Are there no specializations of std::hash for standard containers?

Question

I just found myself a little bit surprised being unable to simply use a

std::unordered_set<std::array<int, 16> > test;

because there does not seem to be a std::hash specialization for std::arrays. Why is that? Or did I simply not find it? If there is indeed none, can the following implementation attempt be simplified?

namespace std
{
    template<typename T, size_t N>
    struct hash<array<T, N> >
    {
        typedef array<T, N> argument_type;
        typedef size_t result_type;

        result_type operator()(const argument_type& a) const
        {
            hash<T> hasher;
            result_type h = 0;
            for (result_type i = 0; i < N; ++i)
            {
                h = h * 31 + hasher(a[i]);
            }
            return h;
        }
    };
}

I really feel this should somehow be part of the standard library.

Answer 1

I'm not sure why the standard library hasn't included this, but Boost has hashing for all sorts of things made up from hashable types. The key function for this is hash_combine, which you are welcome to copy from boost/functional/hash/hash.hpp.

Using hash_combine, Boost derives a range_hash (just combining the hashes of a each element of a range), as well as pair and tuple hashers. The range_hash in turn can be used to hash any iterable container.

Answer 2

Not an answer, but some useful information. The Feb draft of the C++11 standard specifies that std::hash is specialized for these types:

• error_code § 19.5.5
• bitset<N> § 20.5.3
• unique_ptr<T, D> § 20.7.2.36
• shared_ptr<T, D> § 20.7.2.36
• type_index § 20.13.4
• string § 21.6
• u16string § 21.6
• u32string § 21.6
• wstring § 21.6
• vector<bool, Allocator> § 23.3.8
• thread::id § 30.3.1.1

And all these types: § 20.8.12

template <> struct hash<bool>;
template <> struct hash<char>;
template <> struct hash<signed char>;
template <> struct hash<unsigned char>;
template <> struct hash<char16_t>;
template <> struct hash<char32_t>;
template <> struct hash<wchar_t>;
template <> struct hash<short>;
template <> struct hash<unsigned short>;
template <> struct hash<int>;
template <> struct hash<unsigned int>;
template <> struct hash<long>;
template <> struct hash<long long>;
template <> struct hash<unsigned long>;
template <> struct hash<unsigned long long>;
template <> struct hash<float>;
template <> struct hash<double>;
template <> struct hash<long double>;
template<class T> struct hash<T*>;