How do I implement a CString hash function for use with std::unordered_map?

Question

I want to declare :

std::unordered_map<CString, CString> m_mapMyMap;

But when I build I got an error telling me that the standard C++ doesn't provide a hash function for CString, while CString have the (LPCSTR) operator.

How do I properly implement a hash function for CString?

Answer 1

Based on the MS STL implementation for std::string I created the following methods which can be used for std::unordered_set and std::unordered_map :

namespace std {
    template <>
    struct hash<CString>
    {   // hash functor for CString
        size_t operator()(const CString& _Keyval) const
        {   // hash _Keyval to size_t value by pseudorandomizing transform
            return (_Hash_seq((const unsigned char*)(LPCWSTR)_Keyval, _Keyval.GetLength() * sizeof(wchar_t)));
        }
    };

    template <>
    struct hash<CStringA>
    {   // hash functor for CStringA
        size_t operator()(const CStringA& _Keyval) const
        {   // hash _Keyval to size_t value by pseudorandomizing transform
            return (_Hash_seq((const unsigned char*)(LPCSTR)_Keyval, _Keyval.GetLength() * sizeof(char)));
        }
    };
}

Or even more generic:

namespace std {
    template<typename BaseType, class StringTraits>
    struct hash<CStringT<BaseType, StringTraits>> : public unary_function<CStringT<BaseType, StringTraits>, size_t>
    {   // hash functor for CStringT<BaseType, StringTraits>
        typedef CStringT<BaseType, StringTraits> _Kty;

        size_t operator()(const _Kty& _Keyval) const
        {   // hash _Keyval to size_t value by pseudorandomizing transform
            return (_Hash_seq((const unsigned char*)(StringTraits::PCXSTR)_Keyval,
                _Keyval.GetLength() * sizeof(BaseType)));
        }
    };
}

Answer 2

std::unordered_map use std::hash<> that does not use (LPCSTR) operator.

You need to redefine hash function:

template<class T> class MyHash;

template<>
class MyHash<CString> {
public:
    size_t operator()(const CString &s) const
    {
        return std::hash<std::string>()( (LPCSTR)s );
    }
};

std::unordered_map<CString,CString,MyHash> m_mapMyMap;

But for better performance use std::string instead CString for key.

Answer 3

After trying out MrTux's suggestion , I have to say that this doesn't work anymore. std::_HashSeq was removed and std::unary_function was also removed in C++17.

I ended up with another solution that incorporates Microsoft's advice for hash implementations to use std::basic_string_view 's hasher:

namespace std
{
template<typename BaseType, class StringTraits>
struct hash<ATL::CStringT<BaseType, StringTraits>>
{
    size_t operator()(const ATL::CStringT<BaseType, StringTraits>& key) const noexcept
    {
        return hash<basic_string_view<BaseType>>()(
            basic_string_view<BaseType>(
                key.GetString(),
                key.GetLength()));
    }
};
}