Safe reference counting cannot be implemented as a library

[Andrei Alexandrescu via Digitalmars-d]

(Title is borrowed from Hans Boehm's famous "Threads cannot be 
implemented as a library", 
http://www.hpl.hp.com/techreports/2004/HPL-2004-209.pdf.)

We want to implement safe reference counting in D, and the sentiment 
that it can be done in a library (with no or minimal changes to the 
language) is often echoed in the newsgroup. This post explains that 
reference counting can be implemented as a library _only_ if safety is 
forgone. If safety is desired, reference counting must be built into 
language semantics.

The crux of the matter is modular typechecking. Consider the following 
example:

// module widget.d
@safe class Widget {
  void fun() {
    g_widget = this;
  }
}
static Widget g_widget;
// end of module widget.d

This is a perfect valid D class, and safe too. The typechecker assumes 
infinite lifetime for all Widget objects, and allows escaping the 
reference to this from foo() into the global.

Numerous similar examples can be constructed involving more elaborate 
escaping patterns than this simple global assignment.

Now, once the typechecker OKs module widget.d, the summary that all 
other typechecking "sees" is:

@safe class Widget {
  void fun();
}

A library reference counting wrapper a la RC!Widget needs to allow calls 
to fun(). Once that happens, the global will hold an alias to the 
respective Widget object indefinitely, which means as soon as the 
RC!Widget object is released by the reference counting protocol, 
g_widget will become a dangling reference.

It follows that if we want safe reference counting, there must be 
language support for it. One possibility is to attach an attribute to 
the class definition:

@safe @rc class Widget {
  ...
}

Then the compiler is able to enforce more stringent typechecking on 
Widget (for example, address of fields are not allowed to escape) and 
also insert the appropriate reference counting logic. This will be a 
topic discussed on the lifetime list.


Andrei