[SciPy-user] Weave - accessing object data members?

[eric jones]

Hey Fernando,
 
> Hi all,
> 
> I suppose that expecting weave.inline to magically make the following
> work:
> 
> ###############
> from weave import inline
> class bunch: pass
> 
> def oaccess():
>      x=bunch()
> 
>      x.a = 1
> 
>      code = """ // BROKEN!
>      // Try to emulate Python's: print 'x.a',x.a
>      std::cout << "x.a " << x.a << std::endl;
>      """
>      inline(code,['x'])
> 
> oaccess()
> ###############
> 
> 
> is probably a bit too much, right :) ?  It would have to extract all
the
> members of 'x', and make all the right type work in C++.  This sounds
> pretty much near impossible.  

Yep. 

> Things like these are what makes you really
> love the convenience of Python vs C/C++ :)
> 
> But I wonder what the best way to access object members from within
the C
> code is.  So far my typical approach is just to make undotted copies
of
> everything I'll need once inside C/C++:
> 
> a = x.a
> b = x.b
> 
> inline("do_stuff_with(a,b);", ['a','b'])
> 
> But there may be a better way to do it that I've missed.  I pored over
the
> weave tutorial and the auto-generated C++ code, but I couldn't really
find
> any ideas for a better solution.

The attr method will work:

###################################
from weave import inline

class bunch: pass

def oaccess():
     x=bunch()
     x.a = 1
     code = """ 
            // Try to emulate Python's: print 'x.a',x.a
            std::cout << (int)x.attr("a") << (int)x.attr("a") <<
std::endl;
            """
     inline(code,['x'])

oaccess()

The attr method returns a py::object.  Notice you have to cast this
value to an integer before writing it out.  I pretty sure this could
actually be fixed by overloading the << operator for py::object --
please do if you feel comfortable trying it.

eric