Creating the key value for entries in a Map class.

[Lucian Valois]

I am in the process of writing my own map class. Currently the map accepts a key value in the form of a null-terminated string. This key is then converted in to a number using this hash function that I found online:

const int Hash(const wchar_t *pstrString) const
{
     long lHashValue{0};
     wchar_t cKey{0};
     int iIndex{0};
     long lg{0};

          cKey = pstrString[iIndex];

          while (cKey != 0) {
               lHashValue = (lHashValue << 4) + cKey;

               lg = lHashValue & 0xF0000000L;

               if (lg != 0) {
                    lHashValue ^= lg >> 24;
               }

               lHashValue &= ~lg;

               cKey = pstrString[++iIndex];
               }

     return static_cast<int>(lHashValue);
}

I then use the % operator against the key and the dimension of a fixed-size array to determine the index where the the key/data pair is stored. All in all this technique seems to works well and I have a functional map class based upon it.

However, when I examine the STL/C++ standard library I find the map object implemented there will actually accept keys of any type. I find this very puzzling and cannot work out how you would do this.

I can imagine a system where you can supply an integer of any type to be hashed in to a key. I would do it by using a templated union where a fixed array of 8 unsigned char elements share storage with the key material:

template<typename typeKey>
union uKEYMATERIAL{
     typeKey TNumericKey;
     unsigned char aucKey[8]
};

I would then feed the aucKey member into almost the same hash function as above, but specify it goes through the while loop only eight times--the maximum number of bytes used by c++ to store variable data (I think!).

However, I still don't see how you could accept a key of any variable type... Perhaps you have to determine if the key is an array, pointer or integer using runtime-type-information?

[Thomas1965]

Why don't you have a look at the implementatiion of std::hash or even better just use std::hash ?
http://www.cplusplus.com/reference/functional/hash/

[Lucian Valois]

Why don't you have a look at the implementatiion of std::hash or even better just use std::hash ?

Many thanks Thomas1965--I did not realize that the source code was available, I will check it out! Thinking about it a bit more I've started to suspect they rely on the various containers of the STL all supporting an iterator and hash the key through that common interface.

In regards using the standard template library--I guess if I were a professional who got paid for programming then the time-saving would be worth it. I'm a hobbyist though and I write programmes for fun. Plus there comes a point where using these pre-made pieces of code starts to be not that far removed from learning a language in to itself! If you write the programme then, even if you do reuse it at least you know how it (should!) work!