[C++] Native union of struct/table with nested struct is not copyable

[piotrdz]

flatbuffers 1.12.0

Given the following schema:

struct SingleStruct
{
  field: double;
}
struct NestedStruct
{
  nested: SingleStruct;
}
union AnyStruct
{
  SingleStruct,
  NestedStruct
}

I would expect generated native type AnyStructUnion to be copyable for both SingleStruct and NestedStruct.

However, the generated code for AnyStructUnion copy constructor claims otherwise:

inline AnyStructUnion::AnyStructUnion(const AnyStructUnion &u) : type(u.type), value(nullptr) {
  switch (type) {
    case AnyStruct_SingleStruct: {
      value = new SingleStruct(*reinterpret_cast<SingleStruct *>(u.value));
      break;
    }
    case AnyStruct_NestedStruct: {
      FLATBUFFERS_ASSERT(false);  // NestedStruct not copyable.
      break;
    }
    default:
      break;
  }
}

The same problem occurs if NestedStruct is defined as a table.

[aardappel]

That looks like an oversight in the case of NestedStruct. Care to make a PR to fix it?

For tables I am not sure if we generate a copy constructor. That could of course be added, but may be more work. The default one likely wouldn't work since it may contain unique_ptr etc.

[piotrdz]

I created a PR for the nested struct bug.

As for table types, I am hoping that PR #5988 gets merged at some point to address that.

[github-actions]

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