Deprecate GDExtension's object_cast_to and classdb_get_class_tag, in favour of is_class casts.

[Ivorforce]

• Follow-up to Change Object::is_class to take StringName instead of String, for better performance #118582
• Follow-up to Core: Add initial architecture for first-class Object types. Optimize is_class #105793
• Started via Optimize Object::cast_to<T>() using Object::is_class() now that it takes a StringName godot-cpp#1974

Changes the recommended way to cast to object && object->is_class(T::get_class_name_static()) ? object : nullptr, which is simpler and more predictable than the previous class-tag based approach.

We can do this now because is_class became perfectly performant — in fact, it can be faster than class-tag casting by #118582 and #105793

[dsnopek]

Thanks!

I agree 100% with marking these as deprecated, but have some notes about the message text.

[dsnopek]

Thanks, these changes look good to me!

However, it just occurred to me that we should also wrap the implementation and registration of these functions in #ifndef DISABLE_DEPRECATED like we do for other deprecated function (see gdextension_interface.cpp)

[Ivorforce]

You're totally right, I forgot about that part 😅

[dsnopek]

Looks good to me!

Since this only marking something as deprecated (which should be safe) and the replacement is due to a change in Godot 4.7, I think we should target this for Godot 4.7

[Bromeon]

Could you describe the migration path outside of godot-cpp? is_class() is just a class check, it doesn't provide casting functionality.

godotengine/godot-cpp#1974 now uses is_class() + static_cast, but the latter doesn't exist in all languages (C, Go, Rust...). Since bindings can no longer obtain a pointer through object_cast_to(), will the original pointer always be valid? Or is the idea that downcasts are implemented purely on binding side?

[dsnopek]

object_cast_to() never did any actual casting. All it did was check if a cast was possible, and if so, return the same pointer; if not, return nullptr. So, the recommendation here is just to do the check with is_class() instead. Any actual casting has always been handled on the binding side

[Ivorforce]

Could you describe the migration path outside of godot-cpp? is_class() is just a class check, it doesn't provide casting functionality.

godotengine/godot-cpp#1974 now uses is_class() + static_cast, but the latter doesn't exist in all languages (C, Go, Rust...). Since bindings can no longer obtain a pointer through object_cast_to(), will the original pointer always be valid? Or is the idea that downcasts are implemented purely on binding side?

Yes, downcasts are expected to be implemented purely on the binding side. You can expect the pointer to be stable (T * will always be equal to Object *).

[Bromeon]

Thanks, that makes sense.

Should the deprecation message mention that the original pointer can be used? (The previous API was counter-intuitive, could have just returned a bool instead of a pointer).

[Ivorforce]

I added something along the lines, please take a look.

[Bromeon]

Looks good to me, thanks!

Btw, nice branch name 😀

[Repiteo]

Thanks!