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From: esink@turia.dit.upm.es (Eric W. Sink)
Newsgroups: comp.lang.objective-c
Subject: Implementation of Objective C
Message-ID: <1991May27.073640.13936@dit.upm.es>
Date: 27 May 91 07:36:40 GMT
Sender: @dit.upm.es
Reply-To: esink@turia.dit.upm.es (Eric W. Sink)
Followup-To: comp.lang.objective-c
Organization: Dept. Ingenieria de Sistemas Telematicos, dit, upm, Madrid, Spain
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Greetings all !  I recently posted a request for an Objective C
grammar to c.l.objective-c.  Well, I should have phrased my
question better, and I've come up with a few more.  I'm toying around
with the idea of implementing an ObjC compiler, but I currently know
little about the language.  I haven't had a chance to buy Cox's book
yet.  I do have ``Intro to Objective C on the NeXT Machine'', by
Gerrit Huizenga.  The information I'm looking for needs to be a bit
more precise.   Anyone willing to give answers to the following : ?

1.  What new keywords (in the ANSI C sense of a keyword) are added to
ANSI C ?  Which of the following are keywords, and which are not :
@interface, @implementation, @end, @selector, self, super, id, SEL,
nil, Object, new, free

2.  Is Object (the class) declared somewhere, or does the compiler
simply recognize it as already existent ?  If it is declared and
implemented somewhere using Objective C, why can't other root classes
exist ?  What does its declaration look like (I want to know how to
parse a class declaration which has no super class)

3.  I assume that a 'message send' is a primary expression, as it
appears in the ANSI C grammar.  In other words, the following
expressions are all possible :

   [ obj msg][12] = 33; /* assume the method returns (int *) */
   [ obj msg]();        /* assume the method returns a func ptr */
   x = a + *([obj msg]); /* again, method returns (int *) */

I also assume that the id and SEL elements of a message send are
postfix expressions, so the following is legal :

   id foo();
   SEL bar();
   [foo() bar()];

4.  Is a class (factory) object represented differently than an
instance ?  Are messages to a class object dispatched differently ?
How is new implemented ?  What other factory methods are implemented
by Object ?  isa (what does isa return, a factory object ? )?

5.  My biggest question :  How exactly does dispatching work ?  I
assume that every instance is a dynamically allocated region of memory
which contains all the instance variables PLUS a pointer to some data
structure.  I assume for now that this data structure is in fact the
factory object for the class of the instance, and that factory objects
contain the 1. dispatch table as well as a pointer to the 2. super class,
but it doesn't matter, as long as those two items are available
to each instance.  The dispatch table associates message selectors
with methods for the given class, so we take the message being sent,
convert it to a SEL, use the SEL as the index into the table and get
the address of the method.  Arguments have to get pushed onto the
stack (right to left ?), and the receiving object gets pushed (first
or last ?), and we call the method.  My questions :  how do we convert
the method name to a SEL ?  The manner in which this conversion occurs
is very important, because every source file which uses this class
must arrive at the same SEL for the same method name, and most
importantly, that SEL must be the same as the one calculated during
the actual genertion of the dispatch table, which I assume is
generated while compiling the @implementation of the class. (sentence
way to long, sorry)  What happens if the source file we are compiling,
uses many classes, and a given method foo: appears in more than one
place in the class hierarchy (assume that the various uses of foo: are
not inherited versions of the same method, but are quite distinct
methods along totally different lines of inheritance, but simply
happen to have the same name).  Now, how do we know which foo: to use
?  How do we map to a SEL ?  How to we type check arguments ?  How do
we know the type of the resulting message send ?  Is SEL of type
int (a table index requiring scaling) or of type char *, (a table
offset requiring no scaling).
  Consider the following example :  Is this code legal Objective C ?
  Since I see so many problems with it, I'm beginning to speculate
  that ObjC simply prohibits behavior like this...

@interface classone : Object
{
  int x,y;
}
- (long *) foo:(int) v1 :(short) v2;  /* This is foo:: */
@end

@interface classtwo : Object
{
   long a,b,c,d,e,f,g;
}
- bar;
- qwerty:vary;
- (short) foo:(int) a1 :(short) a2;  /* This is a distinct foo:: */
@end

@interface classthree : classone
- (short) foo:(short) v1 : (long) v2; /* Can I override a method with
a new method of different type ? */
@end

int main()
{
  id objekt;
  int k;

  k = [objekt foo:11:22];
  
  /* What is the type of the rhs of this
  assignment ? What is the SEL ? To what do we convert the arguments
  for pushing onto the stack ? */
}

I plead for patience with respect to my terribly trivial questions.
BTW, the few ObjC features exemplified in Huizenga's notes appear to
be enough for a usable language.  I speculate that ObjC doesn't
contain that much more (or else it would be the melting pot that C++
is).  Am I right ?

I really like the looks of Objective C.  I have some experience
implementing oo languages, but not compiled ones.  Separate
compilation does seem to offer some problems.  I was considering
implementing a subset of C++, but I'm leaning towards Objective C
instead.

Thanks in advance for any replies !

Eric


Eric W. Sink                     | "If no one is criticizing |Opinions
Departamento de Telematica       | your work, it is possible |mine -
Universidad Politecnica de Madrid| that you are not doing    |all of
esink@turia.dit.upm.es           | anything." -George Verwer |them.