Does one have to release objects allocated by class functions in NSString

No. In general, unless the class method is name is something like 'alloc' then you aren't responsible for releasing the object. I can't remember the exact rules off hand, and memory management is still something I have to fully get up to speed on.

I suggest you read the memory management document in the 'performance' section of the developer documents though - it explains things pretty well.

As noted, the class methods are autoreleased.

It is the init * methods that you are responsible for.

I believe that Foundation creates and destroys an autorelease pool on each pass through the runloop. Because of the runloop design, your code runs in response to an event (timer, input, etc). Once you return from your event handler, the stack eventually unwinds back up to the runloop, at which point the autorelease pool is drained.

In practice, just be conservative and assume the object will stay alive until the end of your current function/method (unless you create additional autorelease pools yourself).

You only release objects that you own.

Objects that you own are defined as those that you created with

alloc
copy
retain

Objects that are created with the "convenience" methods of a class, namely methods that have terms like "initWith...." or "stringWith.....", etc. are owned by the class object. They are autoreleased, which means they are released after the current run through the run loop.

So if you want to keep a reference to them, either retain them or copy them.

NSString *myString = [[NSString alloc] init]; // Belongs to you
NSString *myString = [NSString stringWithFormat:@"%@", anObject]; //Belongs to NSString