C++运算符重载为非成员函数

#include<iostream>
using namespace std;
class Complex{
public:
    Complex(double r=0.0,double i=0.0):real(r),imag(i){}
    friend Complex operator + (const Complex &c1,const Complex &c2);
    friend Complex operator - (const Complex&c1,const Complex &c2);
    friend ostream & operator <<(ostream & out,const Complex & c);
private:
    double real,imag;
};
Complex operator + (const Complex &c1,const Complex &c2)
{
    return Complex(c1.real+c2.real,c1.imag+c2.imag);
}
Complex operator - (const Complex &c1,const Complex &c2)
{
    return Complex(c1.real-c2.real,c1.imag-c2.imag);
}
ostream & operator <<(ostream & out,const Complex & c)
{
    out<<"("<<c.real<<","<<c.imag<<")";
    return out;
}
int main()
{
    Complex c1(5,4),c2(2,10),c3;
    cout<<"c1="<<c1<<endl;
    cout<<"c2="<<c2<<endl;
    c3=c1-c2;
    cout<<"c3=c1-c2="<<c3<<endl;
    c3=c1+c2;
    cout<<"c3=c1+c2="<<c3<<endl;
    return  0;
}