Queues in C++ are part of the Standard Template Library (STL). A queue is a linear data structure that follows the FIFO (First In, First Out) principle.

What is a Queue?

A queue is a container where:

• The first element added is the first one removed
• Elements are inserted from the back (rear)
• Elements are removed from the front

Why Use Queues?

Queues are useful because they:

• Maintain order of processing
• Work like real-world waiting lines
• Help in task scheduling
• Are used in algorithms like BFS

Real-Life Example

Think of a ticket counter:

• First person in line is served first
• New people join at the end
• No one skips the line

This is exactly how a queue works.

Queue Header File

To use queues in C++, include the <queue> library.

#include <queue>

Declaring a Queue

queue<int> q;

Adding Elements (push)

Elements are added at the back of the queue.

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

Removing Elements (pop)

Elements are removed from the front.

q.pop();

Accessing Elements

Front Element

cout << q.front();

Back Element

cout << q.back();

Example Program of Queue

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

int main() {

    queue<int> q;

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

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

    q.pop();

    cout << "After pop, Front: " << q.front() << endl;

    return 0;
}

Output

Front: 1
Back: 3
After pop, Front: 2

Common Queue Functions

Function	Purpose
push()	Add element at back
pop()	Remove front element
front()	Access first element
back()	Access last element
size()	Get number of elements
empty()	Check if queue is empty

How Queue Works

1. Elements are inserted at the rear
2. Elements wait in order
3. First inserted is first removed
4. Process continues sequentially

Types of Queues

1. Simple Queue

• Follows FIFO
• Basic queue structure

2. Circular Queue

• Last position connects to first
• Efficient memory usage

3. Priority Queue

• Elements are processed based on priority
• Not strictly FIFO

Difference Between Stack and Queue

Stack	Queue
LIFO (Last In First Out)	FIFO (First In First Out)
Insert/remove from top	Insert rear, remove front
Used in recursion	Used in scheduling

Important Points About Queues

• Works on FIFO principle
• Insertion happens at rear
• Deletion happens at front
• No random access allowed

Advantages of Queues

• Simple and efficient
• Maintains order
• Useful in scheduling systems
• Easy to implement using STL

Common Mistakes with Queues

• Trying to access middle elements
• Forgetting to check if queue is empty
• Calling front() on empty queue
• Confusing stack and queue behavior

Real-Life Applications

Queues are used in:

• CPU scheduling
• Print queue systems
• Customer service systems
• Network packet handling
• BFS algorithm in graphs

Why Queues are Important

Queues are important because they:

• Manage tasks in order
• Improve system efficiency
• Help in real-time processing
• Are widely used in system design

Conclusion

Queues in C++ are simple yet powerful STL containers that follow the FIFO principle. They are widely used in real-world applications like scheduling, processing tasks, and algorithm design where order matters.