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

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< cpp‎ | ranges

Defined in header `<ranges>` |  |
template< std::weakly_incrementable W, std::semiregular Bound = std::unreachable_sentinel_t > requires /*weakly-equality-comparable-with*/<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)
Call signature |  |
template< class W > requires /* see below */ constexpr /* see below */ iota( W&& value ); |  | (since C++20)
template< class W, class Bound > requires /* see below */ constexpr /* see below */ iota( W&& value, Bound&& bound ); |  | (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 iota_view< std::decay_t <decltype((e))>>(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

- 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

Member | Definition
`W` `value_` | the beginning value (exposition-only member object*)
`Bound` `bound_` | the sentinel value, may be unreachable (exposition-only member object*)

### Member functions

(constructor) | creates an `iota_view` (public member function)
begin | obtains the beginning iterator of an `iota_view` (public member function)
end | obtains the sentinel denoting the end of an `iota_view` (public member function)
empty | tests whether the `iota_view` is empty (i.e. the iterator and the sentinel compare equal) (public member function)
size (optional) | obtains the size of an `iota_view` (only provided if it is bounded) (public member function)

##### Inherited from std::ranges::view_interface

cbegin (C++23) | returns a constant iterator to the beginning of the range. (public member function of `std::ranges::view_interface<D>`)
cend (C++23) | returns a sentinel for the constant iterator of the range. (public member function of `std::ranges::view_interface<D>`)
operator bool | 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>`)
front | returns the first element in the derived view. Provided if it 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 `bidirectional_range` and `common_range`. (public member function of `std::ranges::view_interface<D>`)
operator[] | returns the `n`th element in the derived view. Provided if it satisfies `random_access_range`. (public member function of `std::ranges::view_interface<D>`)

### Deduction guides

### Nested classes

iterator | the iterator type (exposition-only member class*)
sentinel | 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 > constexpr bool ranges::enable_borrowed_range <ranges::iota_view<W, Bound>> = true; |  | (since C++20)

This specialization of 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 C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 4096 | C++20 | `views::iota` could copy an `iota_view` as-is | forbidden
P2325R3 | C++20 | `iota_view` required that `W` is `semiregular` as `view` required `default_initializable` | only requires that `W` is `copyable`

### See also

iota (C++11) | fills a range with successive increments of the starting value (function template)
ranges::iota (C++23) | fills a range with successive increments of the starting value (algorithm function object)
ranges::repeat_viewviews::repeat (C++23) | a `view` consisting of a generated sequence by repeatedly producing the same value (class template) (customization point object)

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