Re: Where is the elusive jump command?

[Robert Dewar <robert_dewar@my-deja.com>]

In article <8brn4k$p6i$1@slb0.atl.mindspring.net>,
  Richard D Riehle <laoXhai@ix.netcom.com> wrote:
>
> To write such a compiler, it is necessary to include certain
> reserved words and syntactic constructs that "tip off" the
> compiler to possible problems.  The word _aliased_ seems to me
> to be one of those "tip-off the compiler" words.  This can
> later be augmented with a more technical explanation, but it
> works well for most early students when explained  within the
> framework of the above stated goals.

I find this a very odd view of the ALIASED keyword. The reason
for this has nothing to do with the compiler. As in the case
of most C compilers, the compiler could look at your code to
see whether or not you take the address of something. That's
not at all the intention.

The point is that aliasing is generally speaking very bad
practice. The facility for pointing to variables was introduced
in Ada 95 with reluctance when certain convincing examples
were given, including in particular the construction of
static structures involving pointers.

It should be VERY sparingly used, really it should be regarded
like Unchecked_Conversion, a noisy declaration that you intend
to do something rather undesirable, namely in this case the
creation of possibly confusing aliases for the variable.

Note by the way that 'Address can be applied freely to variables
in any case without the ALIASED keyword, so Ada 95 compilers in
any case have to have the circuitry for automatic detection of
which variables have their addresses taken and which do not
(GNAT most certainly has this circuit). Thus ALIASED is not of
the slightest utility to the compiler (in GNAT it is essentially
ignored except for providing the essential semantic checking
associated with this facility).



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