Re: [boost] MPL:Possible Placeholder problem with transform & if_ (?)

From: Aleksey Gurtovoy (agurtovoy_at_[hidden])
Date: 2005-07-28 01:01:02

Brian Braatz writes:
> Either something strange is happening with arg<> and transform or I did
> not write this code correctly. ( :) )

The latter :).

[...]

> typedef mpl::transform
> <
> 	vec_t,
> 	mpl::if_
> 	<
> 		boost::is_same<mpl::_1,	A >,
> 		YES< mpl::_1  >::type,
                       ^^^^^^
> 		NO< mpl::_1  >::type
                      ^^^^^^
> 	>::type 
    ^^^^^^^
>>::type result;

By requesting '::type' on a metafunction you are immediately
instantiating it with the placeholder argument(s), which is not what
you want. Try

    typedef mpl::transform<
          vec_t
        , mpl::if_<
              , boost::is_same<mpl::_1,	A>
              , YES< mpl::_1 >
              , NO< mpl::_1 >
              >
        >::type result;

instead.

HTH,
-- 
Aleksey Gurtovoy
MetaCommunications Engineering