【c++】纯虚函数和抽象类

/*纯虚函数与抽象类-----枯等街寒又何妨2014/01/05*/
#include<iostream>
#include<cmath>
using namespace std;
const double pi=3.14159;
class Shape
{
public:
virtual void show(void)=0;
virtual double area(void)=0;
};

class triangle:public Shape
{
public:
triangle(double=0,double=0,double=0);
void show(void);
double area(void);
private:
double a,b,c;
};

class rectangle :public Shape
{
public:
rectangle(double=0,double=0);
void show(void );
double area(void );
private:
double width,length;
};

class circle :public Shape
{
public:
circle(double =0);
void show(void);
double area(void);
private:
double r;
};

triangle::triangle(double a1,double b1,double c1)
{
a=a1;b=b1;c=c1;
}

void triangle::show(void)
{
cout<<"a="<<a<<"\tb="<<b<<"\tc="<<c<<endl;
}

double triangle::area(void)
{
double s=(a+b+c)/2;
return sqrt(s*(s-a)*(s-b)*(s-c));
}

rectangle::rectangle(double a,double b)
{
length=a;width=b;
}

void rectangle::show(void )
{
cout<<"wigth="<<width<<"\tlength="<<length<<endl;
}

double rectangle::area(void)
{
return (width*length);
}

circle::circle(double a)
{
r=a;
}

void circle::show()
{
cout<<"r="<<r<<endl;
}

double circle::area(void )
{
return pi*r*r;
}

int main()
{
circle r(10);
triangle s(3,4,5);
rectangle l(2,5);
r.show();
s.show();
l.show();
cout<<r.area()<<endl;
cout<<s.area()<<endl;
cout<<l.area()<<endl;
system("pause");
return 1;
}