1. 设计二维空间的点类Dot，并将求两点之间距离的函数声明为点类Dot的友元函数。
   成员变量：
   double x,y;
   成员函数：
   Dot(double xx=0,double yy=0);//构造函数
   ~Dot();//析构函数
   void setX(double xx);//设置点的x坐标
   void setY(double yy);//设置点的y坐标
   double getX();//获取点的x坐标
   double getY();//获取点的y坐标
   void print(); //输出点的坐标
   友元函数：
   friend double distance(Dot d1,Dot d2);2. 求二维空间两点之间的距离：
2. 求二维空间两点之间的距离：
   double distance(Dot d1,Dot d2);
3. 设计main函数进行测试，并求点A（1, 2）、B（3, 5）之间的距离。

#pragma once
class Dot
{
public:
    Dot(double, double);//构造函数
    ~Dot() {};//析构函数
    void setX(double xx);//设置点的x坐标
    void setY(double yy);//设置点的y坐标
    double getX();//获取点的x坐标
    double getY();//获取点的y坐标
    void print();  //输出点的坐标
    friend double distance(Dot d1, Dot d2);

private:
    double x, y;
};

#include "Dot.h"
#include<cmath>
#include<iostream>
using namespace std;

void Dot::setX(double xx) {
    x = xx;

}

void Dot::setY(double yy) {
    y = yy;
}
double Dot::getX() {
    return x;
}
double Dot::getY() {
    return y;
}
void Dot::print() {
    cout << "(" << x << "," << y << ")" << endl;

}

Dot::Dot(double xx, double yy) {
    x = xx;
    y = yy;
}
double distance(Dot a, Dot b) {
    double distance = 0;
    distance = sqrt(pow(b.x - a.x, 2) + pow(b.y - a.y, 2));
    return distance;
}

#include<iostream>
#include"Dot.h"
using namespace std;
int main() {
    Dot a(1, 2); Dot b(3, 5); Dot c(0, 0);
    cout << distance(a, b) << endl;

    system("pause");
    return 0;
}

复数类+复数的加减乘运算(友元函数)

#pragma once
class complex
{
public:
    complex(void);
    complex(double, double);
    friend complex add(complex a,complex b);
    void norm();
    friend complex aqq(complex a,complex b);
    void print();
    void set(double, double);
    complex(complex &c);
private:
    double i, j;
};

#include "complex.h"
#include<math.h>
#include<iostream>
using namespace std;
complex::complex(void)
{
    i = 0;
    j = 0;
}

complex::complex(double i2, double j2) {
    i = i2;
    j = j2;
}

complex::complex(complex &b) {
    i = b.i;
    j = b.j;
}

void complex::norm() {
    double sum;
    sum = sqrt(i*i + j * j);
    cout << "模为：" << sum << endl;
}
void complex::print() {
    cout << i << "i" << j << "j" << endl;
}
void complex::set(double a, double b) {
    i = a;
    j = b;
}
complex add(complex a, complex b) {
    complex y(a.i + b.i, a.j + b.j);
    return y;
}
complex aqq(complex c, complex d) {
    complex y(c.i - d.i, c.j - d.j);
    return y;
}

#include<iostream>
using namespace std;
#include"complex.h"
int main() {
    complex a(3, 4), b(6, 10);
    complex c(a);
    a.print();
    a.norm();
    a.print();
    b.print();
    a.norm();
    a.print();
    c = add(a,b);
    c.print();
    c = aqq(a,b);
    c.print();
    system("pause");
    return 0;

}