定义Point类的无参数构造函数，将point对象的数据成员初始化为0。

#include <iostream>
using namespace std;
class point{
private:
    int x,y;
public:
    point(int a,int b){ x=a;    y=b;}
    int getx(){ return x; }
    int gety(){ return y; }
    point(){ x=0;y=0; } // 显式定义无参构造函数
};
point p1(1,1); // 调用构造函数point(int,int) 
void main (){
    static point p2; // 调用构造函数point()
    point p3,a[10]; // 调用构造函数point()
    point  *p4;
    p4=new point; // 调用构造函数point()
    cout<<"p1: "<<p1.getx()<<","<<p1.gety()<<endl;
    cout<<"p2: "<<p2.getx()<<","<<p2.gety()<<endl;
    cout<<"p3: "<<p3.getx()<<","<<p3.gety()<<endl;
    cout<<"p4: "<<p4->getx()<<","<<p4->gety()<<endl;

}