# std::ranges::views::counted

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< cpp‎ | ranges

Defined in header `<ranges>` |  |
inline constexpr /*unspecified*/ counted = /*unspecified*/; |  | (since C++20)
Call signature |  |
template< class Iterator, class DifferenceType > requires /* see below */ constexpr /*span-or-subrange*/ counted( Iterator&& it, DifferenceType&& count ); |  | (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 `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

Run this code

    #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

ranges::take_view views::take (C++20) | a `view` consisting of the first N elements of another `view` (class template) (range adaptor object)
ranges::subrange (C++20) | combines an iterator-sentinel pair into a `view` (class template)
counted_iterator (C++20) | iterator adaptor that tracks the distance to the end of the range (class template)
ranges::count ranges::count_if (C++20)(C++20) | returns the number of elements satisfying specific criteria (niebloid)

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