std::integer_sequence

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...>;

A helper alias template std::make_integer_sequence is defined to simplify creation of std::integer_sequence and std::index_sequence types 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 = 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)>;

Example

Note: see Possible Implementation in std::apply for another example

Run this code

#include <tuple>
#include <iostream>
#include <array>
#include <utility>

// 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()
{
    std::array<int, 4> array = {1,2,3,4};

    // convert an array into a tuple
    auto tuple = a2t(array);
    static_assert(std::is_same<decltype(tuple),
                               std::tuple<int, int, int, int>>::value, "");

    // print it to cout
    std::cout << tuple << '\n';
}

Output:

(1, 2, 3, 4)

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