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.