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

From: Dave Harris (brangdon_at_[hidden])
Date: 2005-03-16 12:07:14

In-Reply-To: <019d01c52985$a04f54f0$6801a8c0_at_pdimov>
pdimov_at_[hidden] (Peter Dimov) wrote (abridged):
> You also need equal_ptr<MyType>. 

Indeed.

> Also, it is easy to break the above unordered_set my modifying some
> of the MyTypes.

Yes, that's something to be aware of. I use the approach on types which 
are immutable, so it doesn't arise.

> size_t Derived::hash_value() const
> {
>     size_t seed = my_unique_value;
> 
>     hash_combine( seed, Base::hash_value() );
>     hash_combine( seed, m_value );
> 
>     return seed;
> }

That leaves the problem of assigning a different my_unique_value to each 
subclass, bearing in mind that different subclasses may be written by 
different people, teams or organisations. Without some kind of system 
people may just copy and paste code from one class to another and not 
change the value.

-- Dave Harris, Nottingham, UK