C++ Standard Template Library (STL) queue tutorial

The queue in the C++ Standard Template Library (STL) is a container adapter that operates in a First-In-First-Out (FIFO) manner.

Elements are added at the back (end) and removed from the front, making it suitable for scenarios where data needs to be processed in the order it arrives.

Key Features of queue

FIFO Structure: Elements are processed in the order they were added.
Efficient Operations: push, pop, front, and back operations are constant time.
Restricted Access: Only the front and back elements can be accessed directly.

Basic Syntax

#include <queue>
using namespace std;

queue<Type> q;

Let’s explore the queue with examples demonstrating common operations.

1. Basic Operations: push, pop, front, and back

The push function adds an element to the back of the queue, pop removes an element from the front, front retrieves the first element, and back retrieves the last element.

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

int main() {
    queue<int> q;

    // Adding elements
    q.push(10);
    q.push(20);
    q.push(30);

    // Access front and back elements
    cout << "Front element: " << q.front() << endl;
    cout << "Back element: " << q.back() << endl;

    // Remove elements and display them
    cout << "Elements in the queue: ";
    while (!q.empty()) {
        cout << q.front() << " ";
        q.pop();
    }
    cout << endl;

    return 0;
}

Output:

Front element: 10
Back element: 30
Elements in the queue: 10 20 30

2. Checking if the Queue is Empty

The empty function checks if the queue is empty, returning true if it is and false otherwise.

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

int main() {
    queue<int> q;

    // Check if the queue is empty
    if (q.empty()) {
        cout << "The queue is empty." << endl;
    }

    // Add an element and check again
    q.push(5);
    if (!q.empty()) {
        cout << "The queue is not empty." << endl;
    }

    return 0;
}

Output:

The queue is empty.
The queue is not empty.

3. Getting the Size of the Queue

Use the size function to retrieve the number of elements in the queue.

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

int main() {
    queue<int> q;

    // Insert elements
    q.push(10);
    q.push(20);
    q.push(30);

    // Display the size of the queue
    cout << "Size of the queue: " << q.size() << endl;

    return 0;
}

Output:

Size of the queue: 3

4. Using queue with Custom Data Types

You can store custom objects in a queue as long as they have a valid copy constructor and assignment operator.

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

// Custom structure for storing person information
struct Person {
    string name;
    int age;

    Person(string n, int a) : name(n), age(a) {}
};

int main() {
    queue<Person> q;

    // Insert custom objects
    q.push(Person("Alice", 25));
    q.push(Person("Bob", 30));
    q.push(Person("Charlie", 35));

    // Access and display elements
    cout << "People in the queue:" << endl;
    while (!q.empty()) {
        Person p = q.front();
        cout << "Name: " << p.name << ", Age: " << p.age << endl;
        q.pop();
    }

    return 0;
}

Output:

People in the queue:
Name: Alice, Age: 25
Name: Bob, Age: 30
Name: Charlie, Age: 35

5. Processing All Elements in the Queue

You can process each element in the queue by repeatedly using front to access it, and pop to remove it.

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

int main() {
    queue<int> q;

    // Insert elements
    q.push(5);
    q.push(10);
    q.push(15);

    cout << "Processing elements in the queue: ";
    while (!q.empty()) {
        // Process the front element
        int val = q.front();
        cout << val << " ";
        q.pop();  // Remove the element
    }
    cout << endl;

    return 0;
}

Output:

Processing elements in the queue: 5 10 15

6. Using queue with Strings

You can also use queue to manage strings, making it easy to process string data in FIFO order.

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

int main() {
    queue<string> q;

    // Insert strings
    q.push("apple");
    q.push("banana");
    q.push("cherry");

    // Display and remove strings in FIFO order
    cout << "Strings in the queue: ";
    while (!q.empty()) {
        cout << q.front() << " ";
        q.pop();
    }
    cout << endl;

    return 0;
}

Output:

Strings in the queue: apple banana cherry

7. Using queue to Simulate a Simple Task Scheduler

You can use queue to simulate a task scheduler, where tasks are processed in the order they are added.

