Does std::promise internally use std::condition_variable to notify the associated std::future?

Question

My question is does std::promise notify the associated std::future through using a std::condition_variable?

I search the source code of std::promise and found this website. But I didn't see std::promise has std::condition_variable in its member data.

Answer 1

Here's an answer for libc++.

A search for condition_variable in <future> returned exactly one result:

// lines 531 -- 538
class _LIBCPP_TYPE_VIS _LIBCPP_AVAILABILITY_FUTURE __assoc_sub_state
    : public __shared_count
{
protected:
    exception_ptr __exception_;
    mutable mutex __mut_;
    mutable condition_variable __cv_;
    unsigned __state_;

Here __assoc_sub_state is introduced. It is the base class for __assoc_state:

// lines 617 -- 619
template <class _Rp>
class _LIBCPP_AVAILABILITY_FUTURE __assoc_state
    : public __assoc_sub_state

And finally, __assoc_state<_Rp>* is both a member of future<_Rp>:

// lines 1082 -- 1085
template <class _Rp>
class _LIBCPP_TEMPLATE_VIS _LIBCPP_AVAILABILITY_FUTURE future
{
    __assoc_state<_Rp>* __state_;

and a member of promise<_Rp>:

// lines 1360 -- 1363
template <class _Rp>
class _LIBCPP_TEMPLATE_VIS _LIBCPP_AVAILABILITY_FUTURE promise
{
    __assoc_state<_Rp>* __state_;

So yeah, libc++ std::promise internally uses a std::condition_variable to notify the associated std::future.