Protobuf: how nested (custom) optional fields are to be handled in C++?

Question

Reading into Protocol Buffer Basics: C++, found nothing that matches the situation:; with following .proto processed with --cpp_out,

message A {
 required int32 foo = 1;
}
message B {
 optional A data = 1;
}

no obviously looking accessor/setter is generated to set custom optional field (including 'nested types' section which I'm too lazy to put here):

   // accessors -------------------------------------------------------

   // optional .A = 1;
   inline bool has_a() const;
   inline void clear_a();
   static const int kAFieldNumber = 1;
   inline const ::A& a() const;
   inline ::A* mutable_a();
   inline ::A* release_a();

So, how to set B::A to some A instance in C++?

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TEST FILES: .proto, generated results: .h, .cc and some .java

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Upd: in Java, nested fields are set via Builder: see link above for example (look for setData).

Answer 1

Solution: use mutable to modify some returned doodad.

A a;
A.set_foo(1);

B b;
B.mutable_A()->CopyFrom(a);