定义一个复数类，重载“-=”运算符，使这个运算符能直接完成复数的“-=”运算。分别用成员函数与友元函数编写运算符重载函数。在主函数中定义复数对象c1(10,20)、c2(15,30)，进行c2-=c1的复数运算，并输出c1、c2的复数值。

#include <iostream>
using namespace std;

class Complex {
public:
    Complex(double r = 0, double i = 0) : real(r), imag(i) {}
    void print() { cout << real << '+' << imag << 'i' << endl; }
    Complex operator-=(const Complex& c); // 成员函数重载
    friend Complex operator-=(Complex& c1, const Complex& c2); // 友元函数重载
private:
    double real, imag;
};

Complex Complex::operator-=(const Complex& c) {
    real -= c.real;
    imag -= c.imag;
    return *this;
}

Complex operator-=(Complex& c1, const Complex& c2) {
    c1.real -= c2.real;
    c1.imag -= c2.imag;
    return c1;
}

int main() {
    Complex c1(10, 20), c2(15, 30);
    c2 -= c1;
    c1.print();
    c2.print();
    return 0;
}