GNUstep-base is pretty solid if you're looking for an open source Foundation (constructors, destructors, container classes, networking, etc.) that's compatible with Apple's implementation.

If all you're looking for is constructors and destructors, consider ObjFW. https://github.com/Midar/objfw/

ObjFW is much more lightweight, although it does have its own set of conventions. Fortunately, +alloc, -init, +new and -dealloc are all present and accounted for.