Method Mutex

tests/gtest/gtest.h:1966–1969

class Mutex : public MutexBase {
 public:
  Mutex() {
    GTEST_CHECK_POSIX_SUCCESS_(pthread_mutex_init(&mutex_, nullptr));
    has_owner_ = false;
  }
  ~Mutex() {
    GTEST_CHECK_POSIX_SUCCESS_(pthread_mutex_destroy(&mutex_));
  }