NAME¶

std::empty - std::empty

Synopsis¶

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 <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 > (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 empty( C++20)
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 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¶

// 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)