Question

I have managed to get myself confused about some elements of memory management. I am new to objective-c, and memory managed languages generally.

I have read the memory management guidelines, but I remain confused about a couple of things.
1) Is there any need to clean up ivars, and method variables that are not retained by any object. For instance

-(void) someMethod{
int count = 100;
 for (int i=0; i <count;  i++) {
  NSLog(@"Count = %d", i);
 }
    }    

What happens to the “count” var after the method is complete?
If a method allocates lots of temporary variables, do those get removed from memory as long as they are not unreleased, alloc’d objects? Or do i need to set them to nil in some way?

2) If i have a static variable, for instance an NSString, do I have to do anything for that to be removed from memory when the class is dealloced?

3) I have noticed that NSStrings seem to have a retainCount of 2147483647 which wikipedia tells me is the max value for a 32 bit signed integer.

http://en.wikipedia.org/wiki/2147483647

myString retainCount = 2147483647

-(void) someMethod{
NSString *myString = @"testString";
NSLog(@"myString retainCount = %d", [myString retainCount]);
// logs:  myString retainCount = 2147483647
}

What happens to this at the end of the method? Does this memory ever get emptied? The string is not being referenced by anything. My understanding is that the @”” convenience method for NSString returns an autoreleased object, but whats the point of autoreleasing something with a retainCount of 2147483647 anyway? In that case, whats the point of retaining or releasing ANY NSString?

I am well aware that retainCount should be ignored, but it just bugs me not to know what’s going on here.

4) Does this matter at all? I know that the memory associated with an NSString isn’t much to write home about, but I want to be a good memory management citizen, and I’m more interested in best practices than anything else.

Answer 1

Retain/release only matters for objects, not int, bool, float, double or other built-ins. So use it for id or other classes that you hold a pointer to an object. In your example, count doesn’t need to be retained or released. It is allocated on the stack which is automatically cleaned up when the function returns.

You do need to deal with any local objects you alloc. Those are created with a retainCount set to 1, so you need to either hold on to them for later or release them. Most Cocoa functions (that don’t start with copy or alloc) return an object that is autoreleased. This means that they will have release called on them later — you can only hold these after the function if you call retain on them. If you want them to be cleaned up, you don’t need to do anything (calling release would result in too many release calls).

If you have a static variable pointing to an object, then it is not touched when objects of that class are dealloced. If you want it to be released, you have to call release on it. If the static is an int, bool, or other built-in, you don’t (can’t) call release on it. That’s part of the global memory of your app.

NSStrings that are set to string literals should not have release called on them. The retainCount is meaningless for them. That value is also -1 for signed int values.

If you do this — [[NSString alloc] initCallHere:etc] — you have to call release on it. Most of the time you get strings, you don’t use alloc, so you don’t need to call release. If you retain one, you need to call release.

Yes, it does matter. Over time leaks will cause the iPhone to kill your app.