can't initialize a constant using another constant?

can't initialize a constant using another constant?

[Shriramana Sharma]

Dear list,

Quite some time since I wrote. I hope you are all keeping well. :)

A strange thing I ran across this evening. I was not allowed to do:

const double lunarSynodicMonth = 29.53059 ;
const double daysLunarPhaseAngleTakesPerDegree = lunarSynodicMonth / 360.0 ;

I got the following error:

wdf-karanas.c:17: error: initializer element is not constant
make: *** [wdf-karanas.o] Error 1

I would be grateful if anyone could shed some light. TIA.

Shriramana Sharma.

Re: can't initialize a constant using another constant?

[Kristof Provost]

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On 2008-02-26 20:02:06 (+0530), Shriramana Sharma <samjnaa@gmail.com> wrote:
> Dear list,
> 
> Quite some time since I wrote. I hope you are all keeping well. :)
> 
> A strange thing I ran across this evening. I was not allowed to do:
> 
> const double lunarSynodicMonth = 29.53059 ;
> const double daysLunarPhaseAngleTakesPerDegree = lunarSynodicMonth / 360.0 ;
> 
> I got the following error:
> 
> wdf-karanas.c:17: error: initializer element is not constant
> make: *** [wdf-karanas.o] Error 1
> 
> I would be grateful if anyone could shed some light. TIA.
What compiler are you using? 
I did a quick test on g++ 4.1.2 (x86_64) and it seemed to be accepted
without warning.

Kristof
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Re: can't initialize a constant using another constant?

[Kristof Provost]

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On 2008-02-27 08:09:57 (+0000), Kristof Provost <Kristof@sigsegv.be> wrote:
> On 2008-02-26 20:02:06 (+0530), Shriramana Sharma <samjnaa@gmail.com> wrote:
> > A strange thing I ran across this evening. I was not allowed to do:
> > 
> > const double lunarSynodicMonth = 29.53059 ;
> > const double daysLunarPhaseAngleTakesPerDegree = lunarSynodicMonth / 360.0 ;
> > 
> > I got the following error:
> > 
> > wdf-karanas.c:17: error: initializer element is not constant
> > make: *** [wdf-karanas.o] Error 1
> > 
> > I would be grateful if anyone could shed some light. TIA.
> What compiler are you using? 
> I did a quick test on g++ 4.1.2 (x86_64) and it seemed to be accepted
> without warning.
I took a better look. I get the same error with gcc.

It looks like this is allowed in C++ and not in plain C (or at least,
g++ allows it and gcc doesn't).

Kristof
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Re: can't initialize a constant using another constant?

[Glynn Clements]

Shriramana Sharma wrote:

> Quite some time since I wrote. I hope you are all keeping well. :)
> 
> A strange thing I ran across this evening. I was not allowed to do:
> 
> const double lunarSynodicMonth = 29.53059 ;
> const double daysLunarPhaseAngleTakesPerDegree = lunarSynodicMonth / 360.0 ;
> 
> I got the following error:
> 
> wdf-karanas.c:17: error: initializer element is not constant
> make: *** [wdf-karanas.o] Error 1
> 
> I would be grateful if anyone could shed some light. TIA.

In C, "const" is only relevant to pointer targets. Adding the "const"
modifier to a variable has no effect.

You need to change lunarSynodicMonth to a macro (#define) if you want
to be able to use it in an initialiser.

-- 
Glynn Clements <glynn@gclements.plus.com>

Re: can't initialize a constant using another constant?

[Shriramana Sharma]

Thanks to all those who replied. I am very sorry I did not specify the 
compiler version etc. I should have. It's gcc version 4.1.3 20070929 
(prerelease) (Ubuntu 4.1.2-16ubuntu2).

Glynn Clements wrote:
> In C, "const" is only relevant to pointer targets. Adding the "const"
> modifier to a variable has no effect.

I don't understand what you mean. I just tried gcc -o foo foo.c on:

# include <stdio.h>
main () {
         const int i = 1 ;
         i = 2 ;
         printf ( "%d\n", i ) ;
}

and I got:

foo.c: In function ‘main’:
foo.c:6: error: assignment of read-only variable ‘i’

So in what sense are you saying adding const to a variable has no effect?

Shriramana Sharma.
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Re: can't initialize a constant using another constant?

[Glynn Clements]

Shriramana Sharma wrote:

> Thanks to all those who replied. I am very sorry I did not specify the 
> compiler version etc. I should have. It's gcc version 4.1.3 20070929 
> (prerelease) (Ubuntu 4.1.2-16ubuntu2).
> 
> Glynn Clements wrote:
> > In C, "const" is only relevant to pointer targets. Adding the "const"
> > modifier to a variable has no effect.
> 
> I don't understand what you mean. I just tried gcc -o foo foo.c on:
> 
> # include <stdio.h>
> main () {
>          const int i = 1 ;
>          i = 2 ;
>          printf ( "%d\n", i ) ;
> }
> 
> and I got:
> 
> foo.c: In function �main�:
> foo.c:6: error: assignment of read-only variable �i�
> 
> So in what sense are you saying adding const to a variable has no effect?

Sorry; my mistake.

However, although the compiler will prevent modification to const
objects (and if the compiler didn't, the CPU will, as they are stored
in the .rodata section, which is mapped read-only), they are still
considered variables rather than (compile-time) constants, and can't
be used in initialisers or in array dimensions.

In C++, const-qualified objects are treated as compile-time constants.

-- 
Glynn Clements <glynn@gclements.plus.com>
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