NAME

std::ranges::views::counted - std::ranges::views::counted

Synopsis

Defined in header <ranges>
inline constexpr /*unspecified*/ counted = /*unspecified*/; (since C++20)
Call signature
template< class Iterator, class DifferenceType >

requires /* see below */ (since C++20)

constexpr /*span-or-subrange*/ counted( Iterator&& it,
DifferenceType&& count );

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 made explicit
to size_t might be invalid

See also

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