Getting root type of multidimensional variadic std::array

Question

I have nested std::array which dimension is handled by template arguments std::array<type, n, ...>.

I was wondering how to get for example in std::array<std::array<int, 2>, 2>, the type int.

in this case we could just do std::array<std::array<int, 2>, 2>::value_type::value_type, but how can I do this without having to put as many value_types as there are dimensions?

Answer 1

You can use a basic recursive technique:

template <typename T>
struct nested_value_type { using type = T; };

template <typename T, std::size_t N>
struct nested_value_type<std::array<T, N>>
{
    using type = typename nested_value_type<T>::type;
};

Provide an alias template for convenience:

template <typename T>
using nested_value_type_t = typename nested_value_type<T>::type;

And voila:

static_assert(std::is_same_v<
    nested_value_type_t<std::array<int, 1>>,
    int
>);

static_assert(std::is_same_v<
    nested_value_type_t<std::array<std::array<float, 1>, 1>>,
    float
>);

static_assert(std::is_same_v<
    nested_value_type_t<std::array<std::array<std::array<char, 1>, 1>, 1>>,
    char
>);

live example on godbolt.org

---

A bit shorter with C++20's std::type_identity:

template <typename T>
struct nested_value_type : std::type_identity<T> { };

template <typename T>
using nested_value_type_t = typename nested_value_type<T>::type;

template <typename T, std::size_t N>
struct nested_value_type<std::array<T, N>> 
    : std::type_identity<nested_value_type_t<T>> { };