详细分析C++ 多态和虚函数

#include <iostream> 
using namespace std;
 
class Shape {
  protected:
   int width, height;
  public:
   Shape( int a=0, int b=0)
   {
     width = a;
     height = b;
   }
   int area()
   {
     cout << "Parent class area :" <<endl;
     return 0;
   }
};
class Rectangle: public Shape{
  public:
   Rectangle( int a=0, int b=0):Shape(a, b) { }
   int area ()
   { 
     cout << "Rectangle class area :" <<endl;
     return (width * height); 
   }
};
class Triangle: public Shape{
  public:
   Triangle( int a=0, int b=0):Shape(a, b) { }
   int area ()
   { 
     cout << "Triangle class area :" <<endl;
     return (width * height / 2); 
   }
};
// 程序的主函数
int main( )
{
  Shape *shape;
  Rectangle rec(10,7);
  Triangle tri(10,5);
 
  // 存储矩形的地址
  shape = &rec;
  // 调用矩形的求面积函数 area
  shape->area();
 
  // 存储三角形的地址
  shape = &tri;
  // 调用三角形的求面积函数 area
  shape->area();
  
  return 0;
}

class Shape {
  protected:
   int width, height;
  public:
   Shape( int a=0, int b=0)
   {
     width = a;
     height = b;
   }
   virtual int area()
   {
     cout << "Parent class area :" <<endl;
     return 0;
   }
};

class Shape {
  protected:
   int width, height;
  public:
   Shape( int a=0, int b=0)
   {
     width = a;
     height = b;
   }
   // pure virtual function
   virtual int area() = 0;
};