关于“重载运算符和友元函数参数关系”的问题

//重载运算符和友元函数
#include<iostream>
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);             //重载函数作为友元函数
    void display( );

private:
    double real;
    double imag;
};

Complex operator + (Complex &c1,Complex &c2)                        //定义作为友元函数的重载函数
{ return Complex(c1.real + c2.real,c1.imag + c2.imag);}

void Complex::display( )
{ cout << "(" << real << "," << imag <<"i)" << endl;}

int main( )
{
    Complex c1(3,4),c2(5,-10),c3;
    c3 = c1 + c2;
    cout << "c1 = ";c1.display( );
    cout << "c2 = ";c2.display( );
    cout << "c1 + c2 = ";c3.display( );
    system("pause");
    return 0;
}

/*
请问：如果想把类中申明友元函数的 friend 去掉，就是说，把 Complex operator+ ( Complex &c1,Complex &c2) 作
为 Complex 类的成员函数，行不行呢？为什么？你能保证在有两个参数的前提下，不用友元关系，使程序正常运行吗？
*/