Re: Unchecked_Union

[evans@evans.pgh.pa.us (Arthur Evans Jr)]

In article <dewar.825178652@schonberg>, dewar@cs.nyu.edu (Robert Dewar) wrote:

> The following is a description of pragma Unchecked_Union as we are
> implementing it in GNAT. This description is compatible with the
> current Intermetrics usage in their binding generator tool.

This looks most interesting.  Two small comments:

>  Also, equality and inequality operations on unchecked_unions are not
>  available, since there is no discriminant to compare, and in general
>  the compiler does not even know how many bits to compare. It is
>  implementation dependent whether this is detected at compile time
>  as an illegality, or whether it is undetected and considered to be
>  an erroneous construct. In GNAT, a direct comparison is illegal,
>  but GNAT does not attempt to catch the composite case (where two
>  composites are compared that contain an unchecked union component),
>  so such comparisons are simply considered erroneous.

My quibble is with the last quoted word.  I think you could safely say
that this is a bounded error, since the only effect is that the
comparison will give the wrong answer.

>  The layout of the resulting type corresponds exactly to a C union,
>  ...

I wonder whether or not that is a well-defined statement.  That is, is
the layout a a C union specified anywhere, even for a specific C
implementation?  (Maybe this comment is off the wall -- I'm not much of
a C programmer.)

Art Evans

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