Question

I’m probably missing something obvious, but I can’t see any difference between std::condition_variable and std::condition_variable_any. Why do we need both?

Answer 1

std::condition_variable is more specialized, and therefore can be more efficient when you don’t need the flexibility of std::condition_variable_any.

From N3290 §30.5[thread.condition]/1

Class condition_variable provides
a condition variable that can only wait on an object of type unique_lock<mutex>, allowing maximum
efficiency on some platforms. Class condition_variable_any provides a general condition variable that
can wait on objects of user-supplied lock types.

Actually, in LLVM’s libc++, condition_variable_any is implemented using the more specialized condition_variable (which uses pthread_cond_t) on a shared_mutex.