[C++-sig] Re: Making copy of function derived from C++ pure virtual function

[David Abrahams]

Paul F. Kunz wrote:
>>>>>> On Sat, 11 Dec 2004 09:40:20 -0500, David Abrahams
>>>>>> <dave at boost-consulting.com> said:
> 
>> Sufficiently for whom?  It took me 20 minutes of massaging just to 
>> get it to build under vc7.1 (where I have good debugging tools).
> 
> Sorry, I wasn't think about VC++
> 
>> Please reduce your problem to a *simple* test case with as few 
>> classes and functions as possible.  Take out any threading-related 
>> stuff.  There should be one .cpp file and one .py file.  Ideally, 
>> you'd pass me a Jamfile, but I can produce one myself if
>> neccessary.
> 
> Ok, one file, one Python script.   No includes other that C++ 
> standard library.
> 
> ftp://ftp.slac.stanford.edu/users/pfkeb/abrahams/HippoDraw-1.12.7.tar.gz

I'm not surprised by the behavior of this example.
You wrapped a class with a single constructor:

      class_ < FunctionWrap, std::auto_ptr < FunctionWrap > >
	( "FunctionBase",  init < const FunctionBase & > () )

	;

That constructor takes a single FunctionBase& parameter.  Leaving out
the irrelevant Python code, you derive a class:

  from hippo import FunctionBase
  class Linear ( FunctionBase ) :
      def __init__ ( self ) :
          FunctionBase.__init__(self)

But this is how you call a nullary (zero-argument) ctor on a base class!
Don't try

          FunctionBase.__init__(self, self)

it won't work.  There's no C++ FunctionBase object yet that can be
extracted from the 2nd argument.  You just need to add a Default ctor to
FunctionWrap and expose it.  You also need to get rid of
FunctionWrap(PyObject*) -- that's old-style polymorphism.

-- 
Dave Abrahams
Boost Consulting
http://www.boost-consulting.com