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empty

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Defined in header <array>

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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

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

[edit] See also

(C++20)

checks whether a range is empty
(customization point object)[edit]

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