Re: I wish c++ did interfaces better.
On Aug 8, 1:32 pm, Pete Becker <p...@versatilecoding.com> wrote:
On 2008-08-08 00:47:05 -0400, Jerry Coffin <jcof...@taeus.com> said:
In article <2008080710512016807-pete@versatilecodingcom>,
p...@versatilecoding.com says...
On 2008-08-07 10:09:29 -0400, "Chris Becke" <chris.be...@gmail.com> sa=
id:
struct Iv2 : virtual Iv1 {
virtual void method1()=0;};
Actually, this is where the house of cards falls down :-
In a framework that uses interfaces to facilitate
communication between precompiled modules potentially
written in different languages you cannot ever use
'virtual' to extend an interface. The final resulting
vtables MUST be contiguous in memory and using virtual
will fragment them. :-(
Huh? If you don't use virtuals you won't get vtables.
Unless by "vtable" you mean some non-C++ interoperability
mechanism, in which case, it's not particularly pertinent
here.
I think he meant the "virtual" to refer to virtual
inheritance, not virtual functions.
I still have no idea what it means. The compiler lays out
vtables, and makes them work. Doesn't matter if they're
"fragmented" (if that's what happens, I haven't seen it) so
long as the compiler knows how to deal with them.
Well, Chris Becke did mention linking with other languages. If
that's going to work, the compiler has to lay out everything
(including vtables) in a compatible format (and the other
language has to support whatever it is you're trying to
do---virtual functions don't work too well in an `extern "C"',
for example).
It's all very implementation defined. Generally, either the
compilers of both languages concure (and you use `extern
"Whatever"' in C++, to tell the compiler that it has to follow
the conventions of the other language), or you have to use some
external tool such as Corba. In a lot of cases, the only thing
different languages will support is the C API, which means that
anything passed between them must be C compatible. If the other
language does provide the equivalent of an `extern "C++"', it's
possible that it still imposes some restrictions (no templates
would seem likely, but no virtual inheritance, or even no
multiple inheritance, is also a distinct possibility). In some
cases (Java's JNI, for example), the other language defines what
the interface will look like, and your C++ code has to conform;
I'm pretty sure that JNI supports virtual functions in C++, but
I'm also pretty sure that either it doesn't allow multiple
inheritance, or it imposes severe restrictions on it. And
external tools will impose their own constraints.
None of which, of course, is really relevant here. If you're
using tools which impose artificial constraints on the language,
that's not really C++ (and you should seriously think about
changing the tool set).
--
James Kanze (GABI Software) email:james.kanze@gmail.com
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