std::unique_lock::operator=
From cppreference.com
                    
                                        
                    < cpp | thread | unique lock
                    
                                                            
                    | unique_lock& operator=( unique_lock&& other ); | (since C++11) | |
Move assignment operator. Replaces the contents with those of other using move semantics. 
If prior to the call *this has an associated mutex and has acquired ownership of it, the mutex is unlocked.
[edit] Parameters
| other | - | another unique_lockto replace the state with | 
[edit] Return value
*this
[edit] Exceptions
| noexcept specification:   noexcept | (until C++14) | 
| Throws nothing | (since C++14) | 


