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< title > Item 29:认识移动操作的缺点 - Effective Modern C++< / title >
2022-06-30 10:23:03 +08:00
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< 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" > Item 20:当std::shard_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.RRefMovSemPerfF
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< h2 id = "条款二十九假定移动操作不存在成本高未被使用" > < a class = "header" href = "#条款二十九假定移动操作不存在成本高未被使用" > 条款二十九：假定移动操作不存在，成本高，未被使用< / a > < / h2 >
< p > < strong > Item 29: Assume that move operations are not present, not cheap, and not used< / strong > < / p >
< p > 移动语义可以说是C++11最主要的特性。你可能会见过这些类似的描述“移动容器和拷贝指针一样开销小”， ， ， （ ） ， ， ， ， < / p >
< p > 移动语义确实可以做这些事，这把这个特性封为一代传说。但是传说总有些夸大成分。这个条款的目的就是给你泼一瓢冷水，保持理智看待移动语义。< / p >
< p > 让我们从已知很多类型不支持移动操作开始这个过程。为了升级到C++11， ， ， ， （ ） ， ， ， ， ， ， （ < a href = "https://github.com/kelthuzadx/EffectiveModernCppChinese/blob/master/3.MovingToModernCpp/item17.md" > Item17< / a > ） （ ， < a href = "https://github.com/kelthuzadx/EffectiveModernCppChinese/blob/master/3.MovingToModernCpp/item11.md" > Item11< / a > ） ， ， ， < / p >
< p > 即使显式支持了移动操作， ， ， ， ， < / p >
< p > 考虑一下< code > std::array< / code > ， < code > std::array< / code > 本质上是具有STL接口的内置数组。这与其他标准容器将内容存储在堆内存不同。存储具体数据在堆内存的容器， （ ， ） ， ， ： < / p >
< pre > < code class = "language-cpp" > std::vector< Widget> vm1;
//把数据存进vw1
…
//把vw1移动到vw2。以常数时间运行。只有vw1和vw2中的指针被改变
auto vm2 = std::move(vm1);
< / code > < / pre >
< p > < img src = "media/item29_fig1.png" alt = "item29_fig1" / > < / p >
< p > < code > std::array< / code > 没有这种指针实现，数据就保存在< code > std::array< / code > 对象中：< / p >
< pre > < code class = "language-cpp" > std::array< Widget, 10000> aw1;
//把数据存进aw1
…
//把aw1移动到aw2。以线性时间运行。aw1中所有元素被移动到aw2
auto aw2 = std::move(aw1);
< / code > < / pre >
< p > < img src = "media/item29_fig2.png" alt = "item29_fig2" / > < / p >
< p > 注意< code > aw1< / code > 中的元素被< strong > 移动< / strong > 到了< code > aw2< / code > 中。假定< code > Widget< / code > 类的移动操作比复制操作快，移动< code > Widget< / code > 的< code > std::array< / code > 就比复制要快。所以< code > std::array< / code > 确实支持移动操作。但是使用< code > std::array< / code > 的移动操作还是复制操作都将花费线性时间的开销，因为每个容器中的元素终归需要拷贝或移动一次，这与“移动一个容器就像操作几个指针一样方便”的含义相去甚远。< / p >
< p > 另一方面，< code > std::string< / code > 提供了常数时间的移动操作和线性时间的复制操作。这听起来移动比复制快多了，但是可能不一定。许多字符串的实现采用了小字符串优化（< em > small string optimization< / em > ， ） （ ） < code > std::string< / code > 的缓冲区中，并没有存储在堆内存，移动这种存储的字符串并不必复制操作更快。< / p >
< p > SSO的动机是大量证据表明， ， ， ， < / p >
< p > 即使对于支持快速移动操作的类型，某些看似可靠的移动操作最终也会导致复制。< a href = "https://github.com/kelthuzadx/EffectiveModernCppChinese/blob/master/3.MovingToModernCpp/item14.md" > Item14< / a > 解释了原因， ， ， ， ， ， （ ） ， ， < code > noexcept< / code > 。< / p >
< p > 因此， ， ： < / p >
< ul >
< li > < strong > 没有移动操作< / strong > ：要移动的对象没有提供移动操作，所以移动的写法也会变成复制操作。< / li >
< li > < strong > 移动不会更快< / strong > ：要移动的对象提供的移动操作并不比复制速度更快。< / li >
< li > < strong > 移动不可用< / strong > ：进行移动的上下文要求移动操作不会抛出异常，但是该操作没有被声明为< code > noexcept< / code > 。< / li >
< / ul >
< p > 值得一提的是，还有另一个场景，会使得移动并没有那么有效率：< / p >
< ul >
< li > < strong > 源对象是左值< / strong > ：除了极少数的情况外（例如< a href = "https://github.com/kelthuzadx/EffectiveModernCppChinese/blob/master/5.RRefMovSemPerfForw/item25.md" > Item25< / a > ），只有右值可以作为移动操作的来源。< / li >
< / ul >
< p > 但是该条款的标题是假定移动操作不存在， ， ， ， ， ， ， < / p >
< p > 但是，通常，你了解你代码里使用的类型，依赖他们的特性不变性（比如是否支持快速移动操作）。这种情况，你无需这个条款的假设，只需要查找所用类型的移动操作详细信息。如果类型提供了快速移动操作，并且在调用移动操作的上下文中使用对象，可以安全的使用快速移动操作替换复制操作。< / p >
< p > < strong > 请记住：< / strong > < / p >
< ul >
< li > 假定移动操作不存在，成本高，未被使用。< / li >
< li > 在已知的类型或者支持移动语义的代码中，就不需要上面的假设。< / li >
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