Get index of a tuple element's type?

Question

If I have a tuple with different element types like

std::tuple<T0, T1, T2, ...>

And how to get the index of a element type?

template<class T, class Tuple>
struct Index
{
    enum {value = ?;}
};

Thanks.

Answer 1

template <class T, class Tuple>
struct Index;

template <class T, class... Types>
struct Index<T, std::tuple<T, Types...>> {
    static const std::size_t value = 0;
};

template <class T, class U, class... Types>
struct Index<T, std::tuple<U, Types...>> {
    static const std::size_t value = 1 + Index<T, std::tuple<Types...>>::value;
};

See it live at Coliru.

This implementation returns the index of the first occurrence of a given type. Asking for the index of a type that is not in the tuple results in a compile error (and a fairly ugly one at that).

Answer 2

template< size_t I, typename T, typename Tuple_t>
constexpr size_t index_in_tuple_fn(){
    static_assert(I < std::tuple_size<Tuple_t>::value,"The element is not in the tuple");

    typedef typename std::tuple_element<I,Tuple_t>::type el;
    if constexpr(std::is_same<T,el>::value ){
        return I;
    }else{
        return index_in_tuple_fn<I+1,T,Tuple_t>();
    }
}

template<typename T, typename Tuple_t>
struct index_in_tuple{
    static constexpr size_t value = index_in_tuple_fn<0,T,Tuple_t>();
};

The example above avoids generating tons of sub tuples, which makes compilation fail (out of memory) when you call index_in_tuple for large tuples