ODR: A simple question
ebony...@gmail.com
Hi
I encountered a simple but IMO important problem about the C++ linkage
model and One Definition Rule. Why the following code link?
file1.cpp
int x;
file2.cpp
double x;
x was defined in two different translation units with different types.
It breaks the ODR.
The following code is also linked:
file1.cpp
int x;
file2.cpp
extern double x;
It is obvious the type x in declaration and definition are different.
I ran and tested codes in Visual Studio 6.0 and .Net 2005.
Thank you in advance
- Saeed Amrollahi
Pete Becker
>
> I encountered a simple but IMO important problem about the C++ linkage
> model and One Definition Rule. Why the following code link?
>
> file1.cpp
> int x;
>
> file2.cpp
> double x;
>
> x was defined in two different translation units with different types.
> It breaks the ODR.
The language definition doesn't require compilers to diagnose
violations of the ODR. Code like this has undefined behavior. As a
practical matter, recognizing errors like this is expensive, given
C++'s separate compilation model.
--
Pete
Roundhouse Consulting, Ltd. (www.versatilecoding.com) Author of "The
Standard C++ Library Extensions: a Tutorial and Reference
(www.petebecker.com/tr1book)
James Kanze
> On 2008-09-29 06:01:07 -0400, ebony.s...@gmail.com said:
> > I encountered a simple but IMO important problem about the
> > C++ linkage model and One Definition Rule. Why the following
> > code link?
> > file1.cpp
> > int x;
> > file2.cpp
> > double x;
> > x was defined in two different translation units with
> > different types. It breaks the ODR.
> The language definition doesn't require compilers to diagnose
> violations of the ODR. Code like this has undefined behavior. As a
> practical matter, recognizing errors like this is expensive, given
> C++'s separate compilation model.
And the relatively low quality of most linkers. It wouldn't be
very hard to implement, if the linker supported it. For
historical reasons, most linkers don't.
--
James Kanze (GABI Software) email:james...@gmail.com
Conseils en informatique orientée objet/
Beratung in objektorientierter Datenverarbeitung
9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34
ebony...@gmail.com
Sorry for a day delay. I was out of office.
Thank you pete for you answer, but I didn't completely convince.
I thought may be some standard conversions were made (double to int or
vice versa)
I changed double to std::string, but program run. Just when I declare
the variable
with extern, I have to specifiy the type and compiler find the real
type.
Which clause or section of C++ standard draft mention your answer?
Regards,
- Saeed Amrollahi
ebony...@gmail.com
> On Sep 29, 1:17 pm, Pete Becker <p...@versatilecoding.com> wrote:
>
> > On 2008-09-29 06:01:07 -0400, ebony.s...@gmail.com said:
> > > I encountered a simple but IMO important problem about the
> > > C++ linkage model and One Definition Rule. Why the following
> > > code link?
> > > file1.cpp
> > > int x;
> > > file2.cpp
> > > double x;
> > > x was defined in two different translation units with
> > > different types. It breaks the ODR.
> > The language definition doesn't require compilers to diagnose
> > violations of the ODR. Code like this has undefined behavior. As a
> > practical matter, recognizing errors like this is expensive, given
> > C++'s separate compilation model.
>
> And the relatively low quality of most linkers. It wouldn't be
> very hard to implement, if the linker supported it. For
> historical reasons, most linkers don't.
>
> --
> Conseils en informatique orientée objet/
> Beratung in objektorientierter Datenverarbeitung
> 9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34
Sorry for a day delay. I was out of office.
Thank you for your answer. AFAIR, Stroustrup mentioned the poor
quality of linkers in 1st edition
of his book. Where can I find, C++ Linker specific information?
Regards,
Saeed Amrollahi
Triple-DES
> On Sep 29, 2:17 pm, Pete Becker <p...@versatilecoding.com> wrote:
> > On 2008-09-29 06:01:07 -0400, ebony.s...@gmail.com said:
>
> > > I encountered a simple but IMO important problem about the C++ linkage
> > > model and One Definition Rule. Why the following code link?
>
> > > file1.cpp
> > > int x;
>
> > > file2.cpp
> > > double x;
>
> > > x was defined in two different translation units with different types.
> > > It breaks the ODR.
>
> > The language definition doesn't require compilers to diagnose
> > violations of the ODR. Code like this has undefined behavior. As a
> > practical matter, recognizing errors like this is expensive, given
> > C++'s separate compilation model.
>
> Thank you pete for you answer, but I didn't completely convince.
> I thought may be some standard conversions were made (double to int or
> vice versa)
> I changed double to std::string, but program run. Just when I declare
> the variable
> with extern, I have to specifiy the type and compiler find the real
> type.
> Which clause or section of C++ standard draft mention your answer?
>
See 3.2 One definition rule [basic.def.odr]
> Regards,
> - Saeed Amrollahi
James Kanze
> On Sep 29, 4:44 pm, James Kanze <james.ka...@gmail.com> wrote:
> > On Sep 29, 1:17 pm, Pete Becker <p...@versatilecoding.com> wrote:
> > > On 2008-09-29 06:01:07 -0400, ebony.s...@gmail.com said:
> > > > I encountered a simple but IMO important problem about the
> > > > C++ linkage model and One Definition Rule. Why the following
> > > > code link?
> > > > file1.cpp
> > > > int x;
> > > > file2.cpp
> > > > double x;
> > > > x was defined in two different translation units with
> > > > different types. It breaks the ODR.
> > > The language definition doesn't require compilers to
> > > diagnose violations of the ODR. Code like this has
> > > undefined behavior. As a practical matter, recognizing
> > > errors like this is expensive, given C++'s separate
> > > compilation model.
> > And the relatively low quality of most linkers. It wouldn't
> > be very hard to implement, if the linker supported it. For
> > historical reasons, most linkers don't.
> Thank you for your answer. AFAIR, Stroustrup mentioned the poor
> quality of linkers in 1st edition
> of his book.
Yes. C++ was originally designed to use existing linkers, with
a minimum of additional baggage. (There was, IIRC, always a
"pre-linker", which generated the code for dynamic
initialization. But that was it.)
Today, of course, templates require a lot more support, and much
of it could be used to allow greater freedom elsewhere as well.
> Where can I find, C++ Linker specific information?
To tell the truth, I don't know. Linkers seem to be the poor
relative in the language translation area: you'll find all sorts
of information concerning compiler theory and the like, but very
little about linkers.
I think the question has been raised once or twice in
comp.compilers; you might try there. (Not because the question
is off subject here, but because that's where the people who
actually work on these things hang out.)
--
James Kanze (GABI Software) email:james...@gmail.com