Elysian Shadows

Posted: **Wed Apr 21, 2010 6:51 pm**

Hey guys,
I'm reading through my C++ book and right now I'm on virtual functions. I get the syntax and all, but I just don't see a point of using them. Can someone please give me an example where virtual functions would be applied?

Thanks for your help,
EpicAsian

Posted: **Wed Apr 21, 2010 7:12 pm**

The point of using them is so that you can refer to subclass objects through a base class reference and have subclasses' functions have different meanings.
They are useful when you want to design objects that are related to each other.
Ultimately, it allows you to implement a function in different ways.
With virtual functions used, you make a pointer to a base class, and then you assign it to a subclass.

For example:
You might want to draw custom shapes to the screen in your API.
You might have a class named Shape, with virtual functions, and subclasses named Triangle, Rectangle, Circle, Rhombus, et cetera.
These subclasses will have to override the Draw() function defined in the Shape class, so they are drawn to the screen properly.

Now when you want to make a bunch of these shape objects, you can group them together into a vector of pointers to Shape objects.

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vector<Shape*> shapes;

You could then add any type of shape object to that vector like this:

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shapes.push_back(new Triangle());

Now you can call on all of the Shape objects in the vector like this:

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for (Uint32 i = 0; i < shapes.size(); i++)
        shapes[i]->Draw();

So you might be wondering what's the difference between polymorphism and inheritance.
As Wikipedia says: "When a derived object is referred to as being of the base's type, the desired function call behavior is ambiguous."

This basically says that if you don't use virtual functions and try to do the above, the function will just call on the base class's function.

Posted: **Wed Apr 21, 2010 7:47 pm**

So, this would be a good example of using virtual functions?

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//virtual functions 
#include <iostream>

using namespace std;

class shape
{
      public:
             shape() {cout << "shape constructor\n";}
             virtual ~shape() {cout << "shape destructor\n";}
             virtual int draw() {cout << "imma shape!\n";}

};

class triangle: public shape
{
      public:
             triangle() {cout << "triangle constructor\n";}
             virtual ~triangle() {cout << "triangle destructor\n";}
             int draw() {cout << "imma triangle!\n";}
             
};

class square: public shape
{
      public:
             square() {cout << "square constructor\n";}
             virtual ~square() {cout << "square destructor\n";}
             int draw() {cout << "imma square!\n";}
      
};


int main(int argc, char args[])
{
    shape *pTriangle = new triangle;
    cout << endl;
    shape *pSquare = new square;
    cout << endl;
    shape *pShape = new shape;
    cout << endl;
    pTriangle->draw();
    pSquare->draw();
    pShape->draw();
    delete pShape;
    delete pSquare;
    delete pTriangle;
    
    cin.get();
}

output:

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shape constructor
triangle constructor

shape constructor
square constructor

shape constructor

imma triangle!
imma square!
imma shape!
shape destructor
square destructor
shape destructor
triangle destructor
shape destructor

Thanks again for your help,
EpicAsian

Posted: **Wed Apr 21, 2010 7:54 pm**

Yup. A derived/inherited class has access to the protected and public functions of its base/parent class. It can use them as if they were it's own. However what if some of the methods don't really fit for the derived class.

In the example bellow most dogs bark, however a dingo doesn't often bark it howls most of the time. So we override the speak function using a virtual function.

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#include<iostream>
#include<conio.h>

using namespace std;

class Dog
{
public:
 	virtual void speak()
 	{
 	 	cout<<"Bark.";
 	}
};

class Pitbull : public Dog
{};

class Dingo : public Dog
{
public:
 	void speak()
 	{
 	 	cout<<"Howl.";
 	}
};

int main()
{
	Dog dog;
	dog.speak();
	dog.jump();

	Pitbull pitbull;
	pitbull.speak();

	Dingo dingo;
	dingo.speak();

	getch();
};

The pitbull doesn't need it's speak function overridden as it should bark just like most other dogs.

Elysian Shadows

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