Re: Flags in Ada?

[jgv@swl.msd.ray.com (John Volan)]

The way I understand it, applying pragma Pack to a record type will
only guarantee that the record type will be optimized for space.  I
don't think there's anything in the language definition that requires
a compiler to preserve the order of the components.  In fact, RM9X
13.2(8);5.0 states that an implementation may reorder components to
improve the packing.

I grant you that the most straightforward implementation of Pack would
simply pack the components in the order they were originally declared,
and I expect that that's the way GNAT deals with it.  Nevertheless, I
can imagine some super-optimizing compiler that juggled the components
around to get a better fit.  

So if you have an external requirement that each component occupy
specific positions, such as when you're communicating with hardware or
operating-system services, then I think you'd need to use a
representation clause.  You can't just rely on the components being
stored in the order you declared them.

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