NSString stringWithFormat and Memory Mgmt

0
votes

Could someone please advise me memory mgmt is needed or stringWithFormat is autoreleased. I have looked at NSString class reference docs on Apple Developer site, but can't don't see any clarification if a string copy is returned or is 'msg' a pointer only to autoreleased string.

NSString *msg; //pointer declared in interface

- (id) init
{
  //some initialization code..
  //is 'msg' receiving a copy or just a pointer assign
  msg = [NSString stringWithFormat: @"%@ %@", FName, LName];
} 

- (void) dealloc
{
  //release some vars, properties here..

  [msg release]; //is this correct ????     
  //Or should I be only doing: msg = nil;

  [super dealloc];
}
2
objective-cmemory-management
If you are new to Objective-C, you should be using ARC. - bbum
@bbum I disagree to some point, I think he will get a better understanding if he begins without ARC. Besides CF objects still have to be CFRetain and CFReleased, so it is better if he understands the concept from the beginning. - Oscar Gomez
Hopefully, the exposure to CF* APIs should be minimal. And when using CF, there is no autorelease poole without collusion, thus the pattern is different than NS (and a heck of a lot more straightforward). - bbum

2 Answers

1
votes

Well the first problem is that your declared ivar

NSString *msg;

Will lose its value, because [NSSTring stringWithFormat] according to the Cocoa Memory rules will return an autoreleased object. The rule states that any method name that contains alloc, new or copy will return an owned object, that is an object with a retain count of 1, which means the receiver will own that object, any other method will return an autoreleased object. In your case if you did this instead:

msg = [[NSString stringWithFormat: @"%@ %@", FName, LName] retain];

Now you own the object and you can send it a release message in dealloc

You should only send release to objects you own that is to objects you send a retain or copy message, in this case because you received an autorelease object, and you did not send a retain or copy message, therefore you must not sent it a release message. Sending a release message will cause a crash, since msg will be pointing to garbage by that point.

1
votes

If you are using ARC, you don't need to do anything at all in that case. ARC will handle it for you. If you are NOT using ARC then your code may crash as the msg variable will be released automatically at the end of the run loop.

In a non-ARC case you should use:

msg = [[NSString stringWithFormat: @"%@ %@", FName, LName] retain];

- (void) dealloc
{
   [msg release];
   msg = nil;
   ...

Or better, use a property instead.