When release methods local variables in Obj-C - iphone

Question

I'm developing an iPhone application and I've just created this method (it's in a singleton class):
- (NSDictionary *)getLastPosts
{
SBJsonParser *parser = [[SBJsonParser alloc] init];
NSURLRequest *request = [NSURLRequest requestWithURL:
[NSURL URLWithString:http://example.org/last/]];
NSData *response = [NSURLConnection sendSynchronousRequest:request returningResponse:nil error:nil];
NSString *json_string = [[NSString alloc] initWithData:response encoding:NSUTF8StringEncoding];
NSDictionary *data_dict = [parser objectWithString:json_string error:nil];
// release stuff
[parser release];
[request release];
[response release];
[json_string release];
return data_dict;
}
I'm a newbie obj-c developer so I'm not sure of this two things:
Is it correct the four vars release in the method's end?
When should I release the NSDictionary data_dict?
UPDATE 1
If data_dict was NSDictionary *data_dict = [[NSDictionary alloc] init] when I'll should release it?
UPDATE 2
In the caller I have this:
- (void)callerMethod
{
NSDictionary *tmpDict = [mySingleton getLastPosts];
NSLog(#"retain count: %d", [tmpDict retainCount]);
}
and the debug console prints:
retain count: 2
Why "Xcode Analyze" says me these lines?
And why the retain count it's 2?

Answer 1

In general, it is good to release objects you do not need any more.
But remember
- Only things that have alloc, new or copy in their initialization need to be released. Otherwise they are already autoreleased.
So, it is ok to release the parser, not ok to release the request, not ok to release the response, ok to release the json_string.

Answer 2

SBJsonParser *parser = [[SBJsonParser alloc] init];
You called init, then you own the instance and you need to release it.
NSURLRequest *request = [NSURLRequest requestWithURL:
[NSURL URLWithString:http://example.org/last/]];
You called a class method that returns an autoreleased instance which will be added to the autorelease poll.
NSData *response = [NSURLConnection sendSynchronousRequest:request returningResponse:nil error:nil];
Autoreleased.
NSString *json_string = [[NSString alloc] initWithData:response encoding:NSUTF8StringEncoding];
You called init, you will need to release it.
NSDictionary *data_dict = [parser objectWithString:json_string error:nil];
Returned instance, autoreleased.
Thus you just need to release two of them:
[parser release];
[json_string release];
if NSDictionary *data_dict = [[NSDictionary alloc] init] then you would need to autorelease it yourself: the convention is that any instance returned by a method is autoreleased.
By the way by autoreleasing it you make sure that it will be available until the autorelease pool is emptied (unless you call release on it).
To autorelease it:
return [data_dict autorelease];

Answer 3

It is correct to release parser and json_string because these are created with methods containing "alloc". It is incorrect to release the others because they are autoreleased.
You never have to release data_dict in this method, since it is autoreleased.
Please read the Objective-C memory management rules.