# std::integer_sequence

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< cpp‎ | utility

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.

### 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 [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`.

### 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)>; |  |

### 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_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()
    {
        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 (C++20) | creates a `std::array` object from a built-in array (function template)

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