Reddit: SafeD - The Safe Subset of D

[Kevin Bealer]

Kevin Bealer Wrote:

> Simen Kjaeraas Wrote:
> 
> > On Mon, 31 Mar 2008 05:13:50 +0200, Kevin Bealer <kevinbealer at gmail.com>  
> > wrote:
> > 
> > > In languages like D and C++, the garbage collection is conservative, and
> > > this means that any pointer-sized block will be considered a pointer if
> > > it contains a value that is an address of any area that might still be
> > > live.
> > >This means that a few "garbage" blocks can be kept around because they
> > > are in a memory area which is pointed to by some random integer. This
> > > also means that in practice, even for sets of memory blocks that are
> > > not circular and might have an obvious destruction order could not be
> > > guaranteed to be destroyed in the right order, because a random integer
> > > in any of them might make them seem circular, so relying on any policy
> > > that tried to detect circularity would be unreliable at best.
> > 
> > In D, an allocated block can be marked as containing no pointers, and thus
> > will not be scanned for things looking pointers. I don't know how good the
> > GC/compiler is at understanding these things on its own, but at least a
> > programmer can make it more informed.
> > 
> > --Simen
> 
> Yes, and I forgot to mention that at one point the D compiler got an upgrade
> that made it more "precise" (the opposite of "conservative" in GC jargon).  It
> now knows that a dynamically allocated array of a primitive type (such as an
> int[] array or one of the string types) is not a pointer.

Oops -- I meant the data in the array -- the array actually is a pointer/length.

> So if you are dealing with lots of large matrices or lots of string data, you
> should suffer much less from the effects of the accidental retention of blocks.
> I don't think it affects for structures, classes, or stack data, though it would
> probably be straightforward to do this for structures that had no pointers.
> 
> Kevin
>