error for hash function of pair of ints
c++, c++11, hash, stl, unordered-map
Solution
Unfortunately, this program has undefined behavior. C++11 §17.6.4.2.1:
A program may add a template specialization for any standard library template to namespace std only if the declaration depends on a user-defined type and the specialization meets the standard library requirements for the original template and is not explicitly prohibited.
`hash<pair<int,int>>` depends on primitive and standard library types only. This is easily worked around by defining your hash class outside of namespace `std`, and using that hash explicitly in your map declaration:
struct pairhash {
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);
}
};
class abc {
std::unordered_map<std::pair<int,int>, int, pairhash> rules;
};
EDIT: I've used xor to combine the hashes of the pair members here because I'm lazy, but for serious use xor is a fairly crappy hash combining function.
Problem
I have the following class with an `unordered_map` member, and a hash function defined for `pair<int,int>` ``` class abc {public : unordered_map < pair<int,int> , int > rules ; unsigned nodes; unsigned packet ; }; namespace std { template <> class hash < std::pair< int,int> >{ public : size_t operator()(const pair< int, int> &x ) const { size_t h = std::hash<int>()(x.first) ^ std::hash<int>()(x.second); return h ; } }; } ``` But I am getting the following errors : ``` error: invalid use of incomplete type ‘struct std::hash<std::pair<int, int> > error: declaration of ‘struct std::hash<std::pair<int, int> > error: type ‘std::__detail::_Hashtable_ebo_helper<1, std::hash<std::pair<int, int> >, true>’ is not a direct base of ‘std::__detail::_Hash_code_base<std::pair<int, int>, std::pair<const std::pair<int, int>, int>, std::__detail::_Select1st, std::hash<std::pair<int, int> >, std::__detail::_Mod_range_hashing, std::__detail::_Default_ranged_hash, true>’ ```