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From: David Abrahams (dave_at_[hidden])
Date: 2003-03-30 15:13:47

"Jaap Suter" <J.Suter_at_[hidden]> writes:

> Hi,
> I apologize, but once again I'm unable to get a lambda expression working
> with the MPL.
> The code works fine with the Intel and GCC compiler. On MSVC I get the
> following error:
> error C2039: 'lhs_index' : is not a member of 'boost::mpl::arg<N>'
> The error occurs on the line with the boost static constant. It does not
> happen if the template is not instantiated. I have tried using the
> AUX_VOID_SPEC macro, but that didn't really help. I even tried closing my
> own namespace, and doing an AUX_VOID_SPEC inside the MPL namespace, as done
> in
> I also tried deriving predicate from mpl::equal_to that simply compares the
> result to true_c, hoping that mpl::equal_to would give me the lambda support
> for free. That didn't work either.
> Any advice would be greatly appreciated.

I think you could rewrite your code to be clearer (see below) but
that's not really the problem we're having. The problem is that
lambda support requires has_rebind, which instantiates the predicate
on its argument. When the argument is a placeholder (arg<N>), it
doesn't have ::lhs_index or ::rhs_index members, and the predicate
instantiation is an error.

I'm looking at it to see if I can find a way to get has_rebind<T> to
work without instantiating T.


template <class T> struct lhs_index : T::lhs_index
   BOOST_MPL_AUX_LAMBDA_SUPPORT(1, lhs_index, (T));

template <class T> struct rhs_index : T::rhs_index
   BOOST_MPL_AUX_LAMBDA_SUPPORT(1, rhs_index, (T));

template < size_t LhsIndex, size_t RhsIndex, int Sign >
struct signature
    typedef mpl::size_t< LhsIndex > lhs_index;
    typedef mpl::size_t< RhsIndex > rhs_index;
    typedef mpl::int_< Sign > type;

template < class Signature, class LhsIndex, class RhsIndex >
struct predicate
    : mpl::and_<
          mpl::equal_to<lhs_index<Signature>, LhsIndex>
        , mpl::equal_to<rhs_index<Signature>, RhsIndex>
    BOOST_MPL_AUX_LAMBDA_SUPPORT( 3, predicate, (Signature, LhsIndex,RhsIndex) )

// Test code:

typedef mpl::vector< signature< 1, 1, 1 >,
                                signature< 4, 4, 0 >,
                                signature< 5, 5, 0 >,
                                signature< 4, 5, 1 > > signatures;

typedef mpl::find_if< signatures, predicate< mpl::_1, mpl::size_t< 4 >,
mpl::size_t< 5 > > >::type iter_0;

Dave Abrahams
Boost Consulting

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