Derived class returned as a base class from C++ cannot use derived methods

[germandiagogomez]

Hello.

I have been using ChaiScript and I am migrating to Wren because of the fibers stuff that I need badly :)

First, thanks a lot for this project, it is making things much easier so far. I would like to point out one thing that worked in ChaiScript but does not in wrenbind17 that is related to inheritance.

I have registered the base class with its methods and the derived class with all methods, derived and base. I am using module.klass<derived, base>("derived") and registering the funcs I want.

So, let us say I have derived::derivedMethod in C++. The following will not work

// C++ code
shared_ptr<BaseClass> BaseClass::factory(std::string_view className) {
          return some_derived_class_from_base_class;
}

//wren code
_myInstance =  BaseClass.factory("derived")
_myInstance.derivedMethod()

Factory is returning a base class shared pointer but the real instance held is of type derived. In ChaiScript the derived methods will work, but in WrenBind17 the following error is shown:

exception: Runtime error: base does not implement 'derivedMethod()'

Any workaround? Could this be implemented in some way? Maybe I could help, but not sure how much time I will have available. Depends on difficulty. Thanks!

[matusnovak]

Hi @germandiagogomez I am currently busy but I will look at this soon.

[matusnovak]

Hi @germandiagogomez

The module.klass<derived, base>("derived") is used to do upcasting -> https://matusnovak.github.io/wrenbind17/tutorial/custom_types/#66-upcasting This won't solve your problem with calling derived methods.

In order to do that, you will have to:

• Create a base class with a virtual method.
• Create a derived class that overrides this method.
• Register both classes with Wren.
• Register that method twice, for both the base and derived class. (When you register any function to the base class, they are not automatically shown to the derived class.)

Essentially, use polymorphism.

I have added a test that shows this functionality, here: https://github.com/matusnovak/wrenbind17/commit/367bcaba8a772dbe2190f49ff410e4f6cc1d0ed0 Is this what you are looking for?

Automatically inheriting all registered functions from the base class(es) into the derived class is something I would like to implement at some point, but I would like to have it optional. The only problem that keeps me from doing that is that in Wren language you can only inherit from one class at the time [citation needed]. So when you have a C++ class that inherits from multiple classes it gets a little bit tricky.

[germandiagogomez]

Yes, that is exactly what I was looking for. I will close this issue. Thanks!