std::empty - cppreference.com

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

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 std::initializer_list

Return value

1) c.empty()

2) false

3) il.size() == 0

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: