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From: David Brown <david.brown@hesbynett.no>
Newsgroups: comp.lang.c
Subject: Re: constexpr keyword is unnecessary
Date: Sun, 20 Oct 2024 12:55:19 +0200
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On 19/10/2024 23:24, Keith Thompson wrote:
> David Brown <david.brown@hesbynett.no> writes:
>> On 19/10/2024 17:18, Thiago Adams wrote:
>>> Em 10/18/2024 8:54 PM, Keith Thompson escreveu:
>>>> Thiago Adams <thiago.adams@gmail.com> writes:
>>>>> I think constexpr keyword is unnecessary.
>>>>
>>>> Sure, most language features are strictly unnecessary.
>>>>
>>>>> Anything you do with it could/should be done with const.
>>>>
>>>> No, absolutely not.
>>>>
>>> If not, do you have a sample where, using "const" as "constexpr",
>>> would create problems?
>>> The sample I know is VLA.
>>> const int c = 2;
>>> int a[c]; //a is VLA because c is not a constant expression.
>>> But this is not enough to convince me because it is better not to be
>>> a VLA here.
>>>
>>
>> What practical difference would it make?  Can you think of any
>> difference between local variables "a" and "b" defined like this?
>>
>> 	enum { n = 2 };
>> 	const int c = n;
>> 	int a[c];
>> 	int b[n];
>>
>> Can you show any situation where you could use "a" and not "b", or
>> vice versa, or where the meaning would be different?  Can you show any
>> compiler that treats them differently in code generation (assuming a
>> compiler that supports enough of C99 to allow it)?
>>
>> I know of no differences there.  That is enough to convince me that it
>> doesn't matter in the slightest whether it is technically a VLA or
>> not.
> 
> VLAs are optional in C11 and later.  A conforming implementation that
> doesn't support VLAs will accept `int b[n];` and reject `int a[c];`.

Yes.  That's why I said "a compiler that supports enough of C99 to allow 
it".  (I think it was a really bad idea to make VLAs optional in C11 - I 
can't see any justification for it other than pressure from a big 
compiler vendor that has long neglected implementation of C standards.)

> 
> `sizeof a` is not a constant expression, so it can't be used in a case
> label or any other context that requires a constant expression.  (If
> `const int c = n;` made c a constant expression, this would not apply.)
> 

Ah, there you have a difference.  Thanks.

> Generated code for constructs that a compiler acccepts is likely to be
> identical.  For a VLA type with a non-constant length, the compiler must
> implicitly store the size somehow.  For a VLA type with a length that
> the compiler can evaluate at compile time, the compiler is likely to
> generate code equivalent to that for a non-VLA.
> 

Yes, that's the point - and that's why I wonder why Thiago is happy with 
an array with a constant expression size, but dislikes one with a 
non-constant size that is known and fixed at compile time, just because 
it is technically a VLA.