Zero_Fill pragma

["Nick Roberts" <nick.roberts@acm.org>]

Am I too late to make a proposal for a new pragma for
the Ada 2005 Amendment?

It would have the syntax:

    pragma Zero_Fill(simple_name);

It would be applicable to an object or a subtype, and it
would operate on all objects of the given subtype, or on
the given object. The object must be of a composite type.

It would have the effect of causing all the bits in the
object which are unused to have a default initialisation
of 0.

I think the definition of 'in the object' would be any
bit numbered 0 to n-1, where n is the object's size (as
the Size attribute would report).

I think the definition of 'unused' would be any bit
which is not in any of the object's components. If the
place of the component is defined by a record
representation clause, that will define the bits used
by the component, otherwise the bits used by a component
will be chosen by the compiler.

For records, the effect can be achieved by introducing
'dummy' components, but this is a pain and can degrade
readability. For arrays, the effect cannot be achieved
by standard means (other than perhaps adding components
to the array's component subtype, if it is a record
subtype, to bulk the record up to the size of an
integral number of allocation units, and then using
pragma Pack).

Either way, I think the pragma would be useful in real
programs, and that it would be useful for the pragma to
be defined by the language standard. I'm a little
surprised existing compilers don't provide something
like this.

-- 
Nick Roberts