CODEWITHSUNDEEP
c++pybind11
Georgia S
Georgia S

Reputation: 622

PyBind11: Accessing a unique pointer attribute in Python

I have a c++ class that has a unique pointer to an object as an attribute. I would like to be able to access that attribute in Python so I can call its class methods. Here is an example of my setup:

class OuterClass {
public:
   std::unique_ptr<InnerClass> obj;
}

class InnerClass {
public:
    void method();
}

And the pybind11 bindings:

PYBIND11_MODULE(example, m) {
    py::class_<InnerClass>(m, "InnerClass")
        .def("method", &InnerClass::method);

    py::class_<OuterClass>(m, "OuterClass")
        .def_readonly("obj", &OuterClass::obj);
}

However, this fails to compile with the error error: no matching function for call to ‘pybind11::cpp_function::cpp_function(std::unique_ptr<InnerClass> OuterClass::*, pybind11::name, pybind11::is_method, pybind11::sibling)

I would like to be able to do this in python:

outer_obj.obj.method()

The docs for unique ptr does not seem to think this should be a problem as they give an example of a function that returns a unique ptr. Is trying to access a unique ptr attribute the issue? How can I get around this?

Upvotes: 3

Views: 1189

Answers (1)

Piotr Barejko
Piotr Barejko

Reputation: 648

Use def_property with lambda to return the pointer:

class InnerClass {
public:
    void method()
    {
        std::cout << "OK" << std::endl;
    }
};

class OuterClass {
public:
    OuterClass()
        : obj(std::make_unique<InnerClass>())
    {
    }

    std::unique_ptr<InnerClass> obj;
};


PYBIND11_MODULE(example, m) {
    py::class_<InnerClass>(m, "InnerClass")
            .def("method", &InnerClass::method);

    py::class_<OuterClass>(m, "OuterClass", py::dynamic_attr())
            .def(py::init<>())
            .def_property_readonly("obj", [](const OuterClass& o){return o.obj.get();}, py::return_value_policy::reference_internal);
}

Upvotes: 7

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