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Gennaro Prota
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 Posted: Sat Feb 14, 2004 4:53 pm Post subject: Defect Report: does reinterpret_cast<T*>(0) yield a null poi |
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[ note: Forwarded to C++ Committeel -sdc ]
Is reinterpret_cast<T*>(null_pointer_constant) guaranteed to yield the
null pointer value of type T*?
I think a committee clarification is needed. Here's why: 5.2.10
[expr.reinterpret.cast] par. 8 talks of "null pointer value", not
"null pointer constant", so it would seem that
reinterpret_cast<T*>(0)
is a normal int->T* conversion, with an implementation-defined result.
However a little note to 5.2.10 [expr.reinterpret.cast] par. 5 says:
Converting an integral constant expression (5.19) with value zero
always yields a null pointer (4.10), but converting other
expressions that happen to have value zero need not yield a null
pointer.
Where is this supported in normative text? It seems that either the
footnote or paragraph 8 doesn't reflect the intent.
PROPOSED RESOLUTION: I think it would be better to drop the footnote
#64 (and thus the special case for ICEs), for two reasons:
a) it's not normative anyway; so I doubt anyone is relying on the
guarantee it hints at, unless that guarantee is given elsewhere in a
normative part
b) users expect reinterpret_casts to be almost always implementation
dependent, so this special case is a surprise. After all, if one wants
a null pointer there's static_cast. And if one wants reinterpret_cast
semantics the special case requires doing some explicit cheat, such as
using a non-const variable as intermediary:
int v = 0;
reinterpret_cast<T*>(v); // implementation defined
reinterpret_cast<T*>(0); // null pointer value of type T*
const int w = 0;
reinterpret_cast<T*>(w); // null pointer value of type T*
It seems that not only that's providing a duplicate functionality, but
also at the cost to hide what seems the more natural one.
Genny.
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Andrew Koenig
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| Posted: Sun Feb 15, 2004 5:28 pm Post subject: Re: Defect Report: does reinterpret_cast<T*>(0) yield a null |
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"Gennaro Prota" <gennaro_prota (AT) yahoo (DOT) com> wrote
| Quote: | PROPOSED RESOLUTION: I think it would be better to drop the footnote
#64 (and thus the special case for ICEs), for two reasons:
a) it's not normative anyway; so I doubt anyone is relying on the
guarantee it hints at, unless that guarantee is given elsewhere in a
normative part
|
Footnotes are normative.
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Francis Glassborow
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| Posted: Sun Feb 15, 2004 9:32 pm Post subject: Re: Defect Report: does reinterpret_cast<T*>(0) yield a null |
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In article <F3uXb.19485$hR.557901 (AT) bgtnsc05-news (DOT) ops.worldnet.att.net>,
Andrew Koenig <ark (AT) acm (DOT) org> writes
| Quote: | "Gennaro Prota" <gennaro_prota (AT) yahoo (DOT) com> wrote in message
news:ut6s20dk9n2vir774arg0a58nbr86084v7 (AT) 4ax (DOT) com...
PROPOSED RESOLUTION: I think it would be better to drop the footnote
#64 (and thus the special case for ICEs), for two reasons:
a) it's not normative anyway; so I doubt anyone is relying on the
guarantee it hints at, unless that guarantee is given elsewhere in a
normative part
Footnotes are normative.
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Not be standard ISO rules, footnotes, examples and notes are
non-normative.
--
Francis Glassborow ACCU
Author of 'You Can Do It!' see http://www.spellen.org/youcandoit
For project ideas and contributions: http://www.spellen.org/youcandoit/projects
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Gennaro Prota
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| Posted: Mon Feb 16, 2004 3:26 am Post subject: Re: Defect Report: does reinterpret_cast<T*>(0) yield a null |
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On Sun, 15 Feb 2004 21:32:46 +0000 (UTC), [email]francis (AT) robinton (DOT) demon.co.uk[/email]
(Francis Glassborow) wrote:
| Quote: | Not be standard ISO rules, footnotes, examples and notes are
non-normative.
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IIUC any ISO standard must conform to
http://anubis.dkuug.dk/JTC1/SC22/WG9/isodir3.pdf
However Andrew Koenig's comment worries me a bit, because it may mean
some of the authors of the C++ standard could have thought to express
a requirement whereas readers (implementers) will not consider it to
be such.
Genny.
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Francis Glassborow
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| Posted: Mon Feb 16, 2004 6:52 pm Post subject: Re: Defect Report: does reinterpret_cast<T*>(0) yield a null |
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In article <c5tv209dv1t5gudd6b4pkv84k8v6mdkplv (AT) 4ax (DOT) com>, Gennaro Prota
<gennaro_prota (AT) yahoo (DOT) com> writes
| Quote: | On Sun, 15 Feb 2004 21:32:46 +0000 (UTC), [email]francis (AT) robinton (DOT) demon.co.uk[/email]
(Francis Glassborow) wrote:
Not be standard ISO rules, footnotes, examples and notes are
non-normative.
IIUC any ISO standard must conform to
http://anubis.dkuug.dk/JTC1/SC22/WG9/isodir3.pdf
However Andrew Koenig's comment worries me a bit, because it may mean
some of the authors of the C++ standard could have thought to express
a requirement whereas readers (implementers) will not consider it to
be such.
|
I am pretty sure that Andy just misspoke because I know that he is more
familiar than most with the document at the link above.
--
Francis Glassborow ACCU
Author of 'You Can Do It!' see http://www.spellen.org/youcandoit
For project ideas and contributions: http://www.spellen.org/youcandoit/projects
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Andrew Koenig
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| Posted: Mon Feb 16, 2004 9:09 pm Post subject: Re: Defect Report: does reinterpret_cast<T*>(0) yield a null |
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| Quote: | Footnotes are normative.
Not be standard ISO rules, footnotes, examples and notes are
non-normative.
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I'm sorry -- you're right; I misread the table in the ISO Drafting
Directives.
There's a section marked "Normative General [elements in a standard]" that
includes "footnotes" as one of those elements. However, "Footnotes"
(actually "Footnote(s)") is in italic type, and a footnote mentions that
text in italic type represents informative elements.
Which raises the question: If that information is in a footnote, isn't it
just informative?
Fortunately, the directives do answer that question, in subclause 6.5.2, in
which they say clearly that footnotes must not contain requirements. I
don't know why I missed that the first time. Sigh.
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