Access member functions with a pointer to the base class

[Cpp]
/*
* Copyright (c) 2013, computer College, Yantai University
* All rights reserved.
* File name: test. cpp
* Author: Fan Lulu
* Completion date: January 1, May 31, 2013
* Version: v1.0
*
* Input Description: None
* Problem description:
* Program output:
* Problem analysis:
* Algorithm Design: omitted
*/

/*
* Copyright (c) 2013, computer College, Yantai University
* All rights reserved.
* File name: test. cpp
* Author: Fan Lulu
* Completion date: January 1, May 31, 2013
* Version: v1.0
*
* Input Description: None
* Problem description:
* Program output:
* Problem analysis:
* Algorithm Design: omitted
*/
 

First look at a program:

[Cpp]
# Include <iostream>
Using namespace std;
Class Vehicle // Transportation
{
Public:
Void run () const
{
Cout <"run a vehicle" <endl;
}
};
Class Car: public Vehicle // Car
{
Public:
Void run () const
{
Cout <"run a car." <endl;
}
};
Class Airplane: public Vehicle // aircraft
{
Public:
Void run () const
{
Cout <"run a airplane." <endl;
}
};
Int main ()
{
Cout <"(a) directly access member functions using objects:" <endl;
Vehicle v;
V. run ();
Car car;
Airplane airplane;
Car. run ();
Airplane. run ();
Cout <"(B) uses a pointer to the base class to access the member function:" <endl;
Vehicle * vp;
Vp = & car;
Vp-> run ();
Vp = & airplane;
Vp-> run ();
Return 0;
}

# Include <iostream>
Using namespace std;
Class Vehicle // Transportation
{
Public:
Void run () const
{
Cout <"run a vehicle" <endl;
}
};
Class Car: public Vehicle // Car
{
Public:
Void run () const
{
Cout <"run a car." <endl;
}
};
Class Airplane: public Vehicle // aircraft
{
Public:
Void run () const
{
Cout <"run a airplane." <endl;
}
};
Int main ()
{
Cout <"(a) directly access member functions using objects:" <endl;
Vehicle v;
V. run ();
Car car;
Airplane airplane;
Car. run ();
Airplane. run ();
Cout <"(B) uses a pointer to the base class to access the member function:" <endl;
Vehicle * vp;
Vp = & car;
Vp-> run ();
Vp = & airplane;
Vp-> run ();
Return 0;
}

The output result of this program is:

You can see that (B) accessing the member function with a pointer to the base class outputs "run a vehicle .", This is not our purpose.

We can modify the base class to change the subsequent output:

[Cpp]
Class Vehicle // Transportation
{
Public:
Virtual void run () const
{
Cout <"run a vehicle" <endl;
}
};

Class Vehicle // Transportation
{
Public:
Virtual void run () const
{
Cout <"run a vehicle" <endl;
}
}; Add virtual in front of the member function run in the base class to change the output result. Let's take a look at the result:

We can see that the output results have changed in (B.

The reason is that a virtual function is added before the member function run in the base class to make the run function a virtual function. When the two Derived classes Car and Airplane are declared, the virtual function run will be overloaded, the run function in the base class is replaced by a function of the same name in the derived class. Therefore, when the run function is called, The run function in the derived class is called.