[C++-sig] call policies

[David Abrahams]

Stefan Seefeld <stefan.seefeld at orthosoft.ca> writes:

> hi there,
> 
> I'm reading up on call policies but I'm not sure my case is
> covered there:
> 
> I'v got an object handed back from a factory call. I pass
> this object to another object where the second one takes
> over ownership:
> 
> Foo *foo = FooFactory();
> Bar *bar = BarFactory(foo);
> 
> (with either
> 
> Bar *BarFactory(Foo *);
> 
> or
> 
> Bar *BarFactory(auto_ptr<Foo>);
> 
> signature. How do I express that in boost.python ? The
> 'custody_and_wart' policy expects two arguments, the object
   ^^^^^^^^^^^^^^^^

A very interesting re-spelling, to be sure!

> to be consumed as well as the consuming one. In my case
> the consumer is created in the call. Which policy do I need ?

Well, there's no provision in any of the CallPolicy models for
handing ownership for your C++ object away from a Python object. Think
about it: the C++ object is embedded inside some Python object. How do
you wrest its lifetime management away from that object? You can't.

However, if your C++ object is held by smart pointer, you can get
shared or sole ownership back from the Python object. So for example,
if you declare your Foo class_<> with auto_ptr<Foo> as its Holder, the
corresponding Python object can be passed to a wrapped function which
expects an auto_ptr<Foo> argument (which will cause the Python object
to lose ownership), and your FooFactory function can return
auto_ptr<Foo>. I suggested that FooFactory returns auto_ptr<Foo>
explicitly, because manage_new_object uses boost::shared_ptr<> as a
workaround for one broken compiler (Intel C++ 5), though
manage_new_object would work find for any other compiler.

   Bar* BarFactory(std::auto_ptr<Foo> foo);

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


HTH,
Dave

-- 
                    David Abrahams
dave at boost-consulting.com * http://www.boost-consulting.com

Building C/C++ Extensions for Python: Dec 9-11, Austin, TX
http://www.enthought.com/training/building_extensions.html