Re: access aliased obstacle

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

Laurent said

"   A discriminated record isn't a C union type, because the
discriminant is stored in the record in Ada (accessible for reading),
and run-time checks are made on the discriminant when you access
variant parts. The pragma Unchecked_Union makes a simple Ada
discriminated record type match exactly a C union (discriminant not
stored, no checks). It is not in the RM, but all vendors agreed to
implement it. It is documented in the GNAT file "features" I think (or
gnatinfo.txt)."

Not quite right, there is no reason why a discriminant in a variant record
should be stored "in the record" in Ada, that is just a (common)
implementation choice. In fact if you are using sub types extensively
it is attractive to choose an alternative representation where the
discriminants are stored separately,

However, the point about discriminant checks is right of course (although
if one had an implementation which stored discriminants separately, you
would get almost what you wanted simply by suppressing these checks).