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

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

In-Reply-To: <d17nlh$8o3$1_at_[hidden]>
daniel_at_[hidden] (Daniel James) wrote (abridged):
> I'm glad that I'm not the only person who's worried about alignment. 
> But this is a problems for a cross-platform library. Either I'll have 
> to introduce a load of macros to tweak it on various platforms (I'll 
> need help there) or just accept it.

We can probably find something which gives reasonable results across a 
wide range of platforms. Shuffling the bits in a hash_combine-like manner 
would help.

Eg something like:

    template <typename T>
    size_t hash_value( const T *p ) {
        size_t value = reinterpret_cast<size_t>(p);
        return value + value / sizeof(T);
    }

If there's an easy way to turn sizeof(T) into a power of 2, use it. In 
many cases it will already be a power of 2, and I suspect the performance 
difference isn't crucial anyway.

Alternatively we could just use:

        return value + (value >> 3);

which ought to be reasonable for most small objects. It may also be 
reasonable for large objects that are heap-allocated, depending on how the 
heap is implemented. Anyway, I think it is better to do something like 
this than to just return value.

-- Dave Harris, Nottingham, UK