c++ - std::promise 是否在内部使用 std::condition_variable 来通知关联的 std::future？

Question

我的问题是是否通过使用std::promise通知关联？ std::futurestd::condition_variable

std::promise我搜索了这个网站的源代码。但我没有在其成员数据中看到std::promise有。std::condition_variable

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.