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From: Kaz Kylheku <643-408-1753@kylheku.com>
Newsgroups: comp.lang.c
Subject: Re: "not-const" qualifier for C
Date: Thu, 25 Apr 2024 01:47:45 -0000 (UTC)
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Message-ID: <20240424183818.814@kylheku.com>
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On 2024-04-25, Thiago Adams <thiago.adams@gmail.com> wrote:
>
>
> Em 4/24/2024 8:13 PM, Kaz Kylheku escreveu:
>> On 2024-04-24, Thiago Adams <thiago.adams@gmail.com> wrote:
>>> Motivation sample:
>>>
>>> struct X {
>>>        const char* const type;
>>> };
>>>
>>> struct X * make_x(){
>>>     struct X * p = malloc(sizeof *p);
>>>     if (p)
>>>     {
>>>
>>>         p->type = strdup("X");  // *** error, type is const ***
>>>
>>>         if (p->type == NULL)
>>>         {
>>>            free(p);
>>>            p = NULL;
>>>         }
>>>     }
>>>     return p; //ok
>>> }
>> 
>> Different idea: allow all conversions without a cast which only
>> add qualifiers anywhere in the type:
>> 
>>    struct X {
>>      const char* const type;
>>    };
>> 
>>    struct mutable_X {
>>      char* type;
>>    };
>
> In this case the types struct X and struct mutable_X are not 
> convertible.  They are not the same type.

And your mutable keyword is a syntax error in the current C language!

The idea is that we allow "pointer to struct mutable_X" to convert to
"pointer to struct X" without a cast, because the two types are
structurally equivalent, and every element of the destination type at
least as qualified as its counterpart in the source type.

I don't entirely like the idea because it entails structural
equivalence. That has various problems. Ideological: we are used
to "struct foo" and "struct bar" (in the same translation unit)
being different types. Structural equivalence requires complete
types. We could end up with the situation where "foo *" cannot
convert to "bar *" without a cast in one scope in the program
where foo is an incomplete type, but in another scope, foo is
completed in a way that the assignment is compatible.

-- 
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