Does using lists of structs make sense in cocoa?

Question

This question has spawned out of this one. Working with lists of structs in cocoa is not simple. Either use NSArray and encode/decode, or use a C type array and lose the commodities of NSArray. Structs are supposed to be simple, but when a list is needed, one would tend to build a class instead.

When does using lists of structs make sense in cocoa?

I know there are already many questions regarding structs vs classes, and I've read users argue that it's the same answer for every language, but at least cocoa should have its own specific answers to this, if only because of KVC or bindings (as Peter suggested on the first question).

Answer 1

In general, the use of a struct implies the existence of a relatively simple data type that has no logic associated with it nor should have any logic associated with it. Take an NSPoint for instance - it is merely a (x,y) representation. Given this, there are also some issues that arise from it's use. In general, this is OK for this type of data as we usually observe for a change in the point rather than the y-coordinate of a point (fundamentally, (0,1) isn't the same as (1,1) shifted down by 1 unit). If this is an undesirable behavior, it may be a better idea to use a class.

Answer 2

Cocoa has a few common types that are structs, not objects: NSPoint, NSRect, NSRange (and their CG counterparts).

When in doubt, follow Cocoa's lead. If you find yourself dealing with a large number of small, mostly-data objects, you might want to make them structs instead for efficiency.

Using NSArray/NSMutableArray as the top-level container, and wrapping the structs in an NSValue will probably make your life a lot easier. I would only go to a straight C-type array if you find NSArray to be a performance bottleneck, or possibly if the array is essentially read-only.

Answer 3

It is convenient and useful at times to use structs, especially when you have to drop down to C, such as when working with an existing library or doing system level stuff. Sometimes you just want a compact data structure without the overhead of a class. If you need many instances of such structs, it can make a real impact on performance and memory footprint.

Another way to do an array of structs is to use the NSPointerArray class. It takes a bit more thought to set up but it works pretty much just like an NSArray after that and you don't have to bother with boxing/unboxing or wrapping in a class so accessing the data is more convenient, and it doesn't take up the extra memory of a class.

NSPointerFunctions *pf = [[NSPointerFunctions alloc] initWithOptions:NSPointerFunctionsMallocMemory |
                          NSPointerFunctionsStructPersonality |
                          NSPointerFunctionsCopyIn];
pf.sizeFunction = keventSizeFunction;
self.pending = [[NSPointerArray alloc] initWithPointerFunctions:pf];