CPPTM Answers - Exercise 3-3

 // (by Ariel Badichi)
 #include <boost/static_assert.hpp>
 #include <boost/type_traits/is_same.hpp>
 #include <boost/type_traits/add_pointer.hpp>
 #include <boost/mpl/placeholders.hpp>
 #include <boost/mpl/apply.hpp>

 namespace mpl = boost::mpl;
 using namespace mpl::placeholders;

 template<typename F, typename T>
 struct twice : mpl::apply<F, typename mpl::apply<F, T>::type> {};

 int main()
 {
     typedef twice<boost::add_pointer<_>, int>::type ppint;
     typedef twice<boost::add_pointer<_>, ppint>::type ppppint;

     BOOST_STATIC_ASSERT((boost::is_same<ppppint, int ****>::value));

     return 0;
 }

This code doesn't work in versions of Boost prior to 1.32 (I mention this because 1.31 is the standard version you get when you use apt-get in the latest version of Ubuntu).