I have a situation where I have a std::future<some_type> resulting from a call to API A, but need to supply API B with a std::future<void>:

std::future<some_type> api_a();

void api_b(std::future<void>& depend_on_this_event);

In the absence of proposed functionality such as .then() or when_all(), is there any efficient way to throw away the value attached to a std::future<T> and be left only with the underlying std::future<void> representing the event's completion?

Something like the following could work but would be potentially inefficient:

auto f = api_a();
f.wait();
auto void_f = std::async(std::launch::defer, []{});
api_b(void_f);