On 01/13/12 19:44, Kitten, Nicholas wrote:
I've recently been doing a lot mpl::folding (in boost 1.47), and I was disappointed when I realized that a nested fold wouldn't work with a lambda expression passed to the inner call, as in this example which extracts elements of sequences:
using namespace mpl; using namespace mpl::placeholders;
typedef vector< vector<char>, vector<int> > vec_of_vecs; typedef vector< char, int > expected_result;
typedef fold< vec_of_vecs, vector<>, fold< _2, _1, *push_back< _1, _2 >* // wrong - refers to outer arguments >
::type result;
BOOST_MPL_ASSERT(( equal<result, expected_result> ));
After looking around for a solution I came across an old thread<http://lists.boost.org/boost-users/2004/12/9269.php> on the same subject, where it was suggested using mpl::protect might work. Indeed, the following modification does what I want, without any changes to the library:
typedef fold< vec_of_vecs, vector<>, fold< _2, _1, *protect< lambda< push_back< _1, _2 > >::type
* // correct > ::type result;
Hi Nick. I copied your code (removing the * delimiters, which I guess you use just to emphasize the important parts), but couldn't get either to compile. The code I used is attached. Use the #define USE_LAM_PRO to select which version to compile. Could you modify the attached so that it does compile? I'm using gcc-4.7. The error messages I'm getting with gcc-4.7 include: /home/evansl/prog_dev/boost-svn/ro/trunk/boost/mpl/fold.hpp:39:18: required from 'struct boost::mpl::fold<boost::mpl::vector<boost::mpl::vector<char>, boost::mpl::vector<int> >, boost::mpl::vector<mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na>, boost::mpl::fold<mpl_::arg<2>, mpl_::arg<1>, boost::mpl::protect<boost::mpl::lambda<boost::mpl::push_back<mpl_::arg<1>, mpl_::arg<2> > > > > >' nested_placeholders.cpp:45:6: required from here /home/evansl/prog_dev/boost-svn/ro/trunk/boost/mpl/apply_wrap.hpp:141:1: error: no class template named 'apply' in 'struct boost::mpl::protect<boost::mpl::lambda<boost::mpl::push_back<mpl_::arg<1>, mpl_::arg<2> > > >' In file included from /home/evansl/prog_dev/boost-svn/ro/trunk/boost/preprocessor/iteration/detail/iter/forward1.hpp:52:0, from /home/evansl/prog_dev/boost-svn/ro/trunk/boost/mpl/aux_/fold_impl_body.hpp:67, from /home/evansl/prog_dev/boost-svn/ro/trunk/boost/mpl/aux_/fold_impl.hpp:40, from /home/evansl/prog_dev/boost-svn/ro/trunk/boost/mpl/fold.hpp:20, from nested_placeholders.cpp:11: /home/evansl/prog_dev/boost-svn/ro/trunk/boost/mpl/aux_/fold_impl_body.hpp: In instantiation of 'struct boost::mpl::aux::fold_impl<1, boost::mpl::v_iter<boost::mpl::vector<char>, 0l>, boost::mpl::v_iter<boost::mpl::vector<char>, 1l>, boost::mpl::vector<mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na>, boost::mpl::protect<boost::mpl::lambda<boost::mpl::push_back<mpl_::arg<1>, mpl_::arg<2> > > > >': /home/evansl/prog_dev/boost-svn/ro/trunk/boost/mpl/aux_/has_type.hpp:20:1: recursively required from 'const bool boost::mpl::aux::has_type<boost::mpl::fold<boost::mpl::vector<char>, boost::mpl::vector<mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na>, boost::mpl::protect<boost::mpl::lambda<boost::mpl::push_back<mpl_::arg<1>, mpl_::arg<2> > > > >, mpl_::bool_<true> >::value' -regards, Larry