C++程式設計（第三版） 譚浩強 習題12.5 個人設計

習題 12.5 寫一個程式，定義抽象基類Shape，由它派生出5個派生類：Circle、Square、Rectangle、Tarpezoid、Triangle。用虛擬函式分別計算幾種圖形面積，並求它們的和。要求用基類指標陣列，使它的每一個元素指向一個派生類物件。

程式碼塊：

#include <iostream>
#include <iomanip>
using namespace std;
class Shape
{
public:
	Shape(){}
	virtual ~Shape(){}
	virtual double Area() const{return 0;}
	virtual void shapeName() const =0;
};
class Circle: public Shape
{
public:
	Circle(double r){radius=r;}
	~Circle(){}
	virtual double Area() const {return 3.14159*radius*radius;}
	virtual void shapeName() const {cout<<"Circle ";}
protected:
	double radius;
};
class Square: public Shape
{
public:
	Square(double a){x=a;}
	~Square(){}
	virtual double Area() const {return x*x;}
	virtual void shapeName() const {cout<<"Square ";}
protected:
	double x;
};
class Rectangle: public Shape
{
public:
	Rectangle(double a, double b){x=a; y=b;}
	~Rectangle(){}
	virtual double Area() const {return x*y;}
	virtual void shapeName() const {cout<<"Rectangle ";}
protected:
	double x, y;
};
class Trapezoid: public Shape
{
public:
	Trapezoid(double a, double b, double c){x=a; y=b; z=c;}
	~Trapezoid(){}
	virtual double Area() const{return ((x+y)*z)/2;}
	virtual void shapeName() const {cout<<"Trapezoid ";}
protected:
	double x, y, z;
};
class Triangle: public Shape
{
public:
	Triangle(double a, double h){x=a; y=h;}
	~Triangle(){}
	virtual double Area() const {return (x*y)/2;}
	virtual void shapeName() const {cout<<"Triangle ";}
protected:
	double x, y;
};
int main()
{
	Circle circle(5);
	Square square(4);
	Rectangle rectangle(3, 4);
	Trapezoid trapezoid(2.0, 3.0, 4.0);
	Triangle triangle(4, 3);
	double sum=0;
	Shape *pt[5]={&circle, &square, &rectangle, &trapezoid, &triangle};
	Shape **p;
	for (p=pt; p<pt+5; p++){
		(*p)->shapeName();
		cout<<"area="<<(*p)->Area()<<endl;
		sum+=(*p)->Area();
	}
	cout<<"Area sum="<<sum<<endl;
	system("pause");
	return 0;
}