学习笔记-protobuf篇

This means that you can use the nested type's class as if it was the nested class Foo::Bar. However, note that C++ does not allow nested types to be forward-declared. If you want to forward-declare Bar in another file and use that declaration, you must identify it as Foo_Bar.
来源
https://developers.google.com/protocol-buffers/docs/reference/cpp-generated