extern const variable in case label

**thomas** · Oct 2 '08, 09:55 AM

Re: extern const variable in case label

On Oct 2, 5:23 pm, "Christian Meier" <chris@no_spam. comwrote:

Hello NG
>
I have the following code:
>
file1.h:
>
static const int iValue = 5;
>
<EOF>
>
file2.cpp
>
#include <iostream>
#include "file1.h"
>
int main(int args, char* argv[])
{
switch(args) {
case iValue:
std::cout << "Hello\n";
}
>
}
>
<EOF>
>
This works fine as the value of the constant "iValue" is known when
compiling the main()-function in file2.cpp.
I have some more cpp-files which include file1.h. Therefore I have a copyof
"iValue" in each translation unit as it has internal linkage. In the
executable which is made with these object files (translation units) I have
several copies of that variable; all having the same value.
>
Now, I would like to change the linkage of "iValue" to extern so that there
is only one symbol for this constant in my executable.
When I change the keyword "static" to "extern" I have the problem that the
symbol is more than once defined in my executable.
When I only declare the variable "iValue" in file1.h with external linkage
("extern const int iValue;") and define it in a new file called file1.cpp
("extern const iValue = 5;"), then I have no problem with multiple
definitions but I have a new problem in my function main() because the value
of "iValue" is not known when compiling file2.cpp.
>
Do you have any suggestions how I can compile all my code without having
more than one symbol for "iValue" in my program? Or is it not possible what
I am trying to reach?
>
Thanks for all your answers in advance,
Chris

well. I think you should keep in mind that the case value can only be
constant integral values.

**Christian Meier** · Oct 2 '08, 10:15 AM

Re: extern const variable in case label

well. I think you should keep in mind that the case value can only be

constant integral values.

What do you exactly mean with this? Isn't "extern const int iValue" a
constant integral value?

**Martin Eisenberg** · Oct 2 '08, 11:55 AM

Re: extern const variable in case label

Christian Meier wrote:

Do you have any suggestions how I can compile all my code
without having more than one symbol for "iValue" in my program?
Or is it not possible what I am trying to reach?

How about this?

enum { iValue = int(5) };

Martin

--
Quidquid latine scriptum est, altum videtur.

**Christian Meier** · Oct 2 '08, 12:25 PM

Re: extern const variable in case label

"Martin Eisenberg" <martin.eisenbe rg@udo.eduwrote :

Christian Meier wrote:
>

>Do you have any suggestions how I can compile all my code
>without having more than one symbol for "iValue" in my program?
>Or is it not possible what I am trying to reach?

>
How about this?
>
enum { iValue = int(5) };
>

Thanks for your answer!
Ok, you're right, that could be a solution for "int".
To tell a bit more of the truth, I have these constants for quite all
built-in types (double, long int, short...) and IIRC enum values must have
type int.

**James Kanze** · Oct 2 '08, 01:35 PM

Re: extern const variable in case label

On Oct 2, 11:23 am, "Christian Meier" <chris@no_spam. comwrote:

I have the following code:

file1.h:

static const int iValue = 5;

<EOF>

file2.cpp

#include <iostream>
#include "file1.h"

int main(int args, char* argv[])
{
switch(args) {
case iValue:
std::cout << "Hello\n";
}
}
<EOF>

This works fine as the value of the constant "iValue" is known
when compiling the main()-function in file2.cpp.
I have some more cpp-files which include file1.h. Therefore I
have a copy of "iValue" in each translation unit as it has
internal linkage. In the executable which is made with these
object files (translation units) I have several copies of that
variable; all having the same value.

Maybe. Maybe not.

Now, I would like to change the linkage of "iValue" to extern
so that there is only one symbol for this constant in my
executable. When I change the keyword "static" to "extern" I
have the problem that the symbol is more than once defined in
my executable. When I only declare the variable "iValue" in
file1.h with external linkage ("extern const int iValue;") and
define it in a new file called file1.cpp ("extern const iValue
= 5;"), then I have no problem with multiple definitions but I
have a new problem in my function main() because the value of
"iValue" is not known when compiling file2.cpp.

Do you have any suggestions how I can compile all my code
without having more than one symbol for "iValue" in my
program? Or is it not possible what I am trying to reach?

Do you have some valid reason for wanting iValue to have the
same address in every translation unit? It seems strange to me
to give an int identity, especially if you're not going to
change it. And otherwise, what's the problem with what you have
written initially?

--
James Kanze (GABI Software) email:james.kan ze@gmail.com
Conseils en informatique orientée objet/
Beratung in objektorientier ter Datenverarbeitu ng
9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34

**Christian Meier** · Oct 2 '08, 03:35 PM

Re: extern const variable in case label

"James Kanze" <james.kanze@gm ail.comwrote:

Do you have some valid reason for wanting iValue to have the
same address in every translation unit?

No...

It seems strange to me to give an int identity, especially if you're not
going to
change it.

What else would you use for a global integer constant? Would you use a
#define?

And otherwise, what's the problem with what you have written initially?

