Re: [boost] [unordered] request for ways to get more space efficient allocations

From: Thorsten Ottosen (thorsten.ottosen_at_[hidden])
Date: 2008-04-22 02:21:11

Daniel James skrev:
> On 21/04/2008, Thorsten Ottosen <thorsten.ottosen_at_[hidden]> wrote:
>> Daniel James skrev:
>>  > An alternative that has some appeal is letting the user specify a
>>  > policy class to control the bucket count. Although I'm not sure how
>>  > that would play with the standard.
>>
>> Poor, probably.
>>
>>  I guess for cases where I know the intergers fall in the range, say,
>>  [0,64], I can just use vector<optional<unsigned>>.
> 
> Thinking about it some more, I could create a trait class for the hash function.
> 
> template <class T> struct is_good_hash : public boost::mpl::false_ {};
> template <> struct is_good_hash<groovy_hash> : public boost::mpl::true_ {};
> 
> And use power of 2 tables when the trait is true.

As for the integers case, I think the vector approach is actually more 
appropriate. It's harder to use with pointers though. I'd rather just 
see a way to tell the container "have this initial bucket capacity no 
matter what", then I can find a good size that avoids rehashing and 
comes close to the number of needed buckets (for some application 
specific definition of "close" ).

-Thorsten