complex.cpp文件源码：

#ifndef COMPLEX_HPP
#define COMPLEX_HPP
#include <iostream>
#include <cmath>
class Complex {
public:
    Complex() {};//默认构造函数
    ~Complex() {};//析构函数
    Complex(double r, double i = 0) :real(r), imag(i) {};
    Complex(const Complex& c) {
        real = c.real;
        imag = c.imag;
    };
    double get_real() const { return real; }
    double get_imag() const { return imag; }
    void show() const {
        if (imag > 0 && real != 0)
            std::cout << get_real() << " + " << get_imag() << "i";
        else if (imag < 0 && real != 0)
            std::cout << get_real() << get_imag() << "i";
        else if (imag == 0)
            std::cout << get_real();
        else
            std::cout << get_imag()<<"i";
    }
    void add(Complex x);
    friend Complex add(const Complex &x,const Complex &y);
    friend bool is_equal(Complex x, Complex y);
    friend double abs(Complex& x);
private:
    double real, imag;
};

void Complex::add(Complex x) {
    imag += x.imag;
    real += x.real;
}
Complex add( const Complex &x,  const Complex &y) {
    Complex temp;
    temp.real = x.real + y.real;
    temp.imag = x.imag + y.imag;
    return temp;
}
bool is_equal(Complex x, Complex y) {
    if (x.imag == y.imag && x.real == y.real)  return 1;//
    else
        return 0;//
}
double abs(Complex& x) {
    double value;
    value = sqrt(x.imag * x.imag + x.real * x.real);
    return value;
}
#endif

task3.cpp文件源码：

#include "Complex.hpp"
#include <iostream>

int main()
{
using namespace std;

Complex c1(3, -4);
const Complex c2(4.5);
Complex c3(c1);

cout << "c1 = ";
c1.show();
cout << endl;

cout << "c2 = ";
c2.show();
cout << endl;
cout << "c2.imag = " << c2.get_imag() << endl;

cout << "c3 = ";
c3.show();
cout << endl;

cout << "abs(c1) = ";
cout << abs(c1) << endl;

cout << boolalpha;
cout << "c1 == c3 : " << is_equal(c1, c3) << endl;
cout << "c1 == c2 : " << is_equal(c1, c2) << endl;

Complex c4;
c4 = add(c1, c2);
cout << "c4 = c1 + c2 = ";
c4.show();
cout << endl;

c1.add(c2);
cout << "c1 += c2, " << "c1 = ";
c1.show();
cout << endl;
}

测试截图：

User.hpp文件源码

#ifndef USER_HPP
#define USER_HPP
#include <iostream>
#include <regex>
class User {
public:
    User(std::string _name, std::string _passwd = "111111", std::string _email = "") :
        name(_name), passwd(_passwd), email(_email) {n++;};
    void set_email();
    void change_passwd();
    void print_info() const {
        std::cout << "name:\t" << name << std::endl;
        std::cout << "passwd:\t" << "******" << std::endl;
        std::cout << "email:\t" << email << std::endl;
    }
    static void print_n() {
        std::cout << "there are " << n << " users";
    }
private:
    std::string name, passwd, email;
    static int n;
};
int User::n = 0;
void User::set_email() {
    std::string _email;
        std::cout << "Enter email address: ";
        std::cin >> _email;
        if (_email != email)
        {
            email = _email;
            std::cout << "email is set successfully..." << std::endl;
        }
        else
        {
            std::cout << "Incorrect email format.";
        }
}

void User::change_passwd() {
    std::string oldpd;
    std::string newpd;
    int t = 3, time = 3;//t是还可以输老密码的次数，time是还可以输新密码的次数
    std::cout << "Enter old password: ";
    while (t--) {
        std::cin >> oldpd;//用户输入印象中的老密码
        if (oldpd == passwd) //老密码输入正确
        {
            while (time--) {
                std::cout << "Enter new passwd:";
                std::cin >> newpd;
                bool value = regex_match(newpd, std::regex(".{6}.*"));// 正则匹配，密码长度需大于等于6位
                if (value && newpd != passwd) {
                    passwd = newpd;
                    std::cout << "new passwd is set successfully..." << std::endl;
                    break;//新密码输入正确，跳出小循环
                }
                else
                    std::cout << "your passwd is too simple!" << std::endl;
            }
            break;//3次新密码输入机会用完或者新密码输入正确，跳出大循环
        }
        else {
            if (t == 0) {
                std::cout << "password input error. Please try after a while." << std::endl;
                break;
            }
            else std::cout << "password input error. Please re-enter again: ";
        }
    }
}
#endif

task4.cpp

#include "User.hpp"
#include <iostream>

int main()
{
    using namespace std;

    cout << "testing 1......" << endl;
    User user1("Jonny", "92197", "xyz@hotmail.com");
    user1.print_info();

    cout << endl
        << "testing 2......" << endl
        << endl;
    User user2("Leonard");
    user2.change_passwd();
    user2.set_email();
    user2.print_info();

    User::print_n();
}

总结：
1、在task3复数类的hpp文件中，既可以用对象直接调用数据成员，例如：x.real,也可以用对象调用成员函数进而获得数据成员的值例如x.get_real()。而在main函数中则不可以使用前一种调用方式。
2、（1）在task4的User类中最好使用不同的变量来区分不同数据可以输入的次数。
（2）必须定义静态成员函数print_n()已经静态数据成员n以确保n不会在主函数运行过程中被析构函数释放。n的作用域为全局。

3、尚存的问题：不知道怎么对邮箱格式的合法性进行判断，导致无论邮箱输入什么都算合法。