Question

I wrote some code to generate a boost::mpl::vector to use as a lookup table for a factorial function, as a test for a more general library function with which a developer may be able to generate a lookup table in the form of a static array of primitives. The function (which would most probably be implemented as a preprocessor macro definition) would accept the name and size of the array to be initialized, as well the name of a class template to be used as the metafunction to initialize each element i of the array.

I thought that the best way to go about doing this without the use of external scripts would be to

1. Create a boost::mpl::vector, as is done in the code listing below, and push the return value of the user-supplied metafunction for each element of the array to the back of the vector;
2. Use the elements of the vector initialize the static array (perhaps by using a series of macros, the last of which would use the __VARARGS__ macro to accomplish this).

I know neither how I would accomplish (2) nor whether the procedure I describe is a good way of doing what I seek. Here are the following questions for which I would like answers:

1. Is my procedure a good way of accomplishing what I seek? If not, please describe a better procedure which would accomplish the same thing, without the use of external scripts.

2. If my procedure is indeed a good way of accomplishing what I seek, how would I implement (2)?

   I will be sure to post a link to the source file containing library function which I describe once I implement it. The code listing follows below.

   namespace mpl = boost::mpl;

   template <typename x>
   struct factorial:
       mpl::if_<mpl::greater<x, mpl::int_<1>>,
           mpl::multiplies<x, factorial<x::prior>>,
           mpl::int_<1>
       >::type
   {};
   
   template <typename sequence, typename size>
   struct compileTable:
       mpl::if_<mpl::greater<size, mpl::int_<0>>,
           compileTable<
               mpl::push_front<sequence, factorial<size>>::type,
               size::prior
           >,
           sequence
       >::type
   {};
   
   static const int TABLE_SIZE = 13;
   
   typedef compileTable<
       mpl::vector<>,
       mpl::int_<TABLE_SIZE>
   >::type factorialTable;
   
   /*
   ** This is where I am stuck; how would I use the elements
   ** of factorialTable to initialize a static array?
   */
   

Answer 1

http://www.boost.org/doc/libs/1_46_0/libs/preprocessor/doc/index.html

#define MACRO(z, i, data) \
    mpl::at_c<data,i>::value

static const data[] = { BOOST_PP_ENUM(N, MACRO, factorialTable) };