c++ - Passing my compar function to std::multiset with C++11 -

i have std::multiset, stores std::pair. want first attribute have no constraint on uniqueness, want second 1 unique. so, decided pass own function multiset, in order achieve (if not please let me know).

based on this answer, wrote similar function fails, , have no idea why (no idea of λ - , greek :) ).

auto f = [](std::pair<float, int>& a, std::pair<float, int>& b) {   return (a.first < b.first && a.second != b.second); }; 

error:

error: expression ‘#‘lambda_expr’ not supported dump_expr#<expression error>’ not constant-expression sorry, unimplemented: non-static data member initializers error: unable deduce ‘auto’ ‘<expression error>’ 

i think cannot pass lambda (runtime construct) template parameter (compile-time construct). using struct operator() works instead:

#include <set> struct my_compare {   bool operator() (const std::pair<float, int>& a, const std::pair<float, int>& b) {     return (a.first < b.first && a.second != b.second);   }; }; int main(int argc, char ** argv) {   std::multiset<std::pair<float, int>, my_compare> set;   return 0; } 

or, lambda , decltype (as in praetorian's answer):

#include <set>   int main(int argc, char ** argv) {   auto my_compare = [](const std::pair<float, int>& a, const std::pair<float, int>& b) {     return (a.first < b.first && a.second != b.second);   };   std::multiset<std::pair<float, int>, decltype(my_compare)> set(my_compare);   return 0; }