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

From: Peter Dimov (pdimov_at_[hidden])
Date: 2005-03-15 12:37:10

Dave Harris wrote:
> In-Reply-To: <d1409a$kc1$1_at_[hidden]>
> daniel_at_[hidden] (Daniel James) wrote (abridged):
>> Think about how such a class would be stored in the proposed
>> unordered associative containers. If you use unordered_set<base*>
>> then the hash function will be called for the pointer, not the class.
>
> I imagine you would use a hasher which dereferenced the pointer.
>
>    template<typename T>
>    struct hash_ptr {
>        size_t operator()(const T *p) const {
>            return hash_value( *p );
>        }
>    };
>
> And perhaps this should be included in boost too? Eg this:
>
>   unordered_set< shared_ptr<MyType>, hash_ptr<MyType> >
>
> looks reasonable to me.

You also need equal_ptr<MyType>. Also, it is easy to break the above 
unordered_set my modifying some of the MyTypes.

> So it is tempting not to bother, and instead use:
>
>    size_t Derived::hash_value() const {
>        // return combine_hash( Base::hash_value(), m_value );
>        size_t hash = Base::hash_value();
>        combine_hash( hash, m_value );
>        return hash;
>    }
>
> which is correct, but will produce more collisions than if we'd used
> the typeid.

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;
}