queue::empty() and queue::size() in C++ STL

std::queue is a container adapter that follows the First In First Out (FIFO) principle. The empty() and size() member functions are used to check whether a queue contains elements and to determine the number of elements currently stored in the queue.

• empty() checks whether the queue contains any elements.
• size() returns the current number of elements in the queue.

queue::empty()

The empty() function is used to determine whether a queue is empty.

• Returns true if the queue contains no elements.
• Returns false if the queue contains one or more elements.
• Commonly used as the loop condition while processing a queue.

Syntax

queue_name.empty();

Parameters: This function does not take any parameters.

Return Value: Returns a boolean value:

• true if the queue is empty.
• false otherwise.

#include <iostream>
#include <queue>
using namespace std;

int main()
{
    queue<int> q;

    q.push(1);

    if (q.empty())
        cout << "Queue is empty";
    else
        cout << "Queue is not empty";

    return 0;
}

Queue is not empty

queue::size()

The size() function returns the number of elements currently present in the queue.

• Returns the total number of elements in the queue.
• Does not modify the queue.
• Can be used to determine the current length of the queue.

Syntax

queue_name.size();

Parameters: This function does not take any parameters.

Return Value: Returns the number of elements present in the queue.

#include <iostream>
#include <queue>
using namespace std;

int main()
{
    queue<int> q;

    q.push(1);
    q.push(8);
    q.push(3);
    q.push(6);
    q.push(2);

    cout << q.size();

    return 0;
}

5

Example: Processing a Queue Using empty()

The empty() function is commonly used while traversing and processing all elements of a queue.

#include <iostream>
#include <queue>
using namespace std;

int main()
{
    queue<int> q;

    q.push(1);
    q.push(8);
    q.push(3);
    q.push(6);
    q.push(2);

    int sum = 0;

    while (!q.empty()) {
        sum += q.front();
        q.pop();
    }

    cout << sum;

    return 0;
}

20

Explanation: The queue is processed until it becomes empty. During each iteration, the front element is added to the sum and then removed from the queue.

Difference Between queue::empty() and queue::size()

Key Points to Remember

• empty() returns a boolean value indicating whether the queue contains any elements.
• size() returns the current number of elements stored in the queue.
• Both operations execute in constant time, i.e., O(1).
• empty() is generally preferred over size() == 0 when checking whether a queue is empty.