Implementing pure virtual functions : virtual function « Class « C++ Tutorial






  1. C++ Tutorial
  2. Class
  3. virtual function
#include <iostream>
 
 class Shape
 {
 public:
     Shape(){}
     virtual ~Shape(){}
     virtual long GetArea() = 0; 
     virtual long GetPerim()= 0;
     virtual void Draw() = 0;
 private:
 };
 
 void Shape::Draw()
 {
     std::cout << "Abstract drawing mechanism!\n";
 }
 
 class Circle : public Shape
 {
 public:
     Circle(int radius):itsRadius(radius){}
     ~Circle(){}
     long GetArea() { return 3 * itsRadius * itsRadius; }
     long GetPerim() { return 9 * itsRadius; }
     void Draw();
 private:
     int itsRadius;
     int itsCircumference;
 };
 
 void Circle::Draw()
 {
     std::cout << "Circle drawing routine here!\n";
     Shape::Draw();
 }
 
 class Rectangle : public Shape
 {
 public:
     Rectangle(int len, int width):
         itsLength(len), itsWidth(width){}
     virtual ~Rectangle(){}
     long GetArea() { return itsLength * itsWidth; }
     long GetPerim() {return 2*itsLength + 2*itsWidth; }
     virtual int GetLength() { return itsLength; }
     virtual int GetWidth() { return itsWidth; }
     void Draw();
 private:
     int itsWidth;
     int itsLength;
 };
 
 void Rectangle::Draw()
 {
     for (int i = 0; i<itsLength; i++)
     {
         for (int j = 0; j<itsWidth; j++)
             std::cout << "x ";
 
         std::cout << "\n";
     }
     Shape::Draw();
 }
 
 class Square : public Rectangle
 {
 public:
     Square(int len);
     Square(int len, int width);
     ~Square(){}
     long GetPerim() {return 4 * GetLength();}
 };
 
 Square::Square(int len):Rectangle(len,len)
 {}
 
 Square::Square(int len, int width):Rectangle(len,width){
     if (GetLength() != GetWidth())
         std::cout << "Error, not a square... a Rectangle??\n";
 }
 
 int main()
 {
     Shape * sp;
 
     sp = new Circle(5);
     sp->Draw();

     sp = new Rectangle(4,6);
     sp->Draw();

     sp = new Square (5);
     sp->Draw();

     return 0;
 }

Circle drawing routine here!
Abstract drawing mechanism!
x x x x x x
x x x x x x
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Abstract drawing mechanism!
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Abstract drawing mechanism!




9.28.virtual function
9.28.1.A virtual function
9.28.2.Access base class's virtual if derived class does not redefine it
9.28.3.Use virtual functions and polymorphism
9.28.4.A pure virtual function
9.28.5.Multiple virtual member functions called in turn
9.28.6.Class with only virtual methods
9.28.7.Use virtual function to calculate area for different shapes
9.28.8.Use Virtual Functions to change the method behaviour
9.28.9.Why Virtual Functions: one interface, multiple methods
9.28.10.Implementing pure virtual functions