How to use lambda function as hash function in unordered_map?

Question

I wonder if it is possible to use lambda function as custom hash function for unordered_map in C++11? If so, what is the syntax?

Answer 1

#include<unordered_map> #include<string>  int main() {     auto my_hash = [](std::string const& foo) {         return std::hash<std::string>()(foo);     };      std::unordered_map<std::string, int, decltype(my_hash)> my_map(10, my_hash);  } 

You need to pass lambda object to unordered_map constructor, since lambda types are not default constructible.

As @mmocny suggested in comment, it's also possible to define make function to enable type deduction if you really want to get rid of decltype:

#include<unordered_map> #include<string>  template<         class Key,         class T,         class Hash = std::hash<Key>         // skipped EqualTo and Allocator for simplicity > std::unordered_map<Key, T, Hash> make_unordered_map(         typename std::unordered_map<Key, T, Hash>::size_type bucket_count = 10,         const Hash& hash = Hash()) {     return std::unordered_map<Key, T, Hash>(bucket_count, hash); }  int main() {     auto my_map = make_unordered_map<std::string, int>(10,             [](std::string const& foo) {                 return std::hash<std::string>()(foo);             }); }