[C++-sig] pybindgen: Why does pybindgen want to call constructors for a singleton class?

[J. Michael Owen]

Ah, I see!  That does the trick!  Thank you very much.  I was clearly  
confused about what "caller_owns_return" meant  -- I had it backwards.

Mike.

On Jun 22, 2009, at 5:11 PM, Gustavo Carneiro wrote:

>
>
> 2009/6/22 J. Michael Owen <mikeowen at llnl.gov>
> I'm looking at wrapping a C++ singleton with pybindgen, and it seems  
> that if I expose the method for getting the instance to python the  
> generated code wants to call a copy constructor, which seems wrong  
> to me.  If for instance I define a class "A" as a singleton:
>
> class A {
> public:
>  static A* instancePtr() {
>    if (A::mInstancePtr == 0) A::mInstancePtr = new A;
>    return mInstancePtr;
>  }
> private:
>  A();
>  A(const A& rhs);
>  A& operator=(const A& rhs);
>  static A* mInstancePtr;
> };
>
> and I wrap A and its instance method like so:
>
> mod = Module("singleton_example")
> mod.add_include('"singleton_example.hh"')
> x = mod.add_class("A", is_singleton=True)
> x.add_method("instancePtr", retval("A*", caller_owns_return=False),  
> [])
>
> pybindgen generates the following code for the instancePtr method:
>
> PyObject *
> _wrap_PyA_instancePtr(PyA *self)
> {
>    PyObject *py_retval;
>    A *retval;
>    PyA *py_A;
>
>    retval = self->obj->instancePtr();
>    if (!(retval)) {
>        Py_INCREF(Py_None);
>        return Py_None;
>    }
>    py_A = PyObject_New(PyA, &PyA_Type);
>    py_A->obj = new A(*retval);
>    py_retval = Py_BuildValue((char *) "N", py_A);
>    return py_retval;
> }
>
> As you can see it is trying to construct a new object with a copy of  
> the A retval parameter (the line that reads "py_A->obj = new  
> A(*retval);".  This is of course forbidden because all of A's  
> constructors are private -- as a singleton we don't want anyone  
> calling constructors on this object.  Moreover, since I exposed  
> "instancePtr" as returning a pointer I did not expect any copies to  
> be generated anyway -- I'd like to see the pointer "retval" sent  
> back directly (even if this wasn't a singleton).  Am I missing some  
> syntax here to prevent pybindgen from trying to make these copies?
>
> It tries to copy because you say caller_owns_return=False, pybindgen  
> needs to copy the object so that the wrapper can own the object.   
> What you can do here is use caller_owns_return=True instead of  
> False.  It's OK in this case; because of the is_singleton option,  
> the C++ wrapped object will never be freed when the wrapper is  
> destroyed.
>
> -- 
> Gustavo J. A. M. Carneiro
> INESC Porto, Telecommunications and Multimedia Unit
> "The universe is always one step beyond logic." -- Frank Herbert
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