std::integer_sequence
From cppreference.com
| Defined in header <utility>
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template< class T, T... Ints > struct integer_sequence; |
(since C++14) | |
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
| Type | Definition |
value_type
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T
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Member functions
[static] |
returns the number of elements in Ints (public static member function) |
std::integer_sequence::size
static constexpr std::size_t size() noexcept; |
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Returns the number of elements in Ints. Equivalent to sizeof...(Ints).
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...>; |
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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 */>; |
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template< std::size_t N > using make_index_sequence = std::make_integer_sequence<std::size_t, N>; |
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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)>; |
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Notes
| Feature-test macro | Value | Std | Feature |
|---|---|---|---|
__cpp_lib_integer_sequence |
201304L |
(C++14) | Compile-time integer sequences |
Possible implementation
| make_integer_sequence |
|---|
namespace detail {
template<class T, T I, T N, T... integers>
struct make_integer_sequence_helper
{
using type = typename make_integer_sequence_helper<T, I + 1, N, integers..., I>::type;
};
template<class T, T N, T... integers>
struct make_integer_sequence_helper<T, N, N, integers...>
{
using type = std::integer_sequence<T, integers...>;
};
}
template<class T, T N>
using make_integer_sequence = typename detail::make_integer_sequence_helper<T, 0, N>::type;
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Example
See also std::apply possible implementation for another example.
Run this code
#include <array>
#include <cstddef>
#include <iostream>
#include <tuple>
#include <utility>
namespace details {
template <typename Array, std::size_t... I>
constexpr auto array_to_tuple_impl(const Array& a, std::index_sequence<I...>)
{
return std::make_tuple(a[I]...);
}
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 ? ", " : "") << std::get<Is>(t)), ...);
}
}
template <typename T, T... ints>
void print_sequence(int id, std::integer_sequence<T, ints...> int_seq)
{
std::cout << id << ") The sequence of size " << int_seq.size() << ": ";
((std::cout << ints << ' '), ...);
std::cout << '\n';
}
template <typename T, std::size_t N, typename Indx = std::make_index_sequence<N>>
constexpr auto array_to_tuple(const std::array<T, N>& a)
{
return details::array_to_tuple_impl(a, Indx{});
}
template <class Ch, class Tr, class... Args>
auto& operator<<(std::basic_ostream<Ch, Tr>& os, const std::tuple<Args...>& t)
{
os << '(';
details::print_tuple_impl(os, t, std::index_sequence_for<Args...>{});
return os << ')';
}
int main()
{
print_sequence(1, std::integer_sequence<unsigned, 9, 2, 5, 1, 9, 1, 6>{});
print_sequence(2, std::make_integer_sequence<int, 12>{});
print_sequence(3, std::make_index_sequence<10>{});
print_sequence(4, std::index_sequence_for<std::ios, float, signed>{});
constexpr std::array<int, 4> array{1, 2, 3, 4};
auto tuple1 = array_to_tuple(array);
static_assert(std::is_same_v<decltype(tuple1),
std::tuple<int, int, int, int>>, "");
std::cout << "5) tuple1: " << tuple1 << '\n';
constexpr auto tuple2 = array_to_tuple<int, 4,
std::integer_sequence<std::size_t, 1, 0, 3, 2>>(array);
std::cout << "6) tuple2: " << tuple2 << '\n';
}
Output:
1) The sequence of size 7: 9 2 5 1 9 1 6
2) The sequence of size 12: 0 1 2 3 4 5 6 7 8 9 10 11
3) The sequence of size 10: 0 1 2 3 4 5 6 7 8 9
4) The sequence of size 3: 0 1 2
5) tuple1: (1, 2, 3, 4)
6) tuple2: (2, 1, 4, 3)
See also
(C++20) |
creates a std::array object from a built-in array (function template) |
(C++11)(C++17) |
compile-time constant of specified type with specified value (class template) |