Std::empty (3) Linux Manual Page

std::empty – std::empty

Synopsis

Defined in header<iterator>

    template <class C>
    (since C++ 17)

        constexpr auto empty(const C &c) -> decltype(c.empty());
(until C++ 20)

    template <class C>
    (since C++ 20)

        [[nodiscard]] constexpr auto empty(const C &c) -> decltype(c.empty());

template <class T, std::size_t N>
(since C++ 17)

    constexpr bool empty(const T (&array)[N]) noexcept;
(1)(until C++ 20)

    template <class T, std::size_t N>
    (since C++ 20)

        [[nodiscard]] constexpr bool empty(const T (&array)[N]) noexcept;
(2)

    template <class E>
    (since C++ 17)

        constexpr bool empty(std::initializer_list<E> il) noexcept;
(3)(until C++ 20)

    template <class E>
    (since C++ 20)

        [[nodiscard]] constexpr bool empty(std::initializer_list<E> il) noexcept;

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

std::empty (3) Linux Manual Page