Crash when invoking stringWithString on NSMutableString object - iphone

Question

I have the following code running when the user clicks the "Save" button:
- (IBAction) onSaveChangesClick:(id)sender {
NSMutableString *newGroups = [[NSMutableString alloc] init];
for (int i = 0; i < [self.isInGroupArr count]; i++) {
if ([[self.isInGroupArr objectAtIndex:i] boolValue] == YES) {
[newGroups appendString:[[AppDelegate arrayGroups] objectAtIndex:i]];
[newGroups appendString:#","];
}
}
// remove last : ","
if ([newGroups length] > 0)
newGroups = [NSMutableString stringWithString:[newGroups substringToIndex:[newGroups length] - 1]];
self.contact.groups = newGroups;
[newGroups release];
//[[self navigationController] popViewControllerAnimated:YES];
}
self.IsInGroups is BOOL array and arrayGroups is (NSString *) array that holds groups names.
I would like to add the newGroups string to the arrayGroups[i] only if (IsInGroups[i] == YES).
This piece of code generates EXC_BAD_ACCESS. WHY?
Thanks.

Answer 1

newGroups = [NSMutableString stringWithString:[newGroups substringToIndex:[newGroups length] - 1]];
This line generates a leak, then is the cause of the crash.
After this executes, you no longer have a reference on your alloc/inited mutable string, and you have an autoreleased string. So calling release on that string causes a double release somewhere.
EDIT: with solution
Simplest solution: do not add the last ','.
for (int i = 0; i < [self.isInGroupArr count]; i++) {
if ([[self.isInGroupArr objectAtIndex:i] boolValue] == YES) {
[newGroups appendString:[[AppDelegate arrayGroups] objectAtIndex:i]];
if (i != ([self.isInGroupArr count] - 1))
[newGroups appendString:#","];
}
}
Not very elegant, but quite efficient (could avoid taking the count each time though).

Answer 2

Here you create autoreleased instance of NSMutable string.
newGroups = [NSMutableString stringWithString:[newGroups substringToIndex:[newGroups length] - 1]];
so you shouldn't release it, and all will be fine.
Here is improved code:
- (IBAction) onSaveChangesClick:(id)sender {
NSMutableString *newGroups = [[[NSMutableString alloc] init] autorelease];
for (int i = 0; i < [self.isInGroupArr count]; i++) {
if ([[self.isInGroupArr objectAtIndex:i] boolValue] == YES) {
[newGroups appendString:[[AppDelegate arrayGroups] objectAtIndex:i]];
[newGroups appendString:#","];
}
}
// remove last : ","
if ([newGroups length] > 0)
newGroups = [NSMutableString stringWithString:[newGroups substringToIndex:[newGroups length] - 1]];
self.contact.groups = newGroups;
//[[self navigationController] popViewControllerAnimated:YES];
}
Explanation:
Here you allocate memory and retain it.
[[NSMutableString alloc] init]
[NSMutableString stringWithString: returns autoreleased instance of NSMutable string, that we shouldn't release(it does the same as [[[NSMutableString alloc] init] autorelease] + smth more). and you assign it to variable newGroups(so your old value that was stored in this variable lost)
if ([newGroups length] > 0)
newGroups = [NSMutableString stringWithString:[newGroups substringToIndex:[newGroups length] - 1]];
newGroups here is autoreleased, you release it, and it destroys. But as it was autoreleased, autorelease pool tries to release it again and gets exception(because memory is allready free)
[newGroups release];

Answer 3

You have allocated a string(NSMutableString *newGroups = [[NSMutableString alloc] init];
)
and then assigning it with an autorelease string(newGroups = [NSMutableString stringWithString:[newGroups substringToIndex:[newGroups length] - 1]];
).
Which you should never do. See this blog - http://andycodes.tumblr.com/post/947927280/difficult-bug-finally-solved
Comment out [newGroups release]; and code should work fine.
Also always set NSZombieEnabled environment variable and run the code again, have a look at the crash log, you will get exactly which object is causing the crash.