Using custom std::set comparator

Question

I am trying to change the default order of the items in a set of integers to be lexicographic instead of numeric, and I can\'t get the following to compile with g++:

Answer 1

You are using a function where as you should use a functor (a class that overloads the () operator so it can be called like a function).

struct lex_compare {
    bool operator() (const int64_t& lhs, const int64_t& rhs) const {
        stringstream s1, s2;
        s1 << lhs;
        s2 << rhs;
        return s1.str() < s2.str();
    }
};

You then use the class name as the type parameter

set s;

If you want to avoid the functor boilerplate code you can also use a function pointer (assuming lex_compare is a function).

set s(&lex_compare);