執行環境：macOS shell
程式碼：

#include <iostream>
#include <iomanip>
#include <string>

using namespace std;

class point
{
private:
    double fx, fy;
public:
    point();
    point(double x, double y);
    void showpoint();
};

point::point()
{
    fx = 0.0;
    fy = 0.0;
}
point::point(double
 
 x, double y = 5.5)
{
    fx = x;
    fy = y;
}
void point::showpoint()
{
    cout<<fx<<"  "<<fy<<endl;
}

int main ()
{
    point p1;
    cout<<"the fx and fy of p1 : ";
    p1.showpoint();
    point p2(10);
    cout<<"the fx and fy of p2 : ";
    p2.showpoint();
    point p3(1.1
 
,2.0);
    cout<<"the fx and fy of p3 : ";
    p3.showpoint();
    return 0;
}

執行結果：