iphone memory management

Question

I have a method that returns a NSMutableArray:

//implementation of class Students

-(NSMutableArray *)listOfStudents {
  NSMutableArray *students = [[NSMutableArray alloc] init];
  //add objects to students and other operations here
  return students;
}

The problem here is, when and where do I release the object students? If it was an instance variable, I would add a [students release] in the dealloc method, but it's not. The class Students allocated the object, so it owns the new NSMutableArray and it must release it, right? Since I can't release it before return it, the only option I see here is... return it as:

return [students autorelease];

But it doesn't feel right to use autorelease objects on the iPhone. This method will be called many times... and I would like to release the memory as soon as possible. Also, the autorelease pool is in the main function and it looks like it will take a while to clean the mess.

How would you do it?

Answer 1

You're right, autorelease is the standard idiom to use in a situation like this.

UIKit actually creates an autorelease pool at the start of each event cycle and releases it at the end, so your autoreleased objects will get cleared up then; they won't be hanging around forever.

If you had a loop that was calling this method many times within a single event cycle, you might want to create your own autorelease pool inside that loop so that these objects get released at the end of each iteration, rather than building up loads of objects to be released at the end of the current event cycle.

But unless you're doing something like that, or you are encountering another specific out-of-memory situation, UIKit's standard autorelease pools should handle it fine.

Answer 2

I tend to avoid using autorelease wherever I can within my iPhone applications, but there are some cases where it doesn't hurt you that much. If the array that you're generating via this method will be retained for a reasonably long duration, then you won't be sacrificing any performance or memory usage by returning it as an autoreleased result.

However, if you will be using and discarding the returned object quite frequently, you may wish to do something different. I have a naming convention with my methods that if something is prefixed by "generate", like generateStudentsArray, it returns an object that is owned by the caller and must be manually released (like with copy). This helps to avoid using autoreleased objects, but it adds another thing to remember when doing memory management.

Additionally, because memory allocation / deallocation is costly (especially on the iPhone), you may wish to avoid the frequent allocations and deallocations within a method called quite a lot and instead recycle the mutable array by creating it ahead of time and passing it into the method, which only adds to the array's contents.