...

...

template <class type_t, class sequence_t> struct copy_col_if_in_dest : public if_< typename is_same< typename deref<typename boost::mpl::find<sequence_t, type_t>::type >::type, type_t>,

do_copy_col,

...

do_not_copy_col>::type { };

This code has a problem when type_t is not in sequence_t: the past-the-end iterator gets dereferenced. I wrote a short program to do what you want (it's more explicit, it uses mpl::contains<>):

#include <boost/mpl/vector.hpp> #include <boost/mpl/if.hpp> #include <boost/mpl/contains.hpp>

namespace mpl = boost::mpl;

struct do_copy_col {}; struct dont_copy_col {};

template<typename T, typename Seq> struct copy_col_if_in_dest : mpl::if_< mpl::contains<Seq, T>, do_copy_col, dont_copy_col >::type { };

int main() { typedef mpl::vector<char, short, int, long> v; do_copy_col do_copy = copy_col_if_in_dest<int, v>(); dont_copy_col dont_copy = copy_col_if_in_dest<bool, v>(); return 0; }

[Brian Braatz Writes:] Wow. Many THANKS- the mpl::contains<> is REALLY what I was looking for. (Though the exercise of doing it with if_<> was a useful experience) THANK YOU ( I have been burning the morning and afternoon oil here the last few weeks trying to get a lib I am proposing ready for boost people to look at) -I am going to replace the code I had with the code you presented, and make a note of your contribution. THANKS AGAIN! Brian