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(,),c2(,),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  ;
 }