dolfin team mailing list archive

[Johan Hake <johan.hake@xxxxxxxxx>]

On Tuesday 01 December 2009 00:45:50 Anders Logg wrote:
> Would it help to add a new class on the C++ side that is used only for
> passing array data back and forth between C++ and Python? We have had
> this before (SimpleArray) and it would be fairly easy to extend the
> C++ with extra functions in the interface that use SimpleArray instead
> of std::vector.
> 
> Then perhaps we can have one single typemap that hits SimpleArray
> everywhere and converts it to a NumPy array.

Yes, something in that direction is what I had in mind. In addition we could 
also add a foo.array() function to get a NumPy view from this class. This 
would be nice when we do not want to have all the communication through 
typemaps, but actually using the SimpleArray in Python as return argument from 
some function that wants to resize the array.

We would also need some stuff to handle memory management.

I see two fundamental ways such a class could be used:
  1) A replacement for the previous use of double/uint/int*, now std::vector
  2) A replacement for communication using std::vector where resize 
     flexibility is needed.

I think 1, speaks for it self. 2 is where we need to resize any passed vector. 
This goes for GenericMatrix.getrow, foo.intersection, 
GenericFunction.comput_vertex_values. 

> And the work would be to add the extra stuff on the C++ side. The
> advantage would be less complex wrapper code and that Garth and I
> are capable of handling the complexities on the C++ side.

Yes this must be a goal. I agree that the present SWIG situation has grown out 
of hands.

> But what I don't understand is why it would be easier to write a
> typemap for SimpleArray than for std::vector. Both of them use
> contiguous memory.

Yes, but in std::vector it is now way, I suppose, to prevent a vector to 
delete its data when it goes out of scope. This is necessary in a typical in 
typemap.

I will fix the interface of getrow this evening. I was about to do it 
yesterday, but instead I got grumpy :) But a good night sleep makes wonders!


Johan