property for ivar that points to two-dimensional array of pointers to NSStrings - objective-c

Question

I want to create a class that contains a dynamic, two-dimensional c-array of pointers to NSStrings. I know I can simulate a two-dimensional array using an NSArray containing multiple NSArrays, but if possible I'd like to do this using a traditional two-dimensional c-array. ARC won't allow a simple assignment of a pointer to an NSString to an element of a c-array unless you use "__unsafe_unretained":
#interface NumberStringsArray : NSObject
{
#public
NSString * __unsafe_unretained **_array;
}
To avoid memory leaks and to give an object in the class ownership of each NSString assigned to the c-array, I add a pointer to each NSString object to an NSMutableArray. In -(void)dealloc I free the memory acquired to create the two-dimensional c-array.
Here's my question: How do I declare a property based on the _array ivar so that I can refer to the i,j element of the array as "foobar.array[i][j]" rather than "foobar->array[i][j]"?
Later amplification: I did it in a very similar manner to the answerer except for the __bridge stuff. I don't know if that makes a difference. I allocate the two-dimensional array here:
self->_array = (NSString * __unsafe_unretained **)calloc(_columnCount, sizeof(void *));
if (!self->_array)
return nil;
for (UINT16 i = 0; i < _columnCount; i++)
{
self->_array[i] = (NSString * __unsafe_unretained *)calloc(_rowCount, sizeof(void *));
if (!self->_array[i])
{
for (UINT16 a = 0; a < _columnCount; a++)
if (self->_array[a])
free(self->_array[a]);
if (self->_array)
free(self->_array);
return nil;
}
}
I put pointers to the NSString objects into the array using substrings generated from a file of comma-separated values:
NSArray *numbers = [line componentsSeparatedByString: #","];
for (UINT16 i = 0; i < _columnCount; i++)
{
NSString *number = #"";
if (i < [numbers count])
number = [numbers objectAtIndex: i];
//
// save it in owners
//
[self.owners addObject: number];
self->_array[i][j] = number;
}
In -(void)dealloc I free all the memory:
-(void)dealloc
{
for (UINT16 i = 0; i < self.columnCount; i++)
if (self->_array[i])
free(self->_array[i]);
if (self->_array)
free(self->_array);
}

Answer 1

Declare this property:
#property (nonatomic) NSString * __unsafe_unretained **_array;
Then you can allocate the pointers to objects:
_array= (NSString * __unsafe_unretained **) malloc(M*sizeof(CFTypeRef) );
for(NSUInteger i=0; i<M;i++)
{
_array[i]= ((NSString * __unsafe_unretained *) malloc(N*sizeof(CFTypeRef) );
for(NSUInteger j=0; j<N;j++)
{
_array[i][j]= (__bridge NSString*) (__bridge_retained CFTypeRef) [[NSString alloc]initWithCString: "Hello" encoding: NSASCIIStringEncoding];
// I see that you got habit with C so you'll probably like this method
}
}
Then when you don't need it anymore, free the array:
for(NSUInteger i=0; i<M; i++)
{
for(NSUInteger j=0; j<N;j++)
{
CFTypeRef string=(__bridge_transfer CFTypeRef) _array[i][j];
}
free(_array[i]);
}
free(_array);

Answer 2

You can't because you can't declare a concrete object for an Objective-C class. So
NumberStringsArray object;
is not allowed.
You are forced to declare it as
NumberStringsArray *object = [[NumberStringsArray alloc] init.. ];
so you have to access to the ivar through the correct -> operator applied to pointers. Mind that the object.something in Objective-C is just a shorthand for [object something] while in standard C you would use . to access to fields of a concrete struct.

Answer 3

(Note: This addresses the creation/use of the property to access the data, not the way the data should be managed by conventional Objective-C storage management or by ARC. Thinking about that makes my head hurt.)
If you want a read-only C array to "look" like an Objective-C property, declare the property such as #property (readonly, nonatomic) char* myProp; and then, rather than using #synthesize, implement a getter for it along the lines of:
-(char**)myProp {
return myPropPointer;
// Or, if the array is allocated as a part of the instance --
return &myPropArray[0];
}