C++中将双目运算符重载为类的成员函数

/*--------------------------------
功能：双目运算符重载为类的成员函数
运行结果：
2+3i
4+5i
6+8i
-2-2i
----------------------------------
Author: Zhang Kaizhou
Date: 2019-4-9 17:55:03
--------------------------------*/
#include 

using namespace std;

class Complex{ // 定义一个复数类
private:
    double real;
    double imag;
public:
    Complex(double r = 0.0, double i = 0.0):real(r), imag(i){} // 构造函数
    Complex operator +(const Complex & c2) const; // 重载 + 号运算符
    Complex operator -(const Complex & c2) const; // 重载 - 号运算符
    void display() const;
};

Complex Complex::operator +(const Complex & c2) const{ // 形参为一个Complex类型的常引用变量
    return Complex(real + c2.real, imag + c2.imag); // 返回一个临时的无名对象
}

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

void Complex::display() const{
    cout << real;
    if(imag - 0.0 > 1e-8){
        cout << "+" << imag << "i" << endl;
    }else{
        cout << imag << "i" << endl;
    }
}

int main(){
    Complex c1(2, 3), c2(4, 5), c3;
    c1.display();
    c2.display();
    c3 = c1 + c2;
    c3.display();
    c3 = c1 - c2;
    c3.display();

    return 0;
}