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

From: Aleksey Gurtovoy (agurtovoy_at_[hidden])
Date: 2005-01-04 04:17:25

David Abrahams writes:
> Aleksey Gurtovoy wrote:
> > David Abrahams writes:
> >> Aleksey Gurtovoy wrote:
> >>
> >> >     '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".
>
> I understand that you said that, but to me what protect does (prevents
> the recognition and premature substition of placeholders)

It doesn't prevent a "premature substition" of placeholders. It prevents it
once and for all.

> looks very
> similar to what the OP wants.  Also, maybe it's just a failure of my
> imagination, but I have a hard time seeing what the OP wants as "nested
> scopes."

Reciting the rewrite of the OP code from my original reply:

    template< typename Map, typename Entry >
    struct copy_entires // scope #2
        : copy<
              typename Entry::second::entries
            , inserter<Map, insert<_1,_2> >
            >
    {
    };

    typedef fold< // scope #1
          map_types
        , map_types
        , copy_entires<_1,_2>
        >::type result;

--
Aleksey Gurtovoy
MetaCommunications Engineering