std::unordered_set<Key,Hash,KeyEqual,Allocator>::empty

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::unordered_set<int> contains any elements:

#include <unordered_set>
#include <iostream>
 
int main()
{
    std::unordered_set<int> numbers;
    std::cout << "Initially, numbers.empty(): " << numbers.empty() << '\n';
 
    numbers.insert(42);
    numbers.insert(13317); 
    std::cout << "After adding elements, numbers.empty(): " << numbers.empty() << '\n';
}

Output:

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

See also

returns the number of elements
(public member function)

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