std::ranges::views::counted

inline constexpr /*unspecified*/ counted = /*unspecified*/;

template< class Iterator, class DifferenceType >
    requires /* see below */
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.

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

#include <iostream>
#include <ranges>
 
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';
}

1 2 3
2 3 4

Defect reports

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

See also