This lesson explains the concept of multidimensional arrays and pointers pointing to other pointers

We'll cover the following

Multi-dimensional arrays

A multidimensional array allows nesting arrays:

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int grid[3][3];

This allocates 3*3 elements in one memory block.

Note: Even though arrays behave similarly to pointers, a multidimensional array is not a pointer-to-a-pointer.

Here is a visual representation of the multi-dimensional array grid[3][3]:

  • The objects grid, grid[0] and grid[0][0] are always at the same location (but different types).
  • The objects of variable pptr and pointer *pptr, **pptr are at different locations.

When evaluating grid[0][0]

  • The array grid (which is an int[3][3]) is first converted to a pointer of type int( * )[3].
  • Then taking the element at offset 0 yields an object of int[3].
  • Then it is converted to int* again and the element at offset 0 is taken, generating an object of type int&.

Down below is an example code illustrating how to work with multi-dimensional arrays.

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#include <iostream>
using namespace std;
 
int main () {
   // an array with 3 rows and 3 columns.
   int grid[3][3]; 
 
   // setting value of each array element                      
   for ( int i = 0; i < 3; i++ ){
      for ( int j = 0; j < 3; j++ ) {
        grid[i][j] = i+j;
        cout << "grid[" << i << "][" << j << "]: " <<grid[i][j]<< endl;
      }
   }
 
   return 0;
}

Pointer to a pointer

A pointer contains a reference to another variable. It may also point to a pointer:

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int **pptr;

Down below is an illustration demonstrating the concept.

For pptr[0][0], the address stored in pptr is taken and the address stored in that address is taken, ​and it is the result of the expression.

Take a look at the example below to understand this concept better:

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#include <iostream>
using namespace std;
int main() {
  
  int x=1;
  int *ptr1;
  int **ptr2; 
  
  ptr1 = &x;    //getting address of x
  ptr2 = &ptr1;  //getting address of ptr1
  
  cout << "Value of x is: "<<x<<endl;
  
  //let's print the value being pointed to by ptr1
  cout << "The value being pointed to by ptr1 is: "<<*ptr1<<endl;
  
  //let's print the address being pointed to by ptr2
  cout << "The address being pointed to by ptr2 is: "<<*ptr2<<endl;
  
  //let's print the value being pointed by ptr2
  cout << "The value being pointed to by ptr2 is: "<<**ptr2<<endl;
  
  return 0;
}

  • In the example above *ptr2 will give the value of the address at which our number is stored, this will also be the address of ptr1.

  • **ptr2 then further dereferences and gives the value stored at that address which is 1.

Linked List

This allows for the implementation of a linked list:

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class LinkedListOfIntsNode
{
    int value;
    LinkedListOfIntsNode *next_node;
};

Think of a chain of ten LinkedListOfIntsNode, each pointing to its neighbor to the right. You can traverse the list using next_node.

In the next lesson,​ we will discuss how to pass pointers to functions.

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