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6.14 — Pointers to pointers and dynamic multidimensional arrays

This lesson is optional, for advanced readers who want to learn more about C++. No future lessons build on this lesson.

A pointer to a pointer is exactly what you’d expect: a pointer that holds the address of another pointer.

Pointers to pointers

A normal pointer to an int is declared using a single asterisk:

A pointer to a pointer to an int is declared using two asterisks

A pointer to a pointer works just like a normal pointer — you can dereference it to retrieve the value pointed to. And because that value is itself a pointer, you can dereference it again to get to the underlying value. These dereferences can be done consecutively:

The above program prints:

5
5

Note that you can not set a pointer to a pointer directly to a value:

This is because the address of operator (operator&) requires an lvalue, but &value is an rvalue.

However, a pointer to a pointer can be set to null:

Arrays of pointers

Pointers to pointers have a few uses. The most common use is to dynamically allocate an array of pointers:

This works just like a standard dynamically allocated array, except the array elements are of type “pointer to integer” instead of integer.

Two-dimensional dynamically allocated arrays

Another common use for pointers to pointers is to facilitate dynamically allocated multidimensional arrays (see 6.5 -- Multidimensional Arrays [1] for a review of multidimensional arrays).

Unlike a two dimensional fixed array, which can easily be declared like this:

Dynamically allocating a two-dimensional array is a little more challenging. You may be tempted to try something like this:

But it won’t work.

There are two possible solutions here. If the right-most array dimension is a compile-time constant, you can do this:

The parenthesis are required here to ensure proper precedence. In C++11 or newer, this is a good place to use automatic type deduction:

Unfortunately, this relatively simple solution doesn’t work if the right-most array dimension isn’t a compile-time constant. In that case, we have to get a little more complicated. First, we allocate an array of pointers (as per above). Then we iterate through the array of pointers and allocate a dynamic array for each array element. Our dynamic two-dimensional array is a dynamic one-dimensional array of dynamic one-dimensional arrays!

We can then access our array like usual:

With this method, because each array column is dynamically allocated independently, it’s possible to make dynamically allocated two dimensional arrays that are not rectangular. For example, we can make a triangle-shaped array:

In the above example, note that array[0] is an array of length 1, array[1] is an array of length 2, etc…

Deallocating a dynamically allocated two-dimensional array using this method requires a loop as well:

Note that we delete the array in the opposite order that we created it. If we delete array before the array elements, then we’d have to access deallocated memory to delete the array elements. And that would result in undefined behavior.

Because allocating and deallocating two-dimensional arrays is complex and easy to mess up, it’s often easier to “flatten” a two-dimensional array (of size x by y) into a one-dimensional array of size x * y:

Simple math can then be used to convert a row and column index for a rectangular two-dimensional array into a single index for a one-dimensional array:

Passing a pointer by address

Much like we can use a pointer parameter to change the actual value of the underlying argument passed in, we can pass a pointer to a pointer to a function and use that pointer to change the value of the pointer it points to (confused yet?).

However, if we want a function to be able to modify what a pointer argument points to, this is generally better done using a reference to a pointer instead. So we won’t talk about it further here.

We’ll talk more about pass by address and pass by reference in the next chapter.

Pointer to a pointers to a pointer to…

It’s also possible to declare a pointer to a pointer to a pointer:

These can be used to dynamically allocate a three-dimensional array. However, doing so would require a loop inside a loop, and are extremely complicated to get correct.

You can even declare a pointer to a pointer to a pointer to a pointer:

Or higher, if you wish.

However, in reality these don’t see much use because it’s not often you need so much indirection.

Conclusion

We recommend avoiding using pointers to pointers unless no other options are available, because they’re complicated to use and potentially dangerous. It’s easy enough to dereference a null or dangling pointer with normal pointers — it’s doubly easy with a pointer to a pointer since you have to do a double-dereference to get to the underlying value!

6.15 -- An introduction to std::array [2]
Index [3]
6.13 -- Void pointers [4]