std::empty

C++

Language

Headers

Freestanding and hosted implementations

Named requirements

Language support library

Concepts library (C++20)

Diagnostics library

Utilities library

Strings library

Containers library

Algorithms library

Iterators library

Numerics library

Input/output library

Localizations library

Regular expressions library (C++11)

Atomic operations library (C++11)

Thread support library (C++11)

Filesystem library (C++17)

Technical Specifications

Iterator library

Iterator primitives
iterator_traits
input_iterator_tagoutput_iterator_tagforward_iterator_tagbidirectional_iterator_tagrandom_access_iterator_tag
iterator
Iterator adaptors
reverse_iterator move_iterator (C++11) back_insert_iterator front_insert_iterator insert_iterator	make_reverse_iterator (C++14) make_move_iterator (C++11) back_inserter front_inserter inserter
Stream iterators
istream_iterator
ostream_iterator
istreambuf_iterator
ostreambuf_iterator
Iterator operations
advance
distance
prev (C++11)
next (C++11)
Range access
begincbegin (C++11)(C++14)
endcend (C++11)(C++14)
rbegincrbegin (C++14)(C++14)
rendcrend (C++14)(C++14)
Container access
size (C++17)
empty (C++17)
data (C++17)

Defined in header <iterator>
	(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 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.

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