Can we use a virtual function in the class in which the virtual function is defined?

[CPP freshman]

class Parent{
  public:
    virtual int width();
    virtual int height();
    int area(){return width()*height();};
    //[B]Virtual functions are used here to define another function, but this produces an error. [/B] 
//[B]Is is possible to do something like this? [/B]
};

class Child: public Parent{
  int w;
  int h;
  public:
    Child(int a,int b){w=a;h=b;};
    int width(){return w;}
    int height(){return h;}
};

[thepiedpiper]

Hi,

You have a semicolon at the end of this line:

int area(){return width()*height();};

You don't need the last semicolon here. Remove it. The problem is with this semicolon and not with your virtual functions.

[weaknessforcats]

You may not understand the purpose of a virtual function.

A virtual function is used when the derived portion of an object is accessed from a pointer to the base portion of the object. This is what allows polymorphism in C++.

Therefore, you are not allowed to call a virtual function using an object of the class that defines it.

You can, however, call a base class virtual function from a member function in a derved object:

class Base
{
  public:
    virtual void Funct();
};
class Derived :public Base
{
   public:
     virtual void Funct();
};
void Derived::Funct()
{
    Base::Funct();  //OK
}

[CPP freshman]

Thanks for your reply, but I already knew what you just said.

[CPP freshman]

I don't think this is the problem.

[weaknessforcats]

It may be. You are calling derived class functions in the base class.

virtual functions in the base class define an interface to the hierarchy. You make your calls in the derived class. By placing these functions in the base class you force every derived class to support them. Some derived classes may not have length or width so keep the derived functions with the derived class.

This is a place to review the Visitor design pattern. This allows you to acquire a Child* from an object pointed at by a Parent*. This then allows you to call Child functions on Child objects that only have Parent*.

Read this: http://bytes.com/topic/c/insights/67...tterns-visitor

[CPP freshman]

I think I am going to use a template function to do this.

class Parent{
  public:
    virtual int width();
    virtual int height();
   
};
 
class Child: public Parent{
  int w;
  int h;
  public:
    Child(int a,int b){w=a;h=b;};
    int width(){return w;}
    int height(){return h;}
};
template <class T> int area(T){return T.width()*T.height();};

But it's not exactly what I wanted though.

[weaknessforcats]

You need code like this:

Parent* p = new Child(3,4);

Child* c = p->VisitChild();

cout << c->area();

and not a template.

[CPP freshman]

Parent* p = new Child(3,4);
 
Child* c = p->VisitChild();[B]//Could you explain in a little bit more detail please? VisitChild() appears not defined. [/B]
 
cout << c->area();

[CPP freshman]

changed, and worked, though still not exactly what I wanted.

lass Parent{
  public:
    virtual int width();
    virtual int height();
 
};
 
class Child: public Parent{
  int w;
  int h;
  public:
    Child(int a,int b){w=a;h=b;};
    int width(){return w;}
    int height(){return h;}
};
int area(Parent p){return p.width()*p.height();};

[weaknessforcats]

I meant that as a directional example and not a compilable one. Sorry.

A working example is in the article I gave you a link to my Post # 8 above.