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std::ranges::views::counted

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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>,
Otherwise, views::counted(it, count) is ill-formed.

Contents

[edit] 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 potentially-throwing (i.e. noexcept(e) == noexcept(f)).

[edit] 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

Otherwise, no function call operator of __counted_fn participates in overload resolution.

[edit] 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.

[edit] Example

#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

[edit] 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 size_t might be invalid made explicit

[edit] See also

a view consisting of the first N elements of another view
(class template) (range adaptor object) [edit]
combines an iterator-sentinel pair into a view
(class template) [edit]
iterator adaptor that tracks the distance to the end of the range
(class template) [edit]
returns the number of elements satisfying specific criteria
(niebloid) [edit]