Trouble using hash function.

Question

Hi all, I am implementing google's dense hash map in my C++ code. I want to use MurmurHash2 (http://murmurhash.googlepages.com/) as a hash function. But here's the problem. I have tried a lot but cant seem to get the hash function to work. The example shows the use of a default hash function (hash< const char * >).

dense_hash_map < const char * , int, hash < const char*>, eqstr> months;

I would like to replace hash< const char * > by

unsigned int MurmurHash2 ( const char * key, int len, unsigned int seed)

Can any one plz suggest what to do?

thanks!

Answer 1

Clearly you have a mismatch between the signature that the dense_hash_map requires from its hashing function, and the signature that MurmurHash2 provides. You'll have to perform your own "impedence matching" in a function or functor of your own that implements the required signature and internally uses the provided one. This, however, requires the ability to determine the len corresponding to a given const char *, and there's no obvious answer to that. Do you plan to store in your hash map only "null terminated arrays of characters", as in old-fashioned C "pseudo-strings", so that strlen will suffice? or, what else...?

Maybe using std::string (or some other unambiguous C++ type depending on exactly what you're trying to achieve!) instead of those dubious const char *s would be a better start!