Question

What does this line do?
I doubt its a template or expression. (doing those divies and naming a member hpp is a bad idea)

# define BOOST_PP_UPDATE_COUNTER() <boost/preprocessor/slot/detail/counter.hpp>

Full file

# /* **************************************************************************
#  *                                                                          *
#  *     (C) Copyright Paul Mensonides 2005.                                  *
#  *     Distributed under the Boost Software License, Version 1.0. (See      *
#  *     accompanying file LICENSE_1_0.txt or copy at                         *
#  *     http://www.boost.org/LICENSE_1_0.txt)                                *
#  *                                                                          *
#  ************************************************************************** */
#
# /* See http://www.boost.org for most recent version. */
#
# ifndef BOOST_PREPROCESSOR_SLOT_COUNTER_HPP
# define BOOST_PREPROCESSOR_SLOT_COUNTER_HPP
#
# include <boost/preprocessor/slot/detail/def.hpp>
#
# /* BOOST_PP_COUNTER */
#
# define BOOST_PP_COUNTER 0
#
# /* BOOST_PP_UPDATE_COUNTER */
#
# define BOOST_PP_UPDATE_COUNTER() <boost/preprocessor/slot/detail/counter.hpp>
#
# endif

Answer 1

Apparently it does some auto-incrementing magic.

Here’s the documentation.

I have no idea how it works. What I do know is that the magic is in counter.hpp itself; that the definition you posted has brackets in it is just so that you can write the vaguely user-friendly:

#include BOOST_PP_UPDATE_COUNTER()

to invoke said magic.