Runtime typeswitch for typelists as a switch instead of a nested if's ?

Question

This is from TTL:

////////////////////////////////////////////////////////////
//  run-time type switch
template <typename L, int N = 0, bool Stop=(N==length<L>::value) > struct type_switch;

template <typename L, int N, bool Stop>
  struct type_switch
  {
    template< typename F >
      void operator()( size_t i, F& f )
      {
        if( i == N )
        {
          f.operator()<typename impl::get<L,N>::type>();
        }
        else
        {
          type_switch<L, N+1> next;
          next(i, f);
        }
      }
  };

It's used for typeswitching on a TypeList. Question is -- they are doing this via a series of nested if's. Is there a way to do this type switch as a single select statement instead?

Thanks!

Answer 1

I don't think so.

This kind of template metaprogramming is normally done with recursion. Since it all happens at compile-time, I wouldn't be surprised if there's no runtime recursion or condition-checks.

Answer 2

You'll need the preprocessor to generate a big switch. You'll need get<> to no-op out-of-bound lookups. Check the compiler output to be sure unused cases produce no output, if you care; adjust as necessary ;v) .

Check out the Boost Preprocessor Library if you care to get good at this sort of thing…

template <typename L>
  struct type_switch
  {
    template< typename F >
      void operator()( size_t i, F& f )
      {
        switch ( i ) {
         #define CASE_N( N ) \
         case (N): return f.operator()<typename impl::get<L,N>::type>();
         CASE_N(0)
         CASE_N(1)
         CASE_N(2)
         CASE_N(3) // ad nauseam.
      }
  };

Answer 3

You could always use a binary search instead of a linear search. It would be more complicated and more likely to have bugs in it (binary search is surprisingly easy to mess up).

You could also manually expand out N type_switch::operator(), where N is some reasonable upper bound on the number of typelist lengths you will have in your program.