How can I get the index of a type in a variadic class template?

Question

I have a variadic Engine template class:

template  class Engine;

I\'d like to assign a number to each compon

Answer 1

My goal below is to keep things in the compile-time realm as much as possible.

This is an alias to remove some boilerplate. std::integral_constant is a wonderful std type that stores a compile-time determined integer-type:

template
using size=std::integral_constant;

Next, a index_of type, and an index_of_t that is slightly easier to use:

templatestruct types{using type=types;};
templatestruct index_of{};
template
struct index_of>:size<0>{};
template
struct index_of>:size<
  index_of>::value +1
>{};

This alias returns a pure std::integral_constant, instead of a type inheriting from it:

template
using index_of_t = size< index_of>::value >;

Finally our function:

template 
static constexpr index_of_t
ordinal() const {return{};}

it is not only constexpr, it returns a value that encodes its value in its type. size has a constexpr operator size_t() as well as an operator(), so you can use it in most spots that expect integer types seamlessly.

You could also use:

template
using ordinal = index_of_t;

and now ordinal is a type representing the index of the component, instead of a function.