std::ranges::views::counted
| 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) | |
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
Tmodelsinput_or_output_iteratorand decltype((count)) models std::convertible_to<D>,- if
Tmodelscontiguous_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
Tmodelsrandom_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).
- if
- 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'; }
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 to std::size_t might be invalid | made explicit |
See also
| (C++20) |
a view consisting of the first N elements of another view(class template) (range adaptor object) |
| (C++20) |
combines an iterator-sentinel pair into a view (class template) |
| (C++20) |
iterator adaptor that tracks the distance to the end of the range (class template) |
| (C++20)(C++20) |
returns the number of elements satisfying specific criteria (algorithm function object) |