Use virtual functions and polymorphism. : polymorphism « Class « C++ Tutorial

1. C++ Tutorial
2. Class
3. polymorphism

#include <iostream> 
#include <cstring> 
using namespace std; 
 
class Shape { 
  double width; 
  double height; 
 
  char name[20]; 
public: 
  Shape() { 
    width = height = 0.0; 
    strcpy(name, "unknown"); 
  } 
 
  Shape(double w, double h, char *n) { 
    width = w; 
    height = h; 
    strcpy(name, n); 
  } 
 
  Shape(double x, char *n) { 
    width = height = x; 
    strcpy(name, n); 
  } 
 
  void display() { 
    cout << "Width and height are " << width << " and " << height << "\n"; 
  } 
 
  double getWidth() { return width; } 
  double getHeight() { return height; } 
  void setWidth(double w) { width = w; } 
  void setHeight(double h) { height = h; } 
  char *getName() { return name; } 
 
  virtual double area() {   
    cout << "Error: area() must be overridden.\n";  
    return 0.0;  
  }   
 
}; 
 
class Triangle : public Shape { 
  char style[20];
public: 
 
  Triangle() { 
    strcpy(style, "unknown"); 
  } 
 
  Triangle(char *str, double w, double h) : Shape(w, h, "triangle") { 
    strcpy(style, str); 
  } 
   
  Triangle(double x) : Shape(x, "triangle") { 
    strcpy(style, "isosceles");  
  } 
 
  double area() { 
    return getWidth() * getHeight() / 2; 
  } 
 
  void showStyle() { 
    cout << "Triangle is " << style << "\n"; 
  } 
}; 
 
class Rectangle : public Shape { 
public: 
 
  Rectangle(double w, double h) : Shape(w, h, "rectangle") { }  
  
  Rectangle(double x) : Shape(x, "rectangle") { }  
 
  bool isSquare() { 
    if(getWidth() == getHeight()) 
       return true; 
    return false; 
  } 
 
  double area() { 
    return getWidth() * getHeight(); 
  } 
}; 
 
int main() { 
  Shape *shapes[5]; 
  
  shapes[0] = &Triangle("right", 8.0, 12.0);  
  shapes[1] = &Rectangle(10);  
  shapes[2] = &Rectangle(10, 4);  
  shapes[3] = &Triangle(7.0);  
  shapes[4] = &Shape(10, 20, "generic"); 
  
  for(int i=0; i < 5; i++) {  
    cout << "object is " << shapes[i]->getName() << "\n";  
    cout << "Area is " << shapes[i]->area() << "\n\n";  
  }  
 
  return 0; 
}

object is generic
Error: area() must be overridden.
Area is 0

object is generic
Error: area() must be overridden.
Area is 0

object is generic
Error: area() must be overridden.
Area is 0

object is generic
Error: area() must be overridden.
Area is 0

object is generic
Error: area() must be overridden.
Area is 0