C++ queue

🚶 C++ queue (FIFO Data Structure)

std::queue is a container adapter in the C++ STL that follows the FIFO (First In, First Out) principle.
Elements are inserted at the back and removed from the front.

🔹 1. Why Use Queue?

  • Task scheduling

  • Printer queues

  • CPU/process scheduling

  • BFS (Breadth-First Search)

  • Customer service systems

🔹 2. Include Header

#include <queue>

🔹 3. Declaring a Queue

std::queue<int> q;

With underlying container:

std::queue<int, std::deque<int>> q; // default
std::queue<int, std::list<int>> q;

🔹 4. Basic Queue Operations

q.push(10); // insert at back
q.push(20);cout << q.front(); // 10
cout << q.back(); // 20

q.pop(); // removes front

🔹 5. Size and Empty Check

q.size();
q.empty();

🔹 6. Traversing a Queue

❌ No iterators

✔ Use a copy:

queue<int> temp = q;

while (!temp.empty()) {
cout << temp.front() << ” “;
temp.pop();
}

🔹 7. Queue Example: Simple Simulation

queue<string> customers;

customers.push(“Amit”);
customers.push(“Neha”);

while (!customers.empty()) {
cout << customers.front() << ” served\n”;
customers.pop();
}

🔹 8. Queue Example: BFS (Graph)

queue<int> q;
q.push(start);while (!q.empty()) {
int node = q.front();
q.pop();

for (int next : adj[node]) {
if (!visited[next]) {
visited[next] = true;
q.push(next);
}
}
}

🔹 9. Deque vs Queue

Featurequeuedeque
AccessFront & BackFront, Back, Random
InsertBack onlyBoth ends
RemoveFront onlyBoth ends

🔹 10. Queue Time Complexity

OperationComplexity
pushO(1)
popO(1)
front/backO(1)

❌ Common Mistakes

q.pop();
cout << q.front(); // ❌ undefined

✔ Always check:

if (!q.empty())

🔹 11. queue vs priority_queue

queuepriority_queue
FIFOHighest priority first
Order preservedSorted by priority
BFSDijkstra, scheduling

📌 Summary

  • std::queue follows FIFO

  • Insert at back, remove from front

  • No iterators

  • Built on deque by default

  • Used in scheduling and BFS

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