Why can I not release this NSString?

Here is the relevant开发者_运维技巧 code:

NSString *address = [NSString stringWithFormat:@"%@", [defaultServer root]];
address = [address stringByAppendingFormat:@"%@", [defaultServer login]];
address = [address stringByAppendingFormat:@"?%@", [params urlEncodedString]];

NSString *response = [[NSString alloc] init];
response = [self getResponseFromWebAddress:address];

[response release];
[address release];

This code does not error here, but later inside of some Apple Libraries it throws a "message sent to deallocated instance" error. However, if I comment out the line [address release] it works fine. Why? I don't quite understand memory management and this one confuses me.

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This line:

NSString *address = [NSString stringWithFormat:@"%@", [defaultServer root]];

And its following lines all work with autoreleased NSStrings supplied by the stringWithFormat: and stringByAppendingFormat: methods. You're not supposed to release address manually, since the autorelease pool will handle that for you.

And here:

NSString *response = [[NSString alloc] init];
response = [self getResponseFromWebAddress:address];

You are allocating a new NSString, then immediately pointing the response variable to another string which is autoreleased ([self getResponseFromWebAddress:address]). The allocated object no longer has any available pointers, so you can't release it anymore, and therefore it leaks.

To fix both issues, remove the alloc-init line and the release lines. Your address string can also be initialized with just one statement:

// Combine all three arguments into one format string
NSString *address = [NSString stringWithFormat:@"%@%@?%@", 
                        [defaultServer root], 
                        [defaultServer login], 
                        [params urlEncodedString]];

NSString *response = [self getResponseFromWebAddress:address];

// No need to release either variable

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Remember the NARC (new, alloc, retain, copy) rule. You have to release an object only if it was created by one of these messages. Otherwise the object will be autoreleased.