Re: CNI and interface methods

[Bryce McKinlay <bmckinlay@gmail.com>]

On Fri, Apr 3, 2009 at 9:37 AM, Andrew Haley <aph@redhat.com> wrote:
> Stephen Kell wrote:
>
>> I'm using CNI for the first time and have hit a query. What exactly is
>> and isn't supposed to work when invoking interface methods through
>> *object* references (from C++, so I mean pointers), and passing these
>> references around?
>>
>> The CNI chapter in the gcj manual says a lot about interface references,
>> but not about the case of using object references where that object's
>> class implements one or more interfaces. One thing that would appear to
>> be worth mentioning is that clearly, passing them to methods wanting an
>> interface reference won't compile without some sort of cast, since the
>> gcjh-generated headers don't encode the subtyping relationship between
>> interfaces and their implementing classes. I'm guessing a simple static
>> cast isn't the right thing to do though, because I'm getting segfaults
>> when I try.
>
> Right.  To put it simply, we have never got around to implementing Java
> interface calls in CNI.  It wouldn't be enormously difficult, but it
> needs some way to tell the C++ compiler about interface calls.  We could
> have implemented Java interfaces via C++ multiple inheritance but we
> didn't, so we would need to invent some sort of notation to express the
> idea of an interface to the C++ compiler.

Actually we did implement a rather limited form of interface calls in
CNI. Interfaces types are described in CNI headers with __attribute__
((java_interface)), and the C++ compiler knows how to call a method on
a type declared as such.

What is missing in the C++ compiler (and the CNI headers) is knowledge
of interface inheritance, so you have to manually cast interface
references if the method you want to call was declared in a
super-interface.

This limitation is described here:

http://gcc.gnu.org/onlinedocs/gcj/Interfaces.html

Bryce