[boost] TMP runtime access problem

Subject: [boost] TMP runtime access problem
From: Willy Liu (willy5562_at_[hidden])
Date: 2010-02-08 21:02:52

Hi, I wonder that how to access metadata in runtime.
If I pre-compute something and save them into a template ARRAY<int N>,
how can I random access the metadatas in runtime?
What I can only do is binary searching. Is it possible to do in
constant time, or there exist some functions in MPL doing this?

#include<cstdio>

using namespace std;
template<int N>
struct ARRAY
{
       static const int a = ARRAY<N - 1>::a * 2;
};
template<>
struct ARRAY<0>
{
       static const int a = 1;
};
template<int LOWER, int UPPER>
struct BS
{
       static const int MID = UPPER + LOWER >> 1;
       typedef BS<MID + 1, UPPER> GREATER;
       typedef BS<LOWER, MID> LESS;
       static inline int binary_search(int n)
       {
               if(n > MID)
                       return GREATER::binary_search(n);
               else return LESS::binary_search(n);
       }
};
template<int N>
struct BS<N, N>
{
       static const int MID = N;
       typedef ARRAY<MID> result;
       static inline int binary_search(int n)
       {
               return result::a;
       }
};
int main()
{
       int n;
       printf("2^n n:");
       scanf("%d", &n);
       printf("ARRAY<%d>::a = %d\n", n, BS<0, 30>::binary_search(n));
       main();
       return 0;
}