std::ranges::crbegin(3)

🎀 std::ranges::crbegin(3) 🎀

C++ Standard Library — manual name folded in right here like a little name tag, bestie 💖. Okay so std::ranges::crbegin is the const reverse-begin girly, and she has a whole customization-point-object situation going on. Let’s get into it ✨

✨ NAME ✨

std::ranges::crbegin — std::ranges::crbegin

Like… the name is the name, and she said it twice because she’s iconic 💅.

🌸 Synopsis 🌸

Defined in header <ranges> and also defined in header <iterator> — yes, both, she contains multitudes 🦋.

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

    inline constexpr /* unspecified */ crbegin = /*     (since C++20)
unspecified */;
(customization point object)

}
Call signature
template< class T >

    requires /* see below */                            (since C++20)

constexpr /* see below */ auto crbegin( T&& t );

So the call signature is template< class T >, with requires /* see below */ (since C++20), and constexpr /* see below */ auto crbegin( T&& t ); — all up there, no cap 💜.

Here’s the return vibe:

• (until C++23) — returns an iterator to the first element of the const-qualified argument that is treated as a reversed sequence.
• (since C++23) — returns a constant iterator to the first element of the argument that is treated as a reversed sequence.

range-rbegin-rend.svg — that’s the diagram, and she’s serving visuals 🔥.

Let CT be — and this whole definition is tagged (until C++23):

• 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)).

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:

• std::const_iterator<decltype(U)>(U) for some expression U equivalent to ranges::rbegin(/*possibly-const-range*/(t)) — (since C++23).

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. So like… don’t do that unless you enjoy template tears 😭.

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

🛠️ 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

• std::invocable<__crbegin_fn, Args...>,
• std::invocable<const __crbegin_fn, Args...>,
• std::invocable<__crbegin_fn&, Args...>, and
• std::invocable<const __crbegin_fn&, Args...>.

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

🍓 Example 🍓

// Run this code — yes bestie, run it 👇

#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);
    static_assert
    (
        std::ranges::enable_borrowed_range<std::remove_cv_t<decltype(std::span{a})>>
    );
}

🔥 See also 🔥

• ranges::rbegin — returns a reverse iterator to a range; (C++20); (customization point object)
• rbegin — returns a reverse iterator to the beginning of a container or array
• crbegin — (function template); (C++14)

With love from cppreference.com, source/version/date 2024.06.10 — mwah 💋✨