Re: Objective-C vs. gcj (was: Re: Newbie questions)

[Jon Olson <olson at mmsi dot com>]

On Thu, 13 Jan 2000, Maury Markowitz wrote:
>> with GCC in the gcc/objc directory) and is useless without it.  It 
>> handles all the method dispatching and dynamic binding required by the 
>> Obj-C language.
>
>  Right.  That method dispatching and dynamic binding is rather  
>similar to Obj-C's though, and not by coincidence.
>

Java actually has binding semantics much more akin to C++ than
Obj-C.  In addition to the relative unpopularity of Obj-C, C++ was
a much better choice for the CNI interface than Obj-C would have been.
Java doesn't do dynamic binding like does Obj-C, but rather performs
run-time checking of downcasts.

In Obj-C, all bindings are made at run-time via signatures generated
by the compiler.  This is because Obj-C allows a method to be dispatched
on an `id' without any compile time knowledge of whether or not that object
actually implements that method.  Thus, at run-time each object references
its class metadata via an `isa' pointer, and each class references some sort
of hash table or sparse array as in the following diagram:

				Superclass metadata
				        ^
	-----------   isa	        |
	| Obj-C object | ----> Class metadata ---> run-time hash-table
            ----------- 

C++ and Java, on the other hand, are strongly typed and only dispatch
methods to objects known to actually have the method.  Their run-time
representation is typically much different:
 
            ------------- vtable
	| C++/Java object | ----> Method Dispatch Table
	-------------		|
					v
				  Class metadata, if it exists

In C++ and Java, method dispatch (except for interfaces) is performed based
on indices unique only to the class being dispatched;  in Obj-C, method dispatch
must be performed on selectors which are GLOBALLY unique.  Because of Obj-C's
radically different binding semantics, it would be a poor language for
implementing any sort of compile-time binding.

-- 
Jon Olson, Modular Mining Systems
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