adding dllexport in generated headers?

[ojwl...@googlemail.com]

To generate a DLL which uses protocol-buffers as part of its
interface, the autogenerated *.pb.h would need to have some sort of
"__declspec( dllexport )" in each class declaration. Is it possible
to make protoc.exe generate these?

[Kenton Varda]

Yes.  There is an undocumented option to --cpp_out which will do this:

--cpp_out=dllexport_decl=MY_EXPORT_MACRO:output/direcotry

For an example of this, see generate_descriptor_proto.sh, which generates descriptor.pb.{h,cc} from descriptor.proto.

It's undocumented because I'm concerned that exposing protocol buffer objects from DLLs may not be a great idea in general due to ABI compatibility issues -- e.g. if you add a new field and recompile your DLL, every program linked against it will also have to be recompiled.  But, if you know what you're doing, feel free to use it.