🛠️ C++ Queues – Implement FIFO Structures with std::queue

🧲 Introduction – Why Use std::queue in C++

A queue is a First-In, First-Out (FIFO) data structure where elements are inserted at the back and removed from the front. In C++, the Standard Template Library provides std::queue as a container adapter built on top of other sequence containers like deque or list.

🎯 In this guide, you’ll learn:

• How to declare and use std::queue
• Core queue operations: push(), pop(), front(), back()
• Real-world use cases and comparisons with stack
• Best practices for safe and efficient queue handling

📘 What Is a C++ Queue?

A std::queue is a container adapter that allows only limited access—specifically FIFO access to its elements. Elements are added to the rear and removed from the front.

Include the header:

#include <queue>

Declare a queue:

queue<int> q;

💻 Code Examples – With Output

✅ Basic Queue Usage

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

int main() {
    queue<int> q;
    q.push(10);
    q.push(20);
    q.push(30);
    cout << "Front: " << q.front() << endl;
    q.pop();
    cout << "New Front: " << q.front() << endl;
    return 0;
}

🟢 Output:

Front: 10  
New Front: 20

🔧 Queue Member Functions

⚙️ Underlying Container Types

By default, std::queue uses deque. You can also use list:

queue<int, list<int>> q;  // uses list as base

Not all containers are compatible (e.g., vector is not usable).

📊 Queue vs Stack

🛠️ Use Cases for Queues

📥 Print Queues – Jobs processed in order
🧮 Breadth-First Search (BFS) – Graph traversal
⏳ Task Scheduling – Background job queues
📊 Streaming Buffers – Manage continuous data flow
📧 Message Queues – Inter-process communication

💡 Best Practices & Tips

📘 Always check empty() before calling front() or pop()
💡 Use deque for better performance with large queue sizes
⚠️ Avoid using queue for direct element access—it supports only front/back
📦 For priority ordering, use priority_queue instead

📌 Summary – Recap & Next Steps

🔍 Key Takeaways:

• std::queue is a FIFO container adapter for sequential processing
• Offers limited interface to ensure FIFO behavior
• Ideal for scheduling, buffering, and breadth-first logic

⚙️ Real-World Relevance:
Queues are widely used in OS schedulers, data stream handlers, search algorithms, server request queues, and more.

✅ Next Steps:
Learn about C++ Deques, the container behind queue and stack, offering fast front/back access.

❓FAQ – C++ Queues

❓ Can I access middle elements of a queue?
❌ No. queue only allows access to the front and back.

❓ Is queue thread-safe?
❌ No. Use mutexes for concurrent environments or consider thread-safe libraries.

❓ How do I clear a queue?
Loop through while (!q.empty()) q.pop(); since there’s no direct clear() method.

❓ What is the default container in std::queue?
deque.

❓ Should I use list or deque for queues?
deque is generally faster unless you’re optimizing for pointer-based node structures.