std::integer_sequence (3) - Linux Manuals

std::integer_sequence: std::integer_sequence

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
         (public static member function)
[static]

 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...>;

A helper alias template std::make_integer_sequence is defined to simplify creation of std::integer_sequence and std::index_sequence types 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 = 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)>;

Example

Note: see Possible Implementation in std::apply for another example
// Run this code

  #include <tuple>
  #include <iostream>
  #include <array>
  #include <utility>

  // 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()
  {
      std::array<int, 4> array = {1,2,3,4};

      // convert an array into a tuple
      auto tuple = a2t(array);
      static_assert(std::is_same<decltype(tuple),
                                 std::tuple<int, int, int, int>>::value, "");

      // print it to cout
      std::cout << tuple << '\n';
  }

Output:

  (1, 2, 3, 4)