Purpose of the Reset*** methods?

[Giorgi]

If the proto file contains messages with optional fields the classes generated by protogen contain ShouldSerialize*** and Reset*** methods. I understand the purpose of the ShouldSerialize methods but what is the purpose of Reset*** methods?

[mgravell]

basically ShouldSerialize* and Reset* form two parts of a pre-existing pattern-based API used by multiple parts of the framework; we don't use them, but it makes some sense to emit them both; however, we can perhaps make this optional; cross-referencing #1134