[C++-sig] set python attribute from C++

[David Abrahams]

on Thu Jul 17 2008, Gennadiy Rozental <rogeeff-AT-gmail.com> wrote:

> David Abrahams <dave <at> boostpro.com> writes:
>> > Why is that by the way? It seems to me that 
>> > intrusive_ptr is treated like
>> > distant relative to shared_ptr, while the only real 
>> > difference is in storage policy for the counter.
>> 
>> It doesn't have a deleter.
>
> Is there some some strong underlying reason why it's can't
> be designed to support it?

There are 2 "it"s in that sentence and I can't tell what either one
refers to.

>> I'm not entirely convinced that you need your own home 
>> grown smart pointer in the particular code we're 
>> discussing.  Any wrapped T is convertible to 
>> shared_ptr<T>, no matter what method is used to 
>> "hold" the T in the class_<T, ...> declaration.
>
> Not sure i understand what you mean here. Let's say I've 
> exported my class like this:
>
> class<A,my_intrusive_smart_ptr<A>,...>
>
>
> Now you are saying I can write the function foo like this:
>
> void foo( shared_ptr<A> o )
> {
>  ....
> }

Yes.

> What will happened with memory? 

I don't understand the question.  Could you be specific?

> Or custom deleter will somehow make it right?

It will be right.  It Just Works (TM)

> Tn any case it'll be quite confusing for unaware reader.

What can I tell ya?  Sometimes you need to add a few special  C++
functions that will be wrapped solely for exposure to Python.  Maybe
that's what you should do here.

>> > Ok. Here is my latest attempt schematically:
>> >
> ...
>> > class_<Base,intrusive_ptr<Base>,noncopyable)( "Base" )
>>               ^^^^^^^^^^^^^^^^^^^
>> 
>> This automatically ensures that you can 
>> convert Python objects derived from the Python 
>> Base into an intrusive_ptr<Base>. Converting in 
>> the other direction works too, but it produces 
>> a new Python object.
>
> Yeah, it also produces object of wrong type. If I've 
> got bp::object corresponding to the Python class 
> Derived(PythonBase) bp::object constructed from 
> pointer will hold different object of type PythonBase.

Yes.  I notice, by the way, that you didn't specify bases<Base> in your
class_<PythonBase, ...>

>> >      }
>> >
>> > private:
>> >      bp::object m_object;
>> > };
>> >
>> > class_<PythonBase,PythonBase*,
>>                     ^^^^^^^^^^^
>> Very dangerous; could lead to leaks of C++ 
>> objects.  However if you want the C++ object 
>> to own a Python object, it seems like an option.
>> 
>> > ....
>> > ;
>> >
>> > Base*
>> > PythonFactory::create(...) 
>> > {
>> >    bp::object obj = m_py_type();
>> 
>> If m_py_type is the PythonBase Python class, 
>> there is nothing managing
>
> No. m_py_type is bp::object corresponding to 
> the Derived type (there is whole registration 
> phase which I omitted in this discussion)

Then I can't tell whether my statement below is true or not.

>> the C++ PythonBase object's lifetime.
>
> C++ object lifetime is managed by my_intrusive_ptr 
> it gets assigned to  as a result of create function. 
> Not really sure why create function have to return 
> raw pointer instead of smart pointer - it predates me.
>
>> The fact that PythonBase is
>> derived from Base and base is held by an 
>> intrusive_ptr makes no difference.  OTOH, If 
>> m_py_type is the "Base" python class, the following 
>> extract should fail.
>> 
>> > PythonBase* ptr = bp::extract<PythonBase*>( obj );
>
> obj should be of type Derived. This extract should work fine.

OK

>> > On python side I've can now derive from PythonBase:
>> >
>> > class Derived(PythonBase):
>> >     pass
>> >
>> > Now i am seems to be able to create instances of 
>> > Derived class and set their
>> > field from c++. Does this solution make sence?
>> 
>> I'm not 100% certain, but it looks like it will leak 
>> C++ PythonBase instances.  You might try putting some 
>> tracing statements in the ctors and dtors to see if 
>> they are actually going away when they should.
>
> Did just that. Seems to be fine in my unit tests. 
> Not very comprehensive yet though.

OK

-- 
Dave Abrahams
BoostPro Computing
http://www.boostpro.com