std::empty

 

C++

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Feature test macros (C++20)

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Iterator library

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Iterator primitives
input_iterator_tagoutput_iterator_tagforward_iterator_tagbidirectional_iterator_tagrandom_access_iterator_tagcontiguous_iterator_tag (C++20)	iter_value_titer_difference_titer_reference_titer_const_reference_titer_rvalue_reference_titer_common_reference_t (C++20)(C++20)(C++20)(C++23)(C++20)(C++20)	iterator (deprecated in C++17) iterator_traits incrementable_traits (C++20) indirectly_readable_traits (C++20)
Algorithm concepts and utilities
Indirect callable concepts
indirectly_unary_invocableindirectly_regular_unary_invocable (C++20)(C++20)	indirect_unary_predicate (C++20) indirect_binary_predicate (C++20)	indirect_equivalence_relation (C++20) indirect_strict_weak_order (C++20)
Common algorithm requirements
indirectly_movable (C++20) indirectly_movable_storable (C++20) indirectly_copyable (C++20)	indirectly_copyable_storable (C++20)   indirectly_swappable (C++20) indirectly_comparable (C++20)	permutable (C++20) mergeable (C++20) sortable (C++20)
Utilities
indirect_result_t (C++20)	projected (C++20)	projected_value_t (C++26)
Iterator adaptors
reverse_iterator make_reverse_iterator (C++14) move_iterator (C++11) make_move_iterator (C++11) default_sentinel_tdefault_sentinel (C++20)(C++20) unreachable_sentinel_tunreachable_sentinel (C++20)(C++20)	front_insert_iterator back_insert_iterator inserter insert_iterator front_inserter back_inserter move_sentinel (C++20)	common_iterator (C++20) counted_iterator (C++20) basic_const_iterator (C++23) const_iterator (C++23) const_sentinel (C++23) make_const_iterator (C++23) make_const_sentinel (C++23)
Stream iterators istream_iterator    ostream_iterator istreambuf_iterator ostreambuf_iterator	Iterator customization points ranges::iter_move (C++20) ranges::iter_swap (C++20)
Iterator operations advance distance prev (C++11)   next (C++11) ranges::advance (C++20) ranges::distance (C++20) ranges::prev (C++20) ranges::next (C++20)	Range access begincbegin (C++11)(C++14) rbegincrbegin (C++14)(C++14)   endcend (C++11)(C++14) rendcrend (C++14)(C++14)   sizessize (C++17)(C++20) empty (C++17) data (C++17)

 

Defined in header <iterator>
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 container is empty.

1) returns c.empty()

2) returns false

3) returns il.size() == 0

Parameters

c	-	a container with an empty method
array	-	an array of arbitrary type
il	-	an initializer list

Return value

true if the container doesn't have any element.

Exceptions

2,3)

noexcept specification:  

noexcept

  

Notes

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>.

Possible implementation

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; }

Example

Run this code

#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