Objective C shared readonly instance of class

Question

I want to make a readonly instance of a class in Objective C. I have a vector class which is basically floats for x and y position and some methods. In a lot of cases I need a (0, 0)-vector so I was thinking instead of allocating a new one each time that I would have a shared zero vector, something like this:

// Don't want to do this all the time (allocate new vector)
compare(v, [[Vector alloc] initWithCartesian:0:0]);

// Want to do this instead (use a shared vector, only allocate once)
compare(v, [Vector zeroVector]);

// My attempt so far
+ (Vector *)zeroVector {
    static Vector *sharedZeroVector = nil;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        sharedZeroVector = [[self alloc] initWithCartesian:0:0];
    });
    return sharedZeroVector;
}

// The problem
v.x = 3;

This works fine, except that the zero vector is not readonly, which feels kind of silly. As a note I would like to mention that this is more of a want-to-know-how-to kind of question than an actual problem, I don't know if it will make some actual difference.

Answer 1

The common solution is having all instances immutable (see NSNumber, NSDecimalNumber etc.), possibly having a second mutable class (NSString vs NSMutableString or NSArray vs NSMutableArray).

Answer 2

Depends on how your standard vector should work. If you never want to set x and y via property you could just make them readonly:

@property (nonatomic, readonly) NSInteger x;
@property (nonatomic, readonly) NSInteger y;

If some of your vectors should be readwrite you can create a readonly class Vector and derive a class MutableVector:

@interface Vector : NSObject
  @property (nonatomic, readonly) NSInteger x;
  @property (nonatomic, readonly) NSInteger y;
@end

@interface MutableVector : Vector
  @property (nonatomic) NSInteger x;
  @property (nonatomic) NSInteger y;
@end

You would then use Vector for zeroVector and MutableVector for all others.