How to define an optional field in protobuf 3

Question

I need to specify a message with an optional field in protobuf (proto3 syntax). In terms of proto 2 syntax, the message I want to express is something like:

Answer 1

In proto3, all fields are "optional" (in that it is not an error if the sender fails to set them). But, fields are no longer "nullable", in that there's no way to tell the difference between a field being explicitly set to its default value vs. not having been set at all.

If you need a "null" state (and there is no out-of-range value that you can use for this) then you will instead need to encode this as a separate field. For instance, you could do:

message Foo {
  bool has_baz = 1;  // always set this to "true" when using baz
  int32 baz = 2;
}

Alternatively, you could use oneof:

message Foo {
  oneof baz {
    bool baz_null = 1;  // always set this to "true" when null
    int32 baz_value = 2;
  }
}

The oneof version is more explicit and more efficient on the wire but requires understanding how oneof values work.

Finally, another perfectly reasonable option is to stick with proto2. Proto2 is not deprecated, and in fact many projects (including inside Google) very much depend on proto2 features which are removed in proto3, hence they will likely never switch. So, it's safe to keep using it for the foreseeable future.