How to index array of pointers to arrays [queue]? - c++

Question

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
}

Answer 1

Well, it's still arr[3][2].
Although arrays are not pointers, the way we use them is effectively using a pointer because of the way they work and the way their name decays.
x[y] is *(x+y), by definition.
That being said, I would recommend you drop the 2D dynamic allocation (which is poison for your cache) and create one big block of Width×Height chars instead. You can use a little bit of maths to provide 2D indexes over that data.
Also you forgot to free any of that memory. If you use a nice std::vector to implement my suggested 1D data scheme (or even if you hire a vector of vectors, but ew!) then it'll be destroyed for you. Of course if you could do that then you'd probably be using std::queue…