CODEWITHSUNDEEP
pythonc++pybind11weak-referencesreference-lifetimes
VarShrm
VarShrm

Reputation: 1

Giving C++ sole ownership of class object in pybind11

I have an application in which I want to expose a C++ class MyClass to python using pybind. Since I want python to be embedded within c++, I use PYBIND11_EMBEDDED_MODULE to expose my c++ class.

class MyClass
{
    void method1();
    void method2();
}

PYBIND11_EMBEDDED_MODULE(myModule, m)
{
    py::class_<MyClass>(m, "MyPyClass")
    .def(py::init())
    .def("method1()", &MyClass::method1)
    .def("method2()", &MyClass::method2)
}

Now, I want to it to work in such a way that I take an already instantiated instance of MyClass, say myInstance - and create the python instance from this. To achieve this, I use py::cast()

void doPython(MyClass* myInstance)
{
    py::module_ myModule = py::module_::import("myModule");

    py::object myPyInstance = py::cast(myInstance);

    ... // work with myPyInstance further
}

Now, the issue that I'm trying to solve is - I want my c++ application to have sole ownership of myInstance. I want myPyInstance to have nothing to do with the lifetime of myInstance. And that if myInstance is destructed at some point, myPyInstance should know about it and handle that case.

Is there some kind of weak ref workflow that I can use? pybind natively allows for unique_ptr or shared_ptr workflow, but none of those work for my case.

The custom smart pointer workflow sounds promising, and I pursued a solution where the smart pointer held a holder to myInstance. I hoped that .get() on the smart pointer would be called when the individual methods are called, allowing the smart pointer to check whether myInstance is alive or not. But it looks like .get() is called when cast() is called and the result is held onto.

There is return_value_policy::reference but it doesn't handle the case of the object being deleted by the c++ application while still being referenced by python, and this is a crucial case for me.

Perhaps py::capsule? but that turns out to be too verbose

Upvotes: 0

Views: 61

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