问题
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?
回答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).
来源：https://stackoverflow.com/questions/16471051/boostuuidsuuid-as-a-key-in-stdunordered-map