+、-(両目)を複数クラスの友元関数に再ロード

#include
class complex	//     
{
public:	//    
	complex(double r=0.0,double i=0.0){ real=r; imag=i; }	//    
	friend complex operator + (complex c1,complex c2);	//   +       
	friend complex operator - (complex c1,complex c2);	//   -       
	void display();	//      
private:	//      
	double real;
	double imag;
};
			
complex operator +(complex c1,complex c2)	//           
{	 
	return complex(c2.real+c1.real, c2.imag+c1.imag);
}

complex operator -(complex c1,complex c2)	//           
{
	return complex(c1.real-c2.real, c1.imag-c2.imag);
}

void complex::display()
{   cout<