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

From cppreference.com
< cpp‎ | ranges
 
 
 
 
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 C++20)
namespace views {

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

}
(2) (since C++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 (has the same effect as) iota_view(e) and iota_view(e, f) respectively for any suitable subexpressions e and f.

Contents

[edit] Expression-equivalent

Expression e is expression-equivalent to expression f, if e and f have the same effects, either are both potentially-throwing or are both not potentially-throwing (i.e. noexcept(e) == noexcept(f)), and either are both constant subexpressions or are both not constant subexpressions.

[edit] 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.

[edit] 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.

[edit] Member functions

(constructor)
(C++20)
creates a iota_view
(public member function)
begin
(C++20)
obtains the beginning iterator of an iota_view
(public member function)
end
(C++20)
obtains the sentinel denoting the end of an iota_view
(public member function)
size
(C++20)
obtains the size of an iota_view if it is sized
(public member function)
Inherited from std::ranges::view_interface
(C++20)
Returns whether the derived view is empty. Provided if it satisfies forward_range.
(public member function of std::ranges::view_interface<D>) [edit]
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>) [edit]
(C++20)
Returns the first element in the derived view. Provided if it satisfies forward_range.
(public member function of std::ranges::view_interface<D>) [edit]
(C++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>) [edit]
Returns the nth element in the derived view. Provided if it satisfies random_access_range.
(public member function of std::ranges::view_interface<D>) [edit]

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,
                     std::type_identity_t<Bound> bound );
(3) (since C++20)
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, Bound>;
(2) (since C++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::size

constexpr auto size() const

  requires (std::same_as<W, Bound> && /*advanceable*/<W>) ||
           (std::integral<W> && std::integral<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 C++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 interger-like) to the corresponding unsigned version of the argument type.

[edit] 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 C++20)

For any type T, /*is-integer-like*/<T> is true if and only if T is interger-like, and /*is-integer-signed-like*/<T> is true if and only if T is integer-like and capable for representing minus 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.

[edit] Nested classes

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

[edit] Helper templates

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

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

[edit] Example

#include <ranges>
#include <iostream>
 
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) | std::views::take(9))
        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

[edit] 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 use wrong sentinel type corrected
P2325R3 C++20 iota_view required that W is semiregular
as view required default_initializable
only requires that W is copyable

[edit] See also

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