AW: [std-proposals] Base class reflection

Hello, I would like to propose an addition to the standard library, that will allow one to get the immediate base classes of a type, e.g.:

using std::base_class_tuple_t<T> = std::tuple<Bases&...>;

Requires: T is a complete class type.

Result: an std::tuple type, where the tuple element types are the reference types to the immediate base classes of T, if T is a class type with base classes, or the empty tuple type otherwise. The references would have the same cv-qualification as T.

Why: There is currently no way to do this (that I'm aware of) in C++. It would require some compiler magic to implement this type. (note: I don't know if it's customary or not to have compiler magic in the standard library, maybe this could be a keyword instead, similar to typeid)

This is necessary to solve the problem of mixing polymorphism and type erasure. Suppose I have a type erased pointer to some polymorphic object of type D. I don't know about the type D. But I know the type B, which is a base class of D. I would like to get a B* pointer to D. In general, this can't be done as far as I know. (Usually with type erasure you need to know the *exact* original type of the type erased thing)

However, if std::base_class_tuple_t exists, then the type erasure system could be set up so that you could request a type erased pointer to B, and it could search the base classes of D for the type B, and if it finds it then perform the pointer conversion and return a type erased pointer to B, which would could then extract B* from.

It seems like this wouldn't be too hard to implement since the compiler ought to know the immediate base classes of a complete type.

Why reference types? Some of the bases might be pure virtual, and I don't yet know how this might fit into a greater reflection library. It should be possible, at least in principle, to create an object of type std::base_class_tuple_t. Maybe we could also someday add other tuple types to ask other questions about what the compiler knows about a type? Maybe a non-static member tuple?

Billy