April 2018
Intermediate to advanced
303 pages
4h 54m
Chinese
Content preview from Effective Modern C++（中文版）
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262
｜
第
7
章
volatile std::atomic<int>
vai;
//
针对
vai
的操作是原子的，
//
并且不可以被优化掉
如果
vai
对应于由多个线程同时访问的内存映射
I/O
位置，就可能会是有用的。
最后，有些开发人员更喜欢使用
std::atomic
的
load
和
store
成员函数，即使并非必要，
因为这样做可以在源代码中明确地表示所涉及的变量并非“常规”。强调这一事实，
也并非没有理由。访问
std::atomic
型别对象通常比访问非
std::atomic
型别对象慢
得多，我们已经看到
std::atomics
型别对象在使用过程中会阻止编译器对某些类型
的代码重新排序，而这样的重新排序在其他情况下是被允许的。召唤
std::atomic
型
别对象的加载和存储有助于识别出阻碍潜在的可伸缩性之处。从正确性角度来看，如
果本来想要通过某个变量将信息传达到其他线程，却未见它调用
store
（例如，一个指
示数据可用性的标志位），就可能意味着该变量本来应该声明为
std::atomic
，却没有
这样做。
这在很大程度上是一个代码风格问题，因此，这与在
std::atomic
和
volatile
之间
进行的选择有着非常不同的性质。
要点速记
•
std::atomic
用于多线程访问的数据，且不用互斥量。它是撰写并发软件的
工具。
•
volatile
用于读写操作不可以被优化掉的内存。它是在面对特种内存时使
用的工具。
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