Re: [boost][MPL] Numeric metafunctions protocol/infrastructure

From: Cromwell Enage (sponage_at_[hidden])
Date: 2004-09-25 23:45:04

Methinks I found the problem, not in numeric_cast, but
in the numeric ops themselves.

The divides metafunction, for example, inherits from
the apply template nested inside the divides_impl
template.  The problem is that the primary
divides_impl template inherits from if_c, which
doesn't have a nested apply template, although its
argument types do.  The same type of problem occurs in
all other numeric ops, including the comparison ops.

Changing the primary *_impl template for each numeric
op to something like the following code should
vanquish the problem.  (It did for me when I edited
the appropriate files.)

  template <typename Tag1, typename Tag2>
  struct NUMERIC_OP_IMPL
  {
    template <typename N1, typename N2>
    struct apply
    {
      BOOST_STATIC_CONSTANT(
        bool,
        cond =
          BOOST_MPL_AUX_NESTED_VALUE_WKND(int, Tag1) >
          BOOST_MPL_AUX_NESTED_VALUE_WKND(int, Tag2)
      );
      typedef typename
                eval_if_c<
                  cond,
                  identity<N1>,
                  apply_wrap1<
                    BOOST_MPL_AUX_NUMERIC_CAST<
                      Tag1,Tag2
                    >,
                    N1
                  >
                >::type
              n_1;
      typedef typename
                eval_if_c<
                  cond,
                  apply_wrap1<
                    BOOST_MPL_AUX_NUMERIC_CAST<
                      Tag1,Tag2
                    >,
                    N2
                  >,
                  identity<N2>
                >::type
              n_2;
      typedef typename
                apply_wrap2<
                  NUMERIC_OP_IMPL<
                    typename n_1::tag,
                    typename n_2::tag
                  >,
                  n_1,
                  n_2
                >::type
              type;
    };
  };

Some preprocessed files may need similar editing so
that other compilers (which I don't have) will also
succeed.

I may be offline for a while, as I'm about to ride out
yet another hurricane.

                              HTH
                              Cromwell Enage

                
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