From ndevenish at gmail.com  Thu Jun 27 12:32:42 2019
From: ndevenish at gmail.com (Nicholas Devenish)
Date: Thu, 27 Jun 2019 17:32:42 +0100
Subject: [C++-sig] Getting the same python instance back in boost::python?
Message-ID: <CALVyzO9E+PF9U-XyAGkkPMM7yd+juhobfwPxwgjSZfBX2-vNag@mail.gmail.com>

I want to get the same python instance back for subsequent calls to
the C++ function e.g. I want this to work for the C++ structure below:

    import ident_ext
    o = ident_ext.Owner()
    assert o.get() is o.get()

How can I get this behaviour in boost::python? It looked like call
policies was the right sort of place to look, but can't get that to
behave how I want. I can get this to work in pybind11 but not boost
(and unfortunately our project is heavily dependent on boost::python).
My working c++ is:

#include <boost/python.hpp>
#include <iostream>
using namespace boost::python;

class Something {
public:
void name(void) { std::cout << this << std::endl; }
};

class Owner {
Something *_shared = nullptr;

public:
Something &get() {
if (_shared == nullptr) {
_shared = new Something();
}
return *_shared;
}
};

BOOST_PYTHON_MODULE(ident_ext) {
class_<Something>("Something").def("name", &Something::name);
class_<Owner>("Owner").def("get", &Owner::get, return_internal_reference<>());
}

From stefanrin at gmail.com  Fri Jun 28 13:22:55 2019
From: stefanrin at gmail.com (Stefan Ring)
Date: Fri, 28 Jun 2019 19:22:55 +0200
Subject: [C++-sig] Getting the same python instance back in
 boost::python?
In-Reply-To: <CALVyzO9E+PF9U-XyAGkkPMM7yd+juhobfwPxwgjSZfBX2-vNag@mail.gmail.com>
References: <CALVyzO9E+PF9U-XyAGkkPMM7yd+juhobfwPxwgjSZfBX2-vNag@mail.gmail.com>
Message-ID: <CAAxjCEzR5cS4dFUX8T4R9_QAsng+PS6DD380=kN8YPAmSCYvCQ@mail.gmail.com>

On Fri, Jun 28, 2019 at 2:16 PM Nicholas Devenish <ndevenish at gmail.com> wrote:
>
> I want to get the same python instance back for subsequent calls to
> the C++ function e.g. I want this to work for the C++ structure below:
>
>     import ident_ext
>     o = ident_ext.Owner()
>     assert o.get() is o.get()
>
> How can I get this behaviour in boost::python? It looked like call
> policies was the right sort of place to look, but can't get that to
> behave how I want. I can get this to work in pybind11 but not boost
> (and unfortunately our project is heavily dependent on boost::python).
> My working c++ is:
>
> #include <boost/python.hpp>
> #include <iostream>
> using namespace boost::python;
>
> class Something {
> public:
> void name(void) { std::cout << this << std::endl; }
> };
>
> class Owner {
> Something *_shared = nullptr;
>
> public:
> Something &get() {
> if (_shared == nullptr) {
> _shared = new Something();
> }
> return *_shared;
> }
> };
>
> BOOST_PYTHON_MODULE(ident_ext) {
> class_<Something>("Something").def("name", &Something::name);
> class_<Owner>("Owner").def("get", &Owner::get, return_internal_reference<>());
> }

Maybe store a wrapping bp::object in addition to the Something ptr,
then always return the same bp::object? That?s how I did it. Ok, you
would probably have to create a wrapper for Something just for this. I
don?t know how feasible this is.