C++0x: How can I access variadic tuple members by index at runtime?
- by nonoitall
I have written the following basic Tuple template:
template <typename... T>
class Tuple;
template <uintptr_t N, typename... T>
struct TupleIndexer;
template <typename Head, typename... Tail>
class Tuple<Head, Tail...> : public Tuple<Tail...> {
private:
Head element;
public:
template <uintptr_t N>
typename TupleIndexer<N, Head, Tail...>::Type& Get() {
return TupleIndexer<N, Head, Tail...>::Get(*this);
}
uintptr_t GetCount() const {
return sizeof...(Tail) + 1;
}
private:
friend struct TupleIndexer<0, Head, Tail...>;
};
template <>
class Tuple<> {
public:
uintptr_t GetCount() const {
return 0;
}
};
template <typename Head, typename... Tail>
struct TupleIndexer<0, Head, Tail...> {
typedef Head& Type;
static Type Get(Tuple<Head, Tail...>& tuple) {
return tuple.element;
}
};
template <uintptr_t N, typename Head, typename... Tail>
struct TupleIndexer<N, Head, Tail...> {
typedef typename TupleIndexer<N - 1, Tail...>::Type Type;
static Type Get(Tuple<Head, Tail...>& tuple) {
return TupleIndexer<N - 1, Tail...>::Get(*(Tuple<Tail...>*) &tuple);
}
};
It works just fine, and I can access elements in array-like fashion by using tuple.Get<Index() - but I can only do that if I know the index at compile-time. However, I need to access elements in the tuple by index at runtime, and I won't know at compile-time which index needs to be accessed. Example:
int chosenIndex = getUserInput();
cout << "The option you chose was: " << tuple.Get(chosenIndex) << endl;
What's the best way to do this?
