< cpp‎ | ranges
Ranges library
Range access
Range conversions
Range primitives

Dangling iterator handling
Range concepts


Range adaptor objects
Range adaptor closure objects
Helper items
Defined in header <ranges>
inline namespace /* unspecified */ {

    inline constexpr /* unspecified */ crbegin = /* unspecified */;

(since C++20)
(customization point object)
Call signature
template< class T >

    requires /* see below */

constexpr std::input_or_output_iterator auto crbegin( T&& t );
(since C++20)

Returns an iterator to the first element of the const-qualified argument that is treated as a reversed sequence.


Let CT be

  1. const std::remove_reference_t<T>& if the argument is an lvalue (i.e. T is an lvalue reference type),
  2. const T otherwise,

a call to ranges::crbegin is expression-equivalent to ranges::rbegin(static_cast<CT&&>(t)).

The return type models std::input_or_output_iterator in both cases.


[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 ranges::crbegin 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 __crbegin_fn.

All instances of __crbegin_fn are equal. The effects of invoking different instances of type __crbegin_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, ranges::crbegin 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 ranges::crbegin above, __crbegin_fn models

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

[edit] Notes

If the argument is an rvalue (i.e. T is an object type) and ranges::enable_borrowed_range<std::remove_cv_t<T>> is false, the call to ranges::crbegin is ill-formed, which also results in substitution failure.

The return type models std::input_or_output_iterator in all cases.

[edit] Example

#include <iostream>
#include <vector>
#include <iterator>
#include <span>
int main()
    std::vector<int> v = { 3, 1, 4 };
    auto vi = std::ranges::crbegin(v);
    std::cout << *vi << '\n';
    int a[] = { -5, 10, 15 };
    auto ai = std::ranges::crbegin(a);
    std::cout << *ai << '\n';
    // auto x_x = std::ranges::crbegin(std::vector<int>{6,6,6});
    // ill-formed: the argument is an rvalue (see Notes ↑)
    auto si = std::ranges::crbegin(std::span{a}); // OK:
    std::cout << *si << '\n';



[edit] See also

returns a reverse iterator to a range
(customization point object) [edit]
returns a reverse iterator to the beginning of a container or array
(function template) [edit]