How do I overload the operator * when my object is on the right side in C++?

问题

I want to implement "operator * " overloading INSIDE my class, so I would be able to do the following:

Rational a(1, 2), b;
b = 0.5 * a; // b = 1/4

Notice that b is on the right side, is there a way to do such a thing inside "Rational" class?

回答1:

No. You must define operator* as a free function. Of course, you could implement it in terms of a member function on the second argument.

回答2:

Yes:

class Rational {
  // ...
  friend Rational operator*(float lhs, Rational rhs) { rhs *= lhs; return rhs; }
};

Note: this is of course an abuse of the friend keyword. It should be a free function.

回答3:

Answer is no you cannot, but since float value is on left side you may expect that type of result from "0.5 * a" will be double. In that case you may consider doing something about conversion operator. Please note that "pow(a, b)" is added only to illustrate the idea.

  1 #include <stdio.h>
  2 #include <math.h>
  3 
  4 class Complicated
  5 {
  6 public:
  7     Complicated(int a, int b) : m_a(a), m_b(b)
  8     {
  9     }   
 10      
 11     Complicated(double a) : m_a(a)
 12     {
 13     }
 14     
 15     template <typename T> operator T()
 16     {
 17         return (T)(pow(10, m_b) * m_a);
 18     }   
 19     
 20     void Print()
 21     {
 22         printf("(%f, %f)\n", m_a, m_b);
 23     }   
 24     
 25 private:
 26     double m_a;
 27     double m_b;
     28 };  
 29
 30 
 31 int main(int argc, char* argv[])
 32 {
 33     Complicated pr(1, 2);
 34     Complicated c = 5.1 * (double) pr;
 35     c.Print();
 36 }
 37 

来源：https://stackoverflow.com/questions/4846396/how-do-i-overload-the-operator-when-my-object-is-on-the-right-side-in-c