std::ranges::crend(3)

💅 std::ranges::crend(3) — C++ Standard Library realness 💅

okay so bestie, you found a page from the C++ Standard Library manpage set, and the main character is std::ranges::crend. Section 3, C++20 energy, constant-iterator vibes. Let's get into it 💖

🌸 NAME 🌸

std::ranges::crend — yes, literally std::ranges::crend. The manpage was like “std::ranges::crend - std::ranges::crend” and honestly? Iconic. No notes.

🦋 Synopsis 🦋

It's defined in header <ranges> and defined in header <iterator>. Both. Bestie gets two homes 🫶

inline namespace /* unspecified */ {
    inline constexpr /* unspecified */ crend = /* unspecified */;  (since C++20)
}

And that thing right there? It's a (customization point object) ✨

Call signature:

template< class T >
    requires /* see below */  (since C++20)
constexpr /* see below */ auto crend( T&& t );

So what does it actually do? It returns a sentinel for the constant iterator (since C++23) indicating the end of a const-qualified (until C++23) range that is treated as a reversed sequence. That's the whole vibe check 🌀

The docs even have a lil diagram called range-rbegin-rend.svg — yes, that's the file name, don't @ me. Visual aid slay.

Let CT be:

• const std::remove_reference_t<T>& if the argument is an lvalue (i.e. T is an lvalue reference type), (until C++23)
• const T otherwise. (C++23)

A call to ranges::crend is expression-equivalent to ranges::rend(static_cast<CT&&>(t)). Fr fr, that's the equivalence.

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

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

🚨 In all other cases, a call to ranges::crend is ill-formed, which can result in substitution failure when the call appears in the immediate context of a template instantiation. That's the “bestie, this does not compile in this context” moment 💀

If ranges::crend(e) is valid for an expression e, where decltype((e)) is T, then CT models std::ranges::range, and (until C++23) std::sentinel_for<S, I> is true in all cases, where S is decltype(ranges::crend(e)), and I is decltype(ranges::crbegin(e)). Additionally, S models constant-iterator if it models input_iterator. (since C++23)

🎀 Customization point objects 🎀

The name ranges::crend 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 __crend_fn.

All instances of __crend_fn are equal. The effects of invoking different instances of type __crend_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::crend 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::crend above, __crend_fn models:

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

Otherwise, no function call operator of __crend_fn participates in overload resolution. Periodt 💅

🌈 Example 🌈

// Run this code

#include <algorithm>
#include <iostream>
#include <iterator>
#include <vector>

int main()
{
    int a[]{4, 6, -3, 9, 10};
    std::cout << "Array backwards: ";
    namespace ranges = std::ranges;
    ranges::copy(ranges::rbegin(a), ranges::rend(a),
                 std::ostream_iterator<int>(std::cout, " "));
    std::cout << '\n';

    std::cout << "Vector backwards: ";
    std::vector v{4, 6, -3, 9, 10};
    ranges::copy(ranges::rbegin(v), ranges::rend(v),
                 std::ostream_iterator<int>(std::cout, " "));
    std::cout << '\n';
}

⭐ Output ⭐

Array backwards: 10 9 -3 6 4
Vector backwards: 10 9 -3 6 4

💜 See also 💜

• ranges::rend (C++20) — returns a reverse end iterator to a range (customization point object)
• rend (C++14) — returns a reverse end iterator for a container or array (function template)
• crend (C++14) — (function template)

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and that's the page, bestie 🍓 Source/version/date tea: cppreference.com, 2024.06.10, and this was std::ranges::crend(3). Go forth and reverse-iterate responsibly, queen 👑💖