std::map<Key,T,Compare,Allocator>::empty (3) - Linux Manuals

std::map<Key,T,Compare,Allocator>::empty: std::map<Key,T,Compare,Allocator>::empty

NAME

std::map<Key,T,Compare,Allocator>::empty - std::map<Key,T,Compare,Allocator>::empty

Synopsis

bool empty() const; (until C++11)
bool empty() const noexcept; (since C++11)
                                            (until C++20)
[[nodiscard]] bool empty() const noexcept; (since C++20)

Checks if the container has no elements, i.e. whether begin() == end().

Parameters

(none)

Return value

true if the container is empty, false otherwise

Complexity

Constant.

Example

The following code uses empty to check if a std::map<int, int> contains any elements:
// Run this code

  #include <map>
  #include <iostream>
  #include <utility>

  int main()
  {
      std::map<int,int> numbers;
      std::cout << "Initially, numbers.empty(): " << numbers.empty() << '\n';

      numbers.emplace(42, 13);
      numbers.insert(std::make_pair(13317, 123));
      std::cout << "After adding elements, numbers.empty(): " << numbers.empty() << '\n';
  }

Output:

  Initially, numbers.empty(): 1
  After adding elements, numbers.empty(): 0