[C++-sig] Re: smart pointer and polymorphism problem

[Raoul Gough]

Christophe Pradal <christophe.pradal at cirad.fr> writes:

> I use an intrusive refcount pointer as described by Scott Meyers in
> "More effective C++" and improved by adding template casting operator
> (Scott Meyers again, but later).
> Considering RefCountObject as the base class for pointed objects and
> RefCountPtr the smart pointer template, I built a small example
> reproducing the error:
>
> class O: public virtual RefCountObject
> {
> public:
>   O(): RefCountObject() {}
>   virtual ~O() {}
>   virtual int get() = 0;
> };
>
> class A: public O
> {
> public:
>   int _a;
>   A(): O(), _a(1) {}
>   virtual ~A( ) {}
>  int get() {return _a;}
> };
>
> typedef RCPtr<O> OPtr;
> typedef RCPtr<A> APtr;
>
> int foo(const OPtr& p) { return p->get(); }
>
> BOOST_PYTHON_MODULE(test)
> {
>   class_< O, OPtr, boost::noncopyable >("O", no_init);
>   class_< A, APtr, bases<O>,boost::noncopyable >("A", init<>());
>   def( "foo",foo );
> };
>
> In python, the following test fail:
>
>  >>> import test
>  >>> a= test.A()
>  >>> test.foo(a)
> Traceback (most recent call last):
>   File "<stdin>", line 1, in ?
> Boost.Python.ArgumentError: Python argument types in
>     test.foo(A)
> did not match C++ signature:
>     foo(class TOOLS::RefCountPtr<class O>)
>
>
> Any idea?

When there is an implicit conversion between types (other than derived
to base conversions, I mean), you can register it with something like:

boost::python::implicitly_convertible<A, O>();

Of course, in this case you already have this conversion since you
registered O as a base class of A. I notice that foo takes a smart
pointer rather than a reference to O, so maybe you would need this:

boost::python::implicitly_convertible<RefCountPtr<A>, RefCountPtr<O> >();

On the other hand, I'm not sure if that will work because Ptr<A> and
Ptr<O> are only held types and not the actual class_<> types. It's
worth a go I suppose, but you wouldn't have any problem if foo took an
O &.

-- 
Raoul Gough.
export LESS='-X'