Create a function pointer from Typelist subsets

Question

I have a Typelist implementation very similar to this one. If you don't know what a typelist is: in short, it behaves like variadic templates using nested tuples. You can read more about them here.

I would like to build up a function pointer type from a subset of this typelist. The subset is defined by a list of indexes (of any size) and the desired operation does a lookup of these indexes in the typelist and defines the type of a pointer on a function using those types as arguments.

The API would look something like:

#include "typelist.h"
// typelist definition
typedef Typelist<float, Typelist<double, Typelist<int, NullType>>> Pixel;

typedef FunctionFromFields<Pixel, 0, 2>::Type field_0_and_2;
// I want the definition above to be equivalent to:
// typedef void (*field_0_and_2)(float*, int*);

It seems reasonable to assume this is possible since everything is known at compile time, but I have yet to find the right syntax for that.

I don't want to use variadic templates to replace the typelist, however they can be used to define the pointer type.

Has anyone done something similar?

Answer 1

This should be fairly easy. First define a typelist_nth type function (left as an exercise; I assume you have one in your typelist.h):

template<typename TL, int I> struct typelist_nth;

Then use a variadic template to build a function type:

template<typename TL, int... Is> struct FunctionFromFields {
    typedef void (*Type)(typename typelist_nth<TL, Is>::type *...);
};