When is pointer to array useful?

Question

I can declare:

int (*ap)[N];

So ap is pointer to int array of size N. Why is this ever useful? If I pass it to function, what useful thing it can do with it that it could not do with just a normal pointer to the array's contents?

C FAQ say:

2.12: How do I declare a pointer to an array?

Usually, you don't want to.

Answer 1

It's not useful, really. But sometimes pointers to arrays are used e.g. in Microsoft Windows API - I've seen a lot of this stuff there.

Answer 2

You never need to do this since the array name itself is a pointer to the beginning of the array

Answer 3

If you increment the pointer, it will then point to the start of the next group of N elements.

This is not a big deal and it's use is up to the developer.

Answer 4

This will probably careen off into subjective/argumentative, but...

In my opinion, pointers to arrays are in the language because they fell into the language. There are pointers to every other declarable data type, so these are here too. I've never seen anyone get really useful work out of them. Hypothetically, they allow a prototype that demands an array and not a pointer to the element, but ...

Answer 5

It seems pretty useless to me to do a pointer to an array. In C, an array is already a pointer to a block of that data type.

int (*ap)[N];
int **ipp;

are both the same data type (a pointer to a pointer to an Integer). The only difference there is that there is space for N integers allotted for ap.

There's no need to pass an array by a pointer to a function, like, for the purpose of changing the contents of the array within that function because it's already a pointer. As a general rule, I'd say it's unnecessary and just creates an additional need to dereference the pointer to get at the data in the array. But, I'm sure there's a program or algorithm somewhere that could find a legitimate use for it.

Answer 6

There are situations where you want to pass the memory location across programs. e.g a windows API might expect you to pass a pointer to data structure or an array where as you are programming in some other language, say c#. Windows API does not care how the target language handles array's, for windows API it is just a stream of bytes in memory and it will fill it, send it back to you. to avoid cross language type missmatch in some cases we use pointer to array rather than implicit array name as pointer. More over it is not guaranteed that the implicit array name is a long pointer, some compilers might optimize it to be a relative value withing segment. a pointer to an array guarantees that it is of the order of machine register size and you can point to a location anywhere in the available RAM.

Answer 7

A pointer to an array can be used to dynamically allocate a multi-dimensional array N, where N-1 dimensions are known. Below creates a Nx3 array.

int (*ap)[3];
ap = malloc(N * sizeof(*ap));
/* can now access ap[0][0] - ap[N-1][2] */

@Adam E/Cruachan, This is not the same thing as a pointer to a pointer. ap is a single pointer to a block of memory containing three consecutive integers. ap++ will advance the pointer address to the next block of three integers. for int **pp;, pp points to an integer pointer, each of which can point to an integer anywhere in memory.

         +-----+                +------+    +-----+
 ap ---> | int |   vs.  pp ---> | int* | -> | int |
         | int |                +------+    +-----+
         | int |        pp+1 -> | int* | -\
         +-----+                +------+   \   +-----+
 ap+1 -> | int |                  :  :      -> | int |
         | int |                               +-----+
         | int |
         +-----+
           : :  

Answer 8

I think the conclusion from this whole discussion is never. Nobody here really demonstrated a use for this construct where something else wouldn't work in a more comprehensible way.