Question

I’m writing a tool which would benefit from knowing which of a class’ instance variables are declared __weak.

This information must exist somewhere at runtime, but is there any way of accessing it, documented or otherwise? (It’s for a tool, so I don’t care so much about it breaking with updates)

Answer 1

Alright, here is a sample implementation, using a custom object implementation, that does a rudimentary check to see if an iVar is weak or not:

BOOL iVarIsWeak(Class cls, Ivar ivar)
{
    id classInstance = [cls new];

    // our custom base class properly tracks reference counting, no weird voodoo
    id refCounter = [CustomBaseClass new];

    object_setIvar(classInstance, ivar, refCounter);

    if ([refCounter refCount] == 2)
    {
        return NO;
    }

    return YES;
}

The above code is meant to be used with ARC enabled, while the following custom object code is not:

@interface CustomBaseClass : NSObject

+(id) new;
+(id) alloc;
-(id) init;

-(id) retain;
-(void) release;
-(id) autorelease;
-(void) dealloc;

-(id) description;

-(unsigned) refCount;

@end


// easy way to get sizeof
struct CustomBaseClassAsStruct {
    voidPtr isa;
    unsigned volatile refcount;
};

@implementation CustomBaseClass
{
    unsigned volatile  refcount;
}

+(id) new
{
    return [[self alloc] init];
}

+(id) alloc
{
    struct CustomBaseClassAsStruct *results =  malloc(sizeof(struct CustomBaseClassAsStruct));
    results->isa = self;
    results->refcount = 0;
    return (id) results;
}

-(id) init
{
    [self retain];

    return self;
}

-(id) retain
{
    ++refcount;

    return self;
}

-(void) release
{
    if (--refcount == 0)
        [self dealloc];
}

-(id) autorelease
{
    // sample implementation of autorelease
    dispatch_async(dispatch_get_current_queue(), ^{
        [self release];
    });

    return self;
}

-(unsigned) refCount
{
    return refcount;
}

-(void) dealloc
{
    free(self);

    // no call to [super dealloc], we are using custom memory-managment
}

@end

This ONLY works for weak iVars. With a unsafe_unretained variable, it will give a false positive, my best guess for this is because __weak information is saved at runtime whilst unsafe_unretained information is not.

I hope this helps!