boost::uuids::uuid as a key in std::unordered_map?

Question

I'm using clang (CXX='clang++ -std=c++11 -stdlib=libc++') on Mac OS X, with boost 1.53.0.

I want to use uuid as keys in unordered_map, but getting the following errors:

/usr/bin/../lib/c++/v1/type_traits:748:38: error: implicit instantiation of undefined template
      'std::__1::hash<boost::uuids::uuid>'
    : public integral_constant<bool, __is_empty(_Tp)> {};
                                 ^
/usr/bin/../lib/c++/v1/unordered_map:327:54: note: in instantiation of template class
      'std::__1::is_empty<std::__1::hash<boost::uuids::uuid> >' requested here
template <class _Key, class _Tp, class _Hash, bool = is_empty<_Hash>::value

...

/usr/bin/../lib/c++/v1/unordered_map:327:71: error: no member named 'value' in
      'std::__1::is_empty<std::__1::hash<boost::uuids::uuid> >'
template <class _Key, class _Tp, class _Hash, bool = is_empty<_Hash>::value
                                                     ~~~~~~~~~~~~~~~~~^

...

What is it - a bug in Boost, which makes it incompatible with my C++ lib? Or I am doing something wrong? Any workarounds?

Ge

Answer 1

Why bug in boost? You should specialize std::hash template for boost::uuid.

#include <boost/functional/hash.hpp>

namespace std
{

template<>
struct hash<boost::uuids::uuid>
{
    size_t operator () (const boost::uuids::uuid& uid)
    {
        return boost::hash<boost::uuids::uuid>()(uid);
    }
};

}

or, simply create unordered_map with boost::hash par

std::unordered_map<boost::uuids::uuid, T, boost::hash<boost::uuids::uuid>>

or provide hash functor that satisfies requirements of std::hash (thanks to Praetorian).

Distill

+1 Instead of providing an explicit specialization of std::hash you could also create a type (say uuid_hasher) and implement uuid_hasher::operator()(uuid const&). That type would then be the third template argument for the unordered_map – Acquiescence

I think operator() of hash's specialization for your type needs to marked const. Leaving it non-const failed to compile. – Batik