Здравствуйте, _nn_, Вы писали:

Нашел некрасивый хак , value_type можно достать у первого элемента последовательности.
В случае transform — item0::value_type, а в случае joint_view — sequence1_::item0::value_type, и соответственно переделать c_str:

#include <boost/mpl/vector_c.hpp>
#include <boost/mpl/char.hpp>
#include <boost/mpl/transform.hpp>
#include <boost/mpl/plus.hpp>
#include <boost/mpl/string.hpp>
#include <boost/mpl/joint_view.hpp>

#include <iostream>


struct get_value_type_item0
{
    template<typename Sequence>
    struct apply
    {
        typedef typename Sequence::type::item0::value_type value_type;
    };
};

struct get_value_type_sequence1_item0
{
    template<typename Sequence>
    struct apply
    {
        typedef typename Sequence::type::sequence1_::item0::value_type value_type;
    };
};

template<typename Sequence, typename ValueTypeGetter>
struct c_str_tmp
{
    typedef typename boost::mpl::end<Sequence>::type iend;
    typedef typename boost::mpl::begin<Sequence>::type i0;
    #define M0(z, n, data)                                                                      \
    typedef                                                                                     \
        typename boost::mpl::aux_::next_unless<BOOST_PP_CAT(i, n), iend>::type                         \
    BOOST_PP_CAT(i, BOOST_PP_INC(n));
    BOOST_PP_REPEAT(BOOST_MPL_LIMIT_STRING_SIZE, M0, ~)
    #undef M0

    typedef c_str_tmp type;
    typedef typename ValueTypeGetter::template apply<Sequence>::value_type value_type;
    static value_type const value[BOOST_MPL_LIMIT_STRING_SIZE+1];
};

template<typename Sequence, typename ValueTypeGetter>
typename c_str_tmp<Sequence, ValueTypeGetter>::value_type const c_str_tmp<Sequence, ValueTypeGetter>::value[BOOST_MPL_LIMIT_STRING_SIZE+1] =
{
    #define M0(z, n, data)                                                                      \
    boost::mpl::aux_::deref_unless<BOOST_PP_CAT(i, n), iend>::type::value,
    BOOST_PP_REPEAT(BOOST_MPL_LIMIT_STRING_SIZE, M0, ~)
    #undef M0
    '\0'
};

using namespace boost::mpl;

typedef vector_c<char, 'a', 'b', 'c', 'd'> z;
const char* zc = c_str<z>::value;

// Transform
typedef transform<
    z,
    plus<_, char_<1> >
>::type z1;
const char* z1c = c_str_tmp<z1, get_value_type_item0>::value;

// Join View
typedef joint_view<
    z1,
    z1
>::type z1z1;
const char* z1z1c = c_str_tmp<z1z1, get_value_type_sequence1_item0>::value;
    
int main()
{
    std::cout << z1c << "\n"; // bcde
    std::cout << z1z1c << "\n"; // bcdebcde
}