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 )

Formally, if it and count are expressions, T is std::decay_t <decltype( (it) ) > , and D is std::iter_difference_t<T>

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)

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

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...>
• 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

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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Defect reports

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

See also