Re: So You Think You Can Const?

On 08/01/2025 12:25, Ben Bacarisse wrote:

David Brown <david.brown@hesbynett.no> writes:
 

Suppose you have :
>
int v = 123; // Non-const object definition
const int * cp = &v; // Const pointer to non-const data
int * p = (int *) cp; // Cast to non-const pointer
*p = 456; // Change the target data
>
This is allowed, because the original object definition was not a const
definition.
>
However, with this:
>
int v = 123; // Const object definition

Correction:
const int v = 123;

const int * cp = &v; // Const pointer to const data
int * p = (int *) cp; // Cast to non-const pointer
*p = 456; // Undefined behaviour

 I think missed out the crucial "const" on the first line of the second
example!  It's always the way.

Fortunately, Usenet is a self-correcting medium :-)  Thanks for pointing out that mistake, and I hope the OP sees your correction before getting confused by my copy-pasta error.

 

You can make the pointer to non-const, but trying to change an object that
was /defined/ as const is undefined behaviour (even if it was not placed in
read-only memory).
>
When you use dynamic memory, however, you are not defining an object in the
same way.  If you write :
>
const int * cp = malloc(sizeof(int));

 I prefer
      const int *cp = malloc(sizeof *cp);
 

That's a common preference.  Personally, I prefer the former - I think it makes it clearer that we are allocating space for an int.  Hopefully the OP will hang around this group and we'll get a chance to give advice and suggestions on many different aspects of C programming.

you are defining the object "p" as a pointer to type "const int" - but you
are not defining a const int.  You can cast "cp" to "int *" and use that
new pointer to change the value.