Queue STL Implementation

#include <bits/stdc++.h> // Includes <queue> and other standard libraries
using namespace std;     // Uses the standard namespace

int main() {

    queue<int> q; // Declare a queue of integers

    q.push(11); // Add 11 to the back of the queue
    // Queue: [11] (front=11, back=11)

    cout << "Front element of q : " << q.front() << endl; // Access the front element (11)
    cout << "Back element of q : " << q.back() << endl;   // Access the back element (11)

    q.push(15); // Add 15 to the back
    // Queue: [11, 15] (front=11, back=15)
    cout << "Front element of q : " << q.front() << endl; // Front element is still 11
    cout << "Back element of q : " << q.back() << endl;   // Back element is now 15

    q.push(23); // Add 23 to the back
    q.push(30); // Add 30 to the back
    // Queue: [11, 15, 23, 30]

    cout << "Size of queue : " << q.size() << endl; // Get the number of elements (4)

    cout << endl << "Front before pop : " << q.front() << endl; // Front element: 11
    cout << "Back before pop : " << q.back() << endl << endl;   // Back element: 30

    q.pop(); // Remove the front element (11)
    // Queue: [15, 23, 30]

    cout << "Front after pop : " << q.front() << endl; // New front element: 15
    cout << "Back after pop : " << q.back() << endl << endl;   // Back element remains 30

    q.pop(); // Remove 15
    q.pop(); // Remove 23
    // Queue: [30]

    cout << "Size of queue : " << q.size() << endl; // Size is now 1

    q.pop(); // Remove 30
    // Queue: [] (empty)

    // Check if the queue is empty
    if(q.empty()) {
        cout << "Queue is empty!" << endl;
    } else {
        cout << "Queue is not empty!" << endl;
    }

    return 0; // End of program
}

Concept

• Queue: A First-In, First-Out (FIFO) data structure.
• Elements are added to the back (or rear) and removed from the front.
• Analogy: A line of people where the first person in line is the first to be served.

Key Operations

• push(element): Adds an element to the back of the queue.
• pop(): Removes the element from the front of the queue. Does not return the element. You must use front() first to retrieve the value.
• front(): Returns a reference to the element at the front of the queue. Does not remove it.
• back(): Returns a reference to the element at the back of the queue. Does not remove it.
• size(): Returns the number of elements currently in the queue.
• empty(): Returns true if the queue is empty, false otherwise.

Important Notes

• Always check !q.empty() before calling q.front() or q.pop() to avoid undefined behavior (accessing an empty queue).
• std::queue is a container adapter. By default, it uses std::deque as its underlying container. You can specify other sequence containers (like std::list) but not std::vector.

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