Question

Another newbie question, just when I thought I was beginning to get a very
small handle on ios programming. Ugh! I’m following a tutoria from the
appcodeblog.com where I’m building a simple tab bar application utilizing
core data to enter, display, and search vacation destinations. I’ve worked
through the tutorial and have a working app, but I notice when I select the
“Show Destinations” tab I get the following error. The app seems to continue
working, but the error is logged to the console. I’m trying to debug the
issue and understand exactly what is happening, but I just don’t quite
understand what is wrong. I “think” I have an issue with my
ShowDestinations.xib file where I’ve incorrectly hooked up my objects within
the xib. Any help is MUCH appreciated. Thanks in advance for your help and
time.

Here’s the error, “CoreDataTabBarTutorial[1262:207] Failed to call designated
initializer on NSManagedObject class ‘Destination’.

I’m not sure what code to provide so I’ve started out by showing my header
and implementation files ShowDistinationsViewController.h and
ShowDestinationsViewController.m

ShowDistinationsViewController.h

#import <UIKit/UIKit.h>


@interface SearchDestinationsViewController : UIViewController {

    UISearchBar *destinationSearchBar;
    UITableView *searchTableView;

    NSFetchedResultsController *fetchedResultsController;
    NSManagedObjectContext *managedObjectContext;

    NSArray *fetchedObjects;

 }

@property (nonatomic, retain) IBOutlet UISearchBar *destinationSearchBar;
@property (nonatomic, retain) IBOutlet UITableView *searchTableView;

@property (nonatomic, retain) IBOutlet NSFetchedResultsController *fetchedResultsController;
@property (nonatomic, retain) IBOutlet NSManagedObjectContext *managedObjectContext;

@end

ShowDestinationsViewController.m

#import "ShowDestinationsViewController.h"
#import "Destination.h"

@implementation ShowDestinationsViewController

@synthesize destinationsTableView;
@synthesize destinationsArray;
@synthesize fetchedResultsController;
@synthesize managedObjectContext;

// Not sure where the following code came from so I commented it out!!! It didn't seem to break anything when I commented it out
//- (id)initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)nibBundleOrNil
//{
//    self = [super initWithNibName:nibNameOrNil bundle:nibBundleOrNil];
//    if (self) {
//        // Custom initialization
//    }
//    return self;
//}

- (void)dealloc
{
    [destinationsArray release];
    [destinationsTableView release];
    [super dealloc];
}

- (void)didReceiveMemoryWarning
{
    // Releases the view if it doesn't have a superview.
    [super didReceiveMemoryWarning];

    // Release any cached data, images, etc that aren't in use.
}

#pragma mark - View lifecycle

/*
// Implement loadView to create a view hierarchy programmatically, without using a nib.
- (void)loadView
{
}
*/

/*
// Implement viewDidLoad to do additional setup after loading the view, typically from a      nib.
- (void)viewDidLoad
{
    [super viewDidLoad];
}
*/

- (void)viewDidUnload
{
    [super viewDidUnload];
    // Release any retained subviews of the main view.
    // e.g. self.myOutlet = nil;
}

- (BOOL)shouldAutorotateToInterfaceOrientation:(UIInterfaceOrientation)interfaceOrientation
{
    // Return YES for supported orientations
    return (interfaceOrientation == UIInterfaceOrientationPortrait);
}


#pragma mark -
#pragma Data Fetch from Core Data

- (void) viewWillAppear:(BOOL)animated
{

    NSFetchRequest *request = [[NSFetchRequest alloc] init];
    NSEntityDescription *entity = [NSEntityDescription entityForName:@"Destination" inManagedObjectContext:managedObjectContext];
    [request setEntity:entity];
    NSError *error = nil;
    NSMutableArray *mutableFetchResults = [[managedObjectContext executeFetchRequest:request error:&error] mutableCopy];
    if (mutableFetchResults == nil)
    {
        // Handle the error.
        NSLog(@"mutableFetchResults == nil");
    }
    [self setDestinationsArray:mutableFetchResults];
    [request release];
    [destinationsTableView reloadData];
} 


#pragma mark -
#pragma mark Table view data source

- (NSInteger)numberOfSectionsInTableView:(UITableView *)tableView
{
    // Return the number of sections.
    return 1;
}

- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section
{
    // Return the number of rows in the section.
    return [destinationsArray count];
}

// Customize the appearance of table view cells.
- (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];
    }

    // Configure the cell...
    Destination *destination = [[Destination alloc] init];
    destination = (Destination *)[destinationsArray objectAtIndex:indexPath.row];
    cell.textLabel.text = destination.name;
    [destination release];

    return cell;
}

#pragma mark -
#pragma mark Table view delegate

- (void)tableView:(UITableView *)tableView didSelectRowAtIndexPath:(NSIndexPath *)indexPath
{

}

@end

Answer 1

The problem seems to lie in

Destination *destination = [[Destination alloc] init];
destination = (Destination *)[destinationsArray objectAtIndex:indexPath.row];
[destination release];

The first line is unnecessary: In Objective-C, Destination* is a pointer to the object, not the real object. The Destination object you want is presumably already in the array. So you don’t have to create an object to point to, in the line [[Destination alloc] init], which is gone immediately at the next line. What’s going on was

1. [[Destination alloc] init] creates an object a, destination points to a. a is retained by you.
2. (Destination *)[destinationsArray objectAtIndex:indexPath.row] gives you another object b, which is not retained by you. destination now points to b. No one holds a any longer.
3. release is sent to the object pointed to by destination, i.e., to b. This is against the retain-release rule; you should release a, not b!

So, instead, just do

Destination *destination = (Destination *)[destinationsArray objectAtIndex:indexPath.row];

without the release part.

As an advice: always run Analyze (which is available below the Build menu) when you build your project. The analyzer is designed to catch common types of errors, including yours. Correct your code so that all the analyzer warnings go away; you should always regard the analyzer warning as an error on your part.