[boost] Re: [review] hash functions

From: Daniel James (daniel_at_[hidden])
Date: 2005-03-16 13:20:22

Dave Harris wrote:
> In-Reply-To: <012301c5299e$0e2b2f90$6501a8c0_at_pdimov2>
> pdimov_at_[hidden] (Peter Dimov) wrote (abridged):
>>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.
> 
> 
> In this case the computation was correct, and indeed carefully chosen, but 
> produced needlessly many collisions.

We can't tell if it was correct, because you didn't specify the equality 
function for the type. Consider something along the lines of:

     template <class X, class Y>
     bool container_equal(X const& x, Y const& y);

     bool MyType::operator==(MyType const& x) {
         return container_equal(use_a ? a : b, x.use_a ? x.a : x.b);
     }

The types of a and b also matters, the static casts here are a little 
worrying. But the point is, if the types are right, then Peter's 
proposal is ideal in this situation.

And it wasn't actually correct (with the current interface). It called 
hash_value directly, which you shouldn't do in general. I'm going to 
change the documentation to stress this more.

Daniel