The relation between virtual function and pure virtual function

Define a function as a virtual function , Does not mean that the function is not implemented .
It is defined as a virtual function to allow a pointer to the base class to call this function of a subclass .
Define a function as a pure virtual function , It means that the function is not implemented .
A pure virtual function is defined to implement an interface , Play a normative role , The programmer who inherits this class must implement this function .