What good is virtual in a class ?

Question

Possible Duplicate:
C++ Virtual/Pure Virtual Explained

For example i have:

class A {
    private: 
        int i; 
        int j; 

    public: 
        void k (int l, int m) { i=l; j=m; } 
        virtual int n (void); 
};

class B : public A { 
    public: 
        int n (void);
};

What good is this virtual ?

Answer 1

It's to induce polymorphic behaviour, which means that a single contract can be used by a client to invoke different behaviours, without the client having to know about every possible behaviour.

A* a = new B();
a->n(); // Will call B::n();

As a more concrete example, here some fairly stereotypical code:

struct Shape {
    GLfloat transform_[4][4];

    void render() const {
        glPushMatrix();
        glMultMatrixf(&transform_[0][0]);
        draw();
        glPopMatrix();
    }

    virtual void draw() const = 0;
};

struct Line : public Shape {
    GLfloat x1, y1, x2, y2;

    void draw() {
        glBegin();
        glVertex2f(x1, y1);
        glVertex2f(x2, y2);
        glEnd();
    }
};

struct Square : public Shape {
    GLfloat size;

    void draw() {
        GLfloat f = size/2;
        glBegin();
        glVertex2f(-f, -f);
        glVertex2f( f, -f);
        glVertex2f( f,  f);
        glVertex2f(-f,  f);
        glVertex2f(-f, -f);
        glEnd();
    }
};

void renderShapes(Shape* shapes, int nShapes) {
    for (int i = 0; i < nShapes; ++i) {
        shapes[i]->render();
    }
}

(Disclaimer: The above code is neither correct nor complete, and is certainly not to be held up as an example of good graphics code. It's just to illustrate when virtual functions are useful.

Answer 2

I like to use the Chess Game as an example for this:

class ChessPiece
{
    public:
       virtual int Move() = 0;
};
class Queen: public ChessPiece
{
    public:
       virtual int Move() { /* STUFF */ }
};

bool AICode()
{
    int bestMove = 0;
    foreach(ChessPiece* loop = board.Pieces().begin(); loop != board.Pieces().end(); ++loop)
    {
        bestMove = max(bestMove, loop->Move());
    }
 }

The AICode() does not need to know which piece it is looking at.
All it does is ask the piece to see what its best move is. The virtual dispatch mechanism will work out the type of piece and call the correct Move() method.

Answer 3

I refer you to this excellent SO page about polymorphism.