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    std::ranges::empty

    From cppreference.com
     
    C++
     
    Ranges library
    Range adaptors
     
    Defined in header <ranges>
    Defined in header <iterator>
    inline namespace /*unspecified*/ { inline constexpr auto empty = /*unspecified*/; }
    		 (since C++20)
    (customization point object)
    Call signature
    template< class T > requires /* see below */ constexpr bool empty( T&& t );
    		 (since C++20)
    See More

    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.

    Customization point objects

    The name ranges::empty denotes a customization point object, which is a const function object of a literal semiregular class type. See CustomizationPointObject for details.

    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

    (C++17)
    		 checks whether the container is empty
    (function template) [edit]