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.
>
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