c++ - using lambda do define hash function throws exception -

c++ - using lambda do define hash function throws exception -

the next code defines unordered_set. code compiles fine. using lambda function instead of functor throw when calling find:

libc++abi.dylib: terminate called throwing exception

#include <unordered_set> class pair_hash { public: size_t operator() (const std::pair<int, int> &x) const { homecoming std::hash<int>()(x.first) ^ std::hash<int>()(x.second); } }; int main() { std::unordered_set<std::pair<int, int>, pair_hash> temp; temp.find(std::make_pair(0,0)); std::function<std::size_t(std::pair<int , int>)> fpair_hash; fpair_hash = [](const std::pair<int, int>& v) -> std::size_t { homecoming std::hash<int>()(v.first) ^ std::hash<int>()(v.second); }; std::unordered_set<std::pair<int, int>, decltype(fpair_hash)> temp2; //why not work? temp2.find(std::make_pair(0,0)); homecoming 0; }

clang++ -std=c++11 -stdlib=libc++ -o test test.cpp

decltype(fpair_hash) std::function<std::size_t(std::pair<int , int>)> building set empty hash function.

you need provide function constructor of std::unordered_set:

std::unordered_set<std::pair<int, int>, decltype(fpair_hash)> temp2(10, fpair_hash);

this should create work, using std::function have overhead of polymorphic phone call , don't need it:

auto fpair_hash = [](const std::pair<int, int>& v) -> std::size_t { homecoming std::hash<int>()(v.first) ^ std::hash<int>()(v.second); };

finally, hash function not - maps pairs (x, x) 0. perhaps using x * 17 + y * 13 instead of x ^ y cut down probability of collisions.

c++ hash c++11 std unordered-set