You use the assignment operator (operator = ()) whenever an
existing object is to be replaced with a different object.
The copy constructor X(const X&) is used to create a new
instance of an X-object exactly like another.

Notice the subtle difference. Assignment changes an existing
object while construction creates a new object. You can view
assignment as the application of a destructor, to flush away
the existing object, followed by a copy construction, to
make an exact copy of the assigned object.