Why in Objective-C does doing alloc and init in separate statements cause the object to be released according to the Xcode static analyzer?

Question

I\'m making my first steps with Objective-C and have encountered a minor, albeit confusing issue with the static analyzer (Product->Analyze) in XCode 4.1. I have created a

Answer 1

So is it possible that init can return a different pointer than the one passed to it from alloc, rather than just configuring the memory it has been passed?

Yes, that's exactly it. This is also the reason why you don't just call [super init] in your initialisers; instead you call it and assign the result to self because you aren't necessarily going to get the same instance back.

The relevant documentation is in The Objective-C Programming Language.

With this code should I pair [ a release ] with the alloc or [ f release ] with the init?

If you really want to do something like that, you'd have to check that they aren't equal first, otherwise you'd be over-releasing. But the real solution is to not split up alloc and init. That's the idiomatic way of instantiating objects in Objective C and doing it a different way that you have to add extra code for is counterproductive.

Answer 2

Trying to separate calls to +alloc and -init is explicitly recommended against by Apple, per the documentation for +[NSObject alloc]:

An init... method must be used to complete the initialization process. For example:

TheClass *newObject = [[TheClass alloc] init];

and -[NSObject init]:

In some cases, an init method might release the new object and return a substitute. Programs should therefore always use the object returned by init, and not necessarily the one returned by alloc or allocWithZone:, in subsequent code.

The static analyzer is complaining because it expects that +alloc will be paired with -init. Because -init can release the sender, the static analyzer therefore thinks you're trying to call a method on a deallocated instance.

Answer 3

So is it possible that init can return a different pointer than the one passed to it from alloc, rather than just configuring the memory it has been passed?

absolutely.

With this code should I pair [ a release ] with the alloc or [ f release ] with the init?

you would assign the value of the initialized object to f (as you have). at this point, a may be a dangling pointer (if another address is returned). thereore, f should be released.

the explanation for this order is that the object may have opted to return a specialized version/variant of itself, and this reallocation happens along the init... chain.

silly demonstration:

@interface Fraction : NSObject
{
@private
    int numerator;
    int denominator;
}

@end

static Fraction* EvilFraction = ...;

@implementation Fraction

- (id)initWithNumerator:(int)num denominator:(int)den
{
    self = [super init];
    if (nil != self) {
        if (0 == den){
            [self release];
            return [EvilFraction retain];
        }
    }
    return self;
}

@end