[Boost-users] [MPL] Permutations of a List of Types

Subject: [Boost-users] [MPL] Permutations of a List of Types
From: Edward Terchunian (edward.terchunian_at_[hidden])
Date: 2010-12-22 16:18:13

I am trying to create a list containing the permutations of a given type list.

The below code seems to function, though without the intended result,
when I use a specified list instead of generating a new list by
removing from the actual input. This is demonstrated by the difference
between permutation_helper and broken_helper below.

Am I using remove incorrectly, or am I running into a different problem?

#include <boost/mpl/list.hpp>
#include <boost/mpl/transform.hpp>
#include <boost/mpl/fold.hpp>
#include <boost/mpl/push_front.hpp>
#include <boost/mpl/joint_view.hpp>
#include <boost/mpl/remove.hpp>
#include <boost/mpl/assert.hpp>
#include <boost/mpl/equal.hpp>

namespace mpl = boost::mpl;

struct test_type1 {};
struct test_type2 {};
struct test_type3 {};

template< typename T >
struct permutations;

template <typename value>
struct permutations<mpl::list1< value > >: mpl::list1<mpl::list1< value > > {};

template< typename value, typename T>
struct permutation_helper:
 mpl::transform< typename permutations<
     mpl::list1<test_type3> >::type,
   mpl::push_front< mpl::_1, value> > { };

template< typename value, typename T>
struct broken_helper:
 mpl::transform< typename permutations<
     mpl::remove<T, value> >::type,
   mpl::push_front< mpl::_1, value> > { };

template< typename T >
struct permutations:
 mpl::fold< T,
     mpl::list0<>,
     mpl::joint_view< mpl::_1,
        broken_helper<mpl::_2, T > > > { };

typedef mpl::list2<test_type1, test_type2> typelist;
typedef permutations<typelist>::type perms;

int main() {
 BOOST_MPL_ASSERT(( mpl::equal< perms, typelist > ));
 return 0;
}

I used the assert to determine what is being returned from the
function, typelist is not the expected result. This is the message the
assert returns for broken_helper:

testcase.cpp: In function ‘int main()’:
testcase.cpp:45: error: no matching function for call to
‘assertion_failed(mpl_::failed************
boost::mpl::equal<boost::mpl::joint_view<boost::mpl::joint_view<boost::mpl::list0<mpl_::na>,
boost::mpl::l_end>, boost::mpl::l_end>, boost::mpl::list2<test_type1,
test_type2>, boost::is_same<mpl_::arg<-0x00000000000000001>,
mpl_::arg<-0x00000000000000001> > >::************)’

The output using permutation_helper is an actual list:

testcase.cpp: In function ‘int main()’:
testcase.cpp:45: error: no matching function for call to
‘assertion_failed(mpl_::failed************
boost::mpl::equal<boost::mpl::list2<test_type1, test_type2>,
boost::mpl::joint_view<boost::mpl::joint_view<boost::mpl::list0<mpl_::na>,
boost::mpl::l_item<mpl_::long_<1l>,
boost::mpl::l_item<mpl_::long_<2l>, test_type1,
boost::mpl::list1<test_type3> >, boost::mpl::l_end> >,
boost::mpl::l_item<mpl_::long_<1l>,
boost::mpl::l_item<mpl_::long_<2l>, test_type2,
boost::mpl::list1<test_type3> >, boost::mpl::l_end> >,
boost::is_same<mpl_::arg<-0x00000000000000001>,
mpl_::arg<-0x00000000000000001> > >::************)’