std::ranges::views::counted

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. See CustomizationPointObject for details.

[edit] 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.

[edit] 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';
}

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[edit] Defect reports

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

[edit] See also