c++重载>>与<<

输入和输出运算符只能用友元函数重载。
友元函数：指某些虽然不是类成员却能够访问类的所有成员的函数。

#include <iostream>

using namespace std;

class complex{
private:
    double re, im;
public:
    complex(double re, double im){
    this->re = re;
        this->im = im;
  }
    complex(){
        this->re = 0;
        this->im = 0;
    }
    complex operator !();
  complex operator +(const complex& obj);
    friend ostream& operator << (ostream& os, complex c);
  friend istream& operator >> (istream& is, complex& c);
    complex& operator++(){
        this->re += 1;
        this->im += 1;
    }
    const complex operator++(int){
        complex old;
        old.re = this->re;
        old.im = this->im;
        ++(*this);
        return old;
  }
    void operator+=(const complex temp){
        this->re += temp.re;
        this->im += temp.im;
    }
};
complex complex::operator +(const complex& obj){
    complex temp;
    temp.re = obj.re + this->re;
    temp.im = obj.im + this->im;
    return temp;
}
complex complex::operator !(){
    complex temp;
    temp.re = -this->re;
    temp.im = -this->im;
    return temp;
}
ostream& operator<<(ostream& os, complex c){
    os<<c.re;
    if(c.im>0)
        os<<"+"<<c.im<<"i"<<endl;
    else
        os<<c.im<<"i"<<endl;
    return os;
}
istream& operator >> (istream& is, complex& c){
    is>>c.re>>c.im;
    return is;
}
int main(){
  complex obj(1,2),obj1(3,-4);
    obj += obj1;
  cout<<obj;
  cout<<obj1;
  cin >> obj;
    cout << obj++ <<std::endl;
    cout << obj;
    return 0;
}