I'm trying to implement a runtime reflection module, and am having some issues with figuring out the function pointer to invoke an interface method on a class that derives from the one that implements the interface.

For example, given the following code, we can call an interface function using a delegate: http://pastie.org/8255612 - but even with the derived instance passed in it will invoke Bar's implementation as we're using Bar's object.Interface instance and thus vtable. I figured typeid(DerivedBar).interfaces would
return it's own Interface instance for DerivedBar, but it returns an empty array and instead Bar's has to be used.

The ABI page doesn't seem to mention much about inheriting from a class that implements an interface and overrides it's functions. Is there a way to get the function pointer to the overridden version of foo on DerivedBar using Bar's object.Interface instance for Foo? How does the compiler know what method to call in this situation?

I'm hoping to accomplish the below without storing data for DerivedBar:
auto metadata = createMetadata!Foo;
DerivedBar instance = ... // DerivedBar is not known at compile-time.
metadata.invokeMethod("bar", instance, params);

Anyone have any suggestions for this? Essentially trying to
figure out, given an object.Interface instance and an index
within it's vtbl property, how we can get the function pointer
for that method on a class that overrides the method. I assume
this is a better place to ask than the D.learn forum as it's more
of an ABI question than a question about learning D.

On Monday, 2 September 2013 at 06:21:55 UTC, Kapps wrote:
> Anyone have any suggestions for this? Essentially trying to
> figure out, given an object.Interface instance and an index
> within it's vtbl property, how we can get the function pointer
> for that method on a class that overrides the method. I assume
> this is a better place to ask than the D.learn forum as it's more
> of an ABI question than a question about learning D.

Hi,

Maybe not fully hitting your point, as I cannot recall the correct answer anymore.
But I have found using GDB command line and its memory inspection to be quite useful in hunting dereferences in the class info stuff.

See especially the Class Object Layout table at http://dlang.org/abi.html .
Let me know, if you get stuck.

Cheers,
Ludovit

Am 02/09/2013 08:21, schrieb Kapps:
> Anyone have any suggestions for this? Essentially trying to
> figure out, given an object.Interface instance and an index
> within it's vtbl property, how we can get the function pointer
> for that method on a class that overrides the method. I assume
> this is a better place to ask than the D.learn forum as it's more
> of an ABI question than a question about learning D.

I stronlgy recommend that you read the casting routins insde the druntime. They basically contain all the information how to get to the object pointer from a interface.

This might also help:
http://3d.benjamin-thaut.de/?p=25#more-25

Kind Regards
Benjamin Thaut

Awesome, thanks for the help guys, everything seems to work well now. Essentially instead of using the vtable of the initial Interface instance, we can just take the offset of the interface from the instance pointer, which becomes a void*** that contains the actual vtable.

Updated initial code: http://pastie.org/8292653