autoreleasing NSString in class method causing app crash in iOS - objective-c

Question

The error I receive is as follows:
int main(int argc, char *argv[])
{
NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
int retVal = UIApplicationMain(argc, argv, nil, nil); //breakpoint that says Thread 1: Program Received Signal: "EXC_BAD_ACCESS".
[pool release];
return retVal;
}
My two questions can be found at the bottom of this post :)
I am currently working on an assignment for an iOS programming class and have hit a road bump.
I have found a fix, shown below, but it doesn't make sense to me. Check it out:
#implementation MyClass
// This class method takes an (NSMutableArray *) and returns an NSString with its contents printed out.
+ (NSString *)myString:(NSMutableArray)anArray
{
// NSString *myString = [[NSString alloc] init]; OLD CODE THAT CAUSES MEMORY LEAK
NSString *myString = [[[NSString alloc] init] autorelease]; //NEW CODE THAT RELEASES FIRST ALLOCATION OF myString WHEN THE FIRST stringByAppendingFormat: IS CALLED
NSString *vp = VARIABLE_PREFIX; //#defined above to be #"%
for (id object in anArray) {
if ([object isKindOfClass:[NSString class]]) {
if ([object hasPrefix:vp]) {
myString = [myString stringByAppendingFormat:#"%#",[object substringFromIndex:1]];
}else{
myString = [myString stringByAppendingFormat:#"%#",object];
}
}else if ([object isKindOfClass:[NSNumber class]]) {
myString = [myString stringByAppendingFormat:#"%#",object];
}
}
return myString; //shouldn't I autorelease myString right before this line? NO NOT ANY MORE. THIS myString IS NOT THE ORIGINAL THAT I alloc-init, BUT AN AUTORELEASED OBJECT RETURNED BY THE stringByAppendingFormat: message.
}
When I try to send the message [myString autorelease];, the program crashes with the above error. It is working fine now as shown above, but I do not understand why.
Every time I send a message containing the "magic words" alloc, init, copy I have to call release, it don't I? Or are the rules different in a Class method (can the Class itself own a file?). I do not call retain in the object that is calling this file.
Here are my two questions:
Why does this crash when I try to release theDescription using autorelease?
Does my code create a memory leak?
This is my very first question on stack overflow! Thank you for your help!

Answer 1

Why does this crash when I try to release theDescription using autorelease?
Assuming you mean myString, it crashes because myString is already autoreleased. You got it by calling -stringByAppendingFormat:, which returns an autoreleased string. Now, you're probably thinking: "But I created it by calling +alloc, so I should release it." That's true, but NSStrings are immutable, and when you call -stringByAppendingFormat: you get a different string back, and that string is autoreleased. Autoreleasing it a second time is an error.
Does my code create a memory leak?
Yes, but not really. The "leaked" object is the empty string that you allocate in the beginning. You never release that string, so you've got a leak. However, NSString is apparently optimized so that [[NSString alloc] init] returns a singleton, so in this particular case it doesn't make any difference that the empty string isn't released. The other strings that are assigned to myString are all autoreleased, so none of those objects are leaked.

Answer 2

Why does this crash when I try to release theDescription using
With the updated code, the problem is that ypu are reassigning the pointer to myString using the methid which already returns an autoreleases object: stringbyappending, therefore if you call autorelease on this object which is already going to get autoreleased i will crash.
Aditionaly the first assugnment in the alloc init gives a memory leak when ypu reassign with stringbyappendingstring, since you lose the reference to the previously created string with alloc init and therefore you will never be able to release it.
Sorry for the formatting on my iPhone atm =)