c++ - Why is the volatile qualifier used through out std::atomic?

Question

From what I've read from Herb Sutter and others you would think that volatile and concurrent programming were completely orthogonal concepts, at least as far as C/C++ are concerned.

However, in GCC implementation all of std::atomic's member functions have the volatile qualifier. The same is true in Anthony Williams's implementation of std::atomic.

So what's deal, do my atomic<> variables need be volatile or not?

Answer 1

To summarize what others have correctly written:

C/C++ volatile is for hardware access and interrupts. C++11 atomic<> is for inter-thread communication (e.g., in lock-free code). Those two concepts/uses are orthogonal, but they have overlapping requirements and that is why people have often confused the two.

The reason that atomic<> has volatile-qualified functions is the same reason it has const-qualified functions, because it's possible in principle for an object be both atomic<> and also const and/or volatile.

Of course, as my article pointed out, a further source of confusion is that C/C++ volatile isn't the same as C#/Java volatile (the latter is basically equivalent to C++11 atomic<>).