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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, 8 Jan 2025 17:05:24 +0100
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On 08/01/2025 16:16, Ben Bacarisse wrote:
> Julio Di Egidio <julio@diegidio.name> writes:
>> On 07/01/2025 23:11, Kaz Kylheku wrote:
>>> On 2025-01-07, Julio Di Egidio <julio@diegidio.name> wrote:
<snipped>
>>>> To the question, I was reading this, but I am not
>>>> sure what the quoted passage means:
>>>>
>>>> Matt Stancliff, "So You Think You Can Const?",
>>>> <https://matt.sh/sytycc>
>>>> << Your compiler, at its discretion, may also choose
>>>>       to place any const declarations in read-only storage,
>>>>       so if you attempt to hack around the const blocks,
>>>>       you could get undefined behavior. >>
>>
>>> An object defined with a type that is const-qualified
>>> could be put into write-protected storage.
>>
>> What do you/we mean by "object" in this context?  (Sorry, I do have
>> forgotten, the glossary to begin with.)
> 
> An object (in C) is a contiguous region of storage, the contents of
> which can represent values.

Is that regardless of the stack/heap distinction, or is an "object" 
about heap-allocated/dynamic memory only?  --  Anyway, I should in fact 
re-acquaint myself with the language reference instead of asking this 
question.)

>> Overall, I am surmising this and only this might go write-protected:
>>
>>    MyStruct_t const T = {...};
> 
> Yes, though you should extend your concern beyond what might be
> write-protected.  Modifying an object whose type is const qualified is
> undefined, even if the object is in writable storage.

Yes, I am being a bit quick, but I definitely agree with that and indeed 
the priority of "defined behaviour" as a concern.

>> While this one allocates a "byte-array", i.e. irrespective of how the
>> pointer we are assigning it is declared:
>>
>>    MyStruct_t const *pT = malloc(...);
>>
>> Is my understanding (to that point) correct?
> 
> Technically you get an object with no effective type.

OK.

> More relevant to a discussion of const is to ask what you plan to do
> with pT since you can't (without a cast) assign any useful value to the
> allocated object.

Say my program unit implements AVL trees, with (conceptually speaking) 
constructors/destructors, navigation and retrieval, and of course 
manipulation (inserting, deleting, etc.).

My idea (but I would think this is pretty "canonical" and, if it isn't, 
I am missing the mark) is: my public functions take/give "sealed" 
instances (with const members to const data), as the user is not 
supposed to directly manipulate/edit the data, OTOH of course my 
implementation is all about in-place editing...

-Julio