Is int *array[32] a pointer to an array of 32 ints, or an array of 32 pointers to int? Does it matter?

Question

If I write

int *columns[32];

am I defining an array with 32 pointers to ints?
Or is it a pointer to an array of 32 ints?

How do I differentiate between the two? Is there a difference?

Answer 1

Expanding on a comment to another answer:

There's a fairly straightforward procedure for reading C declarations. Start with the leftmost identifier in the declarator and work your way out, remembering that [] and () bind before *. Given the declaration

int *columns[32];

break it down as

     columns                  -- columns
     columns[32]              -- is a 32-element array
    *columns[32]              -- of pointers
int *columns[32]              -- to int.  

If the declaration had been

int (*columns)[32];

then it would break down as

      columns                 -- columns
    (*columns)                -- is a pointer
    (*columns)[32]            -- to a 32-element array
int (*columns)[32]            -- of int.  

This will also help you build up complex declarations. Suppose you wanted to declare an array of pointers to functions returning pointers to arrays of char:

          f                     -- f
          f[N]                  -- is an N-element array
         *f[N]                  -- of pointers
        (*f[N])()               -- to functions
       *(*f[N])()               -- returning pointers
      (*(*f[N])())[M]           -- to M-element arrays
     *(*(*f[N])())[M]           -- of pointers
char *(*(*f[N])())[M];          -- to char

cdecl is a nice tool, but after you'd done this exercise a few times, you shouldn't need it.

Answer 2

When in doubt - ask cdecl

$> cdecl 
Type `help' or `?' for help
cdecl> explain int *columns[32]
declare columns as array 32 of pointer to int

EDIT In response to comments: I found cdecl source on Google Code Search. It requires GNU readline library. I think it shouldn't be a problem to compile it on Mac OS X or Windows.

Answer 3

You are defining an array of 32 pointers.

To define a pointer to an array of 32 ints you have to do

int (*columns)[32];

The former declaration instantiates an array with space for 32 * sizeof(int). On the other hand, the latter instantiates a single uninitialized pointer which you can then use as follows:

int myintegers[32] = {0, 1, 2, ..., 31};
int (*columns)[32];

columns = &myintegers;

printf("%d\n", (*columns)[2]);

I hope I made the difference a little bit clear.

Answer 4

It is an array of 32 pointers to int and yes it does matter.

The C grammar rules specify that array access ([]) binds tighter than dereference (*) and declarations mirror usage.

The declaration int *columns[32]; means that the expression *columns[n] (where n is a number between 0 and 31) is an int. This expression is the same as *(columns[n]). The declaration allocates the space for 32 pointers, but there are no ints allocated and (assuming that this is a function local declaration) none of the pointers are initialized.

Had the declaration been int (*columns)[32]; then the expression (*columns)[n] would have been an int, meaning that the * dereference happens before the array access, so columns would have been a pointer to an array of 32 ints. The declaration would have allocated one pointer, but no arrays of ints.

Answer 5

A test program is illustrative, especially to those of us who aren't language lawyers:

$ gcc -x c -
#include <stdio.h>

int main(void) 
{
    int *columns[32];
    printf("%lu\n%lu\n", sizeof(columns), sizeof(columns[0]));
    return 0;
}
$ ./a.out
128
4
$

It appears to be an array of pointers.

Answer 6

Here are some fun declarations for you:

int *arrayOfIntP[32];
int (*pointerToArrayOf32)[32];

For more fun with multidimensional arrays look at these posts:

Array of pointers to multidimensional arrays

How do I declare a 2d array using new?

Answer 7

One trick is to read from right to left.

Given int* cols[32];

1. You first see cols[32] : this is an array of size 32.
2. Next you see int*: this is a pointer

All that left of array is the type of the elements in the array. So we read it as array of pointers to int (and of count 32).

Answer 8

Refer to Question #5 of A 'C' Test: The 0x10 Best Questions for Would-be Embedded Programmers by Nigel Jones*

---

a) int a; // An integer

b) int *a; // A pointer to an integer

c) int **a; // A pointer to a pointer to an integer

d) int a[10]; // An array of 10 integers

e) int *a[10]; // An array of 10 pointers to integers

f) int (*a)[10]; // A pointer to an array of 10 integers

g) int (*a)(int); // A pointer to a function a that takes an integer argument and returns an integer

h) int (*a[10])(int); // An array of 10 pointers to functions that take an integer argument and return an integer

---

*Alas, the original article on embedded.com can no longer be found on their website.