std::ranges::empty(3)

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

Synopsis
   Defined in header <ranges>
   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 );
   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.
  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)).
  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.
   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.
   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.

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

See also
   empty checks whether the container is empty
   (C++17) (function template)