In proto2, you could use `has_foo()` to check if `foo` is present, even for integer types. You could also specify what the default value should be, so you could

In proto2, you could use `has_foo()` to check if `foo` is present, even for integer types. You could also specify what the default value should be, so you could specify e.g. a default of -1 or some other invalid value, if zero is valid for your app. Unfortunately, proto3 removed both of these features (`has_` and non-zero defaults). I personally think that was a mistake. I'm not sure what proto3 considers idiomatic here. Proto3 is after my time. Cap'n Proto also doesn't support `has_` due to the nature of the encoding, but it does support defaults. So you can set a default of -1 or whatever. Alternatively, you can declare a union like: # (Cap'n Proto syntax) foo :union { unset @0 :Void; value @1 :Int32; } This will take an extra 16 bits on the wire to store the tag, but gets the job done. `unset` will be the default state of the union because it has the lowest ordinal number. I suppose in proto3, you ought to be able to use a `oneof` in a similar way.