object_getInstanceVariable/object_setInstanceVariable in ARC

Question

Why are the Objective-C runtime methods object_getInstanceVariable and object_setInstanceVariable are not available under Automatic Reference Counting and what can I do about it?

object_getInstanceVariable is buggy when the instance variable's size is larger than the development target's pointer size. How can I get around this?

Answer 1

Use the valueForKey: and setValue:forKey: methods instead. These allow you to read/write any instance variable of an object. For primitive-typed instance variables these methods return/take values wrapped as NSNumber or NSValue objects.

You have instance variables larger than the pointer size, maybe struct's? Here are some code fragments showing use with a struct, first let's define a struct:

typedef struct
{
   int i;
   float f;
   char c;
} ThreePrimitives;

and a class with a (private) instance variable:

@interface StructClass : NSObject
...
@end

@implementation StructClass
{
   ThreePrimitives myStruct;
}
...
@end

To set the instance variable:

ThreePrimitives a = { 42, 3.14, 'x' };
NSValue *wrapA = [NSValue value:&a withObjCType:@encode(ThreePrimitives)];
[sc setValue:wrapA forKey:@"myStruct"];

To read the instance variable:

ThreePrimitives b;
NSValue *extracted = [sc valueForKey:@"myStruct"];   
[extracted getValue:&b];

HTH