IOS5 __block variable throws EXC_BAD_ACCESS outside scope

Question

I'm baffled as to why I cannot access my global variable again after it has been through a block. Here is my code:

__block NSString *latitude;
__block NSString *longitude;

CLGeocoder *geoCoder = [[CLGeocoder alloc] init];
[geoCoder geocodeAddressString:location completionHandler:^(NSArray* placemarks, NSError* error) 
{
    for (CLPlacemark* aPlacemark in placemarks)
    {
        CLLocation *latLong = aPlacemark.location;
        latitude = [NSString stringWithFormat:@"%f", latLong.coordinate.latitude];
        longitude = [NSString stringWithFormat:@"%f", latLong.coordinate.longitude];
        //works fine
        NSLog(@"CLLOCATION SSSSSSSSSSSSSSSSSSSSSS LAT: %@, LONG: %@", latitude, longitude);           
    }
}];

//no bueno
NSLog(@"CLLOCATION SSSSSSSSSSSSSSSSSSSSSS LAT: %@, LONG: %@", latitude, longitude);

Now I've tried initializing my NSStrings in different ways:

__block NSString *latitude = @"";
__block NSString *longitude = @"";

and:

__block NSMutableString *latitude = [NSMutableString string];
__block NSMutableString *longitude = [NSMutableString string];

But I just wind up getting empty strings when I am accessing the variables outside of the block.

This is particularly baffling because in Apple's documentation http://developer.apple.com/library/ios/#documentation/cocoa/Conceptual/Blocks/Articles/bxGettingStarted.html#//apple_ref/doc/uid/TP40007502-CH7-SW1

, they're able to set variables outside of blocks, use them, and retrieve them just fine.

Try not to use the <code> tags; highlight the code and select the button that looks like {} on top of the text field. — CodaFi
Just tried. Made it look even worse. Sorry. This is one of the most confusing sites I've ever been on. — John Bowlinger
Stack Overflow uses markdown when writing questions, answers and comments. You can read about how to use it in the "Markdown editing help"-page. If you don't wan't to use markdown there are buttons you can use to style your text as code, bold, italic etc. in the UI where you asked your question (they are right above the text field) — David Rönnqvist

WDUK WDUK · Accepted Answer · 2012-06-07T19:06:50

Okie doke, sounds like a couple of issues (one, none or all may apply, it's hard to tell within the context you've provided):

1 - "But I just wind up getting empty strings when I am accessing the variables outside of the block."

Depends when you're accessing them outside the block. This is because there is no guarantee that latitude and longitude will have been populated at the time of querying them. The block provided is a completion handler for CLGeocoder; it will be called when the geocoder has found locations of interest. Being able to retrieve location data, and to do a search based on that location takes time, and the NSLog statement straight after has a high probability of being called before any 'placemarks' have been found.

2 - *"IOS5 __block variable throws EXC_BAD_ACCESS outside scope"*

You are assigning the following in the block:

latitude = [NSString stringWithFormat:@"%f", latLong.coordinate.latitude];
longitude = [NSString stringWithFormat:@"%f", latLong.coordinate.longitude];

stringWithFormat is a method that returns an autoreleased value, but you're not retaining them anywhere. If you're not using ARC (see https://developer.apple.com/library/ios/#documentation/Cocoa/Conceptual/MemoryMgmt/Articles/MemoryMgmt.html for ARC in memory management terms, and http://developer.apple.com/library/mac/#releasenotes/ObjectiveC/RN-TransitioningToARC/Introduction/Introduction.html generally), then this will cause issues when accessing the values elsewhere, as you're accessing an object that no longer exists. If you are using ARC, then this shouldn't be a problem, as the variables will be strong by default, and retain the value for you.

IOS5 __block variable throws EXC_BAD_ACCESS outside scope

1 Answers