[boost] Re: [review] hash functions

From: Tobias Schwinger (tschwinger_at_[hidden])
Date: 2005-03-11 11:59:00

Peter Dimov wrote:
> Tobias Schwinger wrote:
> 
>> Example: let's say we have a disk-stored hash table and an in-memory
>> hash table for caching. The same objects are stored but it is 
>> desirable to have two different hashing functions. Unfortunately 
>> boost::hash can
>> only be used for at most one of them.
>>
>> If I wanted to implement double hashing I'ld have to duplicate the
>> whole facility (which may include writing a lot of "hash_value2"
>> functions for a grown project) because there can be only one 
>> 'hash_value' per type
>> which can only use one algorithm for combining the hash values, as the
>> current design does not provide a way to get parametrization in there.
>>
>> Looking at the source underlines this impression: hash_combine seems
>> like a forgotten hard wired strategy in something that actually wants
>> to be a facade.
> 
> The next generalization level isn't really about hash functions anymore. 

I agree. That is, given that the "result" (or should I better say 
"context" here ?) is not restricted to std::size_t.

If it's generic enough to factor it out, it's even better, isn't it ?

> You are asking for a generic visitation interface where you give a 
> visitor to a container (or a compound type with nonambiguous state*) and 
> the container applies the visitor to each of its elements. This facility 
> is very useful (not only for hashing, but for serialization, stream 
> output, and others) but I don't believe that it belongs in a utility 
> hash function library.

One could argue that it belongs into Boost to implement things like 
hashing on top of it before they are written...

...but I won't ;-).

Besides we should have hashing in ASAP, at least for political (Java) 
and housekeeping (so other libraries have a default for their template 
parameter lists) reasons.

Regards,

Tobias