difference between standard's atomic bool and atomic flag

std::atomic bool type not guranteed to be lock-free?

Correct. std::atomic may be implemented using locks.

then it's not atomic or what?

std::atomic is atomic whether it has been implemented using locks, or without. std::atomic_flag is guaranteed to be implemented without using locks.

So what's the difference b/w two

The primary difference besides the lock-free guarantee is:

std::atomic_flag does not provide load or store operations.

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and when should I use which?

Usually, you will want to use std::atomic<bool> when you need an atomic boolean variable. std::atomic_flag is a low level structure that can be used to implement custom atomic structures.

std::atomic<T> guarantees that accesses to the variable will be atomic. It however does not says how is the atomicity achieved. It can be using lock-free variable, or using a lock. The actual implementation depends on your target architecture and the type T.

std::atomic_flag on the other hand is guaranteed to be implemented using a lock-free technique.