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

// Custom structure for tasks
struct Task {
    string description;
    int priority;

    Task(string desc, int prio) : description(desc), priority(prio) {}
};

int main() {
    queue<Task> taskQueue;

    // Add tasks to the queue
    taskQueue.push(Task("Process data", 1));
    taskQueue.push(Task("Save file", 2));
    taskQueue.push(Task("Send email", 3));

    // Process tasks in FIFO order
    cout << "Processing tasks:" << endl;
    while (!taskQueue.empty()) {
        Task t = taskQueue.front();
        cout << "Task: " << t.description << ", Priority: " << t.priority << endl;
        taskQueue.pop();
    }

    return 0;
}

Output:

Processing tasks:
Task: Process data, Priority: 1
Task: Save file, Priority: 2
Task: Send email, Priority: 3

8. Clearing All Elements from a Queue

To clear all elements in a queue, use a loop to remove each element with pop.

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

int main() {
    queue<int> q;

    // Insert elements
    q.push(10);
    q.push(20);
    q.push(30);

    // Clear the queue
    while (!q.empty()) {
        q.pop();
    }

    // Verify that the queue is empty
    cout << "Is the queue empty? " << (q.empty() ? "Yes" : "No") << endl;

    return 0;
}

Output:

Is the queue empty? Yes

9. Using queue with Arrays

You can initialize a queue with elements from an array by looping through the array.

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

int main() {
    int arr[] = {1, 2, 3, 4, 5};
    queue<int> q;

    // Insert array elements into the queue
    for (int num : arr) {
        q.push(num);
    }

    // Display elements
    cout << "Elements in the queue: ";
    while (!q.empty()) {
        cout << q.front() << " ";
        q.pop();
    }
    cout << endl;

    return 0;
}

Output:

Elements in the queue: 1 2 3 4 5

10. Using queue with Vectors

You can also initialize a queue with elements from a vector.

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

int main() {
    vector<int> vec = {1, 2, 3, 4, 5};
    queue<int> q;

    // Insert vector elements into the queue
    for (int num : vec) {
        q.push(num);
    }

    // Display elements
    cout << "Elements in the queue: ";
    while (!q.empty()) {
        cout << q.front() << " ";
        q.pop();
    }
    cout << endl;

    return 0;
}

Output:

Elements in the queue: 1 2 3 4 5

Summary Table of queue Operations

Operation	Code Example	Description
Initialize	queue<int> q;	Initializes an empty queue
Add Element	q.push(10);	Adds an element to the back of the queue
Remove Element	q.pop();	Removes the front element
Access Front	q.front();	Accesses the front element
Access Back	q.back();	Accesses the back element
Check if Empty	q.empty();	Returns true if the queue is empty
Get Size	q.size();	Returns the number of elements in the queue
Custom Data Types	queue<CustomType> q;	Stores custom objects
Process All Elements	Use while(!q.empty()) { q.pop(); }	Clears all elements manually
Initialize from Array	for (int x : arr) q.push(x);	Adds elements from an array
Initialize from Vector	for (int x : vec) q.push(x);	Adds elements from a vector

Complete Example: Using Multiple queue Operations

This example demonstrates initializing a queue, adding and removing elements, checking the front and back elements, and clearing the queue.

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

int main() {
    queue<int> q;

    // Adding elements
    q.push(5);
    q.push(10);
    q.push(15);

    // Display front and back elements
    cout << "Front element: " << q.front() << endl;
    cout << "Back element: " << q.back() << endl;

    // Display all elements in the queue
    cout << "Elements in the queue: ";
    while (!q.empty()) {
        cout << q.front() << " ";
        q.pop();
    }
    cout << endl;

    // Check if queue is empty after clearing
    cout << "Is the queue empty? " << (q.empty() ? "Yes" : "No") << endl;

    return 0;
}

Sample Output:

Front element: 5
Back element: 15
Elements in the queue: 5 10 15 
Is the queue empty? Yes

Key Takeaways

queue in C++ provides a simple way to manage a collection of elements in a FIFO manner.
Key operations include push, pop, front, back, empty, and size.
Ideal for tasks like managing buffers, task scheduling, and processing elements sequentially where order matters.

Queue STL

C++ Standard Template Library (STL) queue tutorial

Key Features of queue

Basic Syntax

1. Basic Operations: push, pop, front, and back

Output:

2. Checking if the Queue is Empty

Output:

3. Getting the Size of the Queue

Output:

4. Using queue with Custom Data Types

Output:

5. Processing All Elements in the Queue

Output:

6. Using queue with Strings

Output:

7. Using queue to Simulate a Simple Task Scheduler

Output:

8. Clearing All Elements from a Queue

Output:

9. Using queue with Arrays

Output:

10. Using queue with Vectors

Output:

Summary Table of queue Operations

Complete Example: Using Multiple queue Operations

Sample Output:

Key Takeaways

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