operator overloading,难

return_type operatoroperator_symbol( parameters )
{
  statements
}

Example:

int operator-( Time a, Time b ) { return a.seconds_from( b ) }       there are two types of operator overloading,one is operator function,another is operator member function. as an operator function,if unary, time operator--(time& a); if binary, int operator-(time a,time b); as an operator member function, if unary,then  Fraction& operator++(); // Prefix form Fraction operator++(int unused); // Postfix formif binary,then Fraction& operator+=(const Fraction& right); 有很多advanced topics