If the state associated with *this contains a deferred function, invoke
the deferred function. Otherwise, wait until the asynchronous result
associated with an instance of std::experimental::future is ready.
void wait();
this->valid()
would return true.
If the associated state contains a deferred function invoke the
deferred function and stores the returned value or thrown exception
as the asynchronous result. Otherwise, block until the asynchronous
result associated with *this is ready.
Nothing.
#include <experimental/future>