在这种情况下，为什么我会收到一条消息发送到已释放实例错误？

Question

I am trying to sort a NSMutableArray based on an object it contains. I am getting an error on this line in the code segment below:

InboxItem * ptrInboxItem =  [sortedInboxFaxItems objectAtIndex:[indexPath row]];

#import <UIKit/UIKit.h>
@class InboxItem;


@interface InboxTableViewController : UITableViewController<NSXMLParserDelegate> {
    NSMutableArray *inboxFaxItems;
    NSArray * sortedInboxFaxItems;
    InboxItem *_inboxItem;

    NSMutableData *xmlData;
    NSURLConnection *connectionInprogress;
    NSMutableString *inboxFaxesString;
    UIActivityIndicatorView *activityIndicator;
}

@property(nonatomic,retain) InboxItem * inboxItem;

-(void) loadInbox;

@end

- (void) connectionDidFinishLoading:(NSURLConnection *)connection{
    NSXMLParser *parser = [[NSXMLParser alloc]initWithData:xmlData];
    [parser setDelegate:self];
    [parser parse];
    [parser release];

    //lets sort by messageID
    NSSortDescriptor *sortDescriptor;
    sortDescriptor = [[[NSSortDescriptor alloc] initWithKey:@"messageID" ascending:YES] autorelease];
    NSArray *sortDescriptors = [NSArray arrayWithObject:sortDescriptor];
    sortedInboxFaxItems = [inboxFaxItems sortedArrayUsingDescriptors:sortDescriptors]; 

    [[self tableView] reloadData];
    activityIndicator.stopAnimating;

    [connectionInprogress release];
    connectionInprogress = nil;

    [xmlData release];
    xmlData = nil;
}

// Customize the appearance of table view cells.
- (UITableViewCell *)tableView:(UITableView *)tableView 
         cellForRowAtIndexPath:(NSIndexPath *)indexPath {

    static NSString *CellIdentifier = @"InboxFaxItem";

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

    //I AM GETTING ERROR HERE
    InboxItem * ptrInboxItem =  [sortedInboxFaxItems objectAtIndex:[indexPath row]];

    [[cell textLabel]setText: ptrInboxItem.datetime];
    cell.imageView.image = [UIImage imageNamed:@"document.png"];
    return cell;
}

#pragma mark -
#pragma mark Table view delegate

- (void)tableView:(UITableView *)tableView didSelectRowAtIndexPath:(NSIndexPath *)indexPath {
    // Navigation logic may go here. Create and push another view controller.

    MyManager *sharedManager = [MyManager sharedManager];
    InboxItem * ptrInboxItem =  [sortedInboxFaxItems objectAtIndex:[indexPath row]];

    sharedManager.pages = ptrInboxItem.pages;
    sharedManager.from =ptrInboxItem.from;

    FaxViewController *faxViewController = [[FaxViewController alloc] initWithNibName:@"FaxViewController" bundle:nil];
    faxViewController.messageid=ptrInboxItem.messageID;
    faxViewController.navigationItem.title=@"View Fax";
    [self.navigationController pushViewController:faxViewController animated:YES];
    [faxViewController release];
}

Answer 1

It's because of this line:

sortedInboxFaxItems = [inboxFaxItems sortedArrayUsingDescriptors:sortDescriptors];

You're assigning an object you don't own to an instance variable. Later, when you try to access it, that object has been deallocated, so your instance variable now points to garbage.

You should change that line to this:

[sortedInboxFaxItems release]; // Release any previous value
sortedInboxFaxItems = [[inboxFaxItems sortedArrayUsingDescriptors:sortDescriptors] retain];

All better.