纯虚函数与抽象类

#include <iostream>

using namespace std;
// 这个类是抽象类-抽象数据类型，
// 任何包含一个或多个纯虚函数的类都是抽象类，不应创建这个类的对象，应该继承它，务必覆盖从这个类继承的纯虚函数，
class Shape
{
public:
	Shape() {}
	virtual ~Shape() {}  // virtual 代表的是虚的，
	virtual double GetArea() = 0;  // 虚函数等于0，则为纯虚函数，
	virtual double GetPerim() = 0; // 这个也是纯虚函数，
	virtual void Draw() = 0; // 这个也是纯虚函数，
	
};
// 因为这是一个纯虚函数，可以不写，一般也都是不写，
//void Shape::Draw()
//{
//	cout << "...." ;
//}

class Circle : public Shape
{
public:
	Circle(int radius) : itsRadius(radius) {}
	virtual ~Circle(){}
	double GetArea() { return 3.14 * itsRadius * itsRadius;}
	double GetPerim() { return 2 * 3.14 * itsRadius;}
	void Draw();
private:
	int itsRadius;
};

void Circle::Draw()
{
	cout << "Circle drawing routine here! \n";
}

class Rectangle : public Shape
{
public:
	Rectangle(int len, int width) : itsWidth(width),itsLength(len) {}
	virtual ~Rectangle(){}
	double GetArea() {return itsWidth * itsLength;}
	double GetPerim() { return 2*itsWidth +2 * itsLength;}
	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)
			cout << "x ";
	    cout << "\n";
	}
}

class Square : public Rectangle
{
public:
	Square(int len);
	Square(int len, int width);
	virtual ~Square(){};
	double Getperim() { return 4*GetLength();}
};

Square::Square(int len) : Rectangle(len,len) {}
Square::Square(int len, int width) : Rectangle(len,width) 
{
	if(GetLength() != GetWidth())
		cout << "Error, not a square... a Rectangle??\n";
}

int main()
{
	/*Circle a(5);
	a.Draw();
	Rectangle b(10,6);
	b.Draw();

	Square c(9);
	c.Draw();*/

	int choice;
	bool fQuit = false;
	Shape *sp;
	while (fQuit == false)
	{
		cout << " (1)Circle (2)Rectangle (3)Square (0)Quit: ";
		cin >> choice;
		switch(choice)
		{
		case 1:
			sp = new Circle(5); // 创建指针用new，
			break;
		case 2:
			sp = new Rectangle(3,6);
			break;
		case 3:
			sp = new Square(7);
			break;
		case 4:
			fQuit = true;
			break;
		}
		if(fQuit == false)
		{
			sp->Draw();
			delete sp;
			cout << endl;
		}
	}
	return 0;
}