std::ranges::views::counted

 

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Ranges library

Range access
begin cbegin end cend	rbegin crbegin rend crend	size ssize data cdata	empty
Range conversions
to (C++23)
Range primitives
range_size_trange_difference_t range_value_t	range_reference_trange_const_reference_trange_rvalue_reference_t   (C++23)	iterator_tconst_iterator_tsentinel_t (C++23)
Dangling iterator handling
dangling	borrowed_iterator_t	borrowed_subrange_t
Range concepts
range borrowed_range    sized_range constant_range (C++23)	common_range view viewable_range	input_range output_range forward_range	bidirectional_range random_access_range contiguous_range
Views
view_interface	subrange
Factories
empty_viewviews::empty iota_viewviews::iota	single_viewviews::single basic_istream_viewviews::istream	repeat_viewviews::repeat (C++23)(C++23)
Adaptors
ref_view views::all_tviews::all common_viewviews::common owning_view views::counted reverse_viewviews::reverse zip_viewviews::zip (C++23)(C++23) as_const_viewviews::as_const (C++23)(C++23) as_rvalue_viewviews::as_rvalue (C++23)(C++23)	take_viewviews::take take_while_viewviews::take_while drop_viewviews::drop drop_while_viewviews::drop_while zip_transform_viewviews::zip_transform (C++23)(C++23) adjacent_transform_viewviews::adjacent_transform (C++23)(C++23)   cartesian_product_viewviews::cartesian_product (C++23)(C++23)	elements_viewviews::elements keys_viewviews::keys values_viewviews::values transform_viewviews::transform adjacent_viewviews::adjacent (C++23)(C++23) slide_viewviews::slide (C++23)(C++23) stride_viewviews::stride (C++23)(C++23)	filter_viewviews::filter join_viewviews::join split_viewviews::split lazy_split_viewviews::lazy_split join_with_viewviews::join_with (C++23)(C++23) chunk_viewviews::chunk (C++23)(C++23) chunk_by_viewviews::chunk_by (C++23)(C++23)
Range adaptor objects
Range adaptor closure objects
Helper items
copyable-box	simple-view

 

Defined in header <ranges>
inline constexpr /*unspecified*/ counted = /*unspecified*/;		(since C++20)
Call signature
template< class Iterator, class DifferenceType >     requires /* see below */ constexpr /*span-or-subrange*/ counted( Iterator&& it, DifferenceType&& count );		(since C++20)

A counted view presents a view of the elements of the counted range [i, n) for some iterator i and non-negative integer n.

A counted range [i, n) is the n elements starting with the element pointed to by i and up to but not including the element, if any, pointed to by the result of n applications of ++i.

If n == 0, the counted range is valid and empty. Otherwise, the counted range is only valid if n is positive, i is dereferenceable, and [++i, --n) is a valid counted range.

Formally, if it and count are expressions, T is std::decay_t<decltype((it))>, and D is std::iter_difference_t<T>, then

if T models input_or_output_iterator and decltype((count)) models std::convertible_to<D>,

• if T models contiguous_iterator, then views::counted(it, count) is expression-equivalent to std::span(std::to_address(it), static_cast<std::size_t>(static_cast<D>(count))),
• otherwise, if T models random_access_iterator, then views::counted(it, count) is expression-equivalent to ranges::subrange(it, it + static_cast<D>(count)),
• otherwise, views::counted(it, count) is expression-equivalent to ranges::subrange(std::counted_iterator(it, count), std::default_sentinel).

Otherwise, views::counted(it, count) is ill-formed.

Expression-equivalent

Expression e is expression-equivalent to expression f, if

• e and f have the same effects, and
• either both are constant subexpressions or else neither is a constant subexpression, and
• either both are potentially-throwing or else neither is potentially-throwing (i.e. noexcept(e) == noexcept(f)).

Customization point objects

The name views::counted 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 __counted_fn.

All instances of __counted_fn are equal. The effects of invoking different instances of type __counted_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::counted 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::counted above, __counted_fn models

• std::invocable<__counted_fn, Args...>,
• std::invocable<const __counted_fn, Args...>,
• std::invocable<__counted_fn&, Args...>, and
• std::invocable<const __counted_fn&, Args...>.
  

Otherwise, no function call operator of __counted_fn participates in overload resolution.

Notes

views::counted does not check if the range is long enough to provide all count elements: use views::take if that check is necessary.

Example

Run this code

#include <ranges>
#include <iostream>
 
int main()
{
    const int a[] = {1, 2, 3, 4, 5, 6, 7};
    for(int i : std::views::counted(a, 3))
        std::cout << i << ' ';
    std::cout << '\n';
 
    const auto il = {1, 2, 3, 4, 5};
    for (int i : std::views::counted(il.begin() + 1, 3))
        std::cout << i << ' ';
    std::cout << '\n';
}

Output:

1 2 3 
2 3 4

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
P2393R1	C++20	implicit conversion from an integer-class type to size_t might be invalid	made explicit

See also

ranges::take_viewviews::take (C++20)	a view consisting of the first N elements of another view (class template) (range adaptor object)
ranges::subrange (C++20)	combines an iterator-sentinel pair into a view (class template)
counted_iterator (C++20)	iterator adaptor that tracks the distance to the end of the range (class template)
ranges::countranges::count_if (C++20)(C++20)	returns the number of elements satisfying specific criteria (niebloid)