std::ranges::views::counted(3)

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)