DaniWeb Forum Index > Software Development > C++

What is the point in virtual things???

Please support our C++ advertiser: Intel Parallel Studio Home

Thread Solved

•

Join Date: May 2008

Posts: 51

Reputation: Manutebecker is an unknown quantity at this point

Solved Threads: 2

Manutebecker Manutebecker is offline

Offline

Junior Poster in Training

What is the point in virtual things???

Dec 15th, 2008

After reading up on them in terms of class hierarchy, they seem to just be almost like comments, to remind you that all classes use this sort of function

•

Join Date: Jul 2008

Posts: 2,001

Reputation: ArkM has much to be proud of

Solved Threads: 343

ArkM

Offline

Postaholic

Re: What is the point in virtual things???

Dec 15th, 2008

•

Originally Posted by Manutebecker

After reading up on them in terms of class hierarchy, they seem to just be almost like comments, to remind you that all classes use this sort of function

???!!!
It's a false statement (or I misunderstand the post).

•

Join Date: Oct 2008

Posts: 1,999

Reputation: ddanbe has much to be proud of

Solved Threads: 297

ddanbe

Offline

Posting Virtuoso

Re: What is the point in virtual things???

Dec 15th, 2008

When you mark a method of your class as virtual you give the user of your class the possibility to override that method. If you don't, the metod can't be overridden.

Today is a gift, that's why it is called "The Present".
Make love, no war. Cave ab homine unius libri.
Danny

•

Join Date: Aug 2008

Posts: 206

Reputation: grumpier has a spectacular aura about

Solved Threads: 31

grumpier

Offline

Posting Whiz in Training

Re: What is the point in virtual things???

Dec 15th, 2008

•

Originally Posted by ddanbe

When you mark a method of your class as virtual you give the user of your class the possibility to override that method. If you don't, the metod can't be overridden.

That statement is inaccurate on so many levels that I'm speechless. Non-virtual functions can also be overridden, but the behaviour differs from overridden virtual functions due to "name hiding", as described in the C++ standard.

The point of virtual member functions is that they allow functionality to be specified in a base class, and specialised in a derived class. For example, a generic Shape class might declare a virtual function named Area() to compute it's area, and another virtual function called Name(). Derived classes (which represent particular shapes) override those virtual function to give appropriate results for particular shapes.

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
class Shape
{
     public:
          Shape();
          virtual double Area() const = 0;
          virtual std::string Name() const = 0;
};
 
class Triangle: public Shape
{
     public:
          Triangle(double h, double w) : height(h), width(w) {};
          double Area() const {return 0.5*height*width;};
 
          std::string Name() const {return "Triangle";}
 
     private:
          double height, width;
};
 
class Circle : public Shape
{
     public:
         Circle(double r): radius(r) {};
         virtual double Area() const {return 4.0*atan(1.0)*radius*radius;};
          virtual std::string Name() const {return "Circle";}
      private:
            double radius;
};
class Shape
{
     public:
          Shape();
          virtual double Area() const = 0;
          virtual std::string Name() const = 0;
};

class Triangle: public Shape
{
     public:
          Triangle(double h, double w) : height(h), width(w) {};
          double Area() const {return 0.5*height*width;};

          std::string Name() const {return "Triangle";}

     private:
          double height, width;
};

class Circle : public Shape
{
     public:
         Circle(double r): radius(r) {};
         virtual double Area() const {return 4.0*atan(1.0)*radius*radius;};
          virtual std::string Name() const {return "Circle";}
      private:
            double radius;
};

Now, let's say we have a function that only knows about the Shape class, and wishes to print its name and area. For example;

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
void PrintArea(const Shape &s)
{
    std::cout << s.Name() << " - Area = " << s.Area() << '\n';
}
void PrintArea(const Shape &s)
{
    std::cout << s.Name() << " - Area = " << s.Area() << '\n';
}

This function can be passed any Shape object by reference, and will print out information about the actual shape passed.

This is described as polymorphic behaviour: the class Shape is a polymorphic base class, derived classes modify it's behaviour, but the interface seen by the PrintArea() function is the same for all the classes. The PrintArea() function only receives generic information about a Shape (i.e. it provides a Name() and Area() member function) but, at run time, the behaviours correspond to the actual type of the object passed.

For example;

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
int main()
{
     Triangle tri(2.0, 4.0);
     Circle cir(5.0);
 
      PrintArea(tri);   //   will print "Triangle - Area = 4"
      PrintArea(cir);  //    will print "Circle - Area = 78.5"  (possibly higher precision value)
 
      Shape *s[2];
      s[1] = &cir;
      s[0] = &tri;
 
       // this loop will print out the above lines of output in reverse order
      for (int i = 0; i < 2; ++i)
          PrintArea(s[i]);
}
int main()
{
     Triangle tri(2.0, 4.0);
     Circle cir(5.0);

      PrintArea(tri);   //   will print "Triangle - Area = 4"
      PrintArea(cir);  //    will print "Circle - Area = 78.5"  (possibly higher precision value)

      Shape *s[2];
      s[1] = &cir;
      s[0] = &tri;

       // this loop will print out the above lines of output in reverse order
      for (int i = 0; i < 2; ++i)
          PrintArea(s[i]);
}