Re: Visibility of Indexing aspects

["Randy Brukardt" <randy@rrsoftware.com>]

"Dan'l Miller" <optikos@verizon.net> wrote in message 
news:cc25c794-5986-4f6e-9d5e-0ec20a4f47d0@googlegroups.com...
...
>Ummmmmmmmm, what is so incredibly "expensive" about Element => 
>Self.Data(Index)'Access in OP?

Nothing is expensive about *that*, but it's also very unsafe by itself (this 
either prevents a container from ever reusuing memory, or essentially 
requires being OK with dangling pointers and the erroneousness that they 
cause).

Our requirement was that the user-defined dereference mechanism needs to be 
safe (at least possible to be safe), and that requires the container to be 
able to find out when the dereference ceases to exist. At that point, the 
container can free the underlying memory if need be. (For an extreme 
example, consider a persistent container, where the object only should exist 
in memory so long as it is being used there.) The design that we came up 
with for Ada 2012 uses finalization for this notification; we looked at 
other mechanisms, but they would have had similar overhead.

The Ada containers use the finalization to clear the tampering check (which 
prevents deleting of elements while the dereference exists); other uses are 
possible, of course.

Without these sorts of checks, Ada containers would be no more safe than C 
code. In particular, Ada programmers do not expect checked indexing to be 
unsafe, so syntax of C(I) being unsafe would be a particular surprise.

                                    Randy.