[boost] Re: tuple patch (constructing from a pair)

From: Joel de Guzman (joel_at_[hidden])
Date: 2005-02-14 21:00:35

Howard Hinnant wrote:
> On Feb 14, 2005, at 2:52 PM, Jonathan Turkanis wrote:
> 
>> Howard Hinnant wrote:
>>
>> I'd definitely like some standard way for a type to declare itself  
>> tuple-like.
>> Why didn't it make it into TR1?
>>
>> I use a version of it here, to allow arbitrary tuple-like types to take
>> advantage of a form of tuple i/o:
>>
>> http://www.kangaroologic.com/format_lite/libs/format_lite/doc/? 
>> path=4.2.5
> 
> 
> <nod>  I use is_tuple_like too.  It was in the original proposal.  It  
> got dumped somewhere along the way.  I'm not sure why.  When tuple was  
> first voted into TR1, enable_if was brand new and pretty exotic.  Parts  
> of tuple that benefited from (and depended on) enable_if were dumped.   
> It may be that is_tuple_like got thrown out along with that dump.   
> Imho, tuple did not benefit from that exercise.  We get another shot at  
> it for C++0X though.  So existing practice that is a little better than  
> TR1 can still help.

FYI, Fusion already has the infrastructure to do that. For instance,
MPL sequences are tuple sequences and vice versa. It's a matter of
defining some helper classes that implement something akin to MPL's
TAG dispatch mechanism. Hence, a user can (in theory) make any form
of tuple-like structure play along well.

Alas, the TR1 interface is not sufficiently rich to handle all
forms of tuple like structures. In particular, get<N> assumes
O(1) access, which is obviously not always true. Fusion, like
MPL and STL uses iterators as the underlying and unifying concept.

Cheers,
-- 
Joel de Guzman
http://www.boost-consulting.com
http://spirit.sf.net