Message sent to deallocated instance with ARC using custom getter and setter

Question

I'm trying to implement a custom getter and setter for my custom object HFObject and my app crashed with a Message sent to deallocated instance error despite using ARC.

I've read every single related post, the ones that were written pre-ARC don't apply, and everything else didn't help. I have the zombie object debugger option turned on.

---

Setting up the custom HObject

Within HObject.h I have declared these four properties:

@property (retain) NSString *email;        //Will use custom getter/setter
@property (retain) NSString *firstName;    //Will use custom getter/setter
@property (retain) NSDate *date;           //Will use custom getter/setter
@property (nonatomic, retain) NSMutableDictionary *values;

In the implementation of HObject, I have removed the automatic getting and setting of email. firstName, and date by utilizing @dynamic like so

@dynamic email;
@dynamic firstName;
@dynamic date;

I also allocate the values dictionary in my HObject init

- (id)init {
    self = [super init];

    if (self) {
        self.values = [NSMutableDictionary dictionary];
    }

    return self;
}

---

Implementing Custom Getter & Sender

For my custom getter/setter. I have overridden

- (NSMethodSignature *)methodSignatureForSelector:(SEL)selector

and

- (void)forwardInvocation:(NSInvocation *)invocation

As shown below:

- (NSMethodSignature *)methodSignatureForSelector:(SEL)selector
{
    NSString *sel = NSStringFromSelector(selector);
    if ([sel rangeOfString:@"set"].location == 0) {
        return [NSMethodSignature signatureWithObjCTypes:"v@:@"];
    } else {
        return [NSMethodSignature signatureWithObjCTypes:"@@:"];
    }
}

- (void)forwardInvocation:(NSInvocation *)invocation
{
    NSString *key = NSStringFromSelector([invocation selector]);
    if ([key rangeOfString:@"set"].location == 0) { 
        key = [key substringWithRange:NSMakeRange(3, [key length]-4)]; 

        id obj;   
        [invocation getArgument:&obj atIndex:2];   

        [self.values setObject:obj forKey:key];
    } else {
        id obj = [self.values objectForKey:key];

        [invocation setReturnValue:&obj];
    }
}

What I'm trying to do here is store all of the values of the property into my values dictionary and retrieve them from there as well.

---

App Crashing

In the implementation of my view controller, I try to create a HObject and set values for my properties, and then I log the values dictionary to see all of my values.

- (void)buttonPressed:(id)sender {
    HObject *obj = [[HObject alloc] init];

    NSString *name = @"this is a string object";
    obj.date = [NSDate date];
    obj.email = @"email@website.com";
    obj.firstName = [NSString stringWithFormat:@"%@", name];


    NSLog(@"Values %@", [obj values]);
}

At that point the app crashes and this is my console log

2014-07-27 04:12:37.899 App[61501:60b] Values {
    Date = "2014-07-27 08:12:37 +0000";
    Email = "email@website.com";
    FirstName = "this is a string object";
}
2014-07-27 04:12:37.901 HeadsUp[61501:60b] *** -[CFString release]: message sent to deallocated instance 0x109473fe0

If you can help me from here, I would greatly appreciate it. I am also including my debugging process in case that will help you

---

My debugging Process (Long, skip if you can already help me)

I originally created many of these objects and stored them in an array, and when I do that, as opposed to creating a single object. my app crashed a bit different.

My array:

@property (nonatomic, strong) NSArray *array;

Methods:

- (void)createArray
{
    int i = 1; //number of testobjs

    NSMutableArray *objects = [NSMutableArray arrayWithCapacity:i];

    for (int j = 0; j<i; j++) {
        HFObject *obj = [[User alloc] init];

        NSString *name = @"this is a string object";
        [obj setObject:[NSDate date] forKey:@"Date"];
        obj.email = @"email@website.com";
        obj.firstName = [NSString stringWithFormat:@"%@", name];

        [objects addObject:obj];
    }


    self.array = [NSArray arrayWithArray:objects];

}

- (void)buttonPressed:(id)sender {
    HObject *object = [self.array objectAtIndex:0];

    NSLog(@"Values %@", [object values]);
}

Crash log:

2014-07-27 04:34:02.893 App[61623:60b] *** -[CFString isNSString__]: message sent to deallocated instance 0x1094988f0
(lldb) 

Now this crash log is almost the same as the one before, except this didn't log the values inside of [object values]

Investigating the issue a bit, I looked at the left window (not sure what it is actually called) of the debugger and I saw this:

(Treat HFObject as HObject and dirtyValues as values; I renamed them for presentational purposes)

You can see that under the key @"FirstName" there is no value.

I did several similar tests where I changed the values of the properties I was setting and changed the data types. More often than not, not only did FirstName not have a value, neither did Date. However, the value of email was always present.

After researching about dataTypes, I realized it was because email was a string literal which can't deallocated. On the other hand firstName and date were objects, which can be deallocated.

The crash log refers to a CFString property, which I learned doesn't use ARC. I never created a Core Foundation object, so I set out to found out it was being created in setter by logging the [obj class]:

- (void)forwardInvocation:(NSInvocation *)invocation
{
    NSString *key = NSStringFromSelector([invocation selector]);
    if ([key rangeOfString:@"set"].location == 0) { 
        key = [key substringWithRange:NSMakeRange(3, [key length]-4)]; 

        id obj;   
        [invocation getArgument:&obj atIndex:2];   

        NSLog(@"%@", [obj class]);  //I ADDED THIS LOG

        [self.values setObject:obj forKey:key];
    } else {
        id obj = [self.values objectForKey:key];

        [invocation setReturnValue:&obj];
    }
}

After crashing one more time, I got the obj classes

2014-07-27 04:58:03.893 HeadsUp[61765:60b] __NSDate
2014-07-27 04:58:03.894 HeadsUp[61765:60b] __NSCFConstantString
2014-07-27 04:58:03.894 HeadsUp[61765:60b] __NSCFString
2014-07-27 04:58:03.904 HeadsUp[61765:60b] *** -[__NSDate release]: message sent to deallocated instance 0x109554370
(lldb) 

Here you can see Date is being deallocated for some reason, and my string are now __NSCF strings.

I tried resetting the strings to NSStrings using (__brigde NSString *)obj and every other possible way you can bridge a CF object to ARC, however that didn't work either.

Here is everything I've done. I appreciate any and all help.

Answer 1

The problem is here:

id obj;   
[invocation getArgument:&obj atIndex:2];

getArgument simply copies the object pointer into obj without retaining it. However, since obj is (by default) a __strong variable, it will be released at the end of the current method. To solve the problem, use

__unsafe_unretained id obj;   
[invocation getArgument:&obj atIndex:2];   

Note also that your getter implementation does not work. For example, setFirstName: stores the key in the dictionary using the key "FirstName", but the getter firstName tries to read the value for the key "firstName".

(As already mentioned in a comment, it would probably easier and less error-prone to just override the accessor methods for the three properties separately, instead of dynamic forwarding.)