std::empty

 

C++

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Named requirements

Feature test macros(C++20)

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Concepts library(C++20)

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Metaprogramming library(C++11)

General utilities library

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Iterators library

Ranges library(C++20)

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Technical specifications

Symbols index

External libraries

 

Iterator library

Iterator concepts
indirectly_readable (C++20) indirectly_writable (C++20) weakly_incrementable (C++20) incrementable (C++20) is-integer-like is-signed-integer-like (C++20)(C++20)	sentinel_for (C++20) sized_sentinel_for (C++20) input_iterator (C++20) output_iterator (C++20) input_or_output_iterator (C++20)	forward_iterator (C++20) bidirectional_iterator (C++20) random_access_iterator (C++20) contiguous_iterator (C++20)
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 <array>
Defined in header <deque>
Defined in header <flat_map>
Defined in header <flat_set>
Defined in header <forward_list>
Defined in header <inplace_vector>
Defined in header <iterator>
Defined in header <list>
Defined in header <map>
Defined in header <regex>
Defined in header <set>
Defined in header <span>
Defined in header <string>
Defined in header <string_view>
Defined in header <unordered_map>
Defined in header <unordered_set>
Defined in header <vector>
template<class C > constexprauto empty(const C& c )-> decltype(c.empty());	(1)	(since C++17)
template<class T, std::size_t N > constexprbool empty(const T (&array)[N])noexcept;	(2)	(since C++17)
template<class E > constexprbool 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.

Parameters

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

Return value

1)c.empty()

2)false

3)il.size()==0

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	Value	Std	Feature
__cpp_lib_nonmember_container_access	201411L	(C++17)	std::size(), std::data(), and std::empty()

Possible implementation

First version
template<class C>[[nodiscard]]constexprauto empty(const C& c)-> decltype(c.empty()){return c.empty();}
Second version
template<class T, std::size_t N>[[nodiscard]]constexprbool empty(const T (&array)[N])noexcept{returnfalse;}
Third version
template<class E>[[nodiscard]]constexprbool 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(constauto& 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

ranges::empty (C++20)

checks whether a range is empty (customization point object)