On Jul 22, 2008, at 9:54 PM, Steve Cronin wrote:
I find if I have a method which returns an NSString for example, that often, in the interior of that method I will use an NSMutableString to construct the string I intend to return. My question concerns the actual final return statement.Is there ANY reason to choose A over B? A) return [NSString stringWithString:myWorkingMutableString]; B) return myWorkingMutableString;It just kinda sticks in my craw me that I 'promised' an NSString and using (B) I don't return one. Yeah I do understand inheritance and that a mutable entity isKindOfClass of the base class I looking for reasons like edge cases, compiler optimization, error trapping, portability, etc.. Is there ANY benefit to the costs associated with the explicit declaration of (A)?
Casting up the class hierarchy -- more specific to less specific -- upon return is an oft used implementation pattern. In particular, returning the immutable superclass of the mutable classes from Foundation (as in this case).
There is, however, a very real risk herein.Specifically, consider if myWorkingMutableString was an instance variable of your class. By returning myWorkingMutableString directly, you have created the potential for encapsulation violations in two ways:
(1) Caller grabs a reference to myWorkingMutableString, retains it, and expects the value to remain constant over time. Your class changes the contents and the aforementioned caller explodes.
(2) You return a reference to the instance variable myWorkingMutableString and the caller does something silly and mutates the string. Your class's instance explodes.
If you feel the need to defend against this kind of stuff, you can do something like:
- (NSString *) myWorkingMutableString
{
return [[myWorkingMutableString copy] autorelease];
}
If using GC you can omit the -autorelease bit.
b.bum
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