How do you pass double pointers in a function?

[kamilhassaan]

So, I declared a 2D array using double pointers.

#include <iostream>
using namespace std;
int main()
{
	int **ptr;
	int numofrow = 0;
	int numofcol = 0;
	
	cout << "Number of Rows : ";
	cin >> numofrow;
	cout << "Number of Coloumns : ";
	cin >> numofcol;
	
	
	ptr = new int* [numofrow];
	
	for(int i=0; i<numofrow; i++){
		ptr[i] = new int[numofcol];
	}
	
	for(int r=0; r<numofrow; r++)
	{
		for(int c=0; c<numofcol; c++)
		{
			cin >> ptr[r][c];
		}
	}
	
	cout << endl;
	
	for(int r=0; r<numofrow; r++)
	{
		for(int c=0; c<numofcol; c++)
		{
			cout << ptr[r][c] << " ";
		}
		cout << endl;
	}
}

This works fine.

If I do this;

#include <iostream>
using namespace std;
void data_IO(int **pointer,int numofrow, int numofcol)
{
	for(int r=0; r<numofrow; r++)
	{
		for(int c=0; c<numofcol; c++)
		{
			cin >> pointer[r][c];
		}
	}
	cout << endl;
	
	for(int r=0; r<numofrow; r++)
	{
		for(int c=0; c<numofcol; c++)
		{
			cout << pointer[r][c] << " ";
		}
		cout << endl;
	}
}
int main()
{
	int **ptr;
	int numofrow = 0;
	int numofcol = 0;
	
	cout << "Number of Rows : ";
	cin >> numofrow;
	cout << "Number of Coloumns : ";
	cin >> numofcol;
	
	
	ptr = new int* [numofrow];
	
	for(int i=0; i<numofrow; i++){
		ptr[i] = new int[numofcol];
	}
	data_IO(&ptr , numofrow , numofcol);
	
	return 0;
}

I get an error. How can I fix this?

[Peter87]

You don't want to pass a pointer to ptr so get rid of & in front of it.

[kamilhassaan]

Thanks Peter87.

My teacher said that we should pass perimeters for pointers like this (&ptr). But the case was different here. Can you give some explanations?

[Moschops]

If x is a double, then &x is a pointer to a double

So if ptr is a pointer to a pointer to an int, then &ptr is a pointer to a pointer to a pointer to an int.

If you're meant to pass a pointer to a pointer to an int, do not pass a pointer to a pointer to a pointer to an int.


Don't worry about this too much at this stage. It seems you're being taught advanced material that the majority of C++ programmers never bother with and really don't have much use for. This being C++, we have proper container objects so that we don't have to spend our time laboriously constructing pointers to pointers to pointers and keeping track of memory ourselves.