[C++-sig] Re: return value policy for returning same python object...

[David Abrahams]

"Milind Patil" <milind_patil at hotmail.com> writes:

> I have a scenario where a C++ method that performs inplace
> operation on the object is wrapped using boost python. I
> would like the corresponding python operation to result in the same object
> instead of a new python object. What is the return policy that
> will allow me to do that?
>
> eg. in Y_Wrapper class of some Y class...
>
> const Y&
> do_iadd (int other)
> {
>   *this += Y_FromInt(other);
>    return *this
> }
>
> BOOST_PYTHON_MODULE(hello)
> {
>   ...
> .def("__iadd__", (const Y& Y_Wrapper::*) &do_iadd(int),
                   ^^^^^^^^^^^^^^^^^^^^^^^
This C-style cast is dangerous, and in fact you got it wrong, which is
part of your problem.  No cast should be needed.

> return_value_policy<>(??? which one ???))
> ...
> }
>
> in python...
>
> x = y.hello(0)
> a = x
> x += 1
>
> assert( a is x)
>
> I tried return_existing_object and the copy_... policies. Obviuosly,
> because they all return a new python object, that failed.

It's not obvious to me, though return_existing_object should be
unsafe I would expect it to work.

> Any pointers as to how I can get the desired behaviour?

Well, you are returning a reference to an internal object of the
Python Y object, so return_internal_reference seems entirely appropriate.

> (Notice I cannot do
> .def (self += other<int>())
> because there is no += in the Y class for Y& and int defined.)

Well, that *should* be easily fixable!

> Thanks,
> Milind

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