# std::ranges::empty

From cppreference.com
< cpp‎ | ranges

Defined in header `<ranges>` |  |
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)

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

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