std::ranges::empty(3)

🌸 NAME 🌸

okay so bestie, welcome to std::ranges::empty(3) from the C++ Standard Libary — yes, that’s the manual name exactly as the page has it, no cap 😭. The NAME line is literally:

std::ranges::empty - std::ranges::empty

it’s giving “same name twice but make it iconic” 💅✨.

---

✨ Synopsis ✨

copying the synopsis exactly, because the details are the whole vibe:

          Defined in header <ranges>
          Defined in header <iterator>
          inline namespace /*unspecified*/ {
                                                              (since C++20)
              inline constexpr auto empty = /*unspecified*/;  (customization
       point object)

          }
          Call signature
          template< class T >

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

          constexpr bool empty( T&& t );

So basically, this determines whether or not t has any elements. 🫶

A call to ranges::empty is expression-equivalent to:

1. bool(t.empty()), if that expression is valid.
2. Otherwise, (ranges::size(t) == 0), if that expression is valid.
3. Otherwise, bool(ranges::begin(t) == ranges::end(t)), if that expression is valid and decltype(ranges::begin(t)) models std::forward_iterator.

⚠️ In all other cases, a call to ranges::empty is ill-formed, which can result in substitution failure when ranges::empty(t) appears in the immediate context of a template instantiation. That’s the serious part, bestie — not a drill 💀.

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🧚 Customization point objects 🧚

The name ranges::empty 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 __empty_fn. cute little exposition name, honestly 🎀.

All instances of __empty_fn are equal. The effects of invoking different instances of type __empty_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::empty can be copied freely and its copies can be used interchangeably. lowkey iconic behavior ✨.

Given a set of types Args..., if std::declval<Args>()... meet the requirements for arguments to ranges::empty above, __empty_fn models

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

Otherwise, no function call operator of __empty_fn participates in overload resolution. periodt. 💅

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🌟 Example 🌟

// Run this code

#include <iostream>
#include <ranges>
#include <vector>

template<std::ranges::input_range R>
void print(char id, R&& r)
{
    if (std::ranges::empty(r))
    {
        std::cout << '\t' << id << ") Empty\n";
        return;
    }

    std::cout << '\t' << id << ") Elements:";
    for (const auto& element : r)
        std::cout << ' ' << element;
    std::cout << '\n';
}

int main()
{
    {
        auto v = std::vector<int>{1, 2, 3};
        std::cout << "(1) ranges::empty uses std::vector::empty:\n";
        print('a', v);

        v.clear();
        print('b', v);
    }
    {
        std::cout << "(2) ranges::empty uses ranges::size(initializer_list):\n";
        auto il = {7, 8, 9};
        print('a', il);

        print('b', std::initializer_list<int>{});
    }
    {
        std::cout << "(2) ranges::empty on a raw array uses ranges::size:\n";
        int array[] = {4, 5, 6}; // array has a known bound
        print('a', array);
    }
    {
        struct Scanty : private std::vector<int>
        {
            using std::vector<int>::begin;
            using std::vector<int>::end;
            using std::vector<int>::push_back;
            // Note: both empty() and size() are hidden
        };

        std::cout << "(3) calling ranges::empty on an object w/o empty() or size():\n";
        Scanty y;
        print('a', y);
        y.push_back(42);
        print('b', y);
    }
}

🌈 Output: 🌈

(1) ranges::empty uses std::vector::empty:
        a) Elements: 1 2 3
        b) Empty
(2) ranges::empty uses ranges::size(initializer_list):
        a) Elements: 7 8 9
        b) Empty
(2) ranges::empty on a raw array uses ranges::size:
        a) Elements: 4 5 6
(3) calling ranges::empty on an object w/o empty() or size():
        a) Empty
        b) Elements: 42

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👀 See also 👀

• empty — checks whether the container is empty (C++17) (function template).

xoxo, your Unix bestie 💖 — source/version: http://cppreference.com, date: 2024.06.10, page: std::ranges::empty(3).