Defined in header <iterator> | ||
---|---|---|
(1) | ||
template <class C> constexpr auto empty(const C& c) -> decltype(c.empty()); | (since C++17) (until C++20) | |
template <class C> [[nodiscard]] constexpr auto empty(const C& c) -> decltype(c.empty()); | (since C++20) | |
(2) | ||
template <class T, std::size_t N> constexpr bool empty(const T (&array)[N]) noexcept; | (since C++17) (until C++20) | |
template <class T, std::size_t N> [[nodiscard]] constexpr bool empty(const T (&array)[N]) noexcept; | (since C++20) | |
(3) | ||
template <class E> constexpr bool empty(std::initializer_list<E> il) noexcept; | (since C++17) (until C++20) | |
template <class E> [[nodiscard]] constexpr bool empty(std::initializer_list<E> il) noexcept; | (since C++20) |
Returns whether the given container is empty.
c.empty()
false
il.size() == 0
c | - | a container with an empty method |
array | - | an array of arbitrary type |
il | - | an initializer list |
true
if the container doesn't have any element.
In addition to being included in <iterator>
, std::empty
is guaranteed to become available if any of the following headers are included: <array>
, <deque>
, <forward_list>
, <list>
, <map>
, <regex>
, <set>
, <span>
(since C++20), <string>
, <string_view>
, <unordered_map>
, <unordered_set>
, and <vector>
.
First version |
---|
template <class C> constexpr auto empty(const C& c) -> decltype(c.empty()) { return c.empty(); } |
Second version |
template <class T, std::size_t N> constexpr bool empty(const T (&array)[N]) noexcept { return false; } |
Third version |
template <class E> constexpr bool empty(std::initializer_list<E> il) noexcept { return il.size() == 0; } |
#include <iostream> #include <vector> template <class T> void print(const T& container) { if ( !std::empty(container) ) { std::cout << "Elements:\n"; for ( const auto& element : container ) std::cout << element << '\n'; } else { std::cout << "Empty\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
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