作为类的成员函数，重载运算符只能有一个参数

1 overload a operator of a class, you can only use one para., this pointer is automatically used.

class Rational
{
public:

//not correct since this ponit would be used automatically.
//Rational operator+ (const Rational& lhs, const Rational& rhs);

//correct
Rational operator+ (const Rational& rhs);
}

　　

2 outside the class two para are allowed

Rational operator+(const Rational& lhs, const Rational& rhs))