Thread: Passing a pointer to a pointer to a function.

I'm having minor difficulties understanding pointers and passing them to functions. The program I am trying to make dynamically allocates a two dimensional array assigned to a pointer to a pointer and then I want to pass the pointer to a function I have made. The code is:

Code:

#include <iostream>


using namespace std;
void printTable (int p_p_table, int width, int height);
void createTable (int width, int height);


int **p_p_table;


int main()
{
    int width, height;
    cout << "Please enter the width followed by the height of the table: ";
    cin >> width >> height;
    createTable(width, height);
    printTable(**p_p_table, width, height);






    return 0;
}


void createTable(int width, int height)
{
    p_p_table = new int* [width];
    for (int i = 0; i < width; i++)
    {
        p_p_table = new int* [height];
    }
    for (int i = 0; i < width; i++)
    {
        for (int j = 0; j < height; j++)
        {
            p_p_table [i][j] = i * j;
        }
    }


}


void printTable(int **p_p_table, int width, int height)
{
    for (int i = 0; i < width; i++)
    {
        for (int j = 0; j < height; j++)
        {
            cout << p_p_table [i][j] << " ";
        }
        cout << "\n";
    }
}

When I try to compile it I get an error message: line 15; undefined reference to 'printTable (int, int, int)', and I dont really know what it means, and googling the answer gave answers giving the same error line, but not having the same code.

The asterisks have different meaning when declaring a pointer to pointer and when using it. The declaration "int **p_p_table" tells the compiler that **p_p_table is an int. When using a pointer to pointer, for example, "printTable(**p_p_table, width, height)" the double asterisks double-dereference p_p_table to obtain the int.

Assuming, of course, that p_p_table has been initialised so it points to a valid pointer that points to a valid int. You have not shown any code which does that.

Well, there are several mistakes...

First off, declaring a variable as int** means that you are creating a pointer that points to int*. A pointer must point somewhere, else it cannot be dereferenced (get what it points to). So a "C" dynamic array has a pointer to the first element of an array whose every element is a pointer to the first element in another array. So if you dereference the first pointer, you get the first element of an array of int*, then if you dereference that, you get the first int in that array. Tell me how you can print the array of you fetch one of those ints? I don't know, the compiler doesn't know, the code doesn't know.

Secondly, you should avoid global variables. Make your variable a parameter, or perhaps even a return value.

Thirdly, you don't delete what you new, so you get memory leaks.

And finally, C++ isn't so barbaric so you have to do all this complexity just to get a dynamic 2d array. We have what's called a vector. If you google for 2d vector, I'm pretty sure you will find some good example on how to dynamically allocate a 2d array quickly and easily.