Re: Sugestion to Multiple Inheritance

[Richard Riehle <richard@adaworks.com>]

Lutz Donnerhacke wrote:

> The rationale suggests three solutions in chapter "4.6  Multiple Inheritance":

None of those suggestions in the Rationale satisfies the requirements
for multiple inheritance.  They are simply workarounds.   There is
no MI in Ada, period.   That is not a bad thing, but it is a fact.

MI, as with any inheritance, requires satisfying the Liskov Substitutatility
Priniciple.  None of the workarounds do.    MI implies a corresponding
available of polymorphic types. None of the workarounds do.    MI
implies dispatching, dynamically,  on any derivation of a type in the
inheritance hierarchy.  None of the workarounds do.

In my view, MI is still a research topic.  There is no universally accepted
model, and no foolproof design, at this stage, in any language, that can be
said to solve all the problems.   Eiffel may come the closest to getting it
right, but even with Eiffel,  any discussion of MI requires a long and
detailed understanding of repeated inheritance and other entertaining
issues.    With C++, MI is probably a disaster waiting to happen.

Why would anyone want MI.  MI is most useful if you have a need for
polymorphic dispatching types. Grady Booch suggests that when you need
MI, you really need it.      The frequency with which one needs MI,
contrasted with the risks associated with it should make one pause
before deciding that it should be included in a language design
where software safety is an issue.    Tucker go this one right.

Richard Riehle