Objective C - Access another instance's instance variables?

Question

Is it possible to access another instance's variables, given that we're working in the same class?

Or, in other words, can you do this Java code (which works in Java, I've done it before) in Objective C:

class Matrix {
    private int mat[] = new int[16]; //wouldn't be a pointer in C

    public Matrix (Matrix m){
        for (int i = 0; i < 16; i++){
            this.mat[i] = m.mat[i]; //<-- this here
        }
    }
}

Given that arrays cannot be properties in Objective C, I can't make mat[] into a property. Is there any way to do this then?

Adam Rosenfield · Accepted Answer · 2011-05-07 05:36:30Z

3

You can do it perfectly fine, you just can't make the instance variable (ivar) into a property:

@interface Matrix : NSObject
{
@private
    int mat[16];
}
- (id) initWithMatrix:(Matrix *)m;
@end

@implementation Matrix
- (id) initWithMatrix:(Matrix *)m
{
    if ((self = [super init]))
    {
        for(int i = 0; i < 16; i++)
            mat[i] = m->mat[i];
        // Nota bene: this loop can be replaced with a single call to memcpy
    }
    return self;
}
@end

answered May 7, 2011 at 5:36

Adam Rosenfield

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1 Comment

Tustin2121 Over a year ago

This is the exact thing I was looking for: a way to access it without making it a property (and therefore public). I didn't know you could do a pointer deference in Objective-C and with an Objective-C object. (And thank you for the good note). :)

mportiz08 · Accepted Answer · 2011-05-07 04:49:28Z

3

You could use an NSArray that holds NSNumbers instead of a regular old c int array--then you could use it as a property.

something like this maybe:

self.mat = [NSMutableArray arrayWithCapacity:16];
for(int i = 0; i < 16; i++) {
  [self.mat addObject:[NSNumber numberWithInt:[m.mat objectAtIndex:i]]];
}

edited May 7, 2011 at 4:49

answered May 7, 2011 at 4:43

mportiz08

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1 Comment

Tustin2121 Over a year ago

That seems a bit bloated, plus the eventual goal is to pass the array directly to OpenGL, which would most likely require some conversion with this scheme.

rpetrich · Accepted Answer · 2011-05-07 04:54:43Z

1

The closest analogy would be a readonly property that returns int *:

@interface Matrix : NSObject {
@private
    int values[16];
}
@property (nonatomic, readonly) int *values;
@end

@implementation
- (int *)values
{
    return values;
}
@end

For a Matrix type, you should really be using a struct or Objective-C++; all the method dispatch/ivar lookup will add a lot of overhead to inner loops

answered May 7, 2011 at 4:54

rpetrich

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Comments

hotpaw2 · Accepted Answer · 2011-05-07 04:55:16Z

0

Arrays can't be properties. But pointers to an array of C data types can. Just use an assign property, and check for NULL before indexing it as an array.

answered May 7, 2011 at 4:55

hotpaw2

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