Re: [Boost-users] Re: [MPL] map question

From: Aleksey Gurtovoy (agurtovoy_at_[hidden])
Date: 2004-12-25 16:39:55

David Abrahams writes:
> Aleksey Gurtovoy wrote:
> 
> > 
> >>From http://www.boost.org/libs/mpl/doc/refmanual/protect.html:
> > 
> >     'protect' is an identity wrapper for a Metafunction Class that 
> >     prevents its argument from being recognized as a bind expression.
> > 
> > Actually, the above definition is incomplete; should be:
> > 
> >     'protect' is an identity wrapper for a Metafunction Class that 
> >     prevents its argument from being recognized as a bind/placeholder 
> >     expression.
> > 
> > The main usage for 'protect' is to prevent parametrized metafunction
> > classes from being accidentally treated as a placeholder expression 
> > simply because their arguments happened to embed a placeholder. 
> 
> Hum.  Seems a bit counterintuitive that protect doesn't apply equally
> well to this case.

??? As I tried to explain in my previous reply, 'protect' already has a
well-defined semantics in lambda expressions, and it's quite different 
from what is needed to support "nested scopes". It would be nice to have 
a single magic primitive that would work the way you want it to work 
depending on the particular use case at hand, but I for one have no 
knowledge of how to implement one.

--
Aleksey Gurtovoy
MetaCommunications Engineering