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Pointer-Pointers

Ok, so... now we know what pointers are. So let's learn it over again! A pointer-to-pointer in C is just a pointer variable that holds the address of another pointer.

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This allows you to create complex data structures like arrays of pointers, and to modify pointers indirectly. The syntax is exactly what you would expect:

int value;
int *pointer;
int **pointer_pointer;

Pointers to pointers (or pointers to pointers to pointers to pointers... you get the idea) are like a treasure map or a scavenger hunt. You start at one pointer and keep following the chain of addresses until you get to the final value. Each * follows one address: *pointer_pointer gets the inner int *, and **pointer_pointer gets the final int.

Assignment

Complete the allocate_int function. It accepts a pointer to a pointer to an integer called pointer_pointer, and a raw value. Update the pointer that pointer_pointer points to so it stores the address of newly allocated memory containing value.

Allocate memory for a single integer on the heap, and save its address into a new pointer.
Update the pointer that pointer_pointer is pointing at to hold the newly allocated memory address.
Set the raw value of the integer that pointer_pointer now points to (well, that it points to through 2 address hops) to the value passed in.

Observe: If you take a look at test_does_not_overwrite in the main.c file, you'll notice that while pointer_pointer does indeed unravel and point to the new value, the original value still exists at its spot in memory. We just don't point to it anymore.