IDL41 — IDL should support empty structures

Supporting empty structures is desired to facilitate the mapping from other data-models that may contain them. In addition the type system in XTYPES already supports them but it is not possible to express it in IDL.

We have found many practical situations where people create empty structures. It may be used these are temporary place holders. Or they can be used as markers for specialized classes that represent different types all inheriting from a common base class.

In all these cases when going to IDL it becomes necessary to add a dummy member like:

struct MyEmptyStruct {
    octet __dummy_member;
};

Note that C and C++ support empty structures. So this should not create problems in the language mappings.

Change <struct_def> in BB Extended Data-types

In BB Extended Data-types, structure definition is already modified to support single inheritance (cf. rule (195)). Complement that extension with the ability of defining void structures.