c++ - How can I make a tuple of types that can be accessed by index without pack indexing? - Stack Overflow

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Using C++26's pack indexing, you can create a simple tuple of types like so:

template <typename... Ts>
struct type_tuple {
    template <unsigned Index>
    using get = Ts...[Index];
};

using T = type_tuple<int, float, double>::get<1>; // float

How can this be replicated in earlier C++ versions?

Using C++26's pack indexing, you can create a simple tuple of types like so:

template <typename... Ts>
struct type_tuple {
    template <unsigned Index>
    using get = Ts...[Index];
};

using T = type_tuple<int, float, double>::get<1>; // float

How can this be replicated in earlier C++ versions?

• c++
• parameter-pack
• c++26

Share Improve this question Follow edited Nov 16, 2024 at 23:17 HeliumHydride asked Nov 16, 2024 at 22:58 HeliumHydrideHeliumHydride 8111 silver badge55 bronze badges

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2 Answers 2

Sorted by: Reset to default 5

You can use std::tuple_element. This will work in all C++ versions supporting parameter packs:

template <typename... Ts>
struct type_tuple {

template <std::size_t Index>
using get = typename std::tuple_element<Index, std::tuple<Ts...>>::type;

};

If you are interested how it is done inside std::tuple, then it is as simple as this:

template <std::size_t Index, typename ...T>
struct NthType;
//{};

template <typename T0, typename ...T>
struct NthType<0, T0, T...>
{
    using Type = T0;
};

template <std::size_t Index, typename T0, typename ...T>
struct NthType<Index, T0, T...>
{
    using Type = typename NthType<Index-1, T...>::Type; // skip one type, decrease index
};

template <typename... Ts>
struct type_tuple {
    template <unsigned Index>
    using get = typename NthType<Index, Ts...>::Type;
};

So:

1. Define helper class NthType<Index, T...>
2. Specialize for Index = 0: NthType<0, T0, T...>::Type == T0
3. Make linear search for Nth type: NthType<Idx, T0, T...>::Type == NthType<Idx - 1, T...>

Note, that in last point we skip first type from pack, and decrease the index. In many libraries (including std lib implementations) there are also specialization for Index==1 and Index==2 just for spead up. I am not sure that, but in some compilers I can imagine there are builtin functions to get nth type from a pack - similar to C++26 way.

Before C++11 (before introduction of variadic templates) - there was just 50 or so specialization of such NthType - so it works up to Index==50 (or so...)

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