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iota_view class (C++ Standard Library)

  • Artikkeli

Generates a view of a sequence of elements by repeatedly incrementing an initial value. The sequence can be bounded or unbounded.

Syntax

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>>;

Template parameters

W
The type of the values in the sequence. The specified type must support operator++.

Bound
The type of the end value. If Bound is std::unreachable_sentinel_t (the default value), then the view is unbounded.

View characteristics

For a description of the following entries, see View class characteristics

Characteristic Description
Range adaptor views::iota
Underlying range Not applicable because this view generates its members
Element type Same as the template parameter W
View iterator category Supports input_range up to random_access_range, depending on the type of W
Sized Only if the range has an end value, that is, it isn't an infinite series
Is const-iterable Yes
Common range Only if Bound is the same type as W
Borrowed range Yes

Members

Member functions Description
Constructors C++20 Construct the view.
beginC++20 Get an iterator to the first element.
endC++20 Get the sentinel at the end of the view.
sizeC++20 Get the number of elements.
Inherited from view_interface Description
backC++20 Get the last element.
emptyC++20 Test whether the view is empty.
frontC++20 Get the first element.
operator[]C++20 Get the element at the specified position.
operator boolC++20 Test whether the view isn't empty.

Remarks

A good way to create a iota_view is by using the iota range adaptor. Range adaptors are the intended way to create view classes. The view types are exposed in case you want to create your own custom view type.

The sequence of values can be integral values such as 1,2,3 or 'a', 'b', 'c' or consecutive elements from a range.

This view is typically used to iterate over a series of values. For example:

for (int i : iota_view{1, 10}) // iterate over a view of the integers 1 through 9
{
    std::cout << i << ’ ’; // 1 2 3 4 5 6 7 8 9
}

Requirements

Header: <ranges> (since C++20)

Namespace: std::ranges

Compiler Option: /std:c++20 or later is required.

Constructors

Create an instance of an iota_view.

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

Parameters

value
The starting value for the series.

bound
The bound of the series. It's one greater than the last value in the series.

For information about the template parameter type, see Template parameters.

Remarks

1) Create an iota_view with a starting and ending value determined by the default-initialized template type W.
2) Create an iota_view with the specified starting value and ending value determined by the value-initialized type W.
3) Create a bounded iota_view with the specified starting value and ending value. The ending value is one less than the last value specified.
4) Used to create subviews. For example, std::ranges::iota_view(start, end); where start and end are iterators to the start and end of the subview.

Rather than create this class directly, an easy way to create a iota_view is by using the iota range adaptor. Range adaptors are the intended way to create view classes. The view types are exposed in case you want to create your own custom view type.

If the bound's type is unreachable_sentinel_t, the resulting view is bounded.

When you use one of the constructors that value-initializes the bound, the bound will be the default-initialized value for that type. For example, iota_view<int, int>{} is an empty range of int since the default-constructed int value and bound are both 0. Also, iota_view<int, int>{-4} is -4, -3, -2, -1 because the default-constructed int bound is 0.

iota_view constructor example

// requires /std:c++20, or later
#include <ranges>
#include <iostream>
#include <vector>

int main()
{
    std::ranges::iota_view<int> iv; // create an iota_view with an unbounded range, starting at 0 
    std::ranges::iota_view<int> iv2(5); // create an iota_view with an unbounded range, starting at 5.
    std::ranges::iota_view<int, int> iv3{5, 10}; // create an iota_view with a bounded range, starting at 5 and ending at 9
    
    std::vector<int> v{10, 20, 35, 45, 50, 66, 77, 82, 90, 100};
    auto start = std::ranges::find(v, 35);
    auto end = std::ranges::find(v, 82);
    for (auto &&val : std::ranges::iota_view(start, end))
    {
        std::cout << *val << ' '; // outputs 35 45 50 66 77
    }
}

begin

Get an iterator to the first element in the view.

constexpr auto begin();
constexpr auto begin() const requires range<const V>;

Parameters

None.

Return value

An iterator pointing at the first element in the view.

end

Get the end sentinel at the end of the view.

constexpr auto end();
constexpr auto end() const requires ranges::range<const V>;

Parameters

None.

Return value

If the iota_view is unbounded, returns std::unreachable_sentinel.

If the iota_view is bounded, returns an iterator pointing at the sentinel past the last value in the sequence.

size

Get the number of elements in the view. The iota_view must be bounded.

constexpr auto size() requires
    (same_as<W, Bound> && advanceable <W>) ||
    (integral<W> && integral<Bound>) ||
    sized_sentinel_for<Bound, W>;

Parameters

None.

Return value

The number of elements in the iota_view.

Remarks

You can't call size() on an unbounded iota_view.

See also

<ranges>
iota() range adaptor
std::numerics::iota
view classes