写一个程序，定义一个抽象类Shape,由它派生3类：Square(正方形)、Trapezoid（梯形）和Triangle三角形。用虚函数分别计算几种图形面积、并求它们的和。要求用基类指针数组，使它每一个元素指向一个派生类对象。

#include <iostream.h>

　　class Shape

　　{public:

virtual double area()const=0;

　　};

答案：class Square:public Shape

{public:

Square(double s):side(s){}

double area() const{return side*side;}

private:

double side;

};

class Trapezoid:public Shape

{public:

Trapezoid(double i,double j,double k):a(i),b(j),h(k)

{}

double area() const{return ((a+b)*h/2);}

private:

double a,b,h;

};

class Triangle:public Shape

{public:

Triangle(double i,double j):w(i),h(j)

{}

double area() const{return(w*h/2);}

private:

double w,h;

};

void main()

{Shape *p［5］;

Square se(5);

Trapezoid td(2,5,4);

Triangle te(5,8);

p［0］=&se;

p［1］=&td;

p［2］=&te;

double da=0;

for(int i=0;i<3;i++)

{da+=p［i］->area();}

cout<<"总面积是："<<da<<endl;

}__