const int not constant expression?

[William Payne]

Hello, in my application (which spans approximately 20 source files and a
2-3 thousand lines) I have a few global varialbes, variables that never
change their values. One of them is declared like this:
/* globals.h */
extern const int g_tray_icon_cal lback;

/* globals.cpp */
const int g_tray_icon_cal lback = 4711;

Now, when I tried to use that variable as a case label in switch statement,
I got:
main_dialog_pro cedure.cpp:64: error: case label does not reduce to an
integer
constant

So, I replaced the const int with a #define and it compiles, but why can't I
use a const int? I thought const would be const, but I guess not...

// William Payne

[Andrey Tarasevich]

Re: const int not constant expression?

William Payne wrote:[color=blue]
> Hello, in my application (which spans approximately 20 source files and a
> 2-3 thousand lines) I have a few global varialbes, variables that never
> change their values. One of them is declared like this:
> /* globals.h */
> extern const int g_tray_icon_cal lback;
>
> /* globals.cpp */
> const int g_tray_icon_cal lback = 4711;
>
> Now, when I tried to use that variable as a case label in switch statement,
> I got:
> main_dialog_pro cedure.cpp:64: error: case label does not reduce to an
> integer
> constant
>
> So, I replaced the const int with a #define and it compiles, but why can't I
> use a const int? I thought const would be const, but I guess not...[/color]

You can. But in order to be able to use this 'const int' variable in a
case label, you have to declare it _with_ _initilaizer_ in the same
translation unit where your 'switch' is located.

Just put

const int g_tray_icon_cal lback = 4711;

into your 'globals.h' file and remove the definition from 'globals.cpp'.

--
Best regards,
Andrey Tarasevich
Brainbench C and C++ Programming MVP

[ES Kim]

Re: const int not constant expression?

"William Payne" <mikas493_no_sp am@student.liu. se> wrote in message
news:bieaqv$3n0 $1@news.island. liu.se...[color=blue]
> Hello, in my application (which spans approximately 20 source files and a
> 2-3 thousand lines) I have a few global varialbes, variables that never
> change their values. One of them is declared like this:
> /* globals.h */
> extern const int g_tray_icon_cal lback;
>
> /* globals.cpp */
> const int g_tray_icon_cal lback = 4711;
>
> Now, when I tried to use that variable as a case label in switch statement,
> I got:
> main_dialog_pro cedure.cpp:64: error: case label does not reduce to an
> integer
> constant
>
> So, I replaced the const int with a #define and it compiles, but why can't I
> use a const int? I thought const would be const, but I guess not...
>
> // William Payne
>
>[/color]

It's because g_tray_icon_cal lback defined in globals.cpp has
internal linkage. const objects have internal linkage by default.
Andrey provided a solution. Another is to make it have external
linkage.

/* globals.cpp */
extern const int g_tray_icon_cal lback = 4711;

--
ES Kim

[William Payne]

Re: const int not constant expression?


"Andrey Tarasevich" <andreytarasevi ch@hotmail.com> wrote in message
news:vklcik96v0 fr5f@news.super news.com...[color=blue]
> William Payne wrote:[color=green]
> > Hello, in my application (which spans approximately 20 source files and[/color][/color]
a[color=blue][color=green]
> > 2-3 thousand lines) I have a few global varialbes, variables that never
> > change their values. One of them is declared like this:
> > /* globals.h */
> > extern const int g_tray_icon_cal lback;
> >
> > /* globals.cpp */
> > const int g_tray_icon_cal lback = 4711;
> >
> > Now, when I tried to use that variable as a case label in switch[/color][/color]
statement,[color=blue][color=green]
> > I got:
> > main_dialog_pro cedure.cpp:64: error: case label does not reduce to an
> > integer
> > constant
> >
> > So, I replaced the const int with a #define and it compiles, but why[/color][/color]
can't I[color=blue][color=green]
> > use a const int? I thought const would be const, but I guess not...[/color]
>
> You can. But in order to be able to use this 'const int' variable in a
> case label, you have to declare it _with_ _initilaizer_ in the same
> translation unit where your 'switch' is located.
>
> Just put
>
> const int g_tray_icon_cal lback = 4711;
>
> into your 'globals.h' file and remove the definition from 'globals.cpp'.
>
> --
> Best regards,
> Andrey Tarasevich
> Brainbench C and C++ Programming MVP
>[/color]

Thank you for the help

// William Payne

[Andrey Tarasevich]

Re: const int not constant expression?

ES Kim wrote:[color=blue]
> "William Payne" <mikas493_no_sp am@student.liu. se> wrote in message
> news:bieaqv$3n0 $1@news.island. liu.se...[color=green]
>> Hello, in my application (which spans approximately 20 source files and a
>> 2-3 thousand lines) I have a few global varialbes, variables that never
>> change their values. One of them is declared like this:
>> /* globals.h */
>> extern const int g_tray_icon_cal lback;
>>
>> /* globals.cpp */
>> const int g_tray_icon_cal lback = 4711;
>>
>> Now, when I tried to use that variable as a case label in switch statement,
>> I got:
>> main_dialog_pro cedure.cpp:64: error: case label does not reduce to an
>> integer
>> constant
>>
>> So, I replaced the const int with a #define and it compiles, but why can't I
>> use a const int? I thought const would be const, but I guess not...
>>
>> // William Payne
>>
>>[/color]
>
> It's because g_tray_icon_cal lback defined in globals.cpp has
> internal linkage.[/color]

That's not true. Provided the declaration of 'g_tray_icon_ca llback' in
'globals.h' is visible in 'globals.cpp' (i.e. 'globals.h' is included
into 'globals.cpp'), constant object 'g_tray_icon_ca llback' has external
linkage since it is declared with 'extern' specifier.
[color=blue]
> const objects have internal linkage by default.[/color]

Yes. But in this case it is not "by default". 'g_tray_icon_ca llback' is
declared in 'globals.cpp' with explicit 'extern' specifier.
[color=blue]
> Andrey provided a solution.[/color]

My solution actually changes the linkage of 'g_tray_icon_ca llback' to
internal, meaning that each translation unit will get its own completely
independent instance of 'g_tray_icon_ca llback'.
[color=blue]
> Another is to make it have external
> linkage.
>
> /* globals.cpp */
> extern const int g_tray_icon_cal lback = 4711;[/color]

That's not necessary. The declaration in 'globals.h' is sufficient to
change the linkage of this object.

The whole point of my solution is to make the initializer ('= 4711')
visible in all translation units. Your solution doesn't do that, which
means that it won't solve anything.

--
Best regards,
Andrey Tarasevich
Brainbench C and C++ Programming MVP

[ES Kim]

Re: const int not constant expression?

"Andrey Tarasevich" <andreytarasevi ch@hotmail.com> wrote in message
news:dOmdnVrlSY 3Xm9aiXTWJiQ@co mcast.com...

[snip]

You're right. Thanks for your concrete explanation.

--
ES Kim