[Gambas-devel] Multi-level virtual objects, inheritance and symbol resolution mess

[Benoît Minisini]

Le 05/09/2014 21:26, Tobias Boege a écrit :
> Hi Benoit,
>
> I'm almost done with implementing an adjacency matrix based Graph class
> (GraphMatrix), to then set the Graph interface in stone. See #6451 for
> the code I'll talk about.
>
> Only there is one obstacle to overcome: accessing symbols of a
> GraphMatrix.Vertices[sVertex] object. The situation is as follows:
>
>    - GraphMatrix Inherits Graph (which is an "abstract" class).
>    - .Vertices is a virtual object whose _get method calls back:
>    - Me._getVertex (which is within GraphMatrix and) which returns a:
>    - _Graph_Vertex, a virtual class which is the base of all implementation-
>      specific vertex classes.
>    - In case of GraphMatrix, this is a _Matrix_Vertex virtual class which
>      Inherits _Graph_Vertex.
>
>>From there I want to access the InDegree property which is properly
> implemented in the _Matrix_Vertex class but all I get is ("a" is an existing
> vertex in the graph):
>
>    hGraph.Vertices["a"].InDegree
>    >> (Function GraphMatrix:3)
>
> Indeed, InDegree is the 3rd symbol in _Matrix_Vertex. If I issue
>
>    hGraph.Vertices["a"].InDegree()
>    >> Error: Not enough arguments
>
> The 3rd symbol in GraphMatrix is _getEdge which needs two arguments... If I
> try that:
>
>    hGraph.Vertices["a"].InDegree("b", "a")
>    >> Error: Edge does not exist
>
> which is the error I would expect from my graph if I called the _getEdge
> method! It seems like the InDegree property is not mapped to my
> _Matrix_Vertex.InDegree implementation but to the method of same index in
> GraphMatrix.
>
> _Matrix_Vertex actually is a virtual class obtained via a virtual class
> interface from GraphMatrix. Is the virtual class information lost somewhere
> on the way or do I exceed a limit here?
>
> ---
>
> So, in a nutshell: GraphMatrix.Vertices is a virtual class which calls back
> GraphMatrix._getVertex which returns another virtual object from which I
> want to access a property.
>
> In a graph class (ImageGraph) implemented in Gambas -- where there are no
> virtual classes -- the _ImageGraph_Vertex Inherits _Graph_Vertex is a real
> class. And there it works!
>
> Attached are: the project which uses the native GraphMatrix class and fails,
> and the working ImageGraph Gambas class. I would really like to finish this
> thing soon. If you could take a good look, that would surely be a great help.
>
> Regards,
> Tobi
>

I'm currently looking at it...

But why didn't you do as I told you, i.e. use an internal dispatch 
interface (a structure of function pointers) like I did for the Paint 
class?

Otherwise things are so slow. Calling GB.GetFunction() implies a symbol 
table lookup and signature verification at each call!

I suggest:

- Creating a struct of function pointers that represents the graph node 
interface.

- Adding a pointer on that struct in each graph.

- Use it everywhere you need.

- All classes written in C/C++ inheriting Graph must implement that 
struct of function pointers and put a pointer on it in its objects.

- For classes written in Gambas inheriting Graph, you must create a 
struct of function pointers that calls the Gambas functions you defined 
('_getVertex', '_getEdge', ...). Calls to GB.GetFunction() should be 
done once and cached, the first time you instanciate an object of that 
class.

That way, performance should be acceptable. :-)

By the way: you should use GB_DECLARE_VIRTUAL() instead of GB_DECLARE() 
for declaring virtual classes. No need to use GB_VIRTUAL_CLASS() then. 
And virtual classes name should begin with a dot, not an underscore. I'm 
not sure at the moment it has no impact in the interpreter...

Regards,

-- 
Benoît Minisini