Array of pointers in objective-c

Question

I'm getting confused by pointers in objective-c.

Basically I have a bunch of static data in my code.

static int dataSet0[2][2] = {{0, 1}, {2, 3}};
static int dataSet1[2][2] = {{4, 5}, {6, 7}};

And I want to have an array to index it all.

dataSets[0]; //Would give me dataSet0...

What should the type of dataSets be, and how would I initialize it?

Answer 1

Your index array would be an array of pointers to pointers to int.

So the declaration would look like:

int ** dataset[numOfDataSets] = {dataSet0, dataSet1, ...}

Remember that objective-c is a proper superset of ansi-c, and this question in particular is about the c language, really.

Edit: It's important to remember that in C, arrays are essentially just pointers, and two-dimensional arrays are pointers to pointers.

Edit 2: I think act actually I muffed operator precedence. Should be:

int (** dataset)[numOfDataSets] = {dataSet0, dataSet1, ...}

Answer 2

Pointers to multi-dimensional arrays can be tricky. A typedef can help:

typedef int (*DataSetType)[2];
DataSetType dataSets[] = { dataSet0, dataSet1 /* and so on*/ };

Answer 3

You could use an NSPointerArray.

Answer 4

While it's laid out the same in memory, a pointer to a multidimensional array is different than a pointer to a flat array. The compiler has to convert the [][] index to a flat array index for any multidimensional arrays. Can't mix the two or the distinction for the compiler is lost. You can either use all flat arrays:

  static int dataSet00[2] = {0,1};
  static int dataSet01[2] = {2,3};
  static int * dataSet0[2] = {dataSet00, dataSet01};

  static int dataSet10[2] = {4, 5};
  static int dataSet11[2] = {6, 7};
  static int * dataSet1[2] = {dataSet10, dataSet11};

  static int ** dataSets[2] = {dataSet0, dataSet1};

or one big multidimensional array:

  static int dataSets[2][2][2] = {{{0,1},{2,3}},{{4,5},{6,7}}};

but not a combination of the two unless you clue the compiler in by declaring a special datatype per Jon's suggestion.