From: Tobias Grimm <listaccount@...>
Date: 2004-08-20T09:34:32+09:00
Subject: Overriding virtual C++ method in Ruby

Hi!

Slowly but surly, I'm getting my foot on the ground with embedding Ruby in a C++ 
application.

So far I can call methods of C++ -classes from the ruby script (using SWIG) and 
ruby-stuff from C++ using rb_eval_string_protect.

Now I would like to derive a C++-Class in a Ruby script, override a virtual 
method of the C++ class, return an instance of this new class back to C++ and 
then invoke the overridden method. I've no idea, if this is possible at all.

See this example and what happens:

[C++:]

class Foo
{
     public:
         virtual char *bar(void) { return "C++ implementation" };
};

[Ruby:]

class FooBar < MyModul::Foo
     def bar
         return "Ruby implementation"
     end
end

test = Foo.new
$uglyGlobal = FooBar.new

STDERR.puts("Ruby-Foo:    ", test.bar)
STDERR.puts("Ruby-FooBar: ", $uglyGlobal.bar)

[Back in C++:]

// ...
// initializing ruby and making SWIG-wrapper available
// ...

rb_load_protect(rb_str_new2("test.rb"), Qfalse, &state);
VALUE obj = rb_eval_string_protect("$uglyGlobal", &state);

Foo fooFromCpp = new Foo();
Foo fooFromRuby = *reinterpret_cast<Foo *>(obj);

cerr << "C++-Foo:     " << fooFromCpp.bar() << endl;
cerr << "C++-FooBar:  " << fooFromRuby.bar() << endl;

[The output is:]

Ruby-Foo:    C++ implementation
Ruby-FooBar: Ruby implementation
C++-Foo:     C++ implementation
C++-FooBar:  C++ implementation

Obviously C++ does not see, that the FooBar object returned by Ruby has 
overriden bar(). Am I doing something completely wrong?

I'm not sure how polymorphism is implemented in gcc, but I'm afraid something 
like this is simply not possible, is it?

bye,

Tobias