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 >
< 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， ： （ ） （ ） （ ） ， ， （ ） ， ， ， < code > std::weak_ptr< / code > s容器指向observers， < / p >
< 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 > ） ， ， < 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 >
2022-11-25 13:43:31 +08:00
< 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 = "../4.SmartPointers/item19.html" > 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 = "../4.SmartPointers/item21.html" > Item21< / a > 吧。< / p >
2022-06-30 10:23:03 +08:00
< 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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