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 ) 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

#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