[C++-sig] Need strategic advice designing Boost/Python program

[Craig Finch]

Stefan,
Thank you for your input.  To answer your questions:

> I'm not sure what you mean by 'implement a constructor'. Can you
> elaborate ?
> As a minor point, if you go this route, there is no need to use
> extract<>
> to access values, as within C++, you'll always deal with the (native)
> C++
> interface directly. extract<> is useful to pull out embedded C++
> objects
> from python (wrapper) objects, or to require an explicit conversion /
> cast.

Let's say that I have created classes in C++ that represent the data
needed for my program.  However, I want to build the methods in Python
because they need to be modified easily.  I have gotten this approach
to work for methods except for __init__.  If I try to add a Python
constructor to the class, it can't access the data members of the C++
class.  I don't want to write the constructor in C++ because it's
potentially very complicated, defeating the purpose of writing methods
in Python.  What do you suggest?

Also, thank you for your explanation of <extract>.  It seems like I
will still need to use extract, because I want the Python interface to
use arrays in a Python-like manner.  On Ralf's suggestion, I will look
at  scitbx to see how he handles it.

> > 2. Similar to #1, but instead create Python classes that inherit
> from
> > the C++ base classes, and then add Python methods.  Not sure how to
> get
> > the resulting objects back into C++.
> 
> If you add new methods, or, more generally, attributes, you'll have
> to
> access them through the boost.python 'attr()' method that is part of
> the
> object protocol. However, if these attributes are only added for
> convenience
> (i.e. used within python, but not necessary for the C++ solvers),
> there
> isn't any need to access them from within C++, right ?

Correct--the methods I am building in Python should not need to be
accessible to the C++ solvers.  So, if I have a C++ base class
containing all the "core" data, I can then derive Python classes which
add methods and "extra" data.  In this case, how do I access the "core"
data from C++, since it is now a derived class?

Also--Ralf, thanks for the information about scitbx, and I may have
more questions once I've studied it.  I'll take a look at it and try to
learn from its design, but currently the server 
http://cci.lbl.gov/ seems to be unreachable.

   Craig Finch



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