When do I really need to use atomic instead of bool?

Question

Isn't `atomic` redundant because `bool` is atomic by nature? I don't think it's possible to have a partially modified bool value. When do I really need to use `atomic` instead of `bool`?

Accepted Answer

No type in C++ is "atomic by nature" unless it is an `std::atomic*`-something. That's because the standard says so.

In practice, the actual hardware instructions that are emitted to manipulate an `std::atomic<bool>` may (or may not) be the same as those for an ordinary `bool`, but being atomic is a larger concept with wider ramifications (e.g. restrictions on compiler re-ordering). Furthermore, some operations (like negation) are overloaded on the atomic operation to create a distinctly different instruction on the hardware than the native, non-atomic read-modify-write sequence of a non-atomic variable.

When do I really need to use atomic<bool> instead of bool?

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