# std::ranges::views::counted

< cpp‎ | ranges
 Defined in header  inline constexpr /*unspecified*/ counted = /*unspecified*/; (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 e and f are expressions, T is std::decay_t<decltype((e))>, and D is , then

if T models input_or_output_iterator and decltype((f)) models ,
Otherwise, views::counted(e, f) is ill-formed.

## Contents

### Expression-equivalent

Expression e is expression-equivalent to expression f, if e and f have the same effects, either are both potentially-throwing or are both not potentially-throwing (i.e. noexcept(e) == noexcept(f)), and either are both constant subexpressions or are both not constant subexpressions.

###  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.

###  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

 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::countranges::count_if(C++20)(C++20) returns the number of elements satisfying specific criteria (niebloid)