std::unique_lock::mutex
From cppreference.com
                    
                                        
                    < cpp | thread | unique lock
                    
                                                            
                    | mutex_type* mutex() const; | (since C++11) | |
Returns a pointer to the associated mutex, or a null pointer if there is no associated mutex.
Parameters
(none)
Return value
Pointer to the associated mutex or a null pointer if there is no associated mutex.
Exceptions
noexcept specification:  
noexcept
  Example
| This section is incomplete Reason: no example |