rb.cpp

#include <iostream>
#include <inttypes.h>

template<bool C, typename Ta, typename Tb>
class IfThenElse;

template <typename Ta, typename Tb>
class IfThenElse<true, Ta, Tb> {
public:
  typedef Ta ResultT;
};

template <typename Ta, typename Tb>
class IfThenElse<false, Ta, Tb> {
public:
  typedef Tb ResultT;
};

template <std::size_t S>
class Foobar {
  typedef typename IfThenElse<(S < 256),
    uint8_t,
    typename IfThenElse<(S < 65536),
    uint16_t,
    void>::ResultT >::ResultT indexT;

  indexT index;
  
public:
  Foobar() {
    std::cout << "size of index " << sizeof(index) << "\n";
  }
};

int main(void) {
  Foobar<12> f1;
  Foobar<300> f2;
  Foobar<1000000> f3;
}