std::unique_lock::operator=

From cppreference.com
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_lock to replace the state with

[edit] Return value

*this

[edit] Exceptions

noexcept specification:  
noexcept
  (since C++11)