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Re: Immediate doubles (up to 2^256) and rationals coming to Guile 3

From: Hans Åberg
Subject: Re: Immediate doubles (up to 2^256) and rationals coming to Guile 3
Date: Sat, 8 Jun 2019 15:12:00 +0200

> On 8 Jun 2019, at 11:08, Chris Vine <address@hidden> wrote:
> On Sat, 08 Jun 2019 10:07:45 +0200
> Arne Babenhauserheide <address@hidden> wrote:
> [snip]
>> Wow, I didn’t know that you could do that.
>> However: "The details of that allocation are implementation-defined, and
>> it's undefined behavior to read from the member of the union that wasn't
>> most recently written."
>> Can you guarantee that this works?
> This is C and not C++ and the provision to which you refer does not
> apply.
> Reading from a member of a union other than the one last written to is
> implementation defined in C89/90, and defined in C99 (with Technical
> Corrigendum 3) and above, although it might include a trap
> representation (but wouldn't on any platform supported by guile).  You
> might want to see in particular footnote 95 of C11 (which isn't
> normative but is intended to explain the provisions of § which
> are).
> gcc and clang have always supported type punning through unions.

FYI, the site mentioned above also covers C (there is a link the bottom of the 
C++ page above):

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