std::empty

 

C++

Compiler support

Freestanding and hosted

Language

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Metaprogramming library (C++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (C++17)

Regular expressions library (C++11)

Concurrency support library (C++11)

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) input_or_output_iterator (C++20) sentinel_for (C++20) sized_sentinel_for (C++20)	input_iterator (C++20) output_iterator (C++20) forward_iterator (C++20) bidirectional_iterator (C++20) random_access_iterator (C++20) contiguous_iterator (C++20)
Iterator primitives
iterator_traits input_iterator_tagoutput_iterator_tagforward_iterator_tagbidirectional_iterator_tagrandom_access_iterator_tagcontiguous_iterator_tag (C++20) iterator (deprecated in C++17)	incrementable_traits (C++20) indirectly_readable_traits (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)
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)
Iterator adaptors
reverse_iterator move_iterator (C++11) 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) back_insert_iterator front_insert_iterator insert_iterator	make_reverse_iterator (C++14) make_move_iterator (C++11) default_sentinel_tdefault_sentinel (C++20)(C++20) unreachable_sentinel_tunreachable_sentinel (C++20)(C++20) make_const_iterator (C++23) make_const_sentinel (C++23) back_inserter front_inserter inserter
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) endcend (C++11)(C++14) sizessize (C++17)(C++20)	rbegincrbegin (C++14)(C++14) rendcrend (C++14)(C++14) empty (C++17) data (C++17)

 

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

ranges::empty (C++20)

checks whether a range is empty (customization point object)