[C ++ Primer] class Polymorphism

I. Overview
1) multiple implementation methods of interfaces are polymorphism.
2) polymorphism is a technique that allows you to set a parent object to be equal to one or more other sub-objects, the parent object can operate in different ways based on the features of the sub-objects assigned to it. To put it simply, you can assign a pointer of the subclass type to a pointer of the parent class. Polymorphism is implemented through Virtual functions in C ++.
3) key point: there is also a key to polymorphism: all objects are operated by pointers or references to the base class.

2. Example
1) Common Object operations
[Html]
# Include <iostream>
Using namespace std;
Class {
Public:
Void print ()
{
Cout <"This is A" <endl;
}
};
Class B: public {
Public:
Void print ()
{
Cout <"This is B" <endl;
}
};
Int main () {// for future convenience, the main () code is called main1
A;
B B;
A. print ();
B. print ();
}

Output: This is
This is B
2) perform multi-state object operations: operate subclass objects through parent objects
[Html]
# Include <iostream>
Using namespace std;
Class {
Public:
Void print ()
{
Cout <"This is A" <endl;
}
};
Class B: public {
Public:
Void print ()
{
Cout <"This is B" <endl;
}
};
Int main () {// operate subclass through parent class
A;
B B;
A * p1 = &;
A * p2 = & B;
P1-> print ();
P2-> print ();
}

Output: This is
This is
This is different from what we expected.

3) using polymorphism means adding the virtual
Call corresponding functions based on different class objects. This function is a virtual function.
[Html]
# Include <iostream>
Using namespace std;
Class {
Public:
Virtual void print ()
{
Cout <"This is A" <endl;
}
};
Class B: public {
Public:
Void print ()
{
Cout <"This is B" <endl;
}
};
Int main () {// operate subclass through parent class
A;
B B;
A * p1 = &;
A * p2 = & B;
P1-> print ();
P2-> print ();
}

Iii. virtual functions (advanced tutorial)
Void (A: * fun) (); // defines A function pointer.
A * p = new B;
Fun = & A: fun; // is the address that actually obtains the virtual function? In fact, the address of a piece of code that indirectly obtains the virtual function address
[Html]
# Include <iostream>
Using namespace std;
 
Class {
Public:
Virtual void fun ()
{
Cout <"A: fun" <endl;
}
Virtual void fun2 ()
{
Cout <"A: fun2" <endl;
}
};
Class B: public {
Public:
Void fun ()
{
Cout <"B: fun" <endl;
}
Void fun2 ()
{
Cout <"B: fun2" <endl;
}
};
Void CallVirtualFun (void * pThis, int index = 0)
{
Void (* funptr) (void *);
Long lVptrAddr;
Memcpy (& lVptrAddr, pThis, 4 );
Memcpy (& funptr, reinterpret_cast <long *> (lVptrAddr) + index, 4 );
Funptr (pThis );
}
Int main ()
{
 
Void (A: * fun) (); // defines A function pointer.
A * p = new B;
Fun = & A: fun; // is the address that actually obtains the virtual function? In fact, the address of a piece of code that indirectly obtains the virtual function address
(P-> * fun) (); // B: fun
Fun = & A: fun2;
(P-> * fun) (); // B: fun2

CallVirtualFun (p); // call the virtual function p-> fun ()
CallVirtualFun (p, 1); // call the virtual function p-> fun2 ()
Delete p;
System ("pause ");
Return 0;
}

Author: tianshuai11