Inheritance ：
Inheritance lets us define classes that model relationships among types, sharing what is common and
specializing only that which is inherently different.
那么derived class 就有两种选择，1 。 和base class 有一样的opration 2 。与base class 不一样的，可以自己redefine 的opration。 对于derived class ，它可以redefine 的opration 是被base class 标志为virtual 的operation。
Dynamic Binding ：
Dynamic binding lets us write programs that use objects of any type in an inheritance hierarchy without caring about the objects' specific types 。
In C++, dynamic binding happens when a virtual function is called
hrough a reference (or a pointer) to a base class. The fact that a
reference (or pointer) might refer to either a base- or a derivedclass
object is the key to dynamic binding. Calls to virtual functions
made through a reference (or pointer) are resolved at run time: The
function that is called is the one defined by the actual type of the
object to which the reference (or pointer) refers.
