Re: [Boost-users] hash<pair<...

Subject: Re: [Boost-users] hash<pair<...
From: Anthony Foglia (AFoglia_at_[hidden])
Date: 2009-09-17 18:02:17

John Dlugosz wrote:
> I just moved a project that was using VC8, with TR1 supplied via
> BOOST, to VC9 with its own TR1 implementation.
> 
> I was surprised that a unordered_map failed to compile with a great
> deal of verboseness because it didn't like my key type.  I used a
> pair of two primitive types as the key.  Boost took that just fine.
> 
> But looking at the current Boost documentation for the hash object,
> (<http://www.boost.org/doc/libs/1_40_0/doc/html/boost_tr1/subject_list.html#boost_tr1.subject_list.hash>)
> I see that the hash specializations include primitive types, strings,
> but not tuples.
> 
> Is this something that changed between Boost 1_38 and 1_40, or an
> undocumented extension, or what?  Why would the standard leave out
> that, when everyone and his neighbor will just supply their own
> generic definition.  That is, is there a good reason for not doing
> it?

I think the issue is that TR1 does not (as of yet) require hash for 
pairs.  Boost does have a hash function that works on pairs, in 
Functional/Hash, and that's the default hash used by Boost's unordered 
map, but since they aren't part of TR1, they're not included in the TR1 
namespace.  The Functional/Hash documentation links to a working group 
document which mentions this among other issues with TR1 (as then proposed).

http://www.boost.org/doc/libs/1_40_0/doc/html/hash.html

The document is four years old though.  Someone more up on the C++0x 
development can tell you more, and what the current status is.

-- 
Anthony Foglia
Princeton Consultants
[phone] x233