我在使用该功能时遇到问题：
double area( const Point & a, const Point & b, const Point & c );
此功能的说明是：
Area 是一个非成员函数，它使用其 3 Point 参数进行计算。具体来说，它计算并返回一个三角形的面积，其顶点由 3 个点表示。
任何帮助将不胜感激。谢谢你！
# include <string>
# include <iostream>
# include <cmath>
using namespace std;
class Point{
public:
Point( double x = 0, double y = 0 );
double getX() const;
double getY() const;
Point & setX( double x ); // mutator, returning reference to self
Point & setY( double y );
const Point & output() const;
double distance( const Point & other ) const;
private:
double myX;
double myY;
};
double area( const Point & a, const Point & b, const Point & c );
int main (){
Point a;
Point b;
Point c;
a.setX (2);
a.setY (0);
b.setX (0);
b.setY (1);
c.setX (0);
c.setY (0);
system ("pause");
}//main
Point :: Point( double x, double y){
setX (x);
setY (y);
}// ctr
double Point :: getX() const{
return myX;
}//getX
double Point :: getY() const{
return myY;
}//getY
Point & Point :: setX( double x ){
myX = x;
return *this;
}//setX
Point & Point :: setY( double y ){
myY = y;
return *this;
}//setY
const Point & Point :: output() const{
cout << "(" << myX << "," << myY << ")" << endl;
return *this;
}//output
double Point :: distance( const Point & other ) const{
double d;
d = sqrt(pow(other.myX-myX,2)+pow(other.myY-myY,2));
return d;
}//distance
double area( const Point & a, const Point & b, const Point & c ){
Point area;
double length1, length2, length3;
double s, area1;
length1 = sqrt(pow(a.getX-b.getX,2)+pow(a.getY-b.getY,2));
length2 = sqrt(pow(b.getX-c.getX,2)+pow(b.getY-c.getY,2));
length3 = sqrt(pow(a.getX-c.getX,2)+pow(a.getY-c.getY,2));
s = (length1+length2+length3)/2;
area1 = sqrt(s*(s-length1)(s-length2)(s-length3));
return area1;
}//area