例10.3 將運算符“+”重載為適用于復數加法，重載函數不作為成員函數，而放在類外，作為Complex類的友元函數。
#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);//重載函數作為友元函數
void display( );
 private:
double real;
double imag;
};

Complex operator + (Complex &c1,Complex &c2)          //定義作為友元函數的重載函數
{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( );
}