std::ranges::view::iota, std::ranges::iota_view

< cpp‎ | ranges
template<WeaklyIncrementable W, Semiregular Bound = unreachable_sentinel_t>

    requires _WeaklyEqualityComparableWith<W, Bound>

class iota_view : public view_interface<iota_view<W, Bound>>
(1) (since C++20)
namespace view {

    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 both bounded and unbounded (infinite)
2) view::iota(E) and view::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

Data members


W value_ = W(); /* exposition-only */

the current value


Bound bound_ = Bound(); /* exposition-only */

the bound (defaults to std::unreachable_sentinel_t)

Member functions


iota_view() = default;
constexpr explicit iota_view(W value);
constexpr iota_view(std::type_identity_t<W> value,
                    std::type_identity_t<Bound> bound);
1) Default-initializes value_ and bound_
2) Initializes value_ with value and expects that Bound is either unreachable_­sentinel_­t (the default) or Bound() is reachable from value. 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. This constructor is used to create bounded iota views, e.g. iota(10, 20) yields numbers from 10 to 19.


value - the starting value
bound - the bound


constexpr iterator begin();

Returns the iterator initialized with value_


constexpr sentinel end() const;
constexpr iterator end() const requires Same<W, Bound>;

Returns the iterator initialized with bound_ if this view is bounded, or the sentinel initialized with bound_ if this view is unbounded.


constexpr auto size() const

  requires (Same<W, Bound> && _Advanceable<W>) ||
           (Integral<W> && Integral<Bound>) ||
             SizedSentinel<Bound, W>

  { return bound_ - value_; }

Returns the size of the view if the view is bounded.

Deduction guides

template<class W, class Bound>

    requires (!Integral<W> || !Integral<Bound> || is_signed_v<W> == is_signed_v<Bound>)

  iota_view(W, Bound) -> iota_view<W, Bound>;

Note that the guide protects itself against signed/unsigned mismatch bugs, like view::iota(0, v.size()), where 0 is a (signed) int and v.size() is an (unsigned) std::size_t.

Nested classes


template<class W, class Bound>
struct iota_view<W, Bound>::iterator /* exposition-only */

The return type of iota_view::begin.

This is a RandomAccessIterator if W models _Advanceable, a BidirectionalIterator if W models _Decrementable, a ForwardIterator if W models Incrementable, and InputIterator otherwise.


constexpr explicit iterator(W value);

Initializes exposition-only data member value_ with value. This value will be returned by operator* and incremented by operator++


constexpr W operator*() const noexcept(is_nothrow_copy_constructible_v<W>);

returns the current value, by value (in other words, this is a read-only view)


constexpr iterator& operator++()

Equivalent to

return *this;


constexpr iterator& operator--() requires _Decrementable<W>;

Equivalent to

return *this;


constexpr W operator[](difference_type n) const requires _Advanceable<W>;

Equivalent to

return value_­ + n;

Other members as expected of an iterator.


template<class W, class Bound>
struct iota_view<W, Bound>::sentinel /* exposition-only */

The return type of iota_view::end.


Bound bound_ = Bound();; /* exposition only */

Exposition-only data member holding the sentinel (typically either a number, for a bounded iota view, or an instance of std::unreachable_sentinel_t for an unbounded iota view.


entinel() = default;
constexpr explicit sentinel(Bound bound);

Initializes exposition-only data member bound_ with bound.


friend constexpr bool operator==(const iterator& x, const sentinel& y);
friend constexpr bool operator==(const sentinel& x, const iterator& y);

Equivalent to: x.value_­ == y.bound_­; and return y == x; respectively.


friend constexpr bool operator!=(const iterator& x, const sentinel& y);
friend constexpr bool operator!=(const sentinel& x, const iterator& y);

Equivalent to return !(x == y); and return !(y == x); respectively


#include <ranges>
#include <vector>
#include <iostream>
int main()
  for (int i : std::view::iota{1, 10})
    std::cout << i << ' ';
  std::cout << '\n';
  for (int i : std::view::iota(1) | std::view::take(9))
    std::cout << i << ' ';


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