Re: Fortran's Equivalence

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

Vigar says

<<
I have a BIT_PATTERN : System.Unsigned_32.  The first 4 bits represent
an integer A, the next 17 represent another record B, the last eleven
represent an integer C.  Ada gives something beautiful, in the "use at"
clause, which lets me define a record to superimpose onto the bit
pattern.  The big problem with this approach is that it isn't
guaranteed by the LRM, and I need my application to be portable to
other Ada implementations.  The Ada Quality and Style Guide, Clause
5.9.4, also tells us that we should not use the "use at" clause to do
such things.  I am using the GNAT compiler, 3.07 on the SGI.  The final
system will be delivered for a VAX.>>

Isn't it amazing how Ada's concentration on precision sometimes works to
its disadvantage :-)

Here we have a case where someone is used to misusing a construct in Fortran
(EQUIVALENCE) to do something highly non-portable (go look at the Fortran
standard with the same care that you have looked at the Ada standard!)

Sure the RM does not "guarantee" anything here, because it has very little
to say about representation, but if you are doing low level mucking around
the use of "use at" is exactly equivalent to Fortran EQUIVALENCE, and there
is no reason to be shy of it.

The statement in the AQ&S is a holdover from Ada 83 days, and I see no
justification for it. If "use at" does what you want, use it!