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

UNTESTED:

template 
constexpr int index_of() { return -1; } // type not found

template 
constexpr int index_of() {
    return std::is_same::value ? N : index_of();
}

template 
template 
constexpr int engine::ordinal() {
    return index_of<0, Component, Components...>();
}

I could have used structs, but I find this much cleaner (without all the ::type ugliness).

If you want a compile-time error when the type is not found, change ordinal to:

template 
template 
constexpr int engine::ordinal() {
    static_assert(index_of<0, Component, Components...>()!=-1, "invalid component");
     return index_of<0, Component, Components...>();
}