With my compiler and linker I have these symbols over 10 000 times in my
executable and I was able to reduce the program size a bit with changing
these constants to extern which are not used in case labels.

**James Kanze** · Oct 2 '08, 06:45 PM

Re: extern const variable in case label

On Oct 2, 5:33 pm, "Christian Meier" <chris@no_spam. comwrote:

"James Kanze" <james.ka...@gm ail.comwrote:

Do you have some valid reason for wanting iValue to have the
same address in every translation unit?

No...

It seems strange to me to give an int identity, especially
if you're not going to change it.

What else would you use for a global integer constant? Would you use a
#define?

No. Just a static int const, like you did originally.

And otherwise, what's the problem with what you have written
initially?

With my compiler and linker I have these symbols over 10 000
times in my executable and I was able to reduce the program
size a bit with changing these constants to extern which are
not used in case labels.

That is strange. I've yet to see a compiler where they'd take
any space at all, as long as their address wasn't taken.

--
James Kanze (GABI Software) email:james.kan ze@gmail.com
Conseils en informatique orientée objet/
Beratung in objektorientier ter Datenverarbeitu ng
9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34

**Barry** · Oct 3 '08, 03:05 AM

Re: extern const variable in case label

On 10ÔÂ2ÈÕ, ÏÂÎç5Ê±23·Ö, "Christian Meier" <chris@no_spam. comwrote:

Hello NG
>
I have the following code:
>
file1.h:
>
static const int iValue = 5;
>
<EOF>
>
file2.cpp
>
#include <iostream>
#include "file1.h"
>
int main(int args, char* argv[])
{
switch(args) {
case iValue:
std::cout << "Hello\n";
}
>
}
>
<EOF>
>
This works fine as the value of the constant "iValue" is known when
compiling the main()-function in file2.cpp.
I have some more cpp-files which include file1.h. Therefore I have a copyof
"iValue" in each translation unit as it has internal linkage. In the
executable which is made with these object files (translation units) I have
several copies of that variable; all having the same value.
>
Now, I would like to change the linkage of "iValue" to extern so that there
is only one symbol for this constant in my executable.
When I change the keyword "static" to "extern" I have the problem that the
symbol is more than once defined in my executable.
When I only declare the variable "iValue" in file1.h with external linkage
("extern const int iValue;") and define it in a new file called file1.cpp
("extern const iValue = 5;"), then I have no problem with multiple
definitions but I have a new problem in my function main() because the value
of "iValue" is not known when compiling file2.cpp.
>
Do you have any suggestions how I can compile all my code without having
more than one symbol for "iValue" in my program? Or is it not possible what
I am trying to reach?
>

IIRC, just

const int iValue = 5;

no 'static', no 'extern'

--
Best Regards
Barry

**Christian Meier** · Oct 3 '08, 05:55 AM

Re: extern const variable in case label

"James Kanze" <james.kanze@gm ail.comwrote:

It seems strange to me to give an int identity, especially
if you're not going to change it.

What else would you use for a global integer constant? Would you use a
#define?

No. Just a static int const, like you did originally.

Ok thanks. Probably I will not change the code so it remains a static const
int.

With my compiler and linker I have these symbols over 10 000
times in my executable and I was able to reduce the program
size a bit with changing these constants to extern which are
not used in case labels.

That is strange. I've yet to see a compiler where they'd take
any space at all, as long as their address wasn't taken.

Eventually my assumption was wrong about the more than 10 000 symbols for
this variable.... And I have not checked whether the address is taken.

<Offtopic: UNIX Commands and Compiler version>

But the command "nm MyExecutable | grep iValue | wc -l" has the output
"11059" at this moment.
We use GCC 4.1.0...

</Offtopic>

**Christian Meier** · Oct 3 '08, 05:55 AM

Re: extern const variable in case label

IIRC, just

const int iValue = 5;
no 'static', no 'extern'

IIRC, constants in the file scope have internal linkage (in C++).

**James Kanze** · Oct 3 '08, 09:25 AM

Re: extern const variable in case label

On Oct 3, 7:49 am, "Christian Meier" <chris@no_spam. comwrote:

"James Kanze" <james.ka...@gm ail.comwrote:

With my compiler and linker I have these symbols over 10
000 times in my executable and I was able to reduce the
program size a bit with changing these constants to extern
which are not used in case labels.

That is strange. I've yet to see a compiler where they'd
take any space at all, as long as their address wasn't
taken.

Eventually my assumption was wrong about the more than 10 000
symbols for this variable.... And I have not checked whether
the address is taken.

<Offtopic: UNIX Commands and Compiler version>

But the command "nm MyExecutable | grep iValue | wc -l" has
the output "11059" at this moment.
We use GCC 4.1.0...

</Offtopic>

Interesting. I would have thought 0 occurances (or at most,
only for those where the address was being taken). It's
possible that in debug mode, g++ would generate them, since you
could presumable modify the value with a debugger, but a quick
check with a single module on my system (g++ 4.1.0 under Solaris
and Linux) didn't reveal them. Even in debug mode: "nm a.out |
grep staticConst" (where the actual variable was named
staticConst) didn't have any output. Sun CC did generate the
object in debug mode (as a global, no less, but with a funny
prefix added to the name, presumably to make it distinct), but
not when optimizing was turned on.

