NAME¶

std::ranges::views::iota,std::ranges::iota_view - std::ranges::views::iota,std::ranges::iota_view

Synopsis¶

Defined in header <ranges>
template< std::weakly_incrementable W,

std::semiregular Bound = std::unreachable_sentinel_t >
requires __WeaklyEqualityComparableWith<W, Bound> && (1) (since C++20)
std::copyable<W>

class iota_view : public ranges::view_interface<iota_view<W,
Bound>>
namespace views {

inline constexpr /*unspecified*/ iota = /*unspecified*/; (2) (since C++20)

}
Call signature
template< class W >

requires /* see below */ (since C++20)

constexpr /*see below*/ iota( W&& value );
template< class W, class Bound >

requires /* see below */ (since C++20)

constexpr /*see below*/ iota( W&& value, Bound&& bound );

1) A range factory that generates a sequence of elements by repeatedly incrementing
an initial value. Can be either bounded or unbounded (infinite).
2) views::iota(e) and views::iota(e, f) are expression-equivalent to (has the same
effect as) iota_view(e) and iota_view(e, f) respectively for any suitable
subexpressions e and f.

Expression-equivalent

Expression e is expression-equivalent to expression f, if

* e and f have the same effects, and
* either both are constant subexpressions or else neither is a constant
subexpression, and
* either both are potentially-throwing or else neither is potentially-throwing
(i.e. noexcept(e) == noexcept(f)).

Customization point objects

The name views::iota denotes a customization point object, which is a const function
object of a literal semiregular class type. For exposition purposes, the
cv-unqualified version of its type is denoted as __iota_fn.

All instances of __iota_fn are equal. The effects of invoking different instances of
type __iota_fn on the same arguments are equivalent, regardless of whether the
expression denoting the instance is an lvalue or rvalue, and is const-qualified or
not (however, a volatile-qualified instance is not required to be invocable). Thus,
views::iota can be copied freely and its copies can be used interchangeably.

Given a set of types Args..., if std::declval<Args>()... meet the requirements for
arguments to views::iota above, __iota_fn models

* std::invocable<__iota_fn, Args...>,
* std::invocable<const __iota_fn, Args...>,
* std::invocable<__iota_fn&, Args...>, and
* std::invocable<const __iota_fn&, Args...>.

Otherwise, no function call operator of __iota_fn participates in overload
resolution.

Data members

Typical implementation of iota_view holds two non-static data members: the beginning
value value_ of type W and the sentinel value bound_ of type Bound. The names shown
here are exposition-only.

Member functions¶

constructor creates a iota_view
(C++20) (public member function)
begin obtains the beginning iterator of an iota_view
(C++20) (public member function)
end obtains the sentinel denoting the end of an iota_view
(C++20) (public member function)
size obtains the size of an iota_view if it is sized
(C++20) (public member function)
Inherited from std::ranges::view_interface
empty Returns whether the derived view is empty. Provided if it satisfies
(C++20) sized_range or forward_range.
(public member function of std::ranges::view_interface<D>)
operator bool Returns whether the derived view is not empty. Provided if
(C++20) ranges::empty is applicable to it.
(public member function of std::ranges::view_interface<D>)
front Returns the first element in the derived view. Provided if it
(C++20) satisfies forward_range.
(public member function of std::ranges::view_interface<D>)
back Returns the last element in the derived view. Provided if it satisfies
(C++20) bidirectional_range and common_range.
(public member function of std::ranges::view_interface<D>)
operator[] Returns the nth element in the derived view. Provided if it satisfies
(C++20) random_access_range.
(public member function of std::ranges::view_interface<D>)

std::ranges::iota_view::iota_view

iota_view() requires std::default_initializable<W> = default; (1) (since C++20)
constexpr explicit iota_view( W value ); (2) (since C++20)
constexpr iota_view( std::type_identity_t<W> value, (3) (since C++20)
std::type_identity_t<Bound> bound );
constexpr iota_view( /*iterator*/ first, /*see below*/ last ); (4) (since C++20)

1) Value-initializes value_ and bound_ via their default member initializers (= W()
and = Bound()).
2) Initializes value_ with value and value-initializes bound_. This constructor is
used to create unbounded iota_views, e.g. iota(0) yields numbers 0,1,2...,
infinitely.
3) Initializes value_ with value and bound_ with bound. The behavior is undefined if
std::totally_ordered_with<W, Bound> is modeled and bool(value <= bound) is false.
This constructor is used to create bounded iota views, e.g. iota(10, 20) yields
numbers from 10 to 19.
4) Same as (3), except that value_ is initialized with the W value stored in first,
and

* if W and Bound are the same type, then the type of last is /*iterator*/ and
bound_ initialized with the W value stored in last,
* otherwise, if the iota_view is unbounded (i.e. Bound is
std::unreachable_sentinel_t), then the type of last is
std::unreachable_sentinel_t and bound_ initialized with
std::unreachable_sentinel.
* otherwise, the type of last is /*sentinel*/ and bound_ initialized with the
Bound value stored in last.

