iOS: understanding Problem with release an NSString - iphone

Question

I have following code:
+ (NSDictionary*) JSONRequest: (NSString*)query andWithCredentials:(BOOL)withCredentials
{
if (withCredentials)
{
NSString *username = [LoginHandler GetUsernameFromNSDefaults];
NSString *password = [LoginHandler GetPasswordFromNSDefaults];
NSString *additionalQuery = [NSString stringWithFormat:#"login_username=%#&login_password=%#", username, password];
query = [NSString stringWithFormat:#"%#&%#", query, additionalQuery];
[username release];
[password release];
[additionalQuery release];
}
NSURLRequest *request = [NSURLRequest requestWithURL:
[NSURL URLWithString:query]];
NSData *response = [NSURLConnection sendSynchronousRequest:request returningResponse:nil error:nil];
NSString *jsonString = [[NSString alloc] initWithData:response encoding:NSUTF8StringEncoding];
NSDictionary *results = [jsonString JSONValue];
return results;
[request release];
[response release];
[jsonString release];
[results release];
}
The problem is the Release of the additionalQuery-NSString.
When I run this code, it ends with an BAD-ACCES-EXCEPTION for the
[additionalQuery release];
As soon as I comment this out, the code works fine.
Now, as simple as it is, run my app without this line of code could be fine, but my question is: What do Im wrong?
I generate an NSString in an IF-Clause, then I CAN only release it in the IF-Clause. But why I got a Error there?

Answer 1

Look at your creation of additionalQuery
NSString *additionalQuery = [NSString stringWithFormat:#"login_username=%#&login_password=%#", username, password];
With stringWithFormat you create an autoreleased NSString object. You MUST NOT release it manually according to the Memory Management rules since you don't own it.
You own only things you created with alloc] init..] or something with new.. or create.. in the name and of course if you do a copy such as mutableCopy.
So [additionalQuery release] causes over-releasing an object and thus it is a BAD ACCESS

Answer 2

The problem is that the string instance is created using a class method that starts with the class name (stringWithFormat). By convention, these types of class methods return an autoreleased object, freeing you from worrying about releasing them unless you specifically call retain on the returned object.
If you do want to perform your own memory management on the object, you could change your line:
NSString *additionalQuery = [NSString stringWithFormat:
#"login_username=%#&login_password=%#", username, password];
to either of the following:
NSString *additionalQuery = [[NSString alloc] initWithFormat:
#"login_username=%#&login_password=%#", username, password];
or:
NSString *additionalQuery = [[NSString stringWithFormat:
#"login_username=%#&login_password=%#", username, password] retain];
As an aside, you also have several other issues with this code.
The username variable should not be released because again, by convention, the method you get it from GetUsernameFromNSDefaults should return an autoreleased object. As a general rule of thumb, any method other than an init method should return an autoreleased object. It would become very difficult for a programmer not knowledgable with the codebase to pick it up and modify without following these conventions.
The request variable does not need to be released because it is created with a class method that returns an autoreleased object (requestWithURL). If you wanted it to be retained by your code, either call retain on it, or use the method initWithURL:.
Additionally, the results variable is not retained by you, so there is no need to release it.

Answer 3

You don't have to release it manually, it will get autoreleased. ([NSString stringWithFormat:] vs. [NSString initWithString:])

Answer 4

additionalQuery was never retained, that I can see. (stringWithFormat does an autorelease, so it doesn't count.)

Answer 5

When you create an instance of an object using a convenience method (one that does not begin with new, alloc or copy) the returned object is autoreleased. In other words you do not need to explicitly release it.
When you invoke the stringWithFormat method it returns an autoreleased NSString. You subsequently go on to release this NSString instance...
[additionalQuery release];
This sends the release message to the additionalQuery instance. As it's an autoreleased object it is added to an autorelease pool which lives (usually) on the main event thread. This pool is drained frequently and subsequently sends a release message to each of the objects it contains. Hence when an object is autoreleased the pool will look after sending the release message for you.
Your EXC_BAD_ACCESS here is a result of you releasing the NSString - dropping its retain count to 0 prior to the pool draining. The pool is then drained and attempts to send a message to a deallocated object.

Answer 6

you have specified here [NSString stringWithFormat:#"login_username=%#&login_password=%#", username, password];
means this method will handle allocation and release for your string so "you do not need to release it" hence remove the line [additionalQuery release];
also, u are not allocating string for username and password hence no need to release it .
if you write Nsstring *username = [[NSString alloc]init]; then you need to release it..
for more information regarding Memory Management refer
http://marcelsite.heroku.com/posts/5-iPhone-s-alloc-init-new-retain-release-autorelease-copy-
this will really help you...