C++ Queues

What is a Queue?

A queue is a FIFO (First-In-First-Out) container. Elements are added at the back and removed from the front — like a ticket line. Include <queue>.

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

int main() {
    queue<string> customers;

    customers.push("Alice");    // enqueue
    customers.push("Bob");
    customers.push("Carol");

    cout << "Queue size: " << customers.size() << endl;

    while (!customers.empty()) {
        cout << "Serving: " << customers.front() << endl;
        customers.pop();   // dequeue
    }
    return 0;
}

Output:

Queue size: 3
Serving: Alice
Serving: Bob
Serving: Carol

Queue Methods

Priority Queue

A priority_queue always serves the highest-priority (largest by default) element first:

#include <queue>
priority_queue<int> pq;
pq.push(3); pq.push(1); pq.push(5); pq.push(2);
while (!pq.empty()) {
    cout << pq.top() << " ";   // 5 3 2 1
    pq.pop();
}

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

int main() {
    queue<string> printQueue;
    printQueue.push("Report.pdf");
    printQueue.push("Invoice.docx");
    printQueue.push("Photo.jpg");
    printQueue.push("Presentation.pptx");
    printQueue.push("Letter.txt");

    while (!printQueue.empty()) {
        cout << "Printing: " << printQueue.front() << endl;
        printQueue.pop();
    }
    return 0;
}

Summary

• queue is FIFO: push to back, pop from front.
• Methods: push, pop, front, back, empty, size.
• priority_queue processes the highest-priority element first.