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std::ranges...ews::counted(3) C++ Standard Libarystd::ranges...ews::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<de- cltype((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 poten- tially-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, regard- less 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 in- terchangeably. 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 retroac- tively 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 an- other 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) http://cppreference.com 2022.07.31 std::ranges...ews::counted(3)
NAME | Synopsis | Notes | Example | Output: | See also
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