From: haynberg@yahoo.com
To: "aph@redhat.com" <aph@redhat.com>,
"gcc-help@gcc.gnu.org" <gcc-help@gcc.gnu.org>
Subject: RE: is portable aliasing possible in C++?
Date: Wed, 10 Sep 2014 23:03:00 -0000 [thread overview]
Message-ID: <1410390231.39617.YahooMailNeo@web140202.mail.bf1.yahoo.com> (raw)
> > So it would seem the answer is no, portable aliasing is not possible
> > in C++. Though I wonder if it should be (with a new language
> > feature).
>
> In practice, a union is exactly that, and I've not heard of a compiler
> which doesn't do the right thing. Given that it's explicitly legal in
> C11, I can't see any reason which C++ shouldn't simply adopt the same
> language.
I agree. But in the union case, you're copying bytes. If you don't perform
a copy, for example:
struct msg {
// ...
};
char *get_bytes();
msg *p = reinterpret_cast<msg*>(get_bytes());
if (p->i)
// ...
Then there can't be a portable way to do this, because there's hardware that
doesn't permit unaligned reads (e.g. where you'd get a SIGBUS). So I guess
I retract that "I wonder if" part :-)
Jay Haynberg
next reply other threads:[~2014-09-10 23:03 UTC|newest]
Thread overview: 35+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-09-10 23:03 haynberg [this message]
2014-09-11 8:11 ` Andrew Haley
2014-09-11 23:25 ` haynberg
2014-09-12 8:32 ` Andrew Haley
2014-09-12 22:58 ` haynberg
2014-09-13 7:23 ` Andrew Haley
2014-09-13 11:45 ` Oleg Endo
2014-09-15 2:37 ` Hei Chan
2014-09-15 8:35 ` Andrew Haley
2014-09-15 11:07 ` Hei Chan
2014-09-15 11:21 ` Andrew Haley
2014-09-15 11:29 ` Hei Chan
2014-09-15 11:32 ` Andrew Haley
2014-09-15 11:57 ` Hei Chan
2014-09-15 13:21 ` Andrew Haley
2014-09-15 13:31 ` Hei Chan
2014-09-15 14:11 ` Andrew Haley
2014-09-15 11:27 ` Jonathan Wakely
2014-09-15 12:09 ` Paul Smith
2014-11-02 23:55 ` Hei Chan
2014-11-03 9:34 ` Andrew Haley
-- strict thread matches above, loose matches on Subject: below --
2014-09-09 23:13 haynberg
2014-09-10 8:17 ` Andrew Haley
[not found] <A76FB9DDEDFA994BAF6B77704A4AF465BC2464@xchmbbal502.ds.susq.com>
2014-09-04 16:11 ` Andy Webber
2014-09-04 16:51 ` Andrew Haley
2014-09-04 17:18 ` Andy Webber
2014-09-04 17:23 ` Andrew Haley
2014-09-04 17:44 ` Andy Webber
2014-09-04 17:47 ` Andy Webber
2014-09-04 17:48 ` Andrew Haley
2014-09-04 23:11 ` Jonathan Wakely
2014-09-05 7:16 ` Andrew Haley
2014-09-05 14:19 ` Jason Merrill
2014-09-08 9:33 ` Richard Biener
2014-09-10 14:31 ` Jason Merrill
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