Deallocating memory in a 2D array

Question

Suppose we have:

int** myArray = new int*[100];
for(int i = 0; i < 100; i++){
    myArray[i] = new int[3];
}

What is the appropriate way to deallocate this array (which method below, if either is a correct way to do so)?

1.

delete[] myArray;

2.

for(int i = 0; i < 100; i++){
    for(int j = 0; j < 3; j++){
        delete myArray[i][j];
    }
}
delete[] myArray;

Intuitively it seems like we should do something like 2. since we want all of the memory we allocated to be deleted, but I'm not sure.

Answer 1

You used one loop to create it, you should use one loop to delete it. The order is reversed to the order of allocation:

for(int i = 0; i < 100; i++)
    delete [] myArray[i];              // delete all "rows" in every "column"

delete [] myArray;                     // delete all "columns"

Moreover:

1. is for deleting one-dimensional dynamically allocated array - used to delete "rows" and "columns" above.

2. only resembles how one would delete a 2D array of pointers e.g.:

   int*** myArray = new int**[100];   // (1)
   
   for(int i = 0; i < 100; i++)
   {
       myArray[i] = new int*[3];      // (2)
   
       for(int j = 0; j < 3; j++)
           myArray[i][j] = new int(); // (3)
   }
   
   for(int i = 0; i < 100; i++)
   {
       for(int j = 0; j < 3; j++)
           delete myArray[i][j];      // (3)
   
       delete [] myArray[i];          // (2)
   }
   
   delete [] myArray;                 // (1)
   

   You can see the "reversed" nature of it.

Answer 2

The right method is

for(int i = 0; i < 100; i++)
   delete [] myArray[i];

delete [] myArray;