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标准的 atomic bool 和 atomic flag 之间的区别

[英]difference between standard's atomic bool and atomic flag

原文 2016-09-05 11:08:51 1 2 c++/ multithreading/ c++11/ thread-safety/ atomic

I wasn't aware of the std::atomic variables but was aware about the std::mutex (weird right!) provided by the standard;我不知道std::atomic变量，但知道std::mutex提供的std::mutex （很奇怪！）； however one thing caught my eye: there are two seemingly-same (to me) atomic types provided by the standard, listed below:然而，有一件事引起了我的注意：标准提供了两种看似相同（对我而言）的原子类型，如下所列：

The std::atomic_flag contains the following explanation: std::atomic_flag包含以下解释：

std::atomic_flag is an atomic boolean type. std::atomic_flag是一个原子布尔类型。 Unlike all specializations of std::atomic , it is guaranteed to be lock-free.与std::atomic所有特化不同，它保证是无锁的。 Unlike std::atomic<bool> , std::atomic_flag does not provide load or store operations.与std::atomic<bool> ， std::atomic_flag不提供加载或存储操作。

which I fail to understand.我不明白。 Is std::atomic<bool> not guaranteed to be lock-free? std::atomic<bool>不能保证是无锁的吗？ Then it's not atomic or what?那么它不是原子的还是什么？

So what's the difference between the two and when should I use which?那么两者之间有什么区别，我应该什么时候使用哪个呢？

2 个解决方案

std::atomic bool type not guranteed to be lock-free? std::atomic bool 类型不保证无锁？

Correct.正确。 std::atomic may be implemented using locks. std::atomic可以使用锁来实现。

then it's not atomic or what?那么它不是原子的还是什么？

std::atomic is atomic whether it has been implemented using locks, or without.无论是否使用锁实现， std::atomic都是原子的。 std::atomic_flag is guaranteed to be implemented without using locks. std::atomic_flag保证在不使用锁的情况下实现。

So what's the difference b/w two那么黑白两个有什么区别

The primary difference besides the lock-free guarantee is:除了无锁保证之外的主要区别是：

std::atomic_flag does not provide load or store operations. std::atomic_flag不提供加载或存储操作。

and when should I use which?我什么时候应该使用哪个？

Usually, you will want to use std::atomic<bool> when you need an atomic boolean variable.通常，当您需要原子布尔变量时，您将需要使用std::atomic<bool> 。 std::atomic_flag is a low level structure that can be used to implement custom atomic structures. std::atomic_flag是一个低级结构，可用于实现自定义原子结构。

std::atomic<T> guarantees that accesses to the variable will be atomic. std::atomic<T>保证对变量的访问是原子的。 It however does not says how is the atomicity achieved.然而，它并没有说明如何实现原子性。 It can be using lock-free variable, or using a lock.它可以使用无锁变量，也可以使用锁。 The actual implementation depends on your target architecture and the type T .实际实现取决于您的目标架构和类型T 。