Is calling wait() on a std::future multiple times and from multiple threads safe?

Question

I'm trying to determine when I can safely call wait() on std::future and std::shared_future. I never call get() on the future, and the future is set ready from a call to its corresponding promise's set_value() method.

I want to wait on this future (using wait(), wait_for(), wait_until()) from multiple threads. I also want calls to wait() after promise::set_value() has been called to return immediately.

From http://www.cplusplus.com/reference/future/future/wait/

Calling this member function on a future object that is not valid, produces undefined behavior

Getting a future from a promise starts it off with valid()==true. As far as I can see only future::get() sets valid() back to false. Is this assumption correct? Can I call the wait() family of functions as often as I want? Will calls to wait() after the promise's value has been set return immediately? Can I call them from multiple threads?

Answer 1

Getting a future from a promise starts it off with valid()==true. As far as I can see only future::get() sets valid() back to false. Is this assumption correct?

It can also become invalid by move-assigning an invalid future to it, or by moving the future so its shared state transfers to a different object. It can't become invalid just by waiting though.

Can I call the wait() family of functions as often as I want?

Yes.

Will calls to wait() after the promise's value has been set return immediately?

Yes (it might involve locking a mutex or spinlock to check if the state is ready, then returning, so might involve a small wait to acquire the lock)

Can I call them from multiple threads?

Yes.

wait() is a const member function, so the library is required to ensure it can be called on a single future object from multiple threads without any user-provided synchronisation.