Stack and Queue implementation to array in C++

stackNqueue_C++.cpp

#include <iostream>

template <typename T>
class Stack {
public:
    T *arr = new T[sizeof(T)];
    int top = -1;

    void push(T element)
    {
        this->top = this->top + 1;
        this->arr[this->top] = element;
    }

    T pop()
    {
        if (this->isEmpty()) {
            std::cout << "Stack underflow error" << std::endl;
            return -1;
        } else {
            this->top = this->top - 1;
            return this->arr[this->top + 1];
        }
    }

    bool isEmpty()
    {
        if (this->top < 0)
            return true;
        else
            return false;
    }

    void display()
    {
        for (int i = 0; i <= this->top; i++) {
            std::cout << this->arr[i] << " ";
        }
        std::cout << std::endl;
    }

    void flush()
    {
        delete[] this->arr;
    }
};

template <typename T>
class Queue {
public:
    T arr[100];
    int head = -1;
    int tail = -1;

    void enQueue(T elem)
    {
        if (this->head == -1) {
            this->head = 0;
        }
        this->tail = this->tail + 1;
        this->arr[this->tail] = elem;

    }

    T deQueue()
    {
        if (this->isEmpty()) {
            std::cout << "Queue is empty" << std::endl;
            return -1;
        }
        T element = this->arr[this->head];
        if (this->head >= this->tail) {
            this->head = -1;
            this->tail = -1;
        } else
            this->head = this->head + 1;

        return element;
    }

    bool isEmpty() {
        if (this->head == -1)
            return true;
        else
            return false;
    }

    void display()
    {
        for (int i = this->head; i <= this->tail; i++) {
            std::cout << this->arr[i] << " ";
        }
        std::cout << std::endl;
    }
};

int main() {
    std::cout << "STACK" << std::endl;

    Stack<int> s = Stack<int>();

    for (int i = 0; i <= 10; i++) {
        s.push(i);
    }

    std::cout << "Displaying elements: " << std::endl;
    s.display();
    std::cout << std::endl;
    std::cout << "POP: " << s.pop() << std::endl;
    std::cout << "Displaying elements: " << std::endl;
    s.display();
    std::cout << std::endl;

    std::cout << "QUEUE" << std::endl;
    Queue<int> q = Queue<int>();
    for (int i = 0; i <= 10; i++) {
        q.enQueue(i);
    }

    std::cout << "Displaying elements: " << std::endl;
    q.display();
    std::cout << std::endl;
    std::cout << "POP: " << q.deQueue() << std::endl;
    std::cout << "Displaying elements: " << std::endl;
    q.display();
    std::cout << std::endl;

    return 0;
}