When the assignment hp = hp2; is executed, the copy-assignment operator is used. First, data members of rhs (i.e. hp2) are copyed. Then, data members of rhs (i.e. hp) are deleted. Finally, the copyed data members of rhs are assigned to those of lhs.
When the assignment hp = std::move(hp2); is executed, the move-assignment operator is used. First, std::move() return the rvalue reference to hp2. Second, the data members of the rhs (i.e. rvalue reference to hp2) are moved to those of lhs (i.e. hp1). Finally, the data members of rhs are assigned to valid values for destruction.