What is Queue in C++: Explanation and Example

In C++, a queue is a container adapter that follows the First-In-First-Out (FIFO) principle, where elements are added at the back and removed from the front. It is used to manage data in the order it arrives, like a line of people waiting their turn.

How It Works

A queue in C++ works like a real-life queue or line, such as people waiting at a ticket counter. The first person to get in line is the first one to be served and leave the line. Similarly, the first element added to the queue is the first one to be removed.

Internally, C++ queues use other containers like deque or list to store elements. You can only add elements at the back using push() and remove elements from the front using pop(). This keeps the order intact and prevents random access to elements in the middle.

Example

This example shows how to create a queue of integers, add elements, and remove them in FIFO order.

cpp

#include <iostream>
#include <queue>

int main() {
    std::queue<int> q;

    // Add elements to the queue
    q.push(10);
    q.push(20);
    q.push(30);

    // Process and remove elements
    while (!q.empty()) {
        std::cout << q.front() << " "; // Show front element
        q.pop(); // Remove front element
    }

    return 0;
}

Output

10 20 30 

When to Use

Use a queue when you need to process items in the exact order they arrive. This is common in scenarios like:

• Handling tasks in a printer or CPU scheduler
• Managing requests in web servers
• Simulating real-world lines or waiting lists
• Breadth-first search in graphs

Queues help keep things fair and organized by ensuring first-come, first-served processing.

Key Points

• FIFO order: First element added is first removed.
• Operations: Use push() to add, pop() to remove, and front() to access the first element.
• No random access: You cannot access elements in the middle directly.
• Underlying container: Usually uses deque by default for efficient operations.

Key Takeaways

A queue in C++ follows FIFO, adding at the back and removing from the front.

Use push() to add and pop() to remove elements in order.

Queues are ideal for managing tasks or data that must be processed in arrival order.

You cannot access elements randomly; only the front element is accessible.

The default underlying container is deque, providing efficient operations.