23 struct mutex  struct
31 int mutex::lock_count = 0;  argument
32 int mutex::unlock_count = 0;
34 mutex m;
38     std::shared_lock<mutex> lk(m);  in main()
39     assert(lk.mutex() == &m);  in main()
41     assert(mutex::lock_count == 1);  in main()
42     assert(mutex::unlock_count == 0);  in main()
44     assert(lk.mutex() == nullptr);  in main()
46     assert(mutex::lock_count == 1);  in main()
47     assert(mutex::unlock_count == 0);  in main()