如何实现用于 std::unordered_map 的 CString 哈希函数?

2021-12-27 00:00:00 hash c++ mfc unordered-map

我要声明:

std::unordered_map<CString, CString> m_mapMyMap;

但是当我构建时，我收到一个错误，告诉我标准 C++ 没有为 CString 提供哈希函数，而 CString 有 (LPCSTR) 运算符.

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.

如何正确实现 CString 的哈希函数?

How do I properly implement a hash function for CString?

推荐答案

基于 std::string 的 MS STL 实现，我创建了以下可用于 std 的方法::unordered_set 和 std::unordered_map:

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)));
        }
    };
}

或者更通用:

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)));
        }
    };
}

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