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std::empty

Defined in header `<array>`
Defined in header `<deque>`
Defined in header `<forward_list>`
Defined in header `<iterator>`
Defined in header `<list>`
Defined in header `<map>`
Defined in header `<regex>`
Defined in header `<set>`
Defined in header `<span>`		(since C++20)
Defined in header `<string>`
Defined in header `<string_view>`
Defined in header `<unordered_map>`
Defined in header `<unordered_set>`
Defined in header `<vector>`
	(1)
template <class C> constexpr auto empty(const C& c) -> decltype(c.empty());		(since C++17) (until C++20)
template <class C> [[nodiscard]] constexpr auto empty(const C& c) -> decltype(c.empty());		(since C++20)
	(2)
template <class T, std::size_t N> constexpr bool empty(const T (&array)[N]) noexcept;		(since C++17) (until C++20)
template <class T, std::size_t N> [[nodiscard]] constexpr bool empty(const T (&array)[N]) noexcept;		(since C++20)
	(3)
template <class E> constexpr bool empty(std::initializer_list<E> il) noexcept;		(since C++17) (until C++20)
template <class E> [[nodiscard]] constexpr bool empty(std::initializer_list<E> il) noexcept;		(since C++20)

Returns whether the given range is empty.

1) returns c.empty()

2) returns false

3) returns il.size() == 0

Parameters

c	-	a container or view with an `empty` member function
array	-	an array of arbitrary type
il	-	an initializer list

Return value

true if the range doesn't have any element.

Exceptions

1) May throw implementation-defined exceptions.

Notes

The overload for std::initializer_list is necessary because it does not have a member function empty.

Feature-test macro
`__cpp_lib_nonmember_container_access`

Possible implementation

First version
template <class C> [[nodiscard]] constexpr auto empty(const C& c) -> decltype(c.empty()) { return c.empty(); }
Second version
template <class T, std::size_t N> [[nodiscard]] constexpr bool empty(const T (&array)[N]) noexcept { return false; }
Third version
template <class E> [[nodiscard]] constexpr bool empty(std::initializer_list<E> il) noexcept { return il.size() == 0; }

Example

#include <iostream>
#include <vector>
 
template <class T>
void print(const T& container)
{
    if ( std::empty(container) )
    {
        std::cout << "Empty\n";
    }
    else
    {
        std::cout << "Elements:";
        for ( const auto& element : container )
            std::cout << ' ' << element;
        std::cout << '\n';
    }
}
 
int main() 
{
    std::vector<int> c = { 1, 2, 3 };
    print(c);
    c.clear();
    print(c);
 
    int array[] = { 4, 5, 6 };
    print(array);
 
    auto il = { 7, 8, 9 };
    print(il);
}

Output:

Elements: 1 2 3
Empty
Elements: 4 5 6
Elements: 7 8 9