Conceptual Ques: at what point do C++ class members get copied to Python?

[siddancha]

When one creates bindings for C++ class members using PyBind11, when exactly does the member get copied over to Python? Is it when the object is referenced in Python, or when the object member is referenced?

Consider this simple example:

// C++
class MyClass
{
  public:
    Eigen::MatrixXd bigMatrix;
}

MyClass createBigMatrix();  // implementation creates a big matrix

PYBIND11_MODULE(wrapper, m)
{

    py::class_<MyClass>(m, "MyClass")
        .def_readonly("bigMatrix", &MyClass::bigMatrix)
        ;

    m.def("createBigMatrix", &createBigMatrix);
}

# Python

import wrapper
x = wrapper.createBigImage();  # <-- does MyClass::bigImage get copied from C++ to Python here?
print(x.bigImage);             # <-- or does it get copied here?

In other words, is it sensible to bind members that may never be referenced/used in Python? Or is it best not to because they will be copied regardless, wasting valuable compute time and memory?

Thanks!