Double pointers element access using 2-dimension array syntax in objective-c

Question

I am new to objective-c and am doing something that works for the moment but am afraid might break someday.

In one of my object, I declare a matrix of integer as instance variable. As I do not know the size of the matrix yet, I declared it like this:

int **_matrix;

Later in the code, when I know the amount of lines and columns of my matrix, I initialize it like this:

_matrix = malloc(sizeof(*_matrix) * columns * lines);
for (i = 0; i < columns; i++) {
    _matrix[i] = malloc(sizeof(int) * lines);
    for(j = 0; j < lines; j++)
        _matrix[i][j] = -1;
}

Assuming my allocation is correct (please, feel free to tell me if it is in fact wrong), can I confidently access the elements using the 2-dimension array syntax?

I am asking because from what I read, when you create an actual 2-dimension array the memory is reserved continuously but that it might not be the case using double pointers. Therefore I am afraid that calling

_matrix[i][j]

would actually point to a random place in the memory (and not the place I allocated).

Thank you for your answers!

Answer 1

Yes, it's safe. It is true that two-dimensional arrays on the stack are generally allocated contiguously, but you needn't worry about that. The important thing is that the brackets are evaluated one pair at a time, not as some special [][] operator, and brackets dereference pointers. So it will dereference your first pointer, and then it will dereference the "row" pointer that was yielded by the first dereference.

More info: Array accesses and pointer dereferences are equivalent. The expression x[y] is equivalent to *(x+y). So your code first dereferences the pointer value (matrix + i), which gives you another pointer value (since you had an array of pointers). After this, your code dereferences the pointer (matrix[i] + j) and assigns it the value -1. As you can see, this is precisely what you want it to do.