How to get member uvalue of Derived class, from Base*

[MineBill]

Hello,

I have a vector of Base* and using resolve_type, i can successfully get members of Derived classes but when checking with member.is_gettable_with using that Base*, or a Base& it's always false. It'll only work when providing the type of the Derived class(using static_cast<DerivedA*>(base_ptr)) but at that point, it's not so dynamic any more. Is this not supported yet or am i missing something. I went through the docs multiple times but i couldn't find anything.

In any case, thank you so much for this library!

[MineBill]

After some investigation, it looks like resolve_type and ucast are the only places that take into account that the type might have poly info. Everything else, probably get and is_gettable_with too, will only check with the Base and ignore poly info.

[BlackMATov]

Hello and thanks for your interest in this library!

So, yes, you made a good research. We don't have enough dynamic functions to manage your case.

We can't using instance's poly info in functions like is_gettable_with or is_invocable_with, because it will cause a lot of problems with multiple inheritance, virtual functions, etc. This functions can only rely on the current type of the instance and use upcasts as regular C++ does.

To resolve your problem, I think we should have an additional ucast with dynamic type destination. It can look like this:

template < typename From >
uvalue ucast(From&& from, const class_type& to);

Where From is a pointer or uvalue with a pointer. Also, we have to return uvalue because we don't know static To type.

I believe with this, you can cast your base instance to the most derived class and call whatever you want.

I'm going to think about it. And I will be glad to any suggestions!

[MineBill]

Thank you for taking time to comment!

So, with this additional ucast i should be able to do something like this:

...
class_type klass = resolve_type(base_ptr);
uvalue var = klass.get_member("var").get(ucast(base_ptr, klass));
...

Honestly, i would be more than happy if i could just get a void* to the field/function.

To give you more context, what I'm trying to achieve is to get pointers to members of component classes that derive from a base Component class and give them to the appropriate ImGui::Input* function in imgui. The only "missing" part is getting that pointer. I believe the member_type gives me enough information to figure out the actual type, which i can then cast the pointer to before calling into imgui.

[BlackMATov]

Added new meta_poly_ptr method to META_HPP_ENABLE_POLY_INFO macro.

It returns a pointer of the most derived type, which can be used as an instance for member access. See a little example with comments here: https://github.com/BlackMATov/meta.hpp/blob/main/develop/untests/meta_issues/github_discussion_88.cpp

I believe it should help! :-)

[MineBill]

Wow! Thank you so much for implementing this so quickly! It works as expected now! Thanks!