Re: OO, C++, and something much better!

[dewar@merv.cs.nyu.edu (Robert Dewar)]

Don Harrison says

"No. Ada offers type casting in a couple of ways:

  1) UNCHECKED_CONVERSION and
  2) Address representation clauses (overlays).

Eiffel, by contrast, has no such thing in the standard language. That isn't
to say it isn't needed for some things (and is why some vendors do provide
type conversion between basic types and bits, for example).

I have been trying to think of a safe way of doing this sort of thing but
thus far haven't thought of any. :)"


Well there obviously is no safe way of doing something whose entire purpose
is to allow unsafe operations!

Still I would not characterize these as "type casting" in Ada. Rather I 
would think of them as loopholes, equivalent to calling a C or assembly
routine. Any formal analysis or discussion of the type system of Ada
has to ignore these features, and of course any real language to be used
for low level systems programming needs such features, but I don't think
it is a useful excercise to include these features in the type analysis.

If you like, consider Ada *minus* these two features as the interesting
language to be analyzed. Then understand that in real life programs can
break the bounds of this analysis to the extent they need to.

Note that it is not actually the unchecked conversion that is nasty, since
the fundamental conversion can be modeled as simply a random mapping between
values of one type to values of another type. It is that these operations
can generate abnormal values. 

This may still not be a problem, given that uninitialized values can also
be abnormal, thus:

  a := nasty_unchecked_conversion (b);

is no worse that not assigning to a at all if a is uninitialized.