How to 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 message telling me that standard C ++ does not provide a hash function for CString, while CString has an operator (LPCSTR).

How to implement a hash function for CString?

Answer 1

std :: unordered_map uses std :: hash <>, which does not use a statement (LPCSTR).

You need to override the 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 best performance, use std :: string instead of CString for the key.

Answer 2

MS STL std::string, , std::unordered_set 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)));
        }
    };
}

:

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)));
        }
    };
}