What is a pointer to pointers?

A pointer is a variable like any other. As such, it has memory allocated to it. It is similar to an int in that they both store a value in their memory location. However, whereas an int value is interpreted as a whole number, a pointer's value is interpreted as a memory address. The pointer is said to "point to" the memory address it contains. The type of the pointer determines the type of the variable stored at that memory address. the following is a declaration of a pointer to an integer initialised with the value zero:
int * p = new int(0);

A pointer to a pointer is no different to an ordinary pointer, except the memory address it points to is that of another pointer. The following is an example of a pointer that points to the pointer we've just declared.

int ** pp = p;

Pointer-to-pointer variables are typically used whenever you need to pass a pointer by reference. All pointers are passed by value, so the memory address they contain is passed, not the pointer variable itself. To pass the pointer itself, you must pass a pointer to the pointer, which passes the memory address of the pointer. This then makes it possible to change the memory address stored in that pointer.

Pointer-to-pointer variables are also used when declaring dynamic, multi-dimensional arrays. For instance, a 2-dimensional array requires a pointer-to-pointer to reference the array itself. This pointer points to a 1-dimensional array of pointers, each of which points to a 1-dimensional array of the actual variables in the array. Unlike a static multi-dimensional array, where the entire array occupies contiguous memory, a dynamic multi-dimensional array may not reside in contiguous memory.

Pointer-to-pointer can be extended further to accommodate 3-dimensional arrays, and above. A 3D array requires a pointer-to-pointer-to-pointer, which points to a 1-dimensional array of pointer-to-pointer, each of which references a 2D array, as previously outlined.