# std::ranges::iota_view<W, Bound>::iota_view

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< cpp‎ | ranges‎ | 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 explicit iota_view( std::type_identity_t <W> value, std::type_identity_t <Bound> bound ); | (3) | (since C++20)
constexpr explicit iota_view( /*iterator*/ first, /* see below */ last ); | (4) | (since C++20)

Constructs an `iota_view`.

Overload | Data members
--- | ---
`value_` | `bound_`
(1) | value-initialized | value-initialized
(2) | initialized with value
(3) | initialized with bound
(4) | initialized with first.`value_` | see below

2,3) If any of the following conditions is satisfied, the behavior is undefined:

- Bound() is unreachable from value, unless `Bound` denotes std::unreachable_sentinel_t.
- `W` and `Bound` model `totally_ordered_with`, and bool(value <= bound) is false.

4) If any of the following conditions is satisfied, the behavior is undefined:

- Bound() is unreachable from value, unless `Bound` denotes std::unreachable_sentinel_t.
- `W` and `Bound` model `totally_ordered_with`, and bool(first.`value_``` <= bound) is false.

The type of last and the method of initializing `bound_` are determined by the type `Bound` denotes:

The type `Bound` denotes | The type of last | `bound_`
--- | --- | ---
`W` | `iterator` | initialized with last.`value_`
std::unreachable_sentinel_t | `Bound` | initialized with last
any other type | `sentinel` | initialized with last.`bound_`

### Parameters

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

### Example

Run this code

    #include <cassert>
    #include <iostream>
    #include <iterator>
    #include <ranges>

    int main()
    {
        const auto l = {1, 2, 3, 4};

        auto i1 = std::ranges::iota_view<int, int>(); // overload (1)
        assert(i1.empty() and i1.size() == 0);

        auto i2 = std::ranges::iota_view(1); // overload (2)
        assert(not i2.empty() and i2.front() == 1);
        for (std::cout << "1) "; auto e : i2 | std::views::take(3))
            std::cout << e << ' ';
        std::cout << '\n';

        auto i3 = std::ranges::iota_view(std::begin(l)); // overload (2)
        assert(not i3.empty() and i3.front() == l.begin());
        for (std::cout << "2) "; auto e : i3 | std::views::take(4))
            std::cout << *e << ' ';
        std::cout << '\n';

        auto i4 = std::ranges::iota_view(1, 8); // overload (3)
        assert(not i4.empty() and i4.front() == 1 and i4.back() == 7);
        for (std::cout << "3) "; auto e : i4)
            std::cout << e << ' ';
        std::cout << '\n';

        auto i5 = std::ranges::iota_view(l.begin(), l.end()); // overload (4)
        for (std::cout << "4) "; auto e : i5)
            std::cout << *e << ' ';
        std::cout << '\n';

        auto i6 = std::ranges::iota_view(l.begin(), std::unreachable_sentinel); // (4)
        for (std::cout << "5) "; auto e : i6 | std::views::take(3))
            std::cout << *e << ' ';
        std::cout << '\n';
    }

Output:

    1) 1 2 3
    2) 1 2 3 4
    3) 1 2 3 4 5 6 7
    4) 1 2 3 4
    5) 1 2 3

### 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 | overload (4) might use wrong sentinel type | corrected
P2711R1 | C++20 | overloads (3,4) were not explicit | made explicit

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