Some beginner Objective-C/iPhone questions

Question

I'm just starting out (reading up a lot for the past couple of days). Here's some questions that I have stacked up, hopefully someone can answer them.

1. the (self != nil) check in initializer code. Why do it? To prevent accidental access to some "run-only-once" code that's wrapped up in there? Where from could this accidental access come from? Doing such checks suggest that I don't have control over what's going on.

- (id)init {
    self = [super init]
    if (self != nil) {
    // Code..
    }
    return self;
}

2. How is it that you don't have to free up anything that static methods return? (or this is the idea I've got)

3. How is str = @"Hi there!" different from

str = [[NSString alloc] initWithString:@"Hi there!"];

As I understand, you have to release str in aquired with second method, but not with first? If so, when does the first one get released? Which one is preferable (not minding the typing length)?

4. What is autorelease, if iphone has no garbage collection? I've noticed something called "an autorelease pool" being created in main.m. Is [myObject autorelease]; a way of adding myObject to the nearest wrapping "autorelease pool", which will release it? Basically, some magic to avoid releasing it yourself? Why use it?

Well, thats it for now. Thanks for any answers!

Answer 1

1. In Objective-C, it's possible to return an instance other than self from -init. Classes do this, for example, to enforce a singleton instance, or in the case of class clusters. NSNumber, for example, returns a subclass depending on the type of value passed to its initializer. So when you call [[NSNumber alloc] initWithLong:long_value], NSNumber's -initWithLong: initializer is called after NSNumber's +alloc, but a subclass of NSNumber may be returned to the oringial caller. Thus the pattern

   self = [super init];

   which reassigns self to the value of [super init] so that self points to the actual instance that [super init] returned. If +alloc or the super's init method fails, the result of [super init] may be nil. To avoid, side effects in the case of a failed initialization, the pattern then becomes

   - (id) init {
     if(self = [super init]) {
       // do initialization of instance variables etc.
     }
   
     return self;
   }
   

   Note that you must return self (or nil or an other instance) from the init method. You should assign self to [super init] and you may check for nil before doing more work.

2. You may have to release the return value of a staic method. You should read the Cocoa memory management guide. The rule is generally quite simple: If the method you call has "new", "alloc", or "copy" in its signature, the result belongs to the caller and the caller must call -release on that instance or there will be a memory leak. Of course you should call -retain on anything else (i.e. not from an "alloc","new" or "copy" method) you want to keep a reference to and then call -release or -autorelease when you are done with that instance.

3. str = @"Hi there!", assuming str was declared as NSString *str; assigns the address of the string constant @"Hi there!" to the value of thestrvariable. You do not need to retain or release string constants.str = [[NSString alloc] initWithString:@"Hi there!"];allocates a new string instance. The value ofstrwill be the address of this instance. Each call ofstr = [[NSString alloc] initWithString:@"Hi there!"];again will allocate a new instance. So afterstr2 = [[NSString alloc] initWithString:@"Hi there!"];,str != str2, while afterstr2 = @"Hi There!", str==str2. See this answer as well.

4. -autorelease adds the receiver to the current NSAutoreleasPool. When the pool is drained (usually at the end of the current run loop iteration, or when the pool is manually drained), the pool calls -release on all instances in the pool. If this -release drops the retain count to 0, the object is deallocated (and -dealloc called) just as with any other -release. Using an autorelease pool is generally frowned upon on the iPhone because it may cause you to accumulate many unused instances in the pool before it is drained at the end of the run loop iteration. If you can use -release instead of -autorelease, you generally should. Again, see the Cocoa memory management guide for more info.