std::empty
From cppreference.com
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>
|
||
(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 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
(C++20) |
checks whether a range is empty (customization point object) |