I'm curious. If you compile the following program:

static int const staticConst = 42 ;

int
main()
{
return staticConst ;
}

then do "nm a.out | grep staticConst", what do you get (with and
without optimizing)?

--
James Kanze (GABI Software) email:james.kan ze@gmail.com
Conseils en informatique orientée objet/
Beratung in objektorientier ter Datenverarbeitu ng
9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34

**James Kanze** · Oct 3 '08, 09:35 AM

Re: extern const variable in case label

On Oct 3, 4:56 am, Barry <dhb2...@gmail. comwrote:

[...]

const int iValue = 5;

no 'static', no 'extern'

Both:
int const iValue = 5 ;
and
static int const iValue = 5 ;
mean exactly the same thing to the compiler. The choice of
which one to use is largely an issue of style; some people
prefer making the internal linkage explicit.

This is also one point where C and C++ are not compatible. If
the code is to be used in a header used in both languages, then
the static is necessary.

--
James Kanze (GABI Software) email:james.kan ze@gmail.com
Conseils en informatique orientée objet/
Beratung in objektorientier ter Datenverarbeitu ng
9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34

**Christian Meier** · Oct 3 '08, 11:35 AM

Re: extern const variable in case label

"James Kanze" <james.kanze@gm ail.comwrote:

On Oct 3, 7:49 am, "Christian Meier" <chris@no_spam. comwrote:

"James Kanze" <james.ka...@gm ail.comwrote:

With my compiler and linker I have these symbols over 10
000 times in my executable and I was able to reduce the
program size a bit with changing these constants to extern
which are not used in case labels.
That is strange. I've yet to see a compiler where they'd
take any space at all, as long as their address wasn't
taken.

>

Eventually my assumption was wrong about the more than 10 000
symbols for this variable.... And I have not checked whether
the address is taken.

>

<Offtopic: UNIX Commands and Compiler version>

>

But the command "nm MyExecutable | grep iValue | wc -l" has
the output "11059" at this moment.
We use GCC 4.1.0...

>

</Offtopic>

>
Interesting. I would have thought 0 occurances (or at most,
only for those where the address was being taken).

I don't know in which cases the address is needed. I suppose there are many
other situations than "&iValue".
What about passing it to a function which takes a "const int&" as parameter?
If references are implemented as pointers than the address is needed for
this.
Using this variable in such a way is not unusual in our project. Could this
be the reason for all (or at least almost) the symbols in my executable?

It's possible that in debug mode, g++ would generate them, since you
could presumable modify the value with a debugger, but a quick
check with a single module on my system (g++ 4.1.0 under Solaris
and Linux) didn't reveal them. Even in debug mode: "nm a.out |
grep staticConst" (where the actual variable was named
staticConst) didn't have any output. Sun CC did generate the
object in debug mode (as a global, no less, but with a funny
prefix added to the name, presumably to make it distinct), but
not when optimizing was turned on.
>
I'm curious. If you compile the following program:
>
static int const staticConst = 42 ;
>
int
main()
{
return staticConst ;
}
>
then do "nm a.out | grep staticConst", what do you get (with and
without optimizing)?

I don't get any output for this.

But for an extended version of your code, I got an indication:

$ cat main.cpp
#include <iostream>

static int const staticConst = 42;

void func(const int& ri)
{
std::cout << ri << '\n';
}

int main()
{
func(staticCons t);
return staticConst ;
}
$ g++ -o main main.cpp
$ nm main | grep staticConst
000000000040093 8 r staticConst
$ g++ -O3 -o main main.cpp
$ nm main | grep staticConst
$

Compiling this code without an explicit optimization level results in having
1 symbol staticConst. However, we are compiling everything with -O3.

**Joe Greer** · Oct 3 '08, 12:25 PM

Re: extern const variable in case label

"Christian Meier" <chris@no_spam. comwrote in
news:bb41a$48e6 030d$3e024b42$5 661@news.hispee d.ch:

>
But for an extended version of your code, I got an indication:
>
$ cat main.cpp
#include <iostream>
>
static int const staticConst = 42;
>
void func(const int& ri)
{
std::cout << ri << '\n';
}
>
int main()
{
func(staticCons t);
return staticConst ;
}
$ g++ -o main main.cpp
$ nm main | grep staticConst
000000000040093 8 r staticConst
$ g++ -O3 -o main main.cpp
$ nm main | grep staticConst
$
>
Compiling this code without an explicit optimization level results in
having 1 symbol staticConst. However, we are compiling everything with
-O3.
>
>
>

In its simplest form, the reference takes the address of the variable and
thus you get a symbol. With the optimizer turned on, it probably inlines
the function and you no longer need the variable, thus you don't see it
with -O3.

joe

extern const variable in case label

extern const variable in case label

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