Is using NULL references OK?

Question

I came across this code:

void f(const std::string &s);

And then a call:

f( *((std::string*)NULL) );

A

Answer 1

for the case you can make "empty object", which will play the role of the zero pointer

class Foo
{
static Foo empty;
public:
  static bool isEmpty( const Foo& ref )
  {
    return &ref==∅
  }
}