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	Value	Std	Feature
`__cpp_lib_nonmember_container_access`	201411L	(C++17)	`std::size()`, `std::data()`, and `std::empty()`

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