Re: memory management

[Robert A Duff <bobduff@shell01.TheWorld.com>]

Martin Krischik <krischik@users.sourceforge.net> writes:

> Robert A Duff wrote:
> 
> > alex goldman <hello@spamm.er> writes:
> > 
> >> As I understood from reading the Ada tutorial for C/C++ programmers,
> >> "access" is essentially like C++ smart pointer,
> > 
> > No, Ada access types are just pointers -- no "smarts".
> > To make something like smart pointers, you can use
> > controlled types.
> 
> No again - any paricular Ada implementation is free to implement access
> types as they see fit. With GNAT an access may consist of an pointer to the
> data and a pointer to a dope vector:

That's what I call a "fat pointer".  Still just a pointer (as opposed to
a smart pointer).

When I say "pointer", I don't mean it has to be implemented as a single
machine address.  It could be an offset from some known base address, an
index into an array, or (as you say) a fat pointer -- among other
things.

The same is true of pointers in C and C++ -- an implementation is free
to implement pointers as something other than a machine address.  In
fact, if a C compiler wishes to check array bounds, it pretty much *has*
to use fat pointers.  I know of one C compiler that did just that.  My
point is that "pointer" is not synonymous with "single machine address",
even in C.

By the way, I believe the fat pointers used by GNAT are an option --
there's some way to tell it to use thin pointers for access-to-array.
GNAT uses fat pointers (by default) only when the designated type is an
array, or when the designated type is unknown to the compiler.
Access-to-record, which is far more common, uses thin pointers.
I don't know of any Ada implementation besides GNAT that uses
fat pointers at all.

> http://en.wikipedia.org/wiki/Dope_vector

> Remeber that an Ada compiler may also implement garbage collection.

Yes.  That would typically *not* involve smart pointers.

- Bob