How to Use Queue in C++: Syntax, Example, and Tips

In C++, you use the std::queue container from the <queue> header to create a queue. It works like a line where elements are added at the back using push() and removed from the front using pop().

Syntax

The std::queue is a container adapter that provides FIFO (First In First Out) behavior. You include it with #include <queue>. The main operations are:

• push(value): Adds an element to the back.
• pop(): Removes the front element.
• front(): Accesses the front element.
• back(): Accesses the last element.
• empty(): Checks if the queue is empty.
• size(): Returns the number of elements.

cpp

#include <queue>

std::queue<Type> q;

q.push(value);    // Add element to back
q.pop();          // Remove front element
Type front = q.front();  // Access front element
Type back = q.back();    // Access last element
bool isEmpty = q.empty();
size_t count = q.size();

Example

This example shows how to create a queue of integers, add elements, access the front and back, remove elements, and check if it is empty.

cpp

#include <iostream>
#include <queue>

int main() {
    std::queue<int> q;

    q.push(10);
    q.push(20);
    q.push(30);

    std::cout << "Front element: " << q.front() << "\n";
    std::cout << "Back element: " << q.back() << "\n";
    std::cout << "Queue size: " << q.size() << "\n";

    q.pop(); // removes 10

    std::cout << "After pop, front element: " << q.front() << "\n";
    std::cout << "Is queue empty? " << (q.empty() ? "Yes" : "No") << "\n";

    return 0;
}

Output

Front element: 10
Back element: 30
Queue size: 3
After pop, front element: 20
Is queue empty? No

Common Pitfalls

Common mistakes when using std::queue include:

• Trying to access front() or back() when the queue is empty, which causes undefined behavior.
• Forgetting to pop() elements, leading to memory growth.
• Using pop() expecting it to return the removed element; it does not return anything.

Always check empty() before accessing or popping.

cpp

#include <queue>
#include <iostream>

int main() {
    std::queue<int> q;

    // Wrong: Access front when empty
    // std::cout << q.front(); // Undefined behavior!

    // Right way:
    if (!q.empty()) {
        std::cout << q.front();
    } else {
        std::cout << "Queue is empty.";
    }

    return 0;
}

Output

Queue is empty.

Quick Reference

Remember these key points when using std::queue:

• Use push() to add elements at the back.
• Use pop() to remove elements from the front.
• Use front() and back() to access elements.
• Always check empty() before accessing or popping.
• pop() does not return the removed element.

Key Takeaways

Use std::queue from <queue> for FIFO data structure in C++.

Add elements with push() and remove with pop().

Always check empty() before accessing front() or back().

pop() does not return the removed element; call front() first if needed.

Queue size can be checked with size().