[libavg-devel] Dynamic Node Creation Enhancements

[Nick Hebner]

Well, I spent the past few hours today trying to hack something together,
and have run into a few issues.

After adding typing to Arg, I set off on setting member offsets. I initially
went with my MEMBER_OFFSET() method with the intent of changing it later,
and almost immediately, it dawned on me that we don't need a dummy instance
of the class at all. In fact, all we need is the type and the member, and we
find the offset by taking the address of the member dereferenced from a null
pointer ( e.g. offset = ((int)&((type*)0)->member) ). I even learned that
this operation is built into C/C++ itself with the offsetof(type,member)
operator. Who knew?

Unfortunately, this does not solve our problems because offsetof will only
work on "Plain Ol' Data" (POD) classes (e.g. no virtual bases). This is
indeed a problem causing crashes when I tried it. gcc complains about its
use on node types too.

Another problem that I thought of is member variables of base classes may
not be located in the same spot relative to the this pointer in the derived
class as they were in the base class. e.g. Node::m_ID might be at offset 0
in Node, but offset 50 in RasterNode depending on how the compiler feels
like laying out the memory. Thus, in order to correctly determine the offset
of a member, we would have to do so relative to the most derived class where
private members of base classes are not accessible! I may have made an error
in my thought process here, so please correct me if you disagree with
anything :)

Finally, I have been able to put my finger on my design-reservations about
this feature: it breaks encapsulation.

I have included my changes in case you want to play with them, but I am
close to convinced that what we want to do is not possible. What do you
think?

Nick Hebner
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