Re: Unions Redux

Robert Gamble <rgamble99@xxxxxxxxx> wrote:
On Mar 14, 10:32 pm, p...@xxxxxxxxxxxxxxxxxxxxx (Pierre Asselin)

Annex J is informative, not normative,

Indeed it is not and in this case it is simply erroneous. The
sentence you cited is leftover from a previous version of the
Standard, there is no supporting text in the Standard proper.

Has a defect report been filed ?

[ ... ]
So it seems that [Old Wolf's] code has undefined (not just
unspecified) behavior, by 6.2.6.1(5).

I have no idea how you were able to come to that conclusion from
anything you have cited so far. You appear to have come to a
premature conclusion and then tried, unconvincingly, to make the
evidence fit that conclusion.

Not at all. I was tried to prove from 6.2.6 that O.W. can store
and read the bits of a small positive integer interchangeably
through lvalues of type int and unsigned int, but I could only do
it for the signed --> unsigned direction.

My argument doesn't work the other way. 6.2.6.2(5) says:

A valid (non-trap) object representation of a signed integer
type where the sign bit is zero is a valid object representation
of the corresponding unsigned type, and shall represent
the same value.

The opposite is also true, 6.2.5p9:

"The range of nonnegative values of a signed integer type is a
subrange of the corresponding unsigned integer type, and the
representation of the same value in each type is the same."

Indeed. That takes care of the unsigned --> signed direction,
especially in view of footnote 31. So Old Wolf's example is
conforming after all.

This looks like a small defect in the standard. Section 6.2.6.1
opens with: "The representations of all types are unspecified except
as stated in this subclause". I took "subclause" to mean 6.2.6.1
itself but that doesn't work. It could concievably mean all of
6.2.6, which is titled "Representations of types", but not 6.2.5 .
Yet 5.2.5(9) is needed to close a loophole.


I don't know what C++ has to do with this.

It spelled out the concept of a bit-preserving cast, as opposed to
C's value-preserving (for the most part) cast.


--
pa at panix dot com
.

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