std::empty

Defined in header `<array>`
Defined in header `<deque>`
Defined in header `<flat_map>`		(since C++23)
Defined in header `<flat_set>`		(since C++23)
Defined in header `<forward_list>`
Defined in header `<hive>`		(since C++26)
Defined in header `<inplace_vector>`		(since C++26)
Defined in header `<iterator>`
Defined in header `<list>`
Defined in header `<map>`
Defined in header `<optional>`		(since C++26)
Defined in header `<regex>`
Defined in header `<set>`
Defined in header `<span>`		(since C++20)
Defined in header `<stacktrace>`		(since C++23)
Defined in header `<string>`
Defined in header `<string_view>`
Defined in header `<unordered_map>`
Defined in header `<unordered_set>`
Defined in header `<valarray>`
Defined in header `<vector>`

template< class C > constexpr auto empty( const C& c ) noexcept(noexcept(c.empty())) -> 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)

Returns whether the given range is empty.

1) Returns c.empty().

2) Returns false.

Parameters

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

Return value

1) c.empty()

2) false

Exceptions

1) What and when the underlying c.empty() call throws.

Notes

Feature-test macro	Value	Std	Feature
`__cpp_lib_nonmember_container_access`	`201411L`	(C++17)	`std::size()`, `std::data()`, and `std::empty()`
`__cpp_lib_initializer_list`	`202511L`	(C++26) (DR11)	`data` and `empty` member functions of `std::initializer_list`; removing unnecessary free functions for `std::initializer_list`^[1]

As of 2026-05-27, libstdc++ has not treated the addition and removal of functions from P3016R6 as a defect report against C++11, and only applies the changes since C++26.
The overload for std::initializer_list was necessary before P3016R6 because it did not have a member function empty.

Possible implementation

First version
template<class C> [[nodiscard]] constexpr auto empty(const C& c) noexcept(noexcept(c.empty())) -> 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; }

First version

template<class C>
[[nodiscard]] constexpr auto empty(const C& c) noexcept(noexcept(c.empty()))
    -> 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;
}

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

Defect report

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
P3016R6	C++17	1) non-member `std::empty` overload was provided for `std::initializer_list` 2) `std::empty` was not required to propagate exception specification	1) removed 2) required