Re: constexpr keyword is unnecessary

On 20/10/2024 13:23, Bart wrote:

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.
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I've seen endless exampples where people inadvertently created VLAs, and where they are likely to less efficient.
 

I can't say I have ever seem such as situation.  But of course experiences differ.

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.
 

Yes, a VLA where the size is not known at compile time will be less efficient than when the size is fixed.  It might still be a lot more efficient than alternatives, such as using malloc().  I think VLAs - with run-time sizes - have their uses, but you should be aware of when you use them.

 

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.
 

Yes, that is something I also see as an advantage.