#include <set>
#include <iostream>

struct A {
    int i;
    A() : i(0) {}
    bool operator()(int a, int b)
    {
        ++i;
        return a < b;
    }
};

int main()
{    
    A a;
    std::set<int, A&> s1( {1, 2, 3}, a);
    std::set<int, A&> s2( {4, 5, 6}, a);
    std::cout << a.i;
}