Queue in C++ STL

Last Updated : 21 Aug, 2025

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Queue container follows the FIFO (First In First Out) order of insertion and deletion. According to it, the elements that are inserted first should be removed first. This is possible by inserting elements at one end (called back) and deleting them from the other end (called front) of the data structure.

Syntax

Queue is defined as the std::queue class template inside <queue> header file.

queue<T> q;

where,

T: DataType of elements in the queue.
q: Name assigned to the queue.

Declaration and Initialization

C++

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

int main() {
    //declaration of a queue
    queue<int>q;
    q.push(10);
    q.push(5);
    //Accessing the front and back elements
    cout<<"Front element: "<<q.front()<<endl;
    cout<<"Back element: "<<q.back()<<endl;
    //Removing an element from the front
    q.pop();
    cout<<"Front element after pop: "<<q.front()<<endl;
    return 0;
}

Output

Front element: 10
Back element: 5
Front element after pop: 5

Basic Operations

Here are the basic operations that can be performed on a queue:

1. Inserting Elements

New elements can only be inserted at back of the queue using push() function. The process of inserting elements in a queue is also called enqueue.

Example:

C++

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

int main() {
    queue<int> q;

    // Pushing elements into the queue
    q.push(3);
    q.push(4);
    q.push(5);
    
    return 0;
}

2. Accessing Elements

Only the front and back elements of the queue can be accessed by using front() and back() functions respectively.

Example:

C++

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

int main() {
    queue<int> q;
    q.push(3);
    q.push(4);
    q.push(5);

    // Accessing the front and back elements
    cout << q.front() << endl;
    cout << q.back();
    return 0;
}

Output

3
5

3. Deleting Elements

Elements can only be deleted from the front of the queue using the pop() function. This operation is also called dequeue.

C++

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

int main() {
    queue<int> q;
    q.push(3);
    q.push(4);
    q.push(5);
    
    // Deleting elements from front side
    // of the queue
    q.pop();
    
    while (!q.empty()) {
        cout << q.front() << " ";
        q.pop();
    }
    return 0;
}

Output

4
5

4. empty()

This checks whether the queue is empty. It returns true if the queue has no elements; otherwise, it returns false.

C++

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

int main() {

    queue<int>q;
    if(q.empty()){
        cout<<"Queue is empty "<<endl;
    }
    q.push(100);
    if(!q.empty()){
        cout<<"Queue is not empty. Front element: "<<q.front()<<endl;
    }
    return 0;
}

Output

Queue is empty 
Queue is not empty. Front element: 100

5.Size of queue

The size() function in a queue returns the number of elements currently in the queue. It helps to determine how many items are stored without modifying the queue.

C++

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

int main() {

    queue<int>q;
    q.push(10);
    q.push(5);
    cout<<"Size of queue: "<<q.size()<<endl;
    q.pop();
    cout<<"Size of queue: "<<q.size()<<endl;
    return 0;
}

Output

Size of queue: 2
Size of queue: 1

Pseudo Traversal

Since only the front and back element can be accessed in a queue, we cannot directly traverse it. On the other hand, we can create a copy of the queue, access the front element, and then delete it. By continuing this process until the copied queue is empty, we can effectively traverse all the elements of the queue.

Example:

C++

#include <bits/stdc++.h>
using namespace std;

int main() {
    queue<int> q;
    q.push(3);
    q.push(4);
    q.push(5);
    
    // Create a copy
    queue<int> temp(q);
    
    while(!temp.empty()) {
        cout << temp.front() << " ";
        temp.pop();
    }
    return 0;
}

Output

3 4 5

Time Complexity

The below table lists the time complexity of the basic operations on a queue:

Operation	Time Complexity
Insert an element	O(1)
Delete an element	O(1)
Access front element	O(1)
Access back element	O(1)
Traverse the queue	O(n)

All Member Functions

Following is the list of all member functions of std::queue class in C++:

Functions	Description
front()	Access the front element of the queue.
back()	Access the end element of the queue.
empty()	Check whether a queue is empty or not.
size()	Returns the number of elements in the queue.
push()	Adding an element at the back of the queue.
push_range()	Adding multiple elements at the end of queue.
emplace()	Add the constructs element in top of the queue.
pop()	Delete the front element of the queue.
swap()	Swap two queues.

Queue in C++ STL

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