C++ Accessing Values at pointer of 2D Array - c++

Question

I need a dynamic 2D array of ints, it will represent a standard matrix. The size and elements are read in from a file at runtime.
Taking direction from other stack posts I've setup my array as follows;
void buildArray(ifstream &file, int** 2dArray);
void buildQueue(Queue<int> &Q, int** 2dArray);
int main()
{
int** 2dArray;
Queue<int> Q;
//...
// open file
//...
buildMatrix(file, 2dArray)
buildQueue(Q, 2dArray)
}
void buildArray(ifstream &file, int** 2dArray)
{
int size, element;
while (file.good()) {
file >> size;
2dArray = new int*[size];
for (int i = 0; i < size; i++)
2dArray[i] = new int[size];
// now I should be able to use 2dArray[r][c]
for (int i = 0; i < size; i++) {
for (int j = 0; j < size; j++) {
file >> element;
2dArray[i][j] = element;
}
}
}
Then I need to read the ints stored at each position [r][c] and build a queue. I think my problem is dereferencing the pointers... but I'm not sure.
void buildQueue(Queue<int> &Q, int** 2dArray)
{
int row, column, element;
// size is passed in as well, size is our rows or columns size here
for (int i = 0; i < size; i++) {
for (int j = 0; j < size; j++) {
row = i;
column = j;
element = 2dArray[i][j]; // getting seg fault here!
Q.push_back(row, column, element);
}
}
}
I should add that I know I could use vectors. I'm challenging myself here to responsibly build, use and deallocate a 2D array using pointers.

Answer 1

What is table?
table = new int*[size];
Shouldn't this be
2dArray = new int*[size];
Also return the pointer from the function
Change
void buildArray(ifstream &file, int** 2dArray)
to
int ** buildArray(ifstream &file)
because otherwise buildArray gets a copy of the variable. And this copy now holds the pointer to the allocated memory. The pointer in the calling function never gets updated.
In your original code add a printf 2dArray before and after the buildArray call.
printf("%p, 2dArray);
buildMatrix(file, 2dArray);
printf("%p, 2dArray);
You will see that 2dArray doesn't change. So you are essentially passing an uninitialized
pointer to buildQueue.
buildArray and call should be changed like this
void buildArray(ifstream &file, int** arr)
{
int size, element;
while (file.good()) {
file >> size;
arr = new int*[size];
for (int i = 0; i < size; i++)
arr[i] = new int[size];
for (int i = 0; i < size; i++) {
for (int j = 0; j < size; j++) {
file >> element;
arr[i][j] = element;
}
}
}
return arr;
}
int main()
{
int** 2dArray;
Queue<int> Q;
//...
// open file
//...
2dArray = buildMatrix(file);
buildQueue(Q, 2dArray);
}

Answer 2

If you are allocating the initial array inside a function, you either need to pass it as a triple pointer and change to:
*2dArray = new int*[size],
or return an int**
Otherwise you will lose the pointer to your new memory.
Don't forget, pointers are passed by value. As is, after your buildArray() call, you still will not have a pointer to valid memory.

Answer 3

When you allocate in a function a pointer passed in argument you only change the copy value. In C/C++ when you send a parameter to a function a copy of this is create. So, like the other answers hasaid before, it's possible to return the new value of the pointer changing your void buildArray(...) to int** buildArray(...) or sending a pointer on your pointer (it's look difficult), or finally, you're in C++ so you can send a reference, void buildArray(ifstream &file, int**& 2dArray) also 2dArray will be the real pointer from the main and you should change his value without changing your code.