Re: GCJ and C++ (calling Java from C++)

[Lothar Werzinger <lothar at tradescape dot biz>]

On Wednesday 30 November 2005 16:49, Bryce McKinlay wrote:
> Lothar Werzinger wrote:
> >in the generated headers from gcjh i get this:
> >
> >  class org::eclipse::emf::examples::extlibrary::Addressable
> >
> >  : public ::java::lang::Object
> >
> >  {
> >    ...
> >
> >  class org::eclipse::emf::examples::extlibrary::Library
> >
> >  : public ::java::lang::Object
> >
> >  {
> >    ...
> >
> >  class org::eclipse::emf::examples::extlibrary::impl::LibraryImpl
> >
> >  : public ::org::eclipse::emf::ecore::impl::EObjectImpl
> >
> >  {
> >    ...
>
> C++ doesn't directly support the idea of a Java interface - we'd need to
> make some enhancements to the C++ front end in order to support this.
>
> As a workaround, you can cast your object reference to the interface
> type (using a C++ static_cast, for example), and then call the interface
> method on that. Just be careful that the object you are casting really
> does implement the interface!

I'm not sure what you suggest here. If you look at the C++ code I use you'll 
notice that I call a static Method that returns a Java Object of type 
Library. What goes wrong is if I call any method on the returned object.
As I already get a Library object I don't know what a cast would do different.

> bug 15411 is a different issue, as in that case it can actually generate
> a broken virtual call because C++'s idea of the vtable layout will be
> different from reality. 15411 shouldn't effect you in this case because
> there is no abstract class here.
> Bryce

Well it is similar in the sense that if gcjh does not generate the inheritance 
relationship I suppose it also generates a wrong vtable, as inheritance 
(inheriting virtual methods from interfaces) does affect the vtable - right?

I wonder if we couldn't write a Java program that uses Java reflection to 
generate the (correct) C++ headers. I guess it would probably much easier to 
do that in Java than in C  and we could use gcj to generate the gcjh 
executable from the Java code.

Lothar
-- 
Lothar Werzinger Dipl.-Ing. Univ.
framework & platform architect
Tradescape Inc.
111 West St. John Street, Suite 200
San Jose, Ca 95113
email: lothar@tradescape.biz