std::empty (3) - Linux Manuals

std::empty: std::empty

NAME

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