Is there a way to turn Base class instance into Derived class instance?

[JiauZhang]

// c++ code
class Base {
public:
	...
	Base getitem(int idx);
}

// bind to python
nb::class_<Base>(m, "Base")
	.def("getitem", &Base::getitem);

# python code
from xxx import Base

class Derived(Base):
	def __getitem__(self, idx):
		return self.getitem(idx)

	def some_fn(self, *args):
		...

foo = Derived()
bar = a[0]

Finally, type(foo) is Derived, type(bar) is Base. Is there a way to turn the variable bar into Derived class instance?

[wjakob]

Not if returning by-value. In this case, a copy is made that forgets its Python identity.

[JiauZhang]

Is there any way to create a new python Derived object in c++, and copy Base class instance data to it?

[wjakob]

nb::object inst = nb::type<MyType>(.. constructor binding arguments ..);
MyType &ref = nb::cast<MyType &>(inst);
ref = ...

[wjakob]

Actually, let me fix that.

nb::object inst = nb::module_::import_("my_module").attr("Derived")(.. constructor arguments ..);
MyType &ref = nb::cast<MyType &>(inst);
ref = ...