Интересная инициализация ссылки

class Base
{
public:
    Base(Base *instance, 
         unsigned int NumElems):
      vec(NumElems),
      vec_ref(instance->vec)
    {}
    std::vector<float> vec;
    std::vector<float>& vec_ref;
};
 
class Der : public Base
{
public:
    Der() : Base(nullptr, 5){}
};

#include <iostream>
 
struct Struct
{
  int structMember;
};
 
struct SuperStruct
{
  char offset[8];
  Struct str;
};
 
int main()
{
  SuperStruct* superStruct = nullptr;
  Struct* structPtr = &superStruct->str;
  Struct& structRef = superStruct->str;
  std::cout << "address of struct = 0x" << structPtr << std::endl;
  std::cout << "address of struct = 0x" << &structRef << std::endl;
//  structRef.structMember = 5; // runtime error.
  return 0;
}

class Base
{
public:
    Base(Base *instance, 
         unsigned int NumElems):
      vec(NumElems),
      vec_ref(instance==nullptr?  nullptr : &instance->vec)
    {}
    std::vector<float> vec;
    std::vector<float>* vec_ref;
};
 
class Der : public Base
{
public:
    Der() : Base(nullptr, 5){}
};