Path: utzoo!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!uwm.edu!bionet!ames!uhccux!munnari.oz.au!bruce!trlluna!shiva!summer
From: summer@shiva.trl.oz (Mark Summerfield)
Newsgroups: comp.lang.eiffel
Subject: Re: How to use @ with attribute?
Message-ID: <1159@trlluna.trl.oz>
Date: 9 Mar 90 00:15:57 GMT
References: <18296@cs.yale.edu<
Sender: root@trlluna.trl.oz
Reply-To: summer@shiva.trl.oz (Mark Summerfield)
Organization: Telecom Research Labs., Switched Networks Research Branch
Lines: 48

In article <18296@cs.yale.edu< jellinghaus-robert@CS.Yale.EDU (Rob Jellinghaus) writes:
<
<I want to pass the address of a particular (integer) field of an object to
<a C routine I've written.  The class looks something like this:
<
<class STORED_OBJECT
<	...
<feature
<	hash_value: INTEGER;
<	hash is
<		external
<			hash_object(current_obj: ANY, hash_val: ANY)
<				language "C"
<		do
<			hash_object(Current, @hash)
<		end
<end -- stored_object
<
<Pass 1 on class stored_object
<...
<C-compiling stored_object
<"stored_object.c": line 125: unacceptable operand of &
<es: C-compilation canceled
<
<Why is this?

I had exactly the same problem- I think it's a bug in the compiler which
causes it to generate incorrect C code. The C, if you look at it,
attempts to cast an object structure to type integer (presumably the
integer is the first field of the object structure), and then pass a
pointer to the C routine (in this case hash_object). ie the C code
is (effectively- it doesn't look exactly like this!)
	hash_object( ... , & (int) object )

I don't believe this is a legal C construct. The correct code would
be:
	hash_object( ... , (int *) &object )

If this is a bug (and I'm prepared to be corrected, because I would
like to make it work too!), you may have to work around it by actually
editing the C files (yuk, blechh) but this is utterly impractical if
you've got more than one or two instances of this, and completely defeats
the purpose of the nice incremental build system ISE have provided us
with.

If you find a better solution, let me know.

Mark Summerfield, Telecom Research Laboratories, Australia.