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I am programming an iPhone game which has items laid out on a grid. Each item is represented by an object, and I want to use a 2D array to represent the contents of the grid. Since I will need to be passing the array through methods, I have attempted to declare it as:

MyClass*** grid;

and have a method to return it as:

-(MyClass ***) {
    return grid
}

But before even compiling i get this error:

Pointer to non-const type 'MyClass *' with no explicit ownership.

What does this mean, and why would it happen?

Jumhyn
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  • Are you using automatic reference counting? –  Nov 19 '11 at 21:54
  • ARC requires explicit ownership. Pointers to object pointers have unknown owners, so it won't work. See http://stackoverflow.com/q/7804435/ which is similar. –  Nov 19 '11 at 22:08
  • The suggested answer on that question is to store the NSError** as a simple pointer. However, that is not an option here (I believe). Could you suggest another alternative? – Jumhyn Nov 19 '11 at 22:11
  • You could use manual reference counting, or you can use nested `NSArray`s. If the number of elements in your C-array is known at compile time, you can declare the array as `MyClass *grid[20][20]` and it should work. –  Nov 19 '11 at 22:17
  • But I need to pass it. I was under the impression that passing arrays was forbidden, and to do so you were required to pass a pointer instead. – Jumhyn Nov 19 '11 at 22:21
  • Then you should either use manual reference counting or nested `NSArray`s. –  Nov 19 '11 at 22:23
  • Damn. Thanks for the info, if you post as an answer I'll accept it. – Jumhyn Nov 19 '11 at 22:25

2 Answers2

1

You could just use the standard C 2D array for this,

MyClass *item = (MyClass *)grid[x][y];
Alex
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  • How would I then go about passing/returning `grid` from a method? – Jumhyn Nov 19 '11 at 22:06
  • Remember that objective C is a superset of C, so you can pass C around as you wish. Take a look at http://borkwarellc.wordpress.com/2007/09/06/its-still-just-c-arrays-and-pointers/ – Alex Nov 20 '11 at 10:24
  • So, in answer to your question: - (MyClass *)returnGrid:(MyClass *) grid { // do something to the grid... return grid } – Alex Nov 20 '11 at 10:25
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When using Automatic Reference Counting you cannot have pointers to object pointers that way, since the owner will be unknown.

You can use nested NSArray objects instead, or switch to manual reference counting (you shouldn't do the latter if you already have a large project).

You could also instead have a method like this which the callee will call multiple times (once for each cell in the grid):

- (id)itemAtX:(NSUInteger)x y:(NSUInteger)y {
  // TODO: Add bounds checking.
  return grid[x][y];
}
  • I dont' know about using an NSArray, since I'm attempting to represent a grid of data with it, and NSArrays are dynamically sized. I suppose I could set the objects that I wanted to be filled to `nil`. – Jumhyn Nov 19 '11 at 22:32
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    I'd recommend using NSNull object instead – Alex Nov 20 '11 at 10:18