设计自定义异常类NotTriangleException；设计Triangle（三角形）类； （1）Triangle类基本信息：三个顶点坐标； （2）带三个顶点坐标参数的构造函数；当传入实参为不合理数据时(例如：输入的三角形的顶点不能组成一个三角形)，抛出自定义异常类NotTriangleException的对象； （3）有返回三角形面积的函数getArea()。 （4）各个类的声明放在与类同名的头文件中（*.h），类的实现放在与类同名的源文件中（*.cpp）； （5）只能在主函数中输入输出，各个类的成员函数中不能进行输入输出。 （6）在主函数中，接收用户输入的三角形三个顶点坐标，然后，传给三角形对象的构造函数，并捕获NotTriangleException异常对象。当捕获到异常后，提示用户输入数据错误，并要求用户重新输入。

NotTriangleException.h

#ifndef NOTTRIANGLEEXCEPTION_H
#define NOTTRIANGLEEXCEPTION_H

#include <exception>
#include <string>

class NotTriangleException : public std::exception {
public:
    NotTriangleException(const std::string& msg): message(msg) {}

    virtual const char* what() const noexcept {
        return message.c_str();
    }

private:
    std::string message;
};

#endif // NOTTRIANGLEEXCEPTION_H

Triangle.h

#ifndef TRIANGLE_H
#define TRIANGLE_H

#include "NotTriangleException.h"

class Triangle {
public:
    Triangle(double x1, double y1, double x2, double y2, double x3, double y3);

    double getArea() const;

private:
    double x1, y1, x2, y2, x3, y3;
};

#endif // TRIANGLE_H

Triangle.cpp

#include "Triangle.h"
#include <cmath>

Triangle::Triangle(double x1, double y1, double x2, double y2, double x3, double y3) {
    double a = sqrt(pow(x1 - x2, 2) + pow(y1 - y2, 2));
    double b = sqrt(pow(x1 - x3, 2) + pow(y1 - y3, 2));
    double c = sqrt(pow(x2 - x3, 2) + pow(y2 - y3, 2));
    if (a + b <= c || a + c <= b || b + c <= a) {
        throw NotTriangleException("Invalid triangle coordinates");
    }
    this->x1 = x1;
    this->y1 = y1;
    this->x2 = x2;
    this->y2 = y2;
    this->x3 = x3;
    this->y3 = y3;
}

double Triangle::getArea() const {
    double a = sqrt(pow(x1 - x2, 2) + pow(y1 - y2, 2));
    double b = sqrt(pow(x1 - x3, 2) + pow(y1 - y3, 2));
    double c = sqrt(pow(x2 - x3, 2) + pow(y2 - y3, 2));
    double s = (a + b + c) / 2.0;
    double area = sqrt(s * (s - a) * (s - b) * (s - c));
    return area;
}

main.cpp

#include <iostream>
#include "Triangle.h"

using namespace std;

int main() {
    double x1, y1, x2, y2, x3, y3;
    while (true) {
        cout << "Enter the coordinates of the triangle's vertices:" << endl;
        cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3;
        try {
            Triangle t(x1, y1, x2, y2, x3, y3);
            double area = t.getArea();
            cout << "The area of the triangle is " << area << endl;
            break;
        } catch (NotTriangleException& e) {
            cout << "Error: " << e.what() << endl;
            cout << "Please enter valid coordinates." << endl;
        }
    }

    return 0;
}