#include <iostream>
#include <boost/typeof/typeof.hpp>
#include <boost/preprocessor/cat.hpp>
#include <boost/type_traits/add_const.hpp>
#include <boost/type_traits/add_reference.hpp>
#include <boost/type_traits/remove_reference.hpp>

#define MAKE_CONST(n) \
    if (bool BOOST_PP_CAT(n, _cond) = true) \
        for ( \
            boost::add_reference< \
                boost::add_const< \
                    boost::remove_reference<BOOST_TYPEOF(n)>::type \
                >::type \
            >::type \
                BOOST_PP_CAT(n, _dummy) = n, \
                n = BOOST_PP_CAT(n, _dummy); \
            BOOST_PP_CAT(n, _cond); \
            BOOST_PP_CAT(n, _cond) = false \
        ) \
/**/

int main()
{
    int a = 5;
    MAKE_CONST(a) {
        std::cout << a;
//        a = 3;
    }
}

5