Question

The definitions for std::atomic<> seem to show its obvious usefulness for primitive or perhaps POD-types.

When would you actually use it for classes?

When should you avoid using it for classes?

Answer 1

The operations std::atomic makes available on any trivially copyable type are pretty basic. You can construct and destroy atomic<T>, you can ask if the type is_lock_free(), you can load and store copies of T, and you can exchange values of T in various ways. If that’s sufficient for your purpose then you might be better off doing that than holding an explicit lock.

If those operations aren’t sufficient, if for example you need to atomically perform a sequence operations directly on the value, or if the object is large enough that copying is expensive, then instead you would probably want to hold an explicit lock which you manage to achieve your more complex goals or avoid doing all the copies that using atomic<T> would involve.

// non-POD type that maintains an invariant a==b without any care for
// thread safety.
struct T { int b; }
struct S : private T {
    S(int n) : a{n}, b{n} {}
    void increment() { a++; b++; }
private:
    int a;
};

std::atomic<S> a{{5}}; // global variable

// how a thread might update the global variable without losing any
// other thread's updates.
S s = a.load();
S new_s;
do {
    new_s = s;
    new_s.increment(); // whatever modifications you want
} while (!a.compare_exchange_strong(s, new_s));

As you can see, this basically gets a copy of the value, modifies the copy, then tries to copy the modified value back, repeating as necessary. The modifications you make to the copy can be as complex as you like, not simply limited to single member functions.