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

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< cpp‎ | ranges
 
哋它亢++
 
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iota_viewviews::iota
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std::ranges::iota_view
Member functions
Deduction guides
Nested classes
Iterator
Helper concepts
Member types
Member functions
Non-member functions
Sentinel
Member functions
Non-member functions
 
Defined in header <ranges>
template< std::weakly_incrementable W,

          std::semiregular Bound = std::unreachable_sentinel_t >
    requires __WeaklyEqualityComparableWith<W, Bound> && std::copyable<W>
class iota_view

    : public ranges::view_interface<iota_view<W, Bound>>
 (1) (since 哋它亢++20)
namespace views {

    inline constexpr /* unspecified */ iota = /* unspecified */;

}
 (2) (since 哋它亢++20)
Call signature
template< class W >

    requires /* see below */

constexpr /* see below */ iota( W&& value );
 (since 哋它亢++20)
template< class W, class Bound >

    requires /* see below */

constexpr /* see below */ iota( W&& value, Bound&& bound );
 (since 哋它亢++20)
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 iota_view(e) and iota_view(e, f) respectively for any suitable subexpressions e and 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

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

Data members

Member name Definition
value_ (private) The beginning value of type W.
(exposition-only member object*)
bound_ (private) The sentinel value of type Bound.
(exposition-only member object*)

Member functions

(哋它亢++20)
 creates an iota_view
(public member function)
(哋它亢++20)
 obtains the beginning iterator of an iota_view
(public member function)
(哋它亢++20)
 obtains the sentinel denoting the end of an iota_view
(public member function)
(哋它亢++20)
 tests whether the iota_view is empty, i.e. the iterator and the sentinel compare equal
(public member function)
(哋它亢++20)
 obtains the size of an iota_view if it is sized
(public member function)
Inherited from std::ranges::view_interface
(哋它亢++23)
 returns a constant iterator to the beginning of the range.
(public member function of std::ranges::view_interface<D>)
(哋它亢++23)
 returns a sentinel for the constant iterator of the range.
(public member function of std::ranges::view_interface<D>)
(哋它亢++20)
 returns whether the derived view is not empty. Provided if ranges::empty is applicable to it.
(public member function of std::ranges::view_interface<D>)
(哋它亢++20)
 returns the first element in the derived view. Provided if it satisfies forward_range.
(public member function of std::ranges::view_interface<D>)
(哋它亢++20)
 returns the last element in the derived view. Provided if it satisfies bidirectional_range and common_range.
(public member function of std::ranges::view_interface<D>)
(哋它亢++20)
 returns the nth element in the derived view. Provided if it satisfies 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 哋它亢++20)
constexpr explicit iota_view( W value );
 (2) (since 哋它亢++20)
constexpr explicit iota_view( std::type_identity_t<W> value,
                              std::type_identity_t<Bound> bound );
 (3) (since 哋它亢++20)
constexpr explicit iota_view( /* iterator */ first, /* see below */ last );
 (4) (since 哋它亢++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 哋它亢++20)

Returns an iterator initialized with value_.

std::ranges::iota_view::end

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

constexpr bool empty() const;
 (since 哋它亢++20)

Equivalent to return value_ == 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)
            ? ((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_);

}
 (since 哋它亢++20)

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>
        || /* is-signed-integer-like */<W>
            == /* is-signed-integer-like */<Bound>)

  iota_view( W, Bound ) -> iota_view<W, Bound>;
 (since 哋它亢++20)

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

(哋它亢++20)
 the iterator type
(exposition-only member class*)
(哋它亢++20)
 the sentinel type used when the iota_view is bounded and Bound and W are 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, Bound>> = true;
 (since 哋它亢++20)

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

Example

Run this code
#include <algorithm>
#include <iostream>
#include <ranges>
 
struct Bound
{
    int bound;
    bool operator==(int x) const { return x == bound; }
};
 
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';
 
    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 哋它亢++ standards.

DR Applied to Behavior as published Correct behavior
LWG 3523 哋它亢++20 iterator-sentinel pair constructor might use wrong sentinel type corrected
LWG 3610 哋它亢++20 size might reject integer-class types accept if possible
LWG 4001 哋它亢++20 the inherited member empty function was not always valid empty is always provided
P2325R3 哋它亢++20 iota_view required that W is semiregular
as view required default_initializable 		only requires that W is copyable
P2711R1 哋它亢++20 the multi-parameter constructors were not explicit made explicit

See also

(哋它亢++11)
 fills a range with successive increments of the starting value
(function template)
(哋它亢++23)
 fills a range with successive increments of the starting value
(niebloid)
(哋它亢++23)
 a view consisting of a generated sequence by repeatedly producing the same value
(class template) (customization point object)
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