On Jul 12, 2011, at 2:13 AM, Greg Parker wrote:
> On Jul 11, 2011, at 8:38 PM, Kelvin Chung wrote:
>> It would be nice, for example, to see a hybrid class that can both call methods and receive messages. If an Objective-C class is built upon a C struct, why can't an Objective-C++ class be based on a C++ struct?
>
> Because you'd have to break compatibility with either C++ or Objective-C. The first word of an Objective-C object is its isa pointer. The first word of a C++ object is its virtual table (if any). You can't have both, so somebody would have to break compatibility.
I actually didn't read anywhere that the vtable pointer has to be the first member of a C++ object; in fact I read somewhere that it could be the last, or even different between implementations, or even not use vtables entirely. Based on that, I can't see why you can't have both. I have to acknowledge that in practice things may be different; I wonder why that is.
Another thing that comes into mind: it appears that as long as a C++ class is standard layout, has a nontrivial destructor (but stopping short of trivial), and all of its nonstatic members have the same restrictions (thus stopping short of POD) you could in theory "Objectivize" the C++ class with no problems (think of it as an Objective-C class that extends, from a C++ perspective, objc_object and possibly multiple "compatible" C++ classes). Is there something in the underpinnings of Objective-C++ that, in theory, prevent this?
(Note that I'm not meaning any of this as a criticism, but I probably need more of an understanding of the system before I understand why I can't do this with that.)
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