> But the big problem with this is that you can't ever add a new field to Bar
Yes you can, if it's just an interface and the fields need to exist individuals in each message. If you have a field in bar call "theThing", and it's id is 7 in one message, and 14 in another that's fine, as long as it exists. simple for the compiler to check that all fields in bar exist in any message that "extends/inherits" it. at runtime it's just a interface you can slap on any message that uses it. If you want to add a new field to bar, you have to add it to all the messages individuals, and their ids will be specific to that message.... at least that's how I'd do it.... On Monday, July 20, 2020 at 5:00:31 PM UTC-7 [email protected] wrote: > If you have a struct Foo whose fields are a strict superset of some other > struct Bar (with matching field numbers and everything), then you can > convert one to the other like: > > Foo::Reader foo = ...; > Bar::Reader bar = capnp::AnyStruct::Reader(foo).as<Bar>(); > > (Or vice versa.) > > But the big problem with this is that you can't ever add a new field to > Bar, because it would force you to renumber all the later fields of Foo, > which would be a backwards-incompatible change. > > In general I instead recommend that Foo should contain a field of type > Bar. This seems to suit most use cases just fine. > > BTW, note that RPC interfaces in Cap'n Proto do support inheritance. It's > only structs that don't. > > -Kenton > > On Sun, Jul 19, 2020 at 11:08 PM <[email protected]> wrote: > >> >> I understand real inheritance is a hornets nest or complex weirdness, and >> I don't need that. >> >> but could we add a really simple version? >> >> like all the msgs have a common 5 fields in them. make the common fields >> an interface that all my msgs inherit from? >> >> I don't expect to ever instantiate a concrete version of the "base" >> class. I just want to pass the derived msgs to a function that expects the >> base-type. >> >> -- >> You received this message because you are subscribed to the Google Groups >> "Cap'n Proto" group. >> To unsubscribe from this group and stop receiving emails from it, send an >> email to [email protected]. >> To view this discussion on the web visit >> https://groups.google.com/d/msgid/capnproto/e08ca0bc-b386-4d04-b77d-569be721009do%40googlegroups.com >> >> <https://groups.google.com/d/msgid/capnproto/e08ca0bc-b386-4d04-b77d-569be721009do%40googlegroups.com?utm_medium=email&utm_source=footer> >> . >> > -- You received this message because you are subscribed to the Google Groups "Cap'n Proto" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To view this discussion on the web visit https://groups.google.com/d/msgid/capnproto/43bf6eef-1856-405d-8b6e-db638f36c769n%40googlegroups.com.
