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From: "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com>
Newsgroups: comp.lang.c
Subject: Re: So You Think You Can Const?
Date: Wed, 8 Jan 2025 14:48:06 -0800
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On 1/8/2025 8:05 AM, Julio Di Egidio wrote:
> 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...

Off topic, but for some reason you are making me think of the mutable 
keyword in C++:

https://en.cppreference.com/w/cpp/language/cv

;^)