C++ Queue

1. What Is a Queue?

A Queue is a linear data structure that follows the principle:

FIFO – First In, First Out

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This means:

• The first element added is the first one removed
• Just like a real-life queue (bank line, food line)

Example (Real-Life Analogy)

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People in line:  A → B → C → D
Served first:    A

2. Basic Operations of a Queue

Operation	Description
enqueue	Add an element to the rear
dequeue	Remove an element from the front
front	Get the front element
back	Get the last element
isEmpty	Check if queue is empty
size	Number of elements

3. Queue in C++ (STL)

C++ provides the queue container in the Standard Template Library (STL).

Header File

#include <queue>

Declaration

queue<int> q;

4. Basic Queue Example

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

int main() {
    queue<int> q;

    q.push(10);   // enqueue
    q.push(20);
    q.push(30);

    cout << "Front: " << q.front() << endl; // 10
    cout << "Back: " << q.back() << endl;   // 30

    q.pop(); // dequeue

    cout << "Front after pop: " << q.front() << endl; // 20
    cout << "Size: " << q.size() << endl;

    return 0;
}

5. Queue Functions Explained

Function	Purpose
push(x)	Insert element at rear
pop()	Remove front element
front()	Access first element
back()	Access last element
empty()	Returns true if empty
size()	Returns number of elements

6. Visual Representation

Queue (FIFO)

Front → [10] [20] [30] ← Rear

pop() removes 10

7. Internal Working of STL Queue

• STL queue is implemented using:
  • deque (default)
  • Can also be implemented with list

queue<int, deque<int>> q;
queue<int, list<int>> q;

❗ You cannot access elements randomly (no indexing like arrays).

8. Types of Queues

1. Simple Queue

Basic FIFO structure.

2. Circular Queue

• Rear connects back to front
• Efficient memory usage
• Used in CPU scheduling

3. Priority Queue

Elements served based on priority, not order.

#include <queue>

priority_queue<int> pq;
pq.push(10);
pq.push(50);
pq.push(20);

// Top = 50

Max Heap by default

4. Double-Ended Queue (Deque)

Insert/remove from both ends.

#include <deque>

deque<int> dq;
dq.push_front(10);
dq.push_back(20);

9. Implementing Queue Using Array (Manual)

#define SIZE 5
int arr[SIZE];
int front = -1, rear = -1;

Enqueue

void enqueue(int x) {
    if (rear == SIZE - 1)
        cout << "Queue Overflow";
    else {
        if (front == -1) front = 0;
        arr[++rear] = x;
    }
}

Dequeue

void dequeue() {
    if (front == -1 || front > rear)
        cout << "Queue Underflow";
    else
        front++;
}

10. Implementing Queue Using Linked List

• Each node has:
  • data
  • next

Advantages:

• Dynamic size
• No overflow (until memory full)

11. Time & Space Complexity

Operation	Time Complexity
Enqueue	O(1)
Dequeue	O(1)
Front/Back	O(1)

Space Complexity:

• O(n)

12. Applications of Queue

✔ CPU Scheduling
✔ Breadth-First Search (BFS)
✔ Print Queue
✔ Call Center Systems
✔ Message Queues
✔ Order Processing Systems

13. Queue vs Stack

Stack	Queue
LIFO	FIFO
Push/Pop	Enqueue/Dequeue
Used in recursion	Used in scheduling

14. Common Mistakes

❌ Accessing queue elements using index
❌ Calling front() on empty queue
❌ Forgetting #include <queue>

15. When to Use Queue?

Use a queue when:

• Order of processing matters
• Tasks must be handled sequentially
• Fair scheduling is required

Summary

A queue in C++ is:

• A FIFO data structure
• Provided by STL for efficiency
• Essential for algorithms and system design