boost::mpl::set thinks remove_cv<const T>::type != T

Question

So I'm trying to get a unique list of types, and I want to get rid of all const duplicates that the list I'm passed might contain. I think the following code should work, but my_set contains "int" two times. What am i doing wrong?

#include <boost/mpl/vector.hpp>
#include <boost/mpl/transform.hpp>
#include <boost/type_traits/remove_cv.hpp>
#include <boost/mpl/fold.hpp>
#include <boost/mpl/set.hpp>
#include <boost/mpl/insert.hpp>
#include <boost/mpl/size.hpp>
#include <boost/mpl/assert.hpp>

typedef boost::mpl::vector<float,int,float,const int>::type my_v;

typedef boost::mpl::transform
    < my_v
    , boost::remove_cv<boost::mpl::_1>::type
    >::type my_v2;

typedef boost::mpl::fold
    < my_v2
    , boost::mpl::set0<>
    , boost::mpl::insert
        < boost::mpl::_1
        , boost::mpl::_2
        >
>::type my_set;

BOOST_MPL_ASSERT_RELATION( boost::mpl::size<my_set>::value, ==, 2 ); // Fails

Answer 1

Yes as cv_and_he said, it is because of boost::remove_cv::type.

As add ::type to boost::remove_cv will force evaluation on the expression and make remove_cv evaluate the _1 expression itself but not the underlying argument. Here transform is an recursive function, so we should remove the '::type' to allow lazy evaluation on arguments after the whole transform expression expanded.

see the test result below,

#include <boost/mpl/vector.hpp>
#include <boost/mpl/transform.hpp>
#include <boost/type_traits/remove_cv.hpp>
#include <boost/mpl/fold.hpp>
#include <boost/mpl/set.hpp>
#include <boost/mpl/insert.hpp>
#include <boost/mpl/size.hpp>
#include <boost/mpl/assert.hpp>
#include <iostream>
#include <typeinfo>
#include <cxxabi.h>

typedef boost::mpl::vector<float,int,float,const int>::type my_v;

typedef boost::mpl::transform<my_v,
                              boost::remove_cv<boost::mpl::_1>::type
                              >::type my_v2;

typedef boost::mpl::fold< my_v2,
                          boost::mpl::set0<>,
                          boost::mpl::insert<boost::mpl::_1,
                                             boost::mpl::_2
                                             >
                          >::type my_set;

// BOOST_MPL_ASSERT_RELATION( boost::mpl::size<my_set>::value, ==, 2 ); // Fails

int main()
{
  int status;
  std::cout << abi::__cxa_demangle(typeid(boost::remove_cv<boost::mpl::_1>::type).name(), 0, 0, &status)
            << '\n'
            << abi::__cxa_demangle(typeid(boost::remove_cv<boost::mpl::_1>).name(), 0, 0, &status)
            << '\n'
            << abi::__cxa_demangle(typeid(my_v2).name(), 0, 0, &status)
            << std::endl;
}

output,

mpl_::arg<1>
boost::remove_cv<mpl_::arg<1> >
boost::mpl::v_item<int const, boost::mpl::v_item<float, boost::mpl::v_item<int, boost::mpl::v_item<float, boost::mpl::vector0<mpl_::na>, 0>, 0>, 0>, 0>