C++ - sort algorithm doesnt see my overloaded “<” operator for user-defined type.

Question

Ok so I have 1 user-defined type named fraction and it represents and ordinary fraction with a numerator and denominator. Here is the code:

class Fr         


        

Answer 1

bool operator<(Fraction& first,Fraction& second)

The problem is that your overload of operator< only accepts non-const arguments, but the algorithm is trying to compare a constant reference to a Fraction with the elements in your container.

Since operator< does not modify the contents of the compared objects, it should take them by const reference:

bool operator<(Fraction const & first,Fraction const & second)

Answer 2

Consider this code:

Fraction a, b;
bool res = (a < b);

The second line is essentially a call to

bool res = a.operator<(b);

1. As suggested above - being const correct is going to serve you very well in a long run
2. Define a member bool operator<(const Fraction&) const to make the sorting implementation happy or just go with the first suggestion above - be const-correct with bool operator<(const Fraction& a, const Fraction& b)