Wrapping a C library with its own GC + classes vs refcounted structs

[Laeeth Isharc via Digitalmars-d-learn]

Hi Aldanor.

I wrote a slightly longer reply, but mislaid the file somewhere.

I guess your question might relate to wrapping the HDF5 library - 
something that I have already done in a basic way, although I 
welcome your project, as no doubt we will get to a higher quality 
eventual solution that way.

One question about accurately representing the HDF5 object 
hierarchy.  Are you sure you wish to do this rather than present 
a flattened approach oriented to what makes sense to make things 
easy for the user in the way that is done by h5py and pytables?

In terms of the actual garbage generated by this library - there 
are lots of small objects.  The little ones are things like a 
file access attribute, or a schema for a dataset.  But really the 
total size taken up by the small ones is unlikely to amount to 
much for scientific computing or for quant finance if you have a 
small number of users and are not building some kind of public 
web server.  I think it should be satisfactory for the little 
objects just to wrap the C functions with a D wrapper and rely on 
the object destructor calling the C function to free memory.  On 
the rare occasions when not, it will be pretty obvious to the 
user and he can always call destroy directly.

For the big ones, maybe reference counting brings enough value to 
be useful - I don't know.  But mostly you are either passing data 
to HDF5 to write, or you are receiving data from it.  In the 
former case you pass it a pointer to the data, and I don't think 
it keeps it around.  In the latter, you know how big the buffer 
needs to be, and you can just allocate something from the heap of 
the right size (and if using reflection, type) and use destroy on 
it when done.

So I don't have enough experience yet with either D or HDF5 to be 
confident in my view, but my inclination is to think that one 
doesn't need to worry about reference counting.  Since objects 
are small and there are not that many of them, relying on the 
destructor to be run (manually if need be) seems likely to be 
fine, as I understand it.  I may well be wrong on this, and would 
like to understand the reasons if so.






Laeeth.