#include <iostream>
    #include <set>
 
    typedef std::pair<int, int> pairs;
 
    int main()
    {
       auto compare = [](pairs lhs, pairs rhs) //custom compare lambda function
       {
          if(lhs.first > lhs.second ) lhs = pairs{lhs.second, lhs.first };
          if(rhs.first > rhs.second ) rhs = pairs{rhs.second, rhs.first };
          return lhs< rhs;
       };
 
       std::set<pairs, decltype(compare)> Set(compare);
 
       Set.emplace(std::make_pair(0,1)); // use can also emplace to the Set
       Set.emplace(pairs{0,2});
       Set.emplace(pairs{1,0});
 
       for(const auto& it: Set)
          std::cout << it.first << " " << it.second << std::endl;
    }

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