07 -- declare a multi-state base class as a virtual destructor

1. When the base class is determined to be inherited, it is necessary to declare the Destructor as virtual.

• When the pointer or reference of the returned derived class is used, calling the Destructor is prone to memory leakage.

1. When the base class is used as an abstract class, declare the pure virtual destructor
2. The order of the Destructor ---- The destructor of the derived class is called first (the member of the basic class is first analyzed)
3. The order of the constructor ---- the constructor of the derived class is called first (the base class member is constructed first)