Pass C++ object contained in a smart pointer to Python

Question

I have a class in C++.
I create an object from this class in my C++ code. I want this object to be accessible in Python. I use boost::shared_ptr to keep the object address.

I've checked out some posts about this but wasn't very helpful. I think the best way is to make an object in Python namespace after interpreter initialization and then assign my boost shared_ptr to the created object in Python.

I've wrapped my class using BOOST_PYTHON_MODULE in cpp and tested some ways like namespace["my_module_name_in_python"] = class<"my_class">... to be able to create an object in python and fill it with the shared_ptr.

In summery my question is how's possible to pass a C++ object contained in a shared_ptr to python.

Thanks in advance

Have you seen that: wiki.python.org/moin/boost.python/SimpleExample ? — elmo
– elmo, Commented Sep 18, 2013 at 14:00
Can you dereference the object and pass it by value? Can you pass a weak pointer instead? Do you need python to keep a refcount? — Brian Cain
– Brian Cain, Commented Sep 18, 2013 at 14:15
@Constantinius: it is or I don't understand question. From what I understand question boils down to "how do I expose C++ class to/share object with Python?". And that is all documentation on that wiki. As for handling pointers there is wiki.python.org/moin/boost.python/PointersAndSmartPointers As for "weird" construction there is make_constructor: wiki.python.org/moin/boost.python/HowTo#A.22Raw.22_constructor — elmo
– elmo, Commented Sep 18, 2013 at 14:36
If possible, could you please provide an sscce? As it stands, I am having problems understanding the actual problem needing to be solved. — Tanner Sansbury
– Tanner Sansbury, Commented Sep 18, 2013 at 19:21

Constantinius · Accepted Answer · 2013-09-18 14:10:33Z

1

This is taken from the official boost python documentation.

Lets say you have a C++ class that looks like this:

class Vec2 {
public:
    Vec2(double, double);
    double length();
    double angle();
};

You can derive the Python interface for it like that:

object py_vec2 = class_<Vec2>("Vec2", init<double, double>())
    .def_readonly("length", &Point::length)
    .def_readonly("angle", &Point::angle)
)

The class definition is now stored in the variable py_vec2. Now I can create an instance of said class with:

object vec_instance = py_vec2(3.0, 4.0);

This instance can now be injected to a Python interpreter. E.g set it into a variable of the "__main__" module:

object main_module = import("__main__");
object main_namespace = main_module.attr("__dict__");

main_namespace["vec_instance"] = vec_instance;

answered Sep 18, 2013 at 14:10

Constantinius

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1 Comment

Novin Shahroudi Over a year ago

Cool! I've tried this though I have a problem with this method too. My class contains different types which forces me to define them too. When you define all these in a BOOST_PYTHON_MODULE(<module_name>) everything is fine but with the above mentioned method I don't know how nested objects should be defined. I hope it's been clear.

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