[C++-sig] Function needs ownership of python object that is derived from a C++ class

[David Abrahams]

Thomas Kolb <chilavert123 at gmx.net> writes:

> Hi all,
>
> I have a C++ class Base that is declared as follows:
> 	struct Base
> 	{
> 		virtual int f() = 0;
> 	};
> and a free function call_f that needs ownership of the parameter:
> 	void Moses::call_f(Base* b)
> 	{
> 		std::cout << " b->f() = " << b->f() << std::endl;
> 		delete (b);
> 	}

It should take auto_ptr<Base> as an argument, if possible.

> On pyhton side I derive from Base like that:
>
> class Derived(Base):
> 	def f(self):
> 		return 42
>
>
> of course the following leads to a segmentation fault:
> 	d = Derived()
> 	call_f(d)
>
> First thing I tried was to do this auto_ptr stuff as described in FAQ 
> ("How can I wrap a function which needs to take ownership of a raw 
> pointer"), but this doesn't work because of:
>       Boost.Python.ArgumentError: Python argument types in
>       call_f(Derived)
>            did not match C++ signature:
>       call_f(std::auto_ptr<Moses::Base>)

Did you miss this part?  


  Make sure the C++ object is held by auto_ptr:

  class_<A, std::auto_ptr<A> >("A")
      ...
      ;

In your case, 

  class_<Base, std::auto_ptr<Base> >("Base")
      ...
      ;


> then I tried to wrap call_f as follows below:
> 	call_f_Wrapper(PyObject* p)
> 	{
>               handle<> hP(p);
>               Moses::Base* bP = extract<Moses::Base*>(object(hP));
> 	     call_f(bP);
>          }
> but this leads to segmentation faults too.

I would expect so.

-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com