NAME

std::empty - std::empty

Synopsis

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>
template <class C> (since
constexpr auto empty(const C& c) -> C++17)
decltype(c.empty()); (until
C++20)
template <class C> (since
[[nodiscard]] constexpr auto empty(const C& c) -> C++20)
decltype(c.empty());
template <class T, std::size_t N> (since
constexpr bool empty(const T (&array)[N]) C++17)
noexcept; (until
(1) C++20)
template <class T, std::size_t N> (since
[[nodiscard]] constexpr bool empty(const T C++20)
(&array)[N]) noexcept; (2)
template <class E> (since
constexpr bool empty(std::initializer_list<E> il) C++17)
noexcept; (until
(3) C++20)
template <class E> (since
[[nodiscard]] constexpr bool C++20)
empty(std::initializer_list<E> il) noexcept;

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

// Run this code

#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

See also

ranges::empty checks whether a range is empty
(C++20) (customization point object)