>From: "Gennaro Prota" <[EMAIL PROTECTED]> > On Tue, 28 Jan 2003 10:47:52 -0800, "Andrei Alexandrescu" > <[EMAIL PROTECTED]> wrote: > > >"Peter Dimov" <[EMAIL PROTECTED]> wrote in message > >> While we're at it, is the final verdict that is_base_and_derived<void, X> > >> should be false? What about is_base_and_derived<void, void>? > > > >Well, clearly void is no base. Even if we also define is_super_and_subtype, > >void is hardly a supertype of everything. > > Could you please clarify this? You mean: > > T is a subtype of U <=> ...?... > > T is a supertype of U <=> ...?...
Supertype/subtype is a more general notion than inheritance and base/derived. The Liskov Substitution Principle states: "What is wanted here is something like the following substitution property: If for each object o1 of type S there is an object o2 of type T such that for all programs P defined in terms of T, the behavior of P is unchanged when o1 is substituted for o2 then S is a subtype of T." Notice that it doesn't say anything about "class" or "inheritance", anywhere. If S = T, LSP still holds. Therefore, as Andrei said it, in language theory, such as this, a type may be considered its own supertype/subtype. Supertype/subtype may also have other forms than inheritance. "short" may be considered a subtype of "int", for example (the opposite may or may not be true, depending on if they are of equal size). Likewise, as Kevlin Henney noted in an article (http://www.cuj.com/experts/1901/henney.htm), a class's "const" interface may be considered a subtype of the whole interface. Notice that, as Kevlin notes in the article, for all these cases, LSP is expected by the language: A derived object or pointer/reference is implicitly converted to a Base object or pointer/reference (but going the other way requires a cast), short is convertible to int (and vice versa, in this case), and an object or pointer/reference to an object is implicitly converted to a const object or pointer/reference (but again, going the other way requires a cast). Given that supertype/subtype has such a general meaning, how should an is_super_and_subtype be defined? I guess the proposal mean to define it in terms of inheritance, only, and in that case, it would work like is_base_and_derived, with the addition that a type is its own supertype/subtype (but not its own base/derived type). This is also reflected in Loki: SuperSubclass is like is_super_and_subtype, and SuperSubclassStrict is like is_base_and_derived. Regards, Terje _______________________________________________ Unsubscribe & other changes: http://lists.boost.org/mailman/listinfo.cgi/boost
