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From: Bart <bc@freeuk.com>
Newsgroups: comp.lang.c
Subject: Re: constexpr keyword is unnecessary
Date: Sun, 20 Oct 2024 12:23:41 +0100
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On 20/10/2024 11:45, David Brown wrote:
> On 19/10/2024 21:41, Thiago Adams wrote:
>> Em 10/19/2024 1:03 PM, David Brown escreveu:
>>> 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? 
>>
>> I don't see any practical difference. In theory, the generated code 
>> could be different, but I'm arguing that this doesn't really matter 
>> and, consequently, it's not a good reason to differentiate between 
>> const and constexpr.
>>
> 
> My point was that if there is no practical difference, then there is no 
> reason to object to the VLA.
> 
I've seen endless exampples where people inadvertently created VLAs, and 
where they are likely to less efficient.

It might start off like this:

     const int n = 10;
     int A[n];

Then they change something so that n is not evaluated until runtime 
(maybe it's defined in terms of a parameter). Now the compiler will 
silently generate less efficient code for a VLA, without giving the user 
a chance to use an alternative.


> You can't use this as a reason for arguing that it would have been 
> better for "const" in C to gain the features that are now in C23 
> "constexpr", because this use of "const" was already allowed in C99.  So 
> the "const" vs "constexpr" discussion is an orthogonal issue - I was 
> asking specifically about your comment regarding your apparent dislike 
> of VLA's.

The advantage of constexpr AIUI is that a non-constant initialiser for n 
is not allowed.