Re: C23 thoughts and opinions

On 25/05/2024 13:19, Thiago Adams wrote:

Em 5/25/2024 8:05 AM, David Brown escreveu:

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In C (not C++), defining an object as "constexpr" gives you two things compared to defining it as "const".  One is that its value can be used when you need a constant expression according to the rules of the language (such as for the size of an array in a struct).  The other is that it gives a compile-time error if its initialiser is not itself a constant expression - and that means an extra check and protection against some kinds of programmer errors, and extra information to people reading the code.
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I don't expect it to make a difference in generated code from an optimising compiler, in comparison to objects declared with "const".
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 In my view , for this sample constexpr generates noise.

I don't share that opinion, but I understand it.

It also can make the compilation slower, otherwise, why not everything constexpr by defaul?

That claim, on the other hand, is very strange.  Making everything constexpr by default would be a massive change to the language that would break all but the most negligible of existing code.  And I can think of no particular reason why constexpr would slow down compilation, at least to any measurable degree.

I still didn't find a useful usage for constexpr that would compensate the mess created with const, constexpr.

I don't need a feature to "compensate" for anything to be useful.  I don't need it to be perfect to be useful.  There's a few things about constexpr in C23 that I think are poor decisions, unreasonable restrictions, or suboptimal integration with other language features (like static_assert) - such as the array limitations you've found.  That will mean I can't use constexpr as much as I'd like, or as much as I do in C++.  But even if there is just one situation where I think using constexpr is neater or clearer than using enum, #define, or some other technique, then I will use constexpr in that one situation.  Why are you so insistent on throwing it out completely just because it doesn't do everything you might want?

I already saw ( I don't have it now ) proposals to make const more like constexpr in C. In C++ const is already a constant expression!

No, it is not - but sometimes a const object with particular characteristics can be used in situations where you would otherwise need a constant expression.  I mentioned earlier that I find this convenient in C++ - Keith said it was inconsistent, which is also true.  I think that to a large extent, if C "const" had acquired the additional features of C++ "const" (excluding the different linkage for file-scope "const" objects, since that would be a breaking change) then it would have done everything C23 "constexpr" does today.  I personally would have been fine with that as a solution.  But I fully appreciate that it would have been inconsistent and perhaps hard to specify - you'd would have the situation that /some/ const objects could be used for things like static initialisers, while others could not.

The justification for C was VLA. They should consider VLA not VLA if it has a constant expression. In other words, better break this than create a mess.
#define makes the job of constexpr.
 

#define is one way to make named items that can be used in constant expressions, yes.  But if it can be done using #define or constexpr, I think constexpr is the neater choice.  Opinions can vary - that's my opinion.