const issues

const issues

[Jason Pepas]

this compiles as expected:


#include <stdlib.h>

int main()
{
    const int a = 42;
    const int * const b = &a;
    const int * const c = b;

    return EXIT_SUCCESS;
}


but moving the variables outside of the function causes gcc to die
with "error: initializer element is not constant" on line 5 (the "c =
b" line):


#include <stdlib.h>

const int a = 42;
const int * const b = &a;
const int * const c = b;

int main()
{
    return EXIT_SUCCESS;
}


As best as I can tell, the error is a lie: I can't make that  any more
const than it already is.

ideas?

-jason pepas

Re: const issues

[Dario Saccavino]

Hi Jason,

your code isn't valid C. A global variable can only be initialized
with a compile-time constant, and a const variable isn't a
compile-time constant in C. Compare with the following program:

#include <stdlib.h>

const int a = 42;
const int b = a; // Error

int main()
{
   return 0;
}

See e.g. http://c-faq.com/ansi/constasconst.html

By contrast, in C++ a global const integral variable is a compile-time
constant, and you can use it as an initializer.

   Dario

2008/2/29, Jason Pepas <jasonpepas@gmail.com>:
> this compiles as expected:
>
>
>  #include <stdlib.h>
>
>  int main()
>  {
>     const int a = 42;
>     const int * const b = &a;
>     const int * const c = b;
>
>     return EXIT_SUCCESS;
>  }
>
>
>  but moving the variables outside of the function causes gcc to die
>  with "error: initializer element is not constant" on line 5 (the "c =
>  b" line):
>
>
>  #include <stdlib.h>
>
>  const int a = 42;
>  const int * const b = &a;
>  const int * const c = b;
>
>  int main()
>  {
>     return EXIT_SUCCESS;
>  }
>
>
>  As best as I can tell, the error is a lie: I can't make that  any more
>  const than it already is.
>
>  ideas?
>
>
>  -jason pepas
>

Re: const issues

[Jason Pepas]

On Fri, Feb 29, 2008 at 2:06 AM, Dario Saccavino <kathoum@gmail.com> wrote:
> Hi Jason,
>
>  your code isn't valid C. A global variable can only be initialized
>  with a compile-time constant, and a const variable isn't a
>  compile-time constant in C. Compare with the following program:
>
>
>  #include <stdlib.h>
>
>  const int a = 42;
>  const int b = a; // Error
>
>  int main()
>  {
>    return 0;
>  }
>
>  See e.g. http://c-faq.com/ansi/constasconst.html
>
>  By contrast, in C++ a global const integral variable is a compile-time
>  constant, and you can use it as an initializer.

Ahh, that must have been the source of my confusion.  Thanks!

-jason