Re: calling C++ from IDL - throwing exceptions

[Richard Younger <younger(at)ll.mit.edu>]

Randall Skelton wrote:
> 
> You may want to consider getting Ron Kling's new book on calling C code
> from IDL as he does a nice job of describing DLMs and error trapping with
> them.  I think you will end up needing to write C wrappers around your c++
> in any case.  Ron's book is available at: http://www.rlkling.com/
> 
> The DLM documentation for RSI is adequate as a reference for IDL type
> structures and internals, but don't expect it to be much help when you are
> just starting out!
> 
> Randall
> 
> Ugo_DiGirolamo@InVision.iip.com writes:
> 
> > I'm planning to use IDL as a front end for a C++ dll with call_external.
> > However I couldn't find any way of handling in IDL an exception thrown
> > from the C++ code.
> > I was wondering if anyone have an idea about it (also if to confirm
> > that it's impossible!) or if the DLM way would work better.
> > However, I've no real idea about what DLM is and I found really little
> > clue on it in the RSI documentation.
> > cheers

C++ works fine with the DLMs.  I've tried it out with the MS VC++
compiler (I know, I know, but I'm stuck with it for now), and it works
just fine as long as you make sure to set the C calling convention
(cdecl) in the compiler.  Now, with a DLM that is limited in scope, many
of the _features_ of C++ aren't as useful, but your code that talks to
IDL does just fine in C++.  You can even write your IDL-callable
functions as (static) class members.  

I haven't really tried to write cross-language handlers though.  In your
C++ handler, you can call IDL message routines and pass back debugging
variables, but I'm wondering just what you want to handle in IDL that
was caused in C++, and how you're thinking of handling it.  Just about
all the things I can think of should be handled in their original
language.  If you want user feedback for the handling process, I know
you can take a little input back into C++ with IDL_GetKbrd(), but I
haven't used that for anything but "press any key" pauses, and it
doesn't really return control to IDL.  I suppose if you wanted to, you
could print data or a menu to the log window and have the user select an
option, but that seems to be going backwards.

Regardless, I agree that Ronn's book is a very helpful tutorial for
getting started in DLMs, and includes lots of illuminating examples in
C.  If you can't quite get C++ to work, let me know.

Rich

-- 
Richard Younger                         MIT Lincoln Laboratory
Email: younger@ll.mit.edu