weak_ptr alternative

[Kamil Koczurek via Digitalmars-d-learn]

Is there a way to keep track of objects without owning them? That 
is, could I have a smart pointer that behaves somewhat like this:

WeakPtr!Class wptr = getSomeInstance();
auto obj = wptr.peek; //[1]
if(obj !is null) {
  obj.stuff();
}
[1]: If wptr points to something that is still reachable from 
somewhere else (and wasn't yet marked for collection by GC), 
.peek would return a normal reference of type Class. OTOH, when 
no active reference is present it should return null.