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| From | James Kuyper <jameskuyper@verizon.net> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: John McCarthy died today. |
| Date | 2011-11-14 14:44 -0500 |
| Organization | A noiseless patient Spider |
| Message-ID | <4EC16F88.9000908@verizon.net> (permalink) |
| References | (8 earlier) <20111113084111.100@kylheku.com> <lnobwf4p2d.fsf@nuthaus.mib.org> <20111113191456.538@kylheku.com> <slrn3vfsjc1m4l.34h.ike@sverige.freeshell.org> <20111114101818.819@kylheku.com> |
On 11/14/2011 01:30 PM, Kaz Kylheku wrote:
> On 2011-11-14, Ike Naar <ike@sverige.freeshell.org> wrote:
>> On 2011-11-14, Kaz Kylheku <kaz@kylheku.com> wrote:
>>> One example would be that I can't build the code any more with C++ to check
>>> that all external names are meeting type-safe linkage and the one-definition
>>> rule.
>>>
>>> In C, I can have "extern int x;" in in one module, and "struct foo x = { 0 };"
>>> in another. It will link.
>>>
>>> If you change a C program such that a function has different parameters, but
>>> due to some broken dependencies in your build system, some dependent modules
>>> are not recompiled, you will get a broken link. Of course, it goes away
>>> if you do a complete rebuild, but you could waste time chasing a bug
>>> which isn't there.
>>>
>>> This is fixed in C++: it will catch your broken build system. Type
>>> safe linkage won't let an int (int, char *) call go to an
>>> int (int, char *, char *) function.
>>
>> Using a C++ compiler (GCC 4.1.2), the following:
>>
>> /* a.cxx */
>> extern int x;
>> int main() { return x; }
>>
>> /* b.cxx */
>> struct foo { double d; } x;
>>
>> compiles/links (c++ -ansi -pedantic -Wall a.cxx b.cxx)
>> and runs without any complaints.
>
> Ah yes, g++ is broken in this regard.
>
> But the ODR still kicks in.
There's only one definition of 'foo' in this program; the one in b.cxx.
a.cxx only contains a declaration of the object. Even if this were a
violation of the ODR, section 3.2p3 of the C++ standard says, very
explicitly, "no diagnostic required", which would make it no different
from C in this regard.
The relevant rule is actually in section 3.5p10 of the C++ standard:
"... the types specified by all declarations referring to a given object
or function shall be identical, ...", but in that case, too, it says
very explicitly that "A violation of this rule on type identity does not
require a diagnostic." So again, it's no different from C.
If some C++ compilers provide diagnostics that are not required, that's
a feature of those particular compilers, not a feature of the C++
language itself, and a feature that could have been put into C compilers.
(all citations are from n3035.pdf, which is the closest thing I have to
the actual C++ standard that is readily available. However, I don't
believe that either of these citations are significantly different from
previous versions of the standard, nor from more recent drafts of the
next standard).
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John McCarthy died today. Kaz Kylheku <kaz@kylheku.com> - 2011-10-25 00:44 +0000
Re: John McCarthy died today. Nick Keighley <nick_keighley_nospam@hotmail.com> - 2011-10-25 01:27 -0700
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Re: John McCarthy died today. Kaz Kylheku <kaz@kylheku.com> - 2011-11-08 23:38 +0000
Re: John McCarthy died today. Kaz Kylheku <kaz@kylheku.com> - 2011-11-08 23:18 +0000
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