std::integer_sequence(3)

NAME
std::integer_sequence - std::integer_sequence

Synopsis
   Defined in header <utility>
   template< class T, T... Ints > (since C++14)
   class integer_sequence;
   The class template std::integer_sequence represents a compile-time sequence of
   integers. When used as an argument to a function template, the parameter pack Ints
   can be deduced and used in pack expansion.

Template parameters
   T - an integer type to use for the elements of the sequence
   ...Ints - a non-type parameter pack representing the sequence

Member types
   Member type Definition
   value_type T

Member functions
   size returns the number of elements in Ints
   [static] (public static member function) std::integer_sequence::size
   static constexpr std::size_t size() noexcept;
   Returns the number of elements in Ints. Equivalent to sizeof...(Ints)

Parameters
   (none)

Return value
   The number of elements in Ints.
  Helper templates
   A helper alias template std::index_sequence is defined for the common case where T
   is std::size_t:
   template<std::size_t... Ints>
   using index_sequence = std::integer_sequence<std::size_t, Ints...>;
   Helper alias templates std::make_integer_sequence and std::make_index_sequence are
   defined to simplify creation of std::integer_sequence and std::index_sequence types,
   respectively, with 0, 1, 2, ..., N-1 as Ints:
   template<class T, T N>
   using make_integer_sequence = std::integer_sequence<T, /* a sequence 0, 1, 2, ...,
   N-1 */ >;
   template<std::size_t N>
   using make_index_sequence = std::make_integer_sequence<std::size_t, N>;
   The program is ill-formed if N is negative. If N is zero, the indicated type is
   integer_sequence<T>.
   A helper alias template std::index_sequence_for is defined to convert any type
   parameter pack into an index sequence of the same length:
   template<class... T>
   using index_sequence_for = std::make_index_sequence<sizeof...(T)>;

Notes
   Feature-test macro: __cpp_lib_integer_sequence

Example
   Note: see Possible Implementation in std::apply for another example. // Run this code
 #include <tuple>
 #include <iostream>
 #include <array>
 #include <utility>
 // debugging aid
 template<typename T, T... ints>
 void print_sequence(std::integer_sequence<T, ints...> int_seq)
 {
     std::cout << "The sequence of size " << int_seq.size() << ": ";
     ((std::cout << ints << ' '), ...);
     std::cout << '\n';
 }
 // convert array into a tuple
 template<typename Array, std::size_t... I>
 auto a2t_impl(const Array& a, std::index_sequence<I...>)
 {
     return std::make_tuple(a[I]...);
 }
 template<typename T, std::size_t N, typename Indices = std::make_index_sequence<N>>
 auto a2t(const std::array<T, N>& a)
 {
     return a2t_impl(a, Indices{});
 }
 // pretty-print a tuple
 template<class Ch, class Tr, class Tuple, std::size_t... Is>
 void print_tuple_impl(std::basic_ostream<Ch,Tr>& os,
                       const Tuple& t,
                       std::index_sequence<Is...>)
 {
     ((os << (Is == 0? "" : ", ") << std::get<Is>(t)), ...);
 }
 template<class Ch, class Tr, class... Args>
 auto& operator<<(std::basic_ostream<Ch, Tr>& os,
                  const std::tuple<Args...>& t)
 {
     os << "(";
     print_tuple_impl(os, t, std::index_sequence_for<Args...>{});
     return os << ")";
 }
 int main()
 {
     print_sequence(std::integer_sequence<unsigned, 9, 2, 5, 1, 9, 1, 6>{});
     print_sequence(std::make_integer_sequence<int, 20>{});
     print_sequence(std::make_index_sequence<10>{});
     print_sequence(std::index_sequence_for<float, std::iostream, char>{});
     std::array<int, 4> array = {1, 2, 3, 4};
     // convert an array into a tuple
     auto tuple = a2t(array);
     static_assert(std::is_same_v<decltype(tuple),
                        std::tuple<int, int, int, int>>, "");
     // print it to cout
     std::cout << "The tuple: " << tuple << '\n';
 }

Output:
 The sequence of size 7: 9 2 5 1 9 1 6
 The sequence of size 20: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
 The sequence of size 10: 0 1 2 3 4 5 6 7 8 9
 The sequence of size 3: 0 1 2
 The tuple: (1, 2, 3, 4)

See also
   to_array creates a std::array object from a built-in array
   (C++20) (function template)