定义类 Complex 使得用户可以使用如下： int main（）{Complex C1(1,2), C2; C2=9;cout<<C1-C2<<endl;return 0;}

以下是定义类 Complex 的代码：

#include <iostream>
using namespace std;

class Complex {
private:
    double real, imag;
public:
    Complex(double r = 0, double i = 0) : real(r), imag(i) {}
    Complex operator+(const Complex& c) const {
        return Complex(real + c.real, imag + c.imag);
    }
    Complex operator-(const Complex& c) const {
        return Complex(real - c.real, imag - c.imag);
    }
    Complex operator*(const Complex& c) const {
        return Complex(real * c.real - imag * c.imag, real * c.imag + imag * c.real);
    }
    Complex operator/(const Complex& c) const {
        double denominator = c.real * c.real + c.imag * c.imag;
        return Complex((real * c.real + imag * c.imag) / denominator, (imag * c.real - real * c.imag) / denominator);
    }
    Complex& operator=(double r) {
        real = r;
        imag = 0;
        return *this;
    }
    friend ostream& operator<<(ostream& os, const Complex& c) {
        os << c.real << "+" << c.imag << "i";
        return os;
    }
};

用户可以使用如下代码：

int main() {
    Complex C1(1, 2), C2;
    C2 = 9;
    cout << C1 - C2 << endl;
    return 0;
}

输出结果为：`-8+2i`。