1 class Fraction
 2 {
 3 public:
 4     Fraction(int num, int den=1)
 5     : m_numerator(num), m_denominator(den){ }
 6     operator double() const {
 7         return (double)(m_numerator / m_denominator);
 8     }　　//转换函数
 9 private:
10     int m_numerator;
11     int m_denominator;
12 };

double d=Fraction(2,3)+3.5; //合法的表达式,Fraction会被转换为double进行加运算;