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From: duns1222@w203zrz.zrz.tu-berlin.de (Martin Dunschen)
Newsgroups: comp.lang.eiffel
Subject: Re: passing address of an eiffel feature
Message-ID: <281@mailgzrz.tu-berlin.de>
Date: 6 Mar 91 17:14:22 GMT
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This posting is especially to Greg Economides of Biosystems
Modelling Group at Texas A&M University, e-mail greg@carnivore.tamu.edu

BUT everybody interested, feel free to read on, perhaps read first my
posting from the day before, same subject!

Hi Greg,
I tried to send you a mail, but it turned out that your configuration
is not ready for receiving mail, so I try this way.


thank you for your help regarding our problems passing eiffel features
to foreign routines. I understand the main principle now.

You call the "methods", e. g. the features `item' and `put', from
within the C routine. For this you have to find them, depending on
the object, to which these features belong. In our example we
refer to the features `item' and `put' via the ROUT_PTR eif_rout.
We store the pointers to these features on the variables `put_ptr'
and `item_ptr' for further use.
Ok, so far so good, but I tried your code-examples, with a few minor
changes, and they didn't work. The following questions arose:

1. Do you pass an_array as it is or as @an_array ?

2. How to declare an_array in c_func, as Object or OBJPTR ?

3. How do I know the number/types of parameters to item_ptr and put_ptr?
   Are they according to the syntax in an eiffel-program ?

4. How to declare myvar, as float, or as a pointer to something else ?

Again, I include eiffel and C code, what I got from you with some
changes. (Our C compiler seems not ANSI standardized !)

Excuse me, when I bother you again, but what can I do?

Bye, Martin
CUT HERE ------ CUT HERE ------ CUT HERE ------ CUT HERE ------ CUT HERE ------ 
class WHATEVER 
 
feature
 
	an_array: ARRAY[REAL];
 
	Create is
	   external c_func language "C"
	do
		an_array.Create ( 0, 2);
                an_array.put    (10.,0);
                an_array.put    (20.,1);
                an_array.put    (30.,2);
                output ("before calling c_func:",
                                  an_array.item(0),
                                  an_array.item(1),
                                  an_array.item(2));

		c_func(@an_array);
             -- c_func(an_array);

                output ("after calling c_func:",
                                  an_array.item(0),
                                  an_array.item(1),
                                  an_array.item(2));
	end; -- a_feature
       
        output (str: STRING; r1, r2, r3: REAL) is
        do
                io.putstring (str);
                io.putreal (r1);
                io.putstring (" ");
                io.putreal (r2);
                io.putstring (" ");
                io.putreal (r3);
                io.putstring ("\n")
        end -- output

end; -- class WHATEVER
CUT HERE ------ CUT HERE ------ CUT HERE ------ CUT HERE ------ CUT HERE ------ 
#include "/path/to/Eiffel/files/_eiffel.h"

void c_func (array_ptr)
OBJPTR array_ptr;
/* Object array_ptr */
{
extern ROUT_PTR eif_rout;

/* in the example /path/to/eiffel/examples/eiffel-c/callbacks
   the previous declaration looks like
   extern ROUT_PTR (*eif_rout)();
*/

ROUT_PTR put_ptr, item_ptr;

float myvar; /* is this correct? I'm not shure */

 
/* assigning the eiffel routines to variables `put_ptr' and `item_ptr'
   is useful if these eiffel functions will be used more than once.
   otherwise a one-time call can be made like this:
	(*(eif_rout(array_ptr,"item")))(array_ptr, 3)
	                  arguments to `item' -----^      */

printf ("in c_func, before assigning pointers to eif_rout's\n");
put_ptr = eif_rout(array_ptr, "put");
item_ptr = eif_rout(array_ptr, "item");
printf ("in c_func, after assigning pointers to eif_rout's\n");
 
/* Now, to use these pointers to eiffel functions */
 
printf ("in c_func, before calling item \n");

/* myvar = *(eif_rout(array_ptr,"item"))(array_ptr, 3); */

myvar = (*item_ptr)(array_ptr,3);

printf ("in c_func, after calling item, myvar=%f\n", myvar);

/* maybe you'd make a fortran call here which would change the value of
   myvar....  and then you might want to put it back into the array */
 
myvar = myvar * 2.; /* just an example to change something */

printf ("in c_func, before calling put, myvar=%f\n", myvar);

/* (*(eif_rout(array_ptr,"item")))(array_ptr,myvar, 3); */

(*put_ptr)(array_ptr,myvar,3);

printf ("in c_func, after calling put, myvar=%f\n", myvar);
 
}