能不能写一个接口？急！

#include <iostream>
#include <math.h>
using namespace std;
class Point
{
public:
    double x,y;
    Point(){x=0;y=0;}
    Point(int xx,int yy){x=xx;y=yy;}
};
class Circle
{
public:
    Point center;
    double radius;
    Circle(){}
    Circle(Point p,double r)
    {
        center = p;
        radius = r;
    }
    void fun(Point p)
    {
        double dis = sqrt( (center.x-p.x)*(center.x-p.x) + (center.y-p.y)*(center.y-p.y) );
        if(dis < radius)
            cout << "在圆内"<< endl;
        else if(dis == radius)
            cout << "在圆上"<< endl;
        else
            cout << "在圆外" << endl;
    }
};
class Triangle
{
public:
    Point a,b,c;
    Triangle(){}
    Triangle(Point x,Point y,Point z)
    {
        a =x;b=y;c=z;
        double l1 = sqrt( (a.x-b.x)*(a.x-b.x)+(a.y-b.y)*(a.y-b.y) );
        double l2 = sqrt( (a.x-c.x)*(a.x-c.x)+(a.y-c.y)*(a.y-c.y) );
        double l3 = sqrt( (b.x-c.x)*(b.x-c.x)+(b.y-c.y)*(b.y-c.y) );
        if(l1 +l2 <= l3 || l2+l3 <= l1 || l1+l3<= l2)
            cout << "不能组成三角形";
    }
    double getzc()
    {
        double l1 = sqrt( (a.x-b.x)*(a.x-b.x)+(a.y-b.y)*(a.y-b.y) );
        double l2 = sqrt( (a.x-c.x)*(a.x-c.x)+(a.y-c.y)*(a.y-c.y) );
        double l3 = sqrt( (b.x-c.x)*(b.x-c.x)+(b.y-c.y)*(b.y-c.y) );
        if(l1 +l2 <= l3 || l2+l3 <= l1 || l1+l3<= l2)
        {
            return -1;
        }
        else
            return (l1 + l2 +l3);
    }
};
int main()
{
    Circle c(Point(2,2),1);
    c.fun(Point(1,1));
    Triangle tr(Point(0,0),Point(1,0),Point(0,1));
    if (tr.getzc() < 0)
    {
        cout <<"不构成三角形"<<endl;
    }else
        cout <<"周长="<<tr.getzc()<<endl;
    return 0;
}

参考如下：

#include <iostream>
#include <math.h>
using namespace std;

//抽象类
class MyObject
{
public:
    virtual double getzc()=0;  
};


class Point
{
public:
    double x,y;
    Point(){x=0;y=0;}
    Point(int xx,int yy){x=xx;y=yy;}
};
class Circle :public MyObject
{
public:
    Point center;
    double radius;
    Circle(){}
    Circle(Point p,double r)
    {
        center = p;
        radius = r;
    }
    double getzc()
    {
        return 2*3.1415926*radius;
    }
    void fun(Point p)
    {
        double dis = sqrt( (center.x-p.x)*(center.x-p.x) + (center.y-p.y)*(center.y-p.y) );
        if(dis < radius)
            cout << "在圆内"<< endl;
        else if(dis == radius)
            cout << "在圆上"<< endl;
        else
            cout << "在圆外" << endl;
    }
};
class Triangle :public MyObject
{
public:
    Point a,b,c;
    Triangle(){}
    Triangle(Point x,Point y,Point z)
    {
        a =x;b=y;c=z;
        double l1 = sqrt( (a.x-b.x)*(a.x-b.x)+(a.y-b.y)*(a.y-b.y) );
        double l2 = sqrt( (a.x-c.x)*(a.x-c.x)+(a.y-c.y)*(a.y-c.y) );
        double l3 = sqrt( (b.x-c.x)*(b.x-c.x)+(b.y-c.y)*(b.y-c.y) );
        if(l1 +l2 <= l3 || l2+l3 <= l1 || l1+l3<= l2)
            cout << "不能组成三角形";
    }
    double getzc()
    {
        double l1 = sqrt( (a.x-b.x)*(a.x-b.x)+(a.y-b.y)*(a.y-b.y) );
        double l2 = sqrt( (a.x-c.x)*(a.x-c.x)+(a.y-c.y)*(a.y-c.y) );
        double l3 = sqrt( (b.x-c.x)*(b.x-c.x)+(b.y-c.y)*(b.y-c.y) );
        if(l1 +l2 <= l3 || l2+l3 <= l1 || l1+l3<= l2)
        {
            return -1;
        }
        else
            return (l1 + l2 +l3);
    }
};
int main()
{
    Circle c(Point(2,2),1);
    c.fun(Point(1,1));
    Triangle tr(Point(0,0),Point(1,0),Point(0,1));
    if (tr.getzc() < 0)
    {
        cout <<"不构成三角形"<<endl;
    }else
        cout <<"周长="<<tr.getzc()<<endl;
    return 0;
}

啥意思