第九章——<<>>的重载 （额这是怎么回事）

#include <iostream>using namespace std;class Complex{public:    Complex(){real=0;imag=0;}    friend ostream& operator<< (ostream& output, Complex& c);    friend istream& operator>> (istream& input,Complex& c);    Complex(double r,double i){real=r; imag=i;}    Complex operator+(Complex &c2);    Complex operator-(Complex &c2);    Complex operator-();    Complex operator*(Complex &c2);    Complex operator/(Complex &c2);    void display();private:    double real;    double imag;};//下面定义成员函数ostream& operator<< (ostream& output, Complex& c){    output<<"("<<c.real;    if (c.imag>0)    output<<"+"<<c.imag<<"i)";}istream& operator>> (istream& input,Complex& c){    char ch;    input>>c.real>>ch;    if (ch!='-'||ch!='+')    {        cout<<"请输入+或者-"<<endl;        operator>> (input,c);    }}Complex Complex::operator-(){    return -*this;}Complex Complex::operator+(Complex &c2){    Complex c;    c.real=real+c2.real;    c.imag=imag+c2.imag;    return c;}Complex Complex::operator-(Complex &c2){    Complex c;    c.real=real-c2.real;    c.imag=imag-c2.imag;    return c;}Complex Complex::operator*(Complex &c2){    Complex c;    c.real=real*c2.real-imag*c2.imag;    c.imag=imag*c2.real+real*c2.imag;    return c;}Complex Complex::operator/(Complex &c2){    Complex c;    c.real=(real*c2.real+imag*c2.imag)/(c2.real*c2.real+c2.imag*c2.imag);    c.imag=(imag*c2.real+real*c2.imag)/(c2.real*c2.real+c2.imag*c2.imag);    return c;}//下面定义用于测试的main()函数int main(){    Complex c1(3,4),c2(5,-10),c3;    cout<<"c1=";    cout<<c1<<endl;    cout<<"c2=";    cout<<c2<<endl;    c3=c1+c2;    cout<<"c1+c2=";    cout<<c3<<endl;    c3=c1-c2;    cout<<"c1-c2=";    cout<<c3<<endl;    c3=c1*c2;    cout<<"c1*c2=";    cout<<c3<<endl;    c3=c1/c2;    cout<<"c1/c2=";    cout<<c3<<endl;    return 0;}