Objective-C/iOS : Message sent to deallocated instance

Question

This is my first post on stackoverflow.com so please be kind (rewind) ;)

I have a navigation based application whose purpose is to display blog posts (title) in a Table View (with JSON). The problem I ran into occurred when a cell got out of the screen and then back in. I was getting a EXC_BAD_ACCESS (because I sent a message to a deallocated instance), so I struggled to understand where it came from and I finally found a solution. But the fact is I don't exactly understand how the problem occurs. That's why I need someone to enlighten me, I think this is fundamental understanding !

When the connection to the JSON web service has finished loading, I parse the JSON code to obtain a list of blog posts (recentPosts), then I create a BlogArticle object for each post (blogArticle), store it in a MutableArray iVar (allEntries) and insert a row in the Table View :

- (void)connectionDidFinishLoading:(NSURLConnection *)connection
{
    [connection release];

    NSString *responseString = [[NSString alloc] initWithData:responseData encoding:NSUTF8StringEncoding];
    [responseData release];

    NSError *error;
    SBJsonParser *json = [[SBJsonParser new] autorelease];
    NSDictionary *recentPostsData = [json objectWithString:responseString error:&error];
    [responseString release];


    NSArray *recentPosts = [recentPostsData objectForKey:@"posts"];

    int i = 0;
    for (NSDictionary *post in recentPosts) {
        BlogArticle *blogArticle = [[BlogArticle alloc] initWithDictionary:post];
        [allEntries insertObject:blogArticle atIndex:i];
        [self.tableView insertRowsAtIndexPaths:[NSArray arrayWithObject:[NSIndexPath indexPathForRow:i inSection:0]] withRowAnimation:UITableViewRowAnimationRight];
        i++;
   }
}

Here's the initialisation of the BlogArticle object which turned to be the origin of the problem :

- (id)initWithDictionary:(NSDictionary *)article
{
    if (self = [super init])
    {
        // title = [[[article valueForKey:@"title"] gtm_stringByUnescapingFromHTML] copy];
        // title = [[NSString alloc] initWithString:[[article valueForKey:@"title"] gtm_stringByUnescapingFromHTML]];
        title = [[article valueForKey:@"title"] gtm_stringByUnescapingFromHTML];
    }

    return self;
}

So every Objective-C programmer who isn't as noobish as me is able to tell that title is never allocated before being assigned. If I uncomment one of the two lines above it will work. The program crashes exactly when I try to initialize a cell with that title variable, here :

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {

    static NSString *CellIdentifier = @"Cell";

    UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier];
    if (cell == nil) {
        cell = [[[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:CellIdentifier] autorelease];
    }

    NSLog(@"indexPath.row = %i", indexPath.row);

    // Configure the cell.
    BlogArticle *article = [allEntries objectAtIndex:indexPath.row];

    cell.textLabel.text = article.title;

    return cell;
}

Now, what I need to understand is why it does compile/work without allocating the iVar and where exactly it causes trouble (or where exactly the content of title is released causing the program to crash). Any good resource (noob friendly) about memory management in iOS environment would be much appreciated.

Thanks in advance :)

Unrelated to your problem, but you appear to be leaking blogArticleinstances in your for cycle. — André Morujão
As for memory management, Apple's docs should have all you need to know. Make sure you really understand the "Memory Management Rules" section. — André Morujão
What do you mean by "leaking" ? Allocating memory and never deallocating it ? — teum
Thank you, I'll check that link, even if I find Apple's documentation hard to understand as a beginner (am I the only one ?). — teum
Yes, allocating and never deallocating it. When you first create it, the object's retain count is 1. When you add it to the array, the retain count is further increased (to 2). Eventually, when you remove the object from the array, it will decrease to 1, and it will still that way and never get released because you have no more references to it. So just add a [blogArticle release] after i++ or something. — André Morujão

Nick Moore Nick Moore · Accepted Answer · 2011-03-30T13:42:02

This line

title = [[article valueForKey:@"title"] gtm_stringByUnescapingFromHTML];

is allocating an autoreleased string. Essentially, think of it that an autoreleased string will get released at the end of the method (though it can last longer, it's useful to think of it that way).

You know the string is autoreleased because the name of the method gtm_stringByUnescapingFromHTML does not start with alloc, new, copy or mutableCopy.

You can add retain to this to stop it getting autoreleased:

title = [[[article valueForKey:@"title"] gtm_stringByUnescapingFromHTML] retain];

Now you own the string, and it will not get released until you say so.

The best summary I know of is Apple's own documentation here.

Objective-C/iOS : Message sent to deallocated instance

2 Answers