Invalid use of incomplete type struct std::hash with unordered_map with std::pair of enum class as key

Question

I want to use a unordered_map<std::pair<enum_class,other_enum_class>,std::uint8_t> for managing some pixelmap formats.

Here the minimal code :

#include <unordered_map>
#include <utility>
#include <cstdint> 
#include <iostream>
#include <functional>

enum class PNM : std::uint8_t { PBM, PGM, PPM };
enum class Format : bool      { BIN, ASCII };

struct pair_hash {
public:
    template <typename T, typename U>
    std::size_t operator()(const std::pair<T, U> &x) const { 
        return std::hash<T>()(x.first) ^ std::hash<U>()(x.second); 
    }
};

int main(){

    std::unordered_map<std::pair<PNM, Format>, std::uint8_t, pair_hash> k_magic_number ({
        { { PNM::PBM, Format::BIN   }, 1 }, { { PNM::PGM, Format::BIN   }, 2 }, { { PNM::PPM, Format::BIN   }, 3 },
        { { PNM::PBM, Format::ASCII }, 4 }, { { PNM::PGM, Format::ASCII }, 5 }, { { PNM::PPM, Format::ASCII }, 6 }
    });

    std::cout << k_magic_number[std::make_pair<PNM, Format>(PNM::PBM, Format::BIN)];
}

With GCC I have an error when I try to instantiate the class :

main.cpp:14:24: error: invalid use of incomplete type 'struct std::hash'
return std::hash()(x.first) ^ std::hash()(x.second);
In file included from
/usr/local/include/c++/5.2.0/bits/basic_string.h:5469:0,
from /usr/local/include/c++/5.2.0/string:52,
[...]

With Clang I also have an error :

error: implicit instantiation of undefined template 'std::hash' return std::hash()(x.first) ^ std::hash()(x.second); /usr/local/bin/../lib/gcc/x86_64-unknown-linux-gnu/5.2.0/../../../../include/c++/5.2.0/bits/hashtable_policy.h:1257:16: note: in instantiation of function template specialization 'pair_hash::operator()' requested here [...]

With VS2013 I have no error and the code compile and executes.

What is missing in my code ?

Answer 1

g++-5 gives following errors:

invalid use of incomplete type struct std::hash<PNM>

invalid use of incomplete type struct std::hash<Format>

So, you should just specialize std::hash for PNM and Format.

namespace std {
template<>
struct hash<PNM>
{
   typedef PNR argument_type;
   typedef size_t result_type;

   result_type operator () (const argument_type& x) const
   {
      using type = typename std::underlying_type<argument_type>::type;
      return std::hash<type>()(static_cast<type>(x));
   }
};

template<>
struct hash<Format>
{
   typedef Format argument_type;
   typedef size_t result_type;       

   result_type operator () (const argument_type& x) const
   {
      using type = typename std::underlying_type<argument_type>::type;
      return std::hash<type>()(static_cast<type>(x));
   }
};

}

Or you can write template struct, that will works only for enums with use of SFINAE (not sure, that it's not UB by standard, since it's not specialization actually).

namespace std
{

template<typename E>
struct hash
{
   typedef E argument_type;
   typedef size_t result_type;
   using sfinae = typename std::enable_if<std::is_enum<E>::value>::type;
     
   result_type operator() (const E& e) const
   {
      using base_t = typename std::underlying_type<E>::type;
      return std::hash<base_t>()(static_cast<base_t>(e));
   }
};

}