Objective C two-phase construction of objects

Question

I've been reading up on RAII and single vs. two-phase construction/initialization. For whatever reason, I was in the two-phase camp up until recently, because at some point I must have heard that it's bad to do error-prone operations in your constructor. However, I think I'm now convinced that single-phase is preferable, based on questions I've read on SO and other articles.

My question is: Why does Objective C use the two-phase approach (alloc/init) almost exclusively for non-convenience constructors? Is there any specific reason in the language, or was it just a design decision by the designers?

Answer 1

I have the enviable situation of working for the guy who wrote +alloc back in 1991, and I happened to ask him a very similar question a few months ago. The addition of +alloc was in order to provide +allocWithZone:, which was in order to add memory pools in NeXTSTEP 2.0 where memory was very tight (4M). This allowed the caller to control where objects were allocated in memory. It was a replacement for +new and its kin, which was (and continues to be, though no one uses it) a 1-phase constructor, based on Smalltalk's new. When Cocoa came over to Apple, the use of +alloc was already entrenched, and there was no going back to +new, even though actually picking your NSZone is seldom of significant value.

So it isn't a big 1-phase/2-phase philosophical question. In practice, Cocoa has a single phase construction, because you always do (and always should) call these back-to-back in a single call without a test on the +alloc. You can think of it as a elaborate way of typing "new".

Answer 2

My experience is with c++, but one downside of c++'s one phase initialization is handling of inheritance/virtual functions. In c++, you can't call virtual functions during construction or destruction (well, you can, it just won't do what you expect). A two phase init could solve this (partially. From what I understand, it would get routed to the right class, but the init might not have finished yet. You could still do things with that) (I'm still in favor of the one phase)