编译可以通过但是在输入数字之后就一直无法继续运行怎么回事？（一部分代码）
class Point
{
int x,y;
public:
void set(int a,int b) {x=a;y=b;}
double getx() {return x;}
double gety() {return y;}
};

class PointArray
{
Point a[3];
public:
double getxx() {return a[2].getx()-a[0].getx();}
double getyy() {return a[2].gety()-a[0].gety();}
Point& get1() {return a[0]; }
Point& get2() {return a[1]; }
Point& get3() {return a[2]; }
};

class Polygon
{
PointArray a;
public:
virtual double area()=0;
void setVertices(int a1,int a2,int a3,int a4,int a5=0,int a6=0);
double getlong() {return a.getxx();}
double gethigh() {return a.getyy();}
};

void Polygon::setVertices(int a1,int a2,int a3,int a4,int a5,int a6)
{
a.get1().set(a1,a2); a.get2().set(a3,a4); a.get3().set(a5,a6);
}

class Rectangle:public Polygon
{
public:
double area() {return getlong()*gethigh();}
};