复数加法重载函数不作为成员函数

#include <iostream>

/* run this program using the console pauser or add your own getch, system("pause") or input loop */
using namespace std;
class Complex
{
    public:
        Complex(){
            real=0;
            imag=0;
        }
        Complex(double r,double i){
            real=r;
            imag=i;
        }
        friend Complex operator+(Complex &c1,Complex &c2);
        void display();
    private:
        double real;
        double imag;
};

Complex operator+(Complex &c1,Complex &c2)
{
    return Complex(c1.real+c2.real,c1.imag+c2.imag);
}

void Complex::display()
{
    cout<<"("<<real<<","<<imag<<"i)"<<endl;
}
int main(int argc, char** argv) {
    Complex c1(3,4),c2(5,-10),c3;
    c3=c1+c2;
    cout<<"c1=";c1.display();
    cout<<"c2=";c2.display();
    cout<<"c1+c2=";c3.display();
    return 0;
}