< cpp‎ | ranges
Ranges library
Range access
Range conversions

Range primitives

Dangling iterator handling
Range concepts

Range factories
Range adaptors
Range generators
Range adaptor objects
Range adaptor closure objects
Helper items
(until C++23)(C++23)

Defined in header <ranges>
Defined in header <iterator>
inline namespace /* unspecified */ {

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

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

    requires /* see below */

constexpr /* see below */ 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.

(until C++23)

Returns a constant iterator to the first element of the argument that is treated as a reversed sequence.

(since C++23)


Let CT be

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

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

(until C++23)

If the argument is an lvalue or ranges::enable_borrowed_range<std::remove_cv_t<T>> is true, then a call to ranges::crbegin is expression-equivalent to:

In all other cases, a call to ranges::crbegin is ill-formed, which can result in substitution failure when the call appears in the immediate context of a template instantiation.

(since C++23)

The return type models std::input_or_output_iterator and constant-iterator(since C++23) in all cases.

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] Example

#include <cassert>
#include <iterator>
#include <span>
#include <vector>
int main()
    std::vector<int> v{3, 1, 4};
    auto vi = std::ranges::crbegin(v);
    assert(*vi == 4);
    ++vi; // OK, iterator object is mutable
    assert(*vi == 1);
    // *vi = 13; // Error: underlying element is read-only
    int a[]{-5, 10, 15};
    auto ai = std::ranges::crbegin(a);
    assert(*ai == 15);
    // 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
    assert(*si == 15);

[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]