I have a circular reference. How can I create a weak reference in Objective-C?

Question

I'm working on an iPhone application. I have an object of class Row that needs to release numerous objects of the class Block. Every Block currently has a property that retains an instance variable of class Row.

@interface Block : UIImageView {
  Row *yCoord;
}
@property (nonatomic,retain) Row *yCoord;
@end

Every Row contains an NSMutableArray of these Blocks.

@interface Row : NSObject {
    NSMutableArray *blocks;
}
-(void)addBlock:(Block*)aBlock;
@end

@implementation Row
-(void)addBlock:(Block*)aBlock {
    [blocks addObject:aBlock];
    aBlock.yCoord = self;
}
@end

I understand that this is a circular reference. Apple's documentation states that in order to deallocate an object with a circular reference I need a weak reference instead of a strong reference (a retain property), but it doesn't follow through and explain how exactly I go about doing so. I plan to release and dealloc all Blocks within a Row as well as the Row itself simultaneously. How do I set up a weak reference within each of my Blocks to their "parent" Row?

Answer 1

Edit: Since the asker clarified he's not using garbage collection (iPhone currently does not support it), my advice is to avoid cycles by having only one of the objects retain the other, just as you would do with a delegate. When using properties, use "assign" instead of "retain" to achieve this. For example:

@property (nonatomic,assign) Row *yCoord;

The rest of my answer answer relates to "weak references" in terms of Objective-C 2.0 and GC.

---

When you're working with garbage collection (10.5+), a weak reference is created by prefixing a variable declaration with __weak. When you assign to that variable, the GC (if enabled) keeps track of the reference and will zero it out for you automatically if all strong references to the referenced object disappear. (If GC is not enabled, the __weak attribute is ignored.)

Thus, you can safely modify the above answer to play nicer with garbage collection (currently on 10.5+, and perhaps someday on iPhone) as follows: (See the related Apple docs.)

@property (nonatomic,assign) __weak Row *yCoord;

To quote Chris Hanson (where you can find more detailed information):

"By prefixing an instance variable declaration with __weak, you tell the garbage collector that if it's the only reference to an object that the object should be considered collectable."

I'd clarify that by saying "if there are no non-weak references to an object". As soon as the last strong reference is removed, the object may be collected, and all weak references will be zeroed automatically.

Note: This isn't directly related to creating weak references, but there is also a __strong attribute, but since Objective-C object variables are strong references by default, it is generally used only for raw C pointers to things like structs or primitives that the Garbage Collector will not treat as roots, and will be collected from under you if you don't declare them as strong. (Whereas the lack of __weak can cause retain cycles and memory leaks, the lack of __strong can result in memory stomping and really strange and insidious bugs that occur non-deterministically and can be quite difficult to track down.)

Answer 2

Just change it to assign instead of retain, no more circular references.

@interface Block : UIImageView {
  Row *yCoord;
}
@property (nonatomic,assign) Row *yCoord;
@end

Answer 3

A weak reference is simply an assignment (unless you're talking about Garbage Collection which is a whole separate can of worms, but does not suffer from retain cycles).

Normally, in Cocoa, Row would retain the Block objects (by including them in the NSMutableArray), but Block would not retain Row, each would simply store it in an ivar (with an "assign" property).

As long as Row is careful to release each Block before it is deallocated (ie, its dealloc should release the NSMutableArray which will release the Blocks as long as no one else has any pointers to them) then everything will be deallocated as appropriate.

You can also take the precaution of zeroing the row reference from Blocks before removing the entiries from the array, something like:

- (void) dealloc {
    for (Block* b in _blocks) {
        b.row = nil;
    }
    [_blocks release];
    [super dealloc];
}

where _blocks is the ivar referenced by the blocks property.

Answer 4

Using assign to create weak references can be unsafe in a multithreaded system, particularly when either object can be retained by a third object, and then used to dereference the other object.

Fortunately, this is often a problem of hierarchy, and the object containing the weak reference only cares about the object it refers to for the referred-to object's lifetime. This is the usual situation with a Superior<->Subordinate relationship.

I think that the case in the OP's comment maps to this, with Row = Superior, Block = Subordinate.

In this case, I would use a handle to refer to the Superior from the Subordinate:

// Superior.h

@class Superior;

@interface SuperiorHandle : NSObject {
    @private
        Superior* superior_;
}

// note the deliberate avoidance of "nonatomic"
@property (readonly) Superior *superior;

@end

@interface Superior : NSObject {
    @private
        SuperiorHandle *handle_;
        // add one or more references to Subordinate instances
}

// note the deliberate avoidance of "nonatomic"
@property (readonly) SuperiorHandle *handle;

@end


// Superior.m

#import "Superior.h"

@implementation SuperiorHandle

@synthesize
    superior = superior_;

- (id)initWithSuperior:(Superior *)superior {
    if ((self = [super init])) {
        superior_ = superior; // weak reference
    }
}

- (void)invalidate {
    @synchronized (self) {
        superior_ = nil;
    }
}

- (Superior *)superior {
    @synchronized (self) {
        // retain and autorelease is required to prevent dealloc before we're ready, thanks to AndroidDev for pointing out this mistake
        return [[superior_ retain] autorelease];
    }
}

@end

@implementation Superior

@synthesize
    handle = handle_;

- (id)init {
    if ((self = [super init])) {
        handle_ = [[SuperiorHandle alloc] initWithSuperior:self];
    }
    return self;
}

- (void)dealloc {
    [handle_ invalidate];
    [handle_ release];

    [super dealloc];
}

@end


// Subordinate.h

@class Superior;
@class SuperiorHandle;

@interface Subordinate : NSObject {
    @private
        SuperiorHandle *superior_handle_;
}

@property (readonly) Superior *superior;

@end


// Subordinate.m

#import "Subordinate.h"

#import "Superior.h"

@implementation Subordinate

// no synthesize this time, superior's implementation is special

- (id)initWithSuperior:(Superior *)superior {
    if ((self = [super init])) {
        superior_handle_ = [superior.handle retain];
    }
    return self;
}

- (void)dealloc {
    [superior_handle_ release];

    [super dealloc];
}

- (Superior *)superior { 
    @synchronized (superior_handle_) {
        return superior_handle_.superior; 
    }
}

@end

Some advantages:

1. It's thread safe. There is no way you can have the weak reference contained in Subordinate become an invalid pointer. It may become nil but that is OK.
2. Only the objects themselves need to know about the embedded weak reference. All other objects can treat Subordinate as if it has a regular reference to Superior.