Well here's a tricky one for you: how can I have two ustructs that contain each other?

[Rumbleball]

USTRUCT()
struct FDataA
{
 ...
};

USTRUCT()
struct FDataB
{
...
};

UCLASS()
class A
{
 class B* b;
 FDataA data;
};

UCLASS()
class B
{
 class A* a;
 FDataB data;
};

Missing GENERATED_BODY and public: