virtual functions

[Ceset]

hi guys. well i was working c++ and i learned virtual functions just now. and a question come to mind if i already created the function for all derive classes what good is it to me i create a useless(thats what it seems to me) virtual function in the base class. and i tried when that virtual function doesnt exist it still works fine so what good is virtual functions exctly where is it useful

the sample i was working on

class enemy{
public:
        virtual void attack(){}
};

class ninja:public enemy{
    public:
        void attack(){
            cout << "ninja attack" << endl;
        }
};

class monster:public enemy{
    public:
        void attack(){
            cout << "monster attack" << endl;
        }
};

thx in advance

[Disch]

Virtual functions let you have an 'interface' while leaving the actual 'implementation' details (ie: how it's actually done) to the child class.

This is useful when you have many classes which can be abstracted to a common interface.


An academic example is a 'Shape' class:

class Shape
{
public:
    virtual int area() = 0;  // a pure virtual 'area' function
};
/*
   Shape's area function can't possibly calculate the area of the shape because 
   there's no way to know what kind of shape it is... and each shape's area is computed
   differently
*/

class Rectangle : public Shape
{
    int width;
    int height;
public:
    virtual int area() override
    {
        return width * height;  // <- rectangle area computation
    }
};


class Circle : public Shape
{
    int radius;
public:
    virtual int area() override
    {
        return 2 * pi * radius;  // <- circle area computation
    }
};

This allows you to use and manipulate a 'Shape' reference or pointer without knowing exactly what kind of shape you're dealing with. This allows you to write generic/unassuming code that is easily reusable.

void printArea( Shape& shape )
{
    cout << "The area of this shape is: " << shape.area();
    // we call 'area' here... but we don't know what kind of Shape it is!  So how can it
    //   know which formula to use?
    //
    // That's what being 'virtual' means.  It will figure out what kind of Shape it is, and
    //   call the appropriate function automatically.
}

// Effectively this means the 'printArea' function works with ANY shape... not just circles,
//  not just rectangles... but any and all classes derived from Shape.  Even classes that
//  you have not written yet that you might add in the future!!!

int main()
{
    Rectangle r;
    Circle c;

    // ...

    printArea( r );  // since Rectangle is derived from Shape, we can pass 'r' to this function
    printArea( c );  // same for c, since Circle is derived from Shape
}

Another example would be something like a File class. You can read/write data to a file... but there might be different kinds of files:

- Files that write to disk
- Files that are zipped or otherwise compressed
- Files that are uploaded/downloaded from the internet
- etc


A common 'File' abstract base class would let you use all these different file types with the same code. And having a virtual read/write function would allow the derived types to do the reading/writing in their own specific way.

[Ceset]

ty Disch. even though i dont understand everything u typed. i will work on it. ty again for this detailed answer