LOKI C++: How does 'ScatterHierarchyTag' solve the inheritance ambiguity?

Question

I am going through Chapter 3 of Alexandrescu's Modern C++ Design. It explains the implementation of HierarchyGenerators.h, however changes have been made to the library since then.

I am unable to understand how the ScatterHierarchyTag solves the ambiguity in resolving Field function (the function allows us to access members defined by particular Base<type>, for example Field<int>(Object) would get us a &Base<int> to Object), when the typelist used to generate hierarchy has duplicates. (ex: GenScatterHierarchy<TYPELIST_4(int,int,string,Widget), Base> Object) I see that it adds a unique "type/tag" in each of the branches, right before the root Base<type> class, but still ambiguity is there right?

Thank you in advance.

EDIT:

Chart showing multiple inheritance issue with repeated types in typelist. GSH = GenScatterdHierarchy

I believe the unique tags are inserted seperately, for each of the two connections to Base<int> (marked one by blue line) (same goes for other base classes at root)

Also, I inserted picture to make it easier for everyone to understand. Let me know if it does not fit in stackoverflow's guidelines and I shall remove it.

Answer 1

ScatterHierarchyTag is not removing ambiguity. What it does, is making an ambiguous base class accessible.

Consider the following class hierarchy:

class A {};
class B : public A {};
class C : public A, public B {};

Class C contains two copies of class A (it makes more sense when A is not empty :), one is created because of direct inheritance, another — because of indirect inheritance through B. You can access the second instance converting to B first:

A &indirect_base = static_cast<B>(C_instance);

For the other instance, there is simply no way of accessing it. Because of that, it is called "inaccessible base".

This example embeds naturally in class hierarchy created by GenScatterHierarchy:

1. GenScatterHierarchy<int, Base> is A,
2. GenScatterHierarchy<TYPELIST_3(int,string,Widget)> is B,
3. GenScatterHierarchy(TYPELIST_4(int,int,string,Widget)) is C.

So, instance of Base<int> created for the first int in type list is an inaccessible base. However, if ScatterHierarchyTag is added, we can access it by casting to GenScatterHierarchy<ScatterHierarchyTag<int, TYPELIST_3(int,string,Widget)>, Base> first.