Need for Virtual Function?

Hi,

I have some doubts in Virtual functions.


What is the use of Virtual Functions?

In which scenario, virtual functions will be useful ?

For what real time problems, virtual function is preferred?

Please clear my doubt.


Thanks,
Arun AC

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What is the use of Virtual Functions?

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Read Inheritance — virtual functions.

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Originally Posted by sunkingac

What is the use of Virtual Functions?

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Virtual functions are defined in base classes and overridden in derived classes.

Say the base class is Fruit and it has a virtual print() function. This function can now be overridden in derived classes like Apple and Banana which supplies the specific details about how each fruit looks.

Now say you have a Fruit variable f. By doing f->print() a different fruit will be printed depending on which derived object f points to.

This allows you to write general code using Fruit variables only and it will work with any derived object. If you want to extend your design with say an Orange the old code will work without changes. Orange just overrides print and an orange will appear when f->print() is called (and f is pointing to an Orange object).

Hi all.
A classical example, the devolopment of a graphical library to draw on the screen.
We can think to our library as a set of classes where each class implements a shape: for example, CSquare implements squares, CCircle implements circles, and so on. Each class has some private data members to define the features of the shape (for example, CCircle has center coordinates and radius), some public methods to initialized the graphical mode and a public method "draw" to display the shape on the screen. The initializing methods are common for all graphical classes, so we can make a class CShape that defines these methods, then derive all classes from CShape: this is obvious; now, we add the public method draw to CShape and declare it as virtual, so we can use dynamic binding.
Well, now we can write the code that draws on screen using our graphical classes. What is a drawing? It's a set of shapes, and we get the drawing when we draw all shapes; so we can declare a drawing as an array of pointers to CShape objects. Remenber that a pointer to a class can hold the address of a derived class:

Code:

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CShape *pCircle = new CCircle;

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is right. So the array of pointers to CShape can holds CCircle, CRectangle, and so on, and if we apply the draw method to all objects of the array we'll get our masterpiece:

Code:

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CShape* aMyDrawing[3];  // The painting

// Build an example of contemporary art
aMyDrawing[0] = new CCircle(...);
aMyDrawing[1] = new CTriangle(...);
aMyDrawing[2] = new CSquare(...);

// Now draw it
for( int i = 0; i < 3; i++ )
        aMyDrawing[i]->draw();

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Well, this simply and powerful approach works fine because we've used virtual method with dynamic binding.

I hope that this simply example is useful to understand these OOP key concepts.