How to index array of pointers to arrays [queue]?

I am trying program a queue with arrays in C++.

I used this approach https://stackoverflow.com/a/936709/7104310 as shown below.

My question: How can I index the arrays to fill them?

In a normal 2d-array it would be arr[3][2] for example. But I do not know how to do this with pointers. The question hat not been answered in the Solution upon.

Thank you!

#include <iostream>

#define MAX_SIZE 3

using namespace std;


// ary[i][j] is then rewritten as

//arr[rear*capacity + front]

// Class for queue
class msg_queue
{
    char **arr;     // array to store queue elements
    int capacity;   // maximum capacity of the queue
    int front;      // front points to front element in the queue (if any)
    int rear;       // rear points to last element in the queue
    int count;      // current size of the queue

public:
    msg_queue(int size = MAX_SIZE, int slot_length = MAX_SIZE);     // constructor

    void dequeue();
    void enqueue(char x);
    char peek();
    int size();
    bool isEmpty();
    bool isFull();
};

// Constructor to initialize queue
msg_queue::msg_queue(int size, int slot_length)
{
    arr = new char*[size];
    for (int i = 0; i < size; ++i) {
        arr[i] = new char[slot_length];
    }

    capacity = size;
    front = 0;
    rear = -1;
    count = 0;
}

// Utility function to remove front element from the queue
void msg_queue::dequeue()
{
    // check for queue underflow
    if (isEmpty())
    {
        cout << "UnderFlow\nProgram Terminated\n";
        exit(EXIT_FAILURE);
    }

    cout << "Removing " << arr[front] << '\n';

    front = (front + 1) % capacity;
    count--;
}

// Utility function to add an item to the queue
void msg_queue::enqueue(char item)
{
    // check for queue overflow
    if (isFull())
    {
        cout << "OverFlow\nProgram Terminated\n";
        exit(EXIT_FAILURE);
    }

    cout << "Inserting " << item << '\n';

    rear = (rear + 1) % capacity;
    arr[rear] = item;  //ERROR HERE
    count++;
}

// Utility function to return front element in the queue
char msg_queue::peek()
{
    if (isEmpty())
    {
        cout << "UnderFlow\nProgram Terminated\n";
        exit(EXIT_FAILURE);
    }
    return arr[front]; //ERROR HERE
}