Re: constexpr keyword is unnecessary

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.

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Sure, most language features are strictly unnecessary.
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Anything you do with it could/should be done with const.

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No, absolutely not.
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If not, do you have a sample where, using "const" as "constexpr", would create problems?
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The sample I know is VLA.
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const int c = 2;
int a[c]; //a is VLA because c is not a constant expression.
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But this is not enough to convince me because it is better not to be a VLA here.
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What practical difference would it make?

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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.
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 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.