I was wondering what the differences are between accessing a boolean value using Windows' interlockedXXX functions and using std::atomic_flag.
To my knowledge, both of them are lock-less and you can't set or read an atomic_flag directly. I wonder whether there are more differences.
std::atomic_flag serves basically as a primitive for building other synchronization primitives. In case one needs to set or read, it might make more sense to compare with std::atomic<bool>.
However, there are some additional (conceptual) differences:
With interlockedXXX, you won't get portable code.
interlockedXXX is a function, while std::atomic_flag (as well as std::atomic) is a type. That's a significant difference, since, you can use interlockedXXX with any suitable memory location, such as an element of std::vector. On the contrary, you cannot make a vector of C++ atomic flags or atomic bools, since the corresponding types do not meet the vector value type requirements. 1
You can see the latter difference in the code created by @RmMm, where flag is an ordinary variable. I also added a case with atomic<bool> and you may notice that all the three variants produce the very same assembly:
[1] This problem should be addressed by std::atomic_ref in C++20.