std::empty - cppreference.com (original) (raw)

Defined in header
Defined in header
Defined in header <flat_map>
Defined in header <flat_set>
Defined in header <forward_list>
Defined in header <inplace_vector>
Defined in header
Defined in header
Defined in header
Defined in header
Defined in header
Defined in header
Defined in header
Defined in header <string_view>
Defined in header <unordered_map>
Defined in header <unordered_set>
Defined in header
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.

Contents

[edit] Parameters

c - a container or view with an empty member function
array - an array of arbitrary type
il - an std::initializer_list

[edit] Return value

  1. c.empty()

  2. false

  3. il.size() == 0

[edit] Exceptions

  1. May throw implementation-defined exceptions.

[edit] 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()

[edit] 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; }

[edit] Example

#include #include   template 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 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

[edit] See also