In any case, the type of last is same as decltype(end()).

For (2), (3), and (4), the behavior is undefined if the iota_view is bounded (i.e.
Bound is not std::unreachable_sentinel_t) and bound_ is initialized to a value
unreachable from value_.

Parameters¶

value - the starting value
bound - the bound
first - the iterator denoting the starting value
last - the iterator or sentinel denoting the bound

std::ranges::iota_view::begin

constexpr /*iterator*/ begin() const; (since C++20)

Returns an iterator initialized with value_.

std::ranges::iota_view::end

constexpr auto end() const; (1) (since C++20)
constexpr /*iterator*/ end() const requires std::same_as<W, (2) (since C++20)
Bound>;

1) Returns a sentinel of a specific type (shown as /*sentinel*/ here) initialized
with bound_ if this view is bounded, or std::unreachable_sentinel if this view is
unbounded.
2) Returns an iterator initialized with bound_.

std::ranges::iota_view::size

constexpr auto size() const

requires (std::same_as<W, Bound> && /*advanceable*/<W>) ||
(/*is-integer-like*/<W> && /*is-integer-like*/<Bound>) ||
std::sized_sentinel_for<Bound, W>
{
if constexpr (/*is-integer-like*/<W> && /*is-integer-like*/<Bound>)
return (value_ < 0) (since C++20)
? ((bound_ < 0)
? /*to-unsigned-like*/(-value_) - /*to-unsigned-like*/(-bound_)
: /*to-unsigned-like*/(bound_) + /*to-unsigned-like*/(-value_))
: /*to-unsigned-like*/(bound_) - /*to-unsigned-like*/(value_);
else
return /*to-unsigned-like*/(bound_ - value_);

}

Returns the size of the view if the view is bounded.

The exposition-only concept advanceable is described in this page.

The exposition-only function template to-unsigned-like converts its argument (which
must be integer-like) to the corresponding unsigned version of the argument type.

Deduction guides

template<class W, class Bound>

requires (!/*is-integer-like*/<W> || !/*is-integer-like*/<Bound> || (since C++20)
/*is-signed-integer-like*/<W> == /*is-signed-integer-like*/<Bound>)

iota_view(W, Bound) -> iota_view<W, Bound>;

For any type T, /*is-integer-like*/<T> is true if and only if T is integer-like, and
/*is-signed-integer-like*/<T> is true if and only if T is integer-like and capable
of representing negative values.

Note that the guide protects itself against signed/unsigned mismatch bugs, like
views::iota(0, v.size()), where 0 is a (signed) int and v.size() is an
(unsigned) std::size_t.

Nested classes

iterator the iterator type
(C++20) (exposition-only member class)
sentinel the sentinel type used when the iota_view is bounded and Bound and W are
(C++20) not the same type
(exposition-only member class)

Helper templates

template<std::weakly_incrementable W, std::semiregular Bound>
inline constexpr bool enable_borrowed_range<ranges::iota_view<W, (since C++20)
Bound>> = true;

This specialization of std::ranges::enable_borrowed_range makes iota_view satisfy
borrowed_range.

Example¶

// Run this code

#include <ranges>
#include <iostream>
#include <algorithm>

int main()
{
for (int i : std::ranges::iota_view{1, 10})
std::cout << i << ' ';
std::cout << '\n';

for (int i : std::views::iota(1, 10))
std::cout << i << ' ';
std::cout << '\n';

struct Bound
{
int bound;
bool operator==(int x) const { return x == bound; }
};
for (int i : std::views::iota(1, Bound{10}))
std::cout << i << ' ';
std::cout << '\n';

for (int i : std::views::iota(1) | std::views::take(9))
std::cout << i << ' ';
std::cout << '\n';

std::ranges::for_each(std::views::iota(1, 10), [](int i) {
std::cout << i << ' ';
});
std::cout << '\n';
}

Output:¶

1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9

Defect reports

The following behavior-changing defect reports were applied retroactively to
previously published C++ standards.

DR Applied to Behavior as published Correct behavior
LWG 3523 C++20 iterator-sentinel pair constructor might corrected
use wrong sentinel type
P2325R3 C++20 iota_view required that W is semiregular only requires that W
as view required default_initializable is copyable
LWG 3610 C++20 size might reject integer-class types made accepting if
possible

See also¶

iota fills a range with successive increments of the starting value
(C++11) (function template)