What is the proper way to manage memory of a class variable / method in objective c?

Question

I'm learning Objective - C and coming from a garbage collected world. I am creating a class (static) variable of a dictionary and I am unsure if I am doing it properly for memory management or not. I'm using a convenience method so the object should be auto-released, but I don't really know if I need to release or retain it in my class.

I can't find clear documentation on how class level objects are managed - any advice is appreciated. Thanks.

+(NSDictionary*) polygonNames{        
        NSDictionary* polygonNames = [NSDictionary dictionaryWithObjectsAndKeys:
                                      @"Triangle", @"3",
                                      @"Square", @"4",
                                      @"Square", @"4",
                                      @"Pentagon", @"5",
                                      @"Hexagon", @"6",
                                      @"Heptagon", @"7",
                                      @"Octagon", @"8",
                                      @"Nonagon", @"9",
                                      @"Decagon", @"10",
                                      @"Hendecagon", @"11",
                                      @"Dodecagon", @"12",
                                      nil];
        return polygonNames;

    }

Answer 1

If your data structure is immutable and isn't going to change, you can use a static variable, like so:

+ (NSDictionary *) polygonNames
{
    static NSDictionary *polygonNames = nil;
    if (!polygonNames) polygonNames = [[NSDictionary alloc] initWithObjectsAndKeys:/* objects and keys */];
    return polygonNames;
}

Answer 2

I can recommend the site CocoaDev.com. On which you will find the following rule of thumb:

• If you alloc, retain, or copy it, it's your job to release it. Otherwise it isn't.
• If you alloc, retain, or copy it, it's your job to release it. Otherwise it isn't. Yes: read this again!

Search for "rules of thumb". As I'm not allowed to add hyperlinks..

Basically all class methods named dictionary*, array* and so on will return an auto-released object which you don't have to retain/release.

For further reading I can recommend the "memory management" page.