Re: garbage collection

[Keith Thompson <kst@cts.com>]

"Robert I. Eachus" <eachus@mitre.org> writes:
[...]
>    I could go into all the gory details, but that about sums it up.  The
> validation tests do include several tests to insure that storage is not
> lost in many common situations, but there is no requirement that storage
> be freed the moment their are no more references.  Many Ada compilers
> instead free the storage when the access type goes out of scope for some
> or all types.
> 
>    Why?  Three reasons.  Freeing the storage when the type goes out of
> scope allows for predictable storage reclamation which is very useful in
> real-time code.  Second, the user can easily manage storage for any type
> the way he wants to.  Insisting on "full" garbage collection would limit
> the user's choices.
[...]

BTW, in my experience most access types don't go out of scope until
the program terminates, because they're declared in library-level
packages.  The automated reclamation can only take place if the access
type is declared directly or indirectly within a subprogram or a task
body (there may be other cases).

(I'm not sure how typical my experience is; how common is it to
declare an access type within a subprogram?)

Note that declaring an access *object* within a subprogram doesn't let
the storage it refers to be reclaimed; the access value could have
been copied to an object in an outer scope before the subprogram
completed.

>    And finally, Ada allows return values from functions which are
> unbounded, and for which the storage is "automagically" managed.  If you
> define a function which returns a string in Ada, there is no need for
> explicit alloc and free calls or the equivalent.  So in ninety percent
> or more of the cases where you need garbage collection in other
> languages, you don't even need explicit allocations in Ada.

Right, I think this is the biggest reason that the lack of garbage
collection in most Ada implementations isn't (much of) a problem.

-- 
Keith Thompson (The_Other_Keith) kst@cts.com  <http://www.ghoti.net/~kst>
San Diego Supercomputer Center           <*>  <http://www.sdsc.edu/~kst>
One of the great tragedies of ancient history is that Helen of Troy
lived before the invention of the champagne bottle.