Re: [Boost-users] [mpl] Implementing includes_if

From: Tobias Schwinger (tschwinger_at_[hidden])
Date: 2007-03-20 12:34:57

David Abrahams wrote:
> on Sat Mar 17 2007, Tobias Schwinger <tschwinger-AT-isonews2.com> wrote:
> 
>> The problem comes down to that MPL can't know where one placeholder 
>> expression starts and where another one ends (denoted by uppercase 
>> identifiers, above):
> 
> There is an inherent limitation in that regard.  These kinds of
> problems come up repeatedly, so Aleksey and I agree the library should
> provide a solution.  We had a long discussion on the boost-devel list
> where we talked about the same issues in terms of runtime lambda
> expressions:
> 
>   http://lists.boost.org/Archives/boost/2005/01/78409.php

Oh, thanks!

Incidentally, I provided something very similar to 'outer' (discussed in 
that thread) in the follow-up, called 'defer' (inspired by Chaos).

It's so easy, that it's almost provocative:

     // returns an unspecified placeholder expression whose evaluation
     // results in the original argument, no placeholder substitution
     // applied

     template<typename PlaceholderExpr>
     struct defer
     {
         template<typename IgnoredPlaceholder> struct get
         {
             typedef PlaceholderExpr type;
         };

         typedef typename defer<PlaceholderExpr>::template get<_> type;
     };

     BOOST_MPL_ASSERT((
         is_same<mpl::apply< defer<_1>::type ,int>::type, _1>
     ));

Of course it still needs "typename...type-clutter" in template context, 
otherwise the lambda facility would have to be aware of its existence.

     outer<_1> == typename defer<_1>::type

> 
> As I said, this thread is long (it contains a whole thread attached to
> one message in the outer thread -- how appropriate!)  but it's worth a
> read.
> 
> Incidentally, protect<> doesn't help as it does something different.

Yes, namely nested binds - I figured that out, by now :-).

Regards,
Tobias