Wasn't unique_ptr added as a migration path away from auto_ptr to provide similar functionality more safely? I've never used them but was just reading the history.
But if I understand it right, auto_ptr assignment didn't actually copy the pointer, but instead moved it to a new variable and quietly made the original variable null? And unique_ptr made this operation require an explicit move() call, because the assignment-only style caused too many null pointer bugs.
I'm just curious about this for historical interest.
Yes, it was a design decision of the auto_ptr to have this move semantics and it worked very nicely for us because we used for instance to pass progress monitors down the stack.