Memory leak with NSString - objective-c

Question

I read almost every Question here on SO about memory management that involves NSStrings, but I can't really solve this problem.
#interface:
#property (nonatomic, retain) NSString *criticalTranscription;
#implementation:
viewDidLoad:
criticalTranscription = [[NSString alloc] init];
NSArray *paragraphs = [doc valueForKeyPath:#"critical.text"];
for(int i = 0; i < [paragraphs count]; i++)
{
criticalTranscription = [criticalTranscription stringByAppendingString:[[paragraphs objectAtIndex:i] valueForKey:#"p"]];
criticalTranscription = [criticalTranscription stringByAppendingString:#"\n\n"];
}
[transcription setText:criticalTranscription];
#XIB
A UISegmentedControl with an IBAction linked to:
- (IBAction) changeText:(id)sender
{
if(transcriptionSelector.selectedSegmentIndex == 1)
[transcription setText:diplomaticTranscription];
else
[transcription setText:criticalTranscription];
}
When I change the value of the UISegmentControl (first thing right after loading, nothing else runs), I run into this error (NSZombieEnabled=YES):
2011-07-07 01:10:43.639 Transcribe[404:707] *** -[CFString length]: message sent to deallocated instance 0x1189300
I can't see anything relevant in the backtrace. Without NSZombieEnabled criticalTranscription just points to random arrays or something else. There is no further usage of the variable or any releases.
I ran analyze without any suspicious leaks.
What's the problem?

Answer 1

The problem is that you are overwriting a reference to a string that you own with one that you don't own.
// you own the empty string returned here
criticalTranscription = [[NSString alloc] init];
NSArray *paragraphs = [doc valueForKeyPath:#"critical.text"];
for(int i = 0; i < [paragraphs count]; i++)
{
// immediately overwrite allocated instance (that you own)
criticalTranscription = [criticalTranscription stringByAppendingString:[[paragraphs objectAtIndex:i] valueForKey:#"p"]];
criticalTranscription = [criticalTranscription stringByAppendingString:#"\n\n"];
}
However, don't use this approach, because it pollutes the autorelease pool with unnecessary strings. Instead, use a mutable string and append strings to the single mutable string instance.
Also, in order to utilise the property's built-in memory management, you need to use self.criticalTranscription and not just criticalTranscription. Without the self., you are using the instance variable directly.

Answer 2

In your for loop
criticalTranscription = [criticalTranscription stringByAppendingString:[[paragraphs objectAtIndex:i] valueForKey:#"p"]];
criticalTranscription = [criticalTranscription stringByAppendingString:#"\n\n"];
you are setting criticalTranscription to an autoreleased string object but not retaining it, thus the flaming death.
You could retain it or use a property with #property(nonatomic, copy)NSString *criticalTranscription; and use the property rather than the ivar.

Answer 3

Two problems:
You’re leaking the first instance of the string
Every subsequent value you assign to it is getting autoreleased
To fix it, the easiest way is to change criticalTranscription to an NSMutableString. Then you can do:
criticalTranscription = [[NSMutableString alloc] init];
NSArray *paragraphs = [doc valueForKeyPath:#"critical.text"];
for(int i = 0; i < [paragraphs count]; i++)
{
[criticalTranscription appendString:[[paragraphs objectAtIndex:i] valueForKey:#"p"]];
[criticalTranscription appendString:#"\n\n"];
}
[transcription setText:criticalTranscription];
…alternatively,
[criticalTranscription appendFormat:#"%#\n\n", [[paragraphs objectAtIndex:i] valueForKey:#"p"]];
Also note that you need to call release on criticalTranscription once you’re done with it, either at the end of your -viewDidLoad or in its corresponding -viewDidUnload.