Re: [boost] Re: [review] hash functions

From: Peter Dimov (pdimov_at_[hidden])
Date: 2005-03-15 15:31:55

Dave Harris wrote:
> This was specifically an issue with optional or alternate types. For
> example, I had something similar to:
>
>    size_t MyType::hash_value() const {
>        return use_a ? hash_value(a) : hash_value(b);
>    }
>
> where a and b could be different types.

This is because you ignored the "use_a" member when computing the hash 
value. We can't prevent these mistakes in general; if you omit a member 
(that affects the equivalence relation) when computing the hash value, the 
computation is just wrong.

A hash_value that encodes the type will just hide the error from you if a 
and b have different types; this will bite you (or someone else) later 
if/when b is changed to have the same type as a.