[Boost-users] MPL to get rid of cumbersome switch

From: John Christopher (jcxxr_at_[hidden])
Date: 2006-03-16 20:48:22

Hi;
The program belows compiles and works but I'd like to simplify it, maybe 
using MPL. I spent some fascinating time educating myself on MPL, reading 
C++ Template Metaprogramming, but I am still at sea... to say the least. 
Basically the Functor contains a switch statement that is not convenient 
especially as the number of T classes will increase dramatically.
Many thanks for any help on that
JCR

#include <iostream>
#include <vector>
#include <boost/mpl/vector.hpp>

struct Tbase
{
  Tbase() { std::cout << "Tbase" << std::endl; }
};
struct T0 : public Tbase
{
  T0() { std::cout << "T0" << std::endl; }
};
struct T1 : public Tbase
{
  T1() { std::cout << "T1" << std::endl; }
};
typedef boost::mpl::vector<T0, T1> s;
std::vector<Tbase*> vec;
struct Functor
{
  void operator()(const int& i)
  {
    switch(i)
    {
      case 0:
        vec.push_back(new T0);
        break;
      case 1:
        vec.push_back(new T1);
        break;
    }
    // I'd like to simplify the above swtich statement by writing something 
like:
    vec.push_back(new boost::mpl::at<s,i>::type);
    // but it does not compile and MinGW returns:
    // error: i cannot appear in a constant expression
    // error: template argument 2 is invalid.
  }
};
int main (int argc, char ** argv)
{
  std::vector<int> runtimeVec;
  runtimeVec.push_back(0);
  runtimeVec.push_back(1);
  runtimeVec.push_back(0);
  std::for_each(runtimeVec.begin(), runtimeVec.end(), Functor());
  std::cout << vec.size() << std::endl;
 return 0;
}