Condition variable and shared mutex

Question

I wonder why I can't do this in C++14 (or 17)

std::shared_timed_mutex mutex;
std::unique_lock<std::shared_timed_mutex> lock(mutex);

std::condition_variable var;

while(!some_condition)
    var.wait(lock);

Condition variables only seem to work with std::mutex. But why?

Answer 1

This is defined by the standard to allow for maximum efficiency of the implementation. If you want to use a different lock with condition_variable you need to use condition_variable_any. Note that the condition_variable_any implementation has some overhead though.

Quote from the standard: 30.5 Condition variables

Class condition_variable provides a condition variable that can only wait on an object of type unique_lock<mutex>, allowing maximum effciency on some platforms.