A Queue is a linear data structure that follows the principle:
FIFO – First In, First Out
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This means:
People in line: A → B → C → D
Served first: A| Operation | Description |
|---|---|
enqueue | Add an element to the rear |
dequeue | Remove an element from the front |
front | Get the front element |
back | Get the last element |
isEmpty | Check if queue is empty |
size | Number of elements |
C++ provides the queue container in the Standard Template Library (STL).
#include <queue>queue<int> q;#include <iostream>
#include <queue>
using namespace std;
int main() {
queue<int> q;
q.push(10); // enqueue
q.push(20);
q.push(30);
cout << "Front: " << q.front() << endl; // 10
cout << "Back: " << q.back() << endl; // 30
q.pop(); // dequeue
cout << "Front after pop: " << q.front() << endl; // 20
cout << "Size: " << q.size() << endl;
return 0;
}| Function | Purpose |
|---|---|
push(x) | Insert element at rear |
pop() | Remove front element |
front() | Access first element |
back() | Access last element |
empty() | Returns true if empty |
size() | Returns number of elements |
Queue (FIFO)
Front → [10] [20] [30] ← Rear
pop() removes 10queue is implemented using: deque (default)listqueue<int, deque<int>> q;
queue<int, list<int>> q;❗ You cannot access elements randomly (no indexing like arrays).
Basic FIFO structure.
Elements served based on priority, not order.
#include <queue>
priority_queue<int> pq;
pq.push(10);
pq.push(50);
pq.push(20);
// Top = 50
Max Heap by default
Insert/remove from both ends.
#include <deque>
deque<int> dq;
dq.push_front(10);
dq.push_back(20);
#define SIZE 5
int arr[SIZE];
int front = -1, rear = -1;void enqueue(int x) {
if (rear == SIZE - 1)
cout << "Queue Overflow";
else {
if (front == -1) front = 0;
arr[++rear] = x;
}
}void dequeue() {
if (front == -1 || front > rear)
cout << "Queue Underflow";
else
front++;
}datanextAdvantages:
| Operation | Time Complexity |
|---|---|
| Enqueue | O(1) |
| Dequeue | O(1) |
| Front/Back | O(1) |
Space Complexity:
✔ CPU Scheduling
✔ Breadth-First Search (BFS)
✔ Print Queue
✔ Call Center Systems
✔ Message Queues
✔ Order Processing Systems
| Stack | Queue |
|---|---|
| LIFO | FIFO |
| Push/Pop | Enqueue/Dequeue |
| Used in recursion | Used in scheduling |
❌ Accessing queue elements using index
❌ Calling front() on empty queue
❌ Forgetting #include <queue>
Use a queue when:
A queue in C++ is:
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View Comments
Nice explanation of the basic operations. One thing worth adding is how the `empty()` function is often used to prevent errors when trying to dequeue from an empty queue. It’s a small detail, but it can really help prevent runtime issues in more complex programs.