Queues (std::queue)

Queues (std::queue)

A queue works like a ticket counter line: the first person in line is served first. That is FIFO - First In, First Out.

Create a queue

#include <queue>

std::queue<int> line;

Add at the back, look at the ends

line.push(10);   // line is now {10}
line.push(20);   // line is now {10, 20}
std::cout << line.front() << "\n";   // 10, the oldest
std::cout << line.back() << "\n";    // 20, the newest

• push(x) adds to the back.
• front() reads the oldest element (the one that goes next).
• back() reads the newest element just arrived.

pop removes from the front

line.pop();   // removes 10, the front

Like a stack, pop() returns nothing - read front() first if you need the value.

Drain the queue in order

while (!line.empty()) {
    std::cout << line.front() << "\n";
    line.pop();
}

This prints the elements in the order they were added, oldest first.

TL;DR

• A queue follows FIFO: push to the back, leave the front.
• push(x) / front() / back() / pop() are the queue basics.
• pop() removes the oldest element and returns nothing.
• empty() and size() report the state of the line.
• Read front() before pop() in a loop to serve the queue.