Objective-c – Returning nil in the init in Objective C code

iphoneobjective c

In the case, if we return nil in the init method, what's happening with retain count and who is going to release this object?

As I undertand as soon as we called alloc (which will happen before init), the retain count will become 1. Now, init is called and let say for some reason it can't initialize the object, so it returns nil.

And it looks like now we have the object with retain count equal 1 and nobody has reference to it to call release.

Should we call [self autorelease] in init for such case or do something else?

Best Answer

See Allocating and Initializing Objects in the documentation.

Specifically, if you have an error in your initializer, then you release self and return nil:

- init
{
    self = [super init];
    if (self) {
         if (... failed to initialize something in self ...) {
             [self release];
             return nil;
         }
    }
    return self;
}

Now, consider what happens when you call [super init] and it returns nil. The above pattern has already been employed, what was self has been released, and the nil return indicates that the instance is gone. There is no leak and everyone is happy.

This is also a valid pattern (assume that self is an instance of MyClass):

- init
{
    self = [super init];
    if (self) {
        ... normal initialization here ...
    } else {
        self = [MyClass genericInstanceWhenInitializationGoesBad];
        self = [self retain];
    }
    return self;
 }

Since init is expected to return something that is retained (by implication of being chained from +alloc), then that [self retain], though it looks goofy, is actually correct. The self = [self retain] is just being extra defensive in case MyClass overrides retain to do something weird.

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