20 General utilities library [utilities]

20.3 Compile-time integer sequences [intseq]

20.3.1 In general [intseq.general]

1

The library provides a class template that can represent an integer sequence. When used as an argument to a function template the parameter pack defining the sequence can be deduced and used in a pack expansion.

2

[ Example:

template<class F, class Tuple, std::size_t... I>
  decltype(auto) apply_impl(F&& f, Tuple&& t, index_sequence<I...>) {
    return std::forward<F>(f)(std::get<I>(std::forward<Tuple>(t))...);
  }

template<class F, class Tuple>
  decltype(auto) apply(F&& f, Tuple&& t) {
    using Indices = make_index_sequence<std::tuple_size_v<std::decay_t<Tuple>>>;
    return apply_impl(std::forward<F>(f), std::forward<Tuple>(t), Indices());
  }

 — end example ] [ Note: The index_sequence alias template is provided for the common case of an integer sequence of type size_t.  — end note ]

20.3.2 Class template integer_sequence [intseq.intseq]

namespace std {
  template<class T, T... I>
  struct integer_sequence {
    using value_type = T;
    static constexpr size_t size() noexcept { return sizeof...(I); }
  };
}
1

T shall be an integer type.

20.3.3 Alias template make_integer_sequence [intseq.make]

template<class T, T N> using make_integer_sequence = integer_sequence<T, see below>;

1

If N is negative the program is ill-formed. The alias template make_integer_sequence denotes a specialization of integer_sequence with N template non-type arguments. The type make_integer_sequence<T, N> denotes the type integer_sequence<T, 0, 1, ..., N-1>. [ Note: make_integer_sequence<int, 0> denotes the type integer_sequence<int>  — end note ]