struct element orderin vs. align

[Norman Diamond]

> >Sigh,
> >        until you give me a subclass mechanism that ensures that the
> >representation of two data types share a common prefix, I'll rely on this
> >compiler hack.
> 
> You already have it:
> 
> struct {
> 	common part;
> 	union {
> 		distinct part1;
> 		distinct part2;
> 		...
> 	};
> };
> The distinct parts can of course be structures.
> This is the only guaranteed portable way to do this.

Nope; you're only guaranteed a common portion, which might or might not be
a prefix.  If some one really needs certain fields mapped to the beginning
of a structure, they have to take advantage of knowing how the compiler will
map the data.  If a person can depend on "part" being at the beginning of
that struct, then they can also depend on the ordering in every structure.

This is a very hard problem -- assuming that one wants a correct solution.

-- Norman Diamond

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