Question

struct B {};  // B contains data members
struct D : B {};  // D doesn't contain ANY data member

B g_b;  // global object
D& fun ()  // want to return by reference ONLY
{
  return <???>(g_b);  // how ???
}

[Note: I want to avoid overloading constructor (or assignment) such as D(const B&).]

Answer 1

That is undefined behavior.
You can use dynamic_cast for performing safe down casting of Base class pointer/reference to derived class pointer/reference. It returns a null in case of pointers or throws an exception in case of references.