The Inheritance of C++

Question

Recently, I am learning the Inheritance. Here is my code.

#include <iostream>


using namespace std;


class Pad{

public:

    void Show(){
        cout<<"this is pad ";
    }  

};

class Decorator: public Pad{
protected:
    Pad* mpad;
public:
    Decorator(Pad* ipad){
        mpad = ipad;
    }   

    void Show(){
        mpad->Show();

    }
};


class CoverDecorator: public Decorator{

protected:

public:
    CoverDecorator(Pad* ipad):Decorator(ipad){

    }

    void Show(){
        mpad->Show();
        cout<<"with Cover";
    }

};


int main()
{

    Pad* ipad = new Pad;

    Pad* Coverpad = new CoverDecorator(ipad);   
    Coverpad->Show();


    return 0;
}

But I found that it printed out "this is pad" instead of "this is pad with Cover" . It means that in Coverpad->Show(), it implements the function void Show() of class Pad instead of implementing the void Show() of class CoverDecorator.

Why and How do I make it implement the void Show() of class CoverDecorator ?

Thanks!

Answer 1

All you need to do is make the base function virtual:

class Pad{

public:

    virtual void Show(){
        cout<<"this is pad ";
    }  

};

It is also important to note that when creating a virtual function, although the it is not needed, it is good practice to label the derived functions with the override keyword:

class Decorator: public Pad{
protected:
    Pad* mpad;
public:
    Decorator(Pad* ipad){
        mpad = ipad;
    }   

    void Show() override {
        mpad->Show();

    }
};

Declaring a function as virtual basically means that that function will not bind statically at compile time, and instead bind dynamically during the runtime of the program (this allows polymorphism).

Answer 2

With C++ inheritance, declaring certain methods within the base and derived classes as virtual means they are subject to change within the derived classes and will be called as such.

virtual <return type> <name>(){}