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From: Julio Di Egidio <julio@diegidio.name>
Newsgroups: comp.lang.c
Subject: Re: So You Think You Can Const?
Date: Wed, 15 Jan 2025 13:53:22 +0100
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On 08/01/2025 13:25, David Brown wrote:
> 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.

LOOK AT THESE TWO PIECES OF NAZI-RETARDED SPAMMING SHIT.

-Julio

>>> 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.
>>
>