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std::integer_sequence(3) C++ Standard Libary 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 para- meter 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 com- mon 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_in- dex_sequence are defined to simplify creation of std::integer_sequence and std::in- dex_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 indi- cated type is integer_sequence<T>. A helper alias template std::index_sequence_for is defined to con- vert 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_in- dex_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) http://cppreference.com 2022.07.31 std::integer_sequence(3)
NAME | Synopsis | Template parameters | Member types | Member functions | Parameters | Return value | Notes | Example | Output: | See also
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