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std::integer_sequence

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Compile-time rational arithmetic
Compile-time integer sequences
integer_sequence
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Defined in header <utility>
template< class T, T... Ints >
class 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.

Contents

[edit] Template parameters

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

[edit] Member types

Member type Definition
value_type T

[edit] Member functions

size
[static]
 returns the number of elements in Ints
(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.

[edit] 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)>;

[edit] Notes

Feature-test macro Value Std Feature
__cpp_lib_integer_sequence 201304L (C++14) Compile-time integer sequences

[edit] 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)

[edit] See also

(C++20)
 creates a std::array object from a built-in array
(function template) [edit]
(C++11)(C++17)
 compile-time constant of specified type with specified value
(class template) [edit]
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