> >From what I've seen, supporting both garbage collection and true stack
> >variables is a difficult task.
> Why is that?
Because stack variables can refer to heap variables and heap variables can
refer to stack variables. The garbage collector needs to be smart enough to
handle all cases correctly. For example, let's say we have the following
two classes...
class A
{
public B b_member;
}
class B
{
public A a_member;
}
....and the following declarations and function...
B global_b; // Creates an instance of B with an A member, both
on the stack.
B global_b2; // Creates an instance of B with an A member, both on
the stack.
void f()
{
A a; // Creates an instance of A with a B member, both on
the stack.
B b = new B(); // Creates an instance of B with an A member, both on
the heap.
// 4 objects have now been created in this function, an A and B on the
stack, and an A and B on the heap.
a.b_member = b; // Stack variable a now references heap variable
b.
b.a_member = a; // Heap variable b now references stack variable
a.
global_b.a_member = a; // Stack variable global_b now references
stack variable a.
global_b2 = b; // Stack variable global_b2 now references heap
variable b.
// What happens to the six objects we've created when we leave the
scope of f()?
}
This implies several things.
1) Garbage collection is optional because stack variables are not garbage
collected.
2) Garbage collection knows every object created, how every object was
created (stack or heap), and how every object is referenced by other
objects.
3) The stack interacts seemlessly with garbage collection. When we leave f
() in the above example, referencing global_b.a_member needs to produce an
error or exception.
4) Garbage collection interacts seemlessly with the stack. In the above
example, b can never be garbage collected for the life of the program
because the stack variable global_b2 references it and global_b2 never goes
out of scope.
It's not impossible to write a garbage collector that can handle this, I've
done it myself. This is why I've been asking these questions about Parrot.
I want to know if I can use Parrot as a VM for a language I've designed.
I'm currently generating C++ code. I'd like to use Parot, but I need true
stack variables and 100% deterministic garbage collection.
I don't know of any language that has these features. Java doesn't. C++
doesn't (unless you roll your own and restrict what programmers can do), C#
doesn't.
Dave
"Melvin Smith"
<[EMAIL PROTECTED] To: [EMAIL PROTECTED]
m.com> cc: [EMAIL PROTECTED], Simon
Cozens <[EMAIL PROTECTED]>
Subject: Re: How Powerful Is Parrot? (A
Few More Questions)
01/25/02 12:00
PM
>From what I've seen, supporting both garbage collection and true stack
>variables is a difficult task.
Why is that?
-Melvin Smith
IBM :: Atlanta Innovation Center
[EMAIL PROTECTED] :: 770-835-6984
