[boost] Re: [review] hash functions

From: Daniel James (daniel_at_[hidden])
Date: 2005-03-15 17:31:13

Dave Harris wrote:

> When I wrote it seemed there were still unresolved issues. In fact, I 
> don't believe anyone has yet posted an acceptable hash_value for pointers. 
> Presumably we want to shift out bits which are always 0 for alignment 
> reasons.

I'm glad that I'm not the only person who's worried about alignment. But 
this is a problems for a cross-platform library. Either I'll have to 
introduce a load of macros to tweak it on various platforms (I'll need 
help there) or just accept it.

It's going to be impossible to write a perfect hash function for 
pointers. I plan to start with the proposed reinterpret_cast version, 
get it working on platforms where we know it's broken (if there are any) 
and only then worry about tweaking it for specific platforms.

>>>For example, the proposal has no specialisation for chars at all. I 
>>>can't tell if that is an oversight, or whether it is a design
>>>aim [...]
>>
>>I don't think it's either. Peter?
> 
> 
> Peter has now said it is a design aim. That even you weren't sure 
> illustrates my point :-)

Oh, this is definitely Peter's design. I obviously like it, since I 
decided to follow it.

>>Perhaps it would be better spelt as:
>>
>>template <class It>
>>void hash_combine(std::size_t& seed, It begin, It end);
> 
> 
> Yes, I considered that. I wasn't sure whether overloading the name would 
> help or hinder - hash_range and hash_combine are not substitutable for 
> each other.

Oh yes, I didn't think that one through. It would be wrong.

Daniel