RE: [boost] Re: mpl/loki

From: Drazen DOTLIC (Drazen.Dotlic_at_[hidden])
Date: 2003-07-11 13:58:07

> > Btw, VC7.1 does not seem to like identity in this (and 
> David's) scenario
> > - it complains that it does not have inner typedef to type, 
> even though
> > it does. 
> 
> Can you please post a small example which fails?

Well, your own :) from few hours ago, used something like this (btw, I
am using CVS snapshot as of, say, 2 weeks ago):

template<typename Result, TypeVector>
class Useful
{
public:
      typedef typename type_at_non_strict_c<TypeVector, 0>::type Param1;
      typedef typename type_at_non_strict_c<TypeVector, 1>::type Param2;
      typedef typename type_at_non_strict_c<TypeVector, 2>::type Param3;
}

Blatant copy from your mail plus small typo (size_t_c to size_t only)
corrected:

template <class S, class N>
    struct type_at_non_strict
      : mpl::apply_if<
            mpl::greater<mpl::size<S>,N>
          , mpl::at<S,N>
          , mpl::identity<mpl::void_>
        >
    {};

    template <class S, std::size_t N>
    struct type_at_non_strict_c
      : type_at_non_strict<S, mpl::size_t<N> >
    {};

If I do not replace identity with void_ only, I get the following (nice
VC71 elaborate error report):
c:\projects\tools\boost\boost\mpl\apply_if.hpp(39) : error C2039: 'type'
: is not a member of 'boost::mpl::identity<boost::mpl::void_>'
c:\projects\tools\boost\boost\mpl\identity.hpp(44) : see declaration of
'boost::mpl::identity<boost::mpl::void_>'
c:\projects\mylib\MetaProgramming.h(26) : see reference to class
template instantiation 'boost::mpl::apply_if<C,F1,F2>' being compiled
with
[
 
C=boost::mpl::greater<boost::mpl::size<boost::mpl::vector<bool>>,boost::
mpl::size_t<1>>,
    F1=boost::mpl::at<boost::mpl::vector<bool>,boost::mpl::size_t<1>>,
    F2=boost::mpl::identity<boost::mpl::void_>
]
c:\projects\mylib\MetaProgramming.h(34) : see reference to class
template instantiation 'type_at_non_strict<TypeContainer,N>' being
compiled
with
[
    TypeContainer=boost::mpl::vector<bool>,
    N=boost::mpl::size_t<1>
]
c:\projects\mylib\Useful.h(37) : see reference to class template
instantiation 'type_at_non_strict_c<TypeContainer,N>' being compiled
with
[
    TypeContainer=boost::mpl::vector<bool>,
    N=1
]
c:\projects\mylib\Used.h(45) : see reference to class template
instantiation 'Useful<Result,TypeVector>' being compiled
with
[
    Result=bool,
    TypeVector=boost::mpl::vector<bool>
]

I tried to replace company/project specific names with generic ones,
hope I did not ruin the example.

Drazen