定义一个点类Point，定义一个三角形类Triangle，给两个类设计合适不带参数的构造函数，在主程序中，创建一个Triangle类的对象cyl并采用默认值初始化，通过调用成员函数area得到面积并输出。

下面是代码示例：

#include <iostream>
#include <cmath>
using namespace std;

class Point {
private:
    double x, y;
public:
    Point() { x = 0.0; y = 0.0; }
    Point(double xval, double yval) { x = xval; y = yval; }
    double getX() { return x; }
    double getY() { return y; }
};

class Triangle {
private:
    Point p1, p2, p3;
public:
    Triangle() {}
    Triangle(Point p1val, Point p2val, Point p3val) {
        p1 = p1val;
        p2 = p2val;
        p3 = p3val;
    }
    double area() {
        double a = sqrt(pow(p2.getX() - p1.getX(), 2) + pow(p2.getY() - p1.getY(), 2));
        double b = sqrt(pow(p3.getX() - p2.getX(), 2) + pow(p3.getY() - p2.getY(), 2));
        double c = sqrt(pow(p1.getX() - p3.getX(), 2) + pow(p1.getY() - p3.getY(), 2));
        double s = (a + b + c) / 2;
        return sqrt(s * (s - a) * (s - b) * (s - c));
    }
};

int main() {
    Point p1(0, 0);
    Point p2(1, 0);
    Point p3(0, 1);
    Triangle tri(p1, p2, p3);
    cout << "The area of the triangle is: " << tri.area() << endl;
    return 0;
}

输出结果为：

The area of the triangle is: 0.5