Atomically replaces the underlying
desired. The operation is a read-modify-write operation. Memory is affected according to the value of
|desired||-||value to assign|
|order||-||memory order constraints to enforce|
 Return value
The value of the atomic variable before the call.
All changes to the
atomic_weak_ptr object itself, and all associated use_count increments, are guaranteed to be performed atomically. Associated
use_count decrements take place after the atomic operation, but are not required to be part of it. Any associated destruction or deallocation operations take place after the atomic operation and are not part of it.