Re: [boost] hash_combine vs hash_value

Subject: Re: [boost] hash_combine vs hash_value
From: Topher Cooper (topher_at_[hidden])
Date: 2011-03-25 14:01:13

Your method implies a special status to the first element, which it 
should not have.  Of course, because ordering makes a difference the 
first element is special because it is the first, but the second element 
is special because it is the second, and so on.  Every element should be 
treated uniformly.

Code should always be written, to the extent possible, to reflect 
concept and intent to the greatest extent possible.  Is it really 
conceptually necessary that any combining algorithm should be just as 
good if the hash of the initial value can be treated as if it were the 
combination of a sequence of values?

(My own preference would have been for a combiner that was a struct 
which would be initialized, called on each value to be included and then 
finalized when the final hash value is extracted -- a more general 
interface which could be implemented to do precisely what this one 
does.  But I didn't speak up when this was being discussed, so I don't 
have anything to complain about).

Topher

On 3/25/2011 9:57 AM, Michael Goldshteyn wrote:
> After reading the explanation of how hash_combine should be used, I am 
> left wondering. Why is the recommendation for hash_combine different 
> from what one would intuitively expect?
>
> Namely, given:
>
> struct MyStruct
> {
>  int a;
>  int b;
>  int c;
> };
>
> The recommendation via examples and documentation is:
>
> size_t hash(const MyStruct &s)
> {
>  size_t hashValue=0;
>
>  // Combine hashes of all elements
>  hash_combine(hashValue,s.a);
>  hash_combine(hashValue,s.b);
>  hash_combine(hashValue,s.c);
>
>  return hashValue;
> }
>
> Rather than what I would think is the expected correct approach:
>
> size_t hash(const MyStruct &s)
> {
>  // For a composite hash, start by hashing the first element
>  size_t hashValue=hash_value(s.a);
>
>  // Then, combine with hashes of the remaining elements
>  hash_combine(hashValue,s.b);
>  hash_combine(hashValue,s.c);
>
>  return hashValue;
> }
>
> Thanks,
>
> Michael Goldshteyn