C++ で抽象クラスを使用してインターフェイスを実装する

class A {
 public:
  virtual void b() = 0;  // function "b" in class "A" is an example of a pure
                         // virtual function
};

#include <iostream>

using namespace std;
class Shape {
 protected:
  int length;
  int height;

 public:
  virtual int Area() = 0;

  void getLength() { cin >> length; }

  void getHeight() { cin >> height; }
};

class Rectangle : public Shape {
 public:
  int Area() { return (length * height); }
};

class Triangle : public Shape {
 public:
  int Area() { return (length * height) / 2; }
};

int main() {
  Rectangle rectangle;
  Triangle triangle;

  cout << "Enter the length of the rectangle: ";
  rectangle.getLength();
  cout << "Enter the height of the rectangle: ";
  rectangle.getHeight();
  cout << "Area of the rectangle: " << rectangle.Area() << endl;

  cout << "Enter the length of the triangle: ";
  triangle.getLength();
  cout << "Enter the height of the triangle: ";
  triangle.getHeight();
  cout << "Area of the triangle: " << triangle.Area() << endl;

  return 0;
}

Enter the length of the rectangle: 2
Enter the height of the rectangle: 4
Area of the rectangle: 8
Enter the length of the triangle: 4
Enter the height of the triangle: 6
Area of the triangle: 12