pointer cast costs

/**
 * Nonempty base class
 */
struct Base
{
  int v;

  void do_something();

  static void do_something(Base* self);
};

/**
 * Child class that introduces a vtable pointer
 *
 * The combination of nonempty base and vtable in child causes an offset
 * between pointers to Base and pointers to Child.
 *
 * Example:
 * Child* child = NOT NULL;
 * Base* base = child;
 * reinterpret_cast<intptr_t>(child) != reinterpret_cast<intptr_t>(base);
 */
struct Child: Base
{
  virtual ~Child() = default;

  Base* self_as_base();
};

/**
 * Unconditional conversion because `this` must not be NULL
 */
Base* Child::self_as_base()
{ return this; }

/**
 * Conditional conversion because child may be NULL
 */
Base* child_to_base_ptr(Child* child)
{
  return child;
}
/**
 * Unconditional conversion because references must not be NULL
 */
Base& child_to_base_ref(Child& child)
{
  return child;
}
/**
 * Unconditional conversion because `this` must not be NULL
 */
void call_do_something(Child** children, int count)
{
  for(int i = 0; i < count; ++i)
    children[i]->do_something();
}
/**
 * Conditional conversion because child may be NULL
 */
void call_do_something_static_ptr(Child** children, int count)
{
  for(int i = 0; i < count; ++i)
    Base::do_something(children[i]);
}
/**
 * Unconditional conversion because references must not be NULL
 */
void call_do_something_static_ref(Child** children, int count)
{
  for(int i = 0; i < count; ++i)
    Base::do_something(&static_cast<Base&>(*children[i]));
}