Re: [boost] [MPL] Bug with default lambda expressions?

22 May 2013

      On 05/21/13 18:04, Larry Evans wrote:
...
On 05/21/13 15:43, Louis Dionne wrote:
...
Hi,
Some higher order algorithms in the MPL have a default for the lambda
expression they accept. A good example is `boost::mpl::equal`:
template  >
    struct equal;
This works fine most of the time, but I was recently bitten by the following:
template 
    struct find_vector
        : find_if<
            VectorOfVectors, equal
        >
    { };
typedef find_vector<
        vector<
            vector,
            vector
        >,
        vector
    >::type ThisWillBreak;
What happens here is that the `equal` expression inside
`find_vector` really is `equal >` because
of the default value for the predicate to `equal`. When the lambda is
evaluated, the placholders inside the inner `is_same<_1, _2>` expression
are replaced too, which yields unexpected results.
Using
template  >::type>
    struct equal;
or equivalently
template  >
    struct equal;
fixes the issue. Also note that all of the unit tests of the MPL still pass
with these changes. Is the current behavior intended, or should I submit
a patch?
Regards,
Louis Dionne
This sounds like a problem similar to the one here:
http://article.gmane.org/gmane.comp.lib.boost.devel/227344
Maybe reading that thread would give you some ideas for
a solution.
OOPS.  I should have read further and noted your solution using lambda.
Sorry for noise.

However, with the attached and your changes and gcc4.8 I'm getting
errors:

bug_with_default_lambda.cpp:18:12:   required from 'struct
find_vector,
boost::mpl::vector >, boost::mpl::vector >'
bug_with_default_lambda.cpp:30:6:   required from here
/home/evansl/prog_dev/boost-svn/ro/boost_1_53_0/boost/mpl/aux_/preprocessed/gcc/apply_wrap.hpp:36:8:
error: no class template named 'apply' in 'struct
boost::mpl::equal, mpl_::arg<1>,
boost::mpl::protect, mpl_::arg<-1>, mpl_::arg<-1> >, 0> >'
 struct apply_wrap1
        ^
In file included from
/home/evansl/prog_dev/boost-svn/ro/boost_1_53_0/boost/mpl/not.hpp:19:0,
                 from
/home/evansl/prog_dev/boost-svn/ro/boost_1_53_0/boost/mpl/assert.hpp:17,
                 from
/home/evansl/prog_dev/boost-svn/ro/boost_1_53_0/boost/mpl/aux_/na_assert.hpp:23,
                 from
/home/evansl/prog_dev/boost-svn/ro/boost_1_53_0/boost/mpl/arg.hpp:25,
                 from
/home/evansl/prog_dev/boost-svn/ro/boost_1_53_0/boost/mpl/placeholders.hpp:24,
                 from
/home/evansl/prog_dev/boost-svn/ro/boost_1_53_0/boost/mpl/apply.hpp:24,
                 from
/home/evansl/prog_dev/boost-svn/ro/boost_1_53_0/boost/mpl/aux_/iter_fold_if_impl.hpp:22,
                 from
/home/evansl/prog_dev/boost-svn/ro/boost_1_53_0/boost/mpl/equal.hpp:17,
                 from bug_with_default_lambda.cpp:10:
.
.
.

The mpl/equal.hpp contains your suggested modification:

template<
      typename BOOST_MPL_AUX_NA_PARAM(Sequence1)
    , typename BOOST_MPL_AUX_NA_PARAM(Sequence2)
  #ifdef LJE_USE_LAMBDA_PREDICATE_IS_SAME
    , typename Predicate = typename lambda >::type
  #else
    , typename Predicate = is_same<_,_>
  #endif
    >
struct equal
    : aux::msvc_eti_base<
          typename aux::equal_impl::type
        >::type
{
    BOOST_MPL_AUX_LAMBDA_SUPPORT(2,equal,(Sequence1,Sequence2))
};

Could you supply a complete test case so I can see what I might
be doing wrong to reproduce your solution?

TIA.

-regards,
Larry