| 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: noexceptHelper 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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