对"<<"和">>"重载的函数形式如下

istream& operator >> (istream & , 自定义类 &) ;

ostream& operator << (ostream & , 自定义类 &) ;

只能将重载">>"和"<<"函数作为友元函数或普通函数，而不能将它们定义为成员函数

 1 #include <iostream.h>
 2 
 3 //using namespace std;
 4 
 5 class Complex
 6 {
 7 public:
 8     friend ostream& operator<< (ostream& , Complex&);
 9     friend istream& operator>> (istream& , Complex&);
10 private:
11     double real;
12     double imag;
13 };
14 
15 ostream& operator<< (ostream& output , Complex& c)
16 {
17     output << "(" << c.real ;
18     if(c.imag >= 0) output << "+" ;
19     output << c.imag << "i)" << endl;
20     return output ;
21 }
22 
23 istream& operator>> (istream& input , Complex& c)
24 {
25     cout << "please input real part and imaginary part of complex number:";
26     input >> c.real >> c.imag ;
27     return input;
28 }
29 
30 int main(void)
31 {
32     Complex c1 , c2 ;
33     cin >> c1 >> c2 ;
34     cout << "c1 = " << c1 << endl;
35     cout << "c2 = " << c2 << endl;
36     return 0 ;
37 }

编译系统把 "cout << c"解释为：

operator << (cout , c)

即以把cout 和 c 作为实参 ，调用下面的operator<< 函数

ostream& operator<< (ostream& output , Complex& c)