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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();

Returns the number of elements in Ints. Equivalent to sizeof...(Ints).

Parameters

(none).

Return value

The number of elements in Ints.

Exceptions

noexcept specification:
noexcept

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

#include <tuple>
#include <iostream>
#include <array>
#include <utility>
 
// Convert array into a tuple
template<typename Array, std::size_t... I>
decltype(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>>
decltype(auto) a2t(const std::array<T, N>& a)
{
    return a2t_impl(a, Indices());
}
 
// pretty-print a tuple (from http://stackoverflow.com/a/6245777/273767)
 
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...>)
{
    using swallow = int[]; // guaranties left to right order
    (void)swallow{0, (void(os << (Is == 0? "" : ", ") << std::get<Is>(t)), 0)...};
}
 
template<class Ch, class Tr, class... Args>
decltype(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)

Example

This example shows how a std::tuple can be converted into arguments for a function invocation, see std::experimental::apply.

#include <iostream>
#include <tuple>
#include <utility>
 
template<typename Func, typename Tup, std::size_t... index>
decltype(auto) invoke_helper(Func&& func, Tup&& tup, std::index_sequence<index...>)
{
    return func(std::get<index>(std::forward<Tup>(tup))...);
}
 
template<typename Func, typename Tup>
decltype(auto) invoke(Func&& func, Tup&& tup)
{
    constexpr auto Size = std::tuple_size<typename std::decay<Tup>::type>::value;
    return invoke_helper(std::forward<Func>(func),
                         std::forward<Tup>(tup),
                         std::make_index_sequence<Size>{});
}
 
void foo(int a, const std::string& b, float c)
{
    std::cout << a << " , " << b << " , " << c << '\n';
}
 
int main()
{
    auto args = std::make_tuple(2, "Hello", 3.5);
    invoke(foo, args);
}

Output:

2 , Hello , 3.5

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