std::integer_sequence
Defined in header <utility>
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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
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T
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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(); |
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Returns the number of elements in Ints
. Equivalent to sizeof...(Ints)
Parameters
(none)
Return value
The number of elements in Ints
.
Exceptions
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...>; |
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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 */ >; |
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template<std::size_t N> using make_index_sequence = make_integer_sequence<std::size_t, N>; |
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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)>; |
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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[]; // guarantees 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