22 struct mutex  struct
30 int mutex::lock_count = 0;  argument
31 int mutex::unlock_count = 0;
33 mutex m;
37     std::unique_lock<mutex> lk(m);  in main()
38     assert(lk.mutex() == &m);  in main()
40     assert(mutex::lock_count == 1);  in main()
41     assert(mutex::unlock_count == 0);  in main()
43     assert(lk.mutex() == nullptr);  in main()
45     assert(mutex::lock_count == 1);  in main()
46     assert(mutex::unlock_count == 0);  in main()