Re: C23 thoughts and opinions

Thiago Adams <thiago.adams@gmail.com> writes:

On 23/05/2024 09:17, David Brown wrote:

If I try to be precise about the terms "constant expression", "integer
constant expression", etc., I suspect I will get the details wrong
unless I spend a lot of time checking carefully.  So I hope it is good
enough for me to be a bit lazy and quote the error messages from gcc
(with "-std=c23 -Wpedantic").
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With this code, compilation fails "initialiser element is not a
constant" for y.
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      int x = 100;
      int y = x / 20;
      int zs[y];
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With this code, compilation fails because the zs is actually a VLA, and
"variably modified 'zs' at file scope" is not allowed.
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      const int x = 100;
      const int y = x / 20;
      int zs[y];
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This code, however, is fine:
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      constexpr int x = 100;
      constexpr int y = x / 20;
      int zs[y];
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This also works, even for older standards:
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      enum { x = 100 };
      enum { y = x / 20 };
      int zs[y];
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But constexpr works for other types, not just "int" which is the type of
all enumeration constants.  (And "enum" constants are a somewhat weird
way to get this effect - "constexpr" looks neater.)
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And in general, I like to be able to say, to the compiler and to people
reading the code, "this thing is really fixed and constant, and stop
compiling if you think I am wrong" rather than just "I promise I won't
change this thing - or if I do, I don't mind the nasal daemons".

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We can write:
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#define X 100
#define Y ((X) / 20)

 Neither of which convey type information.
 

int zs[Y];
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I cannot see a good justification for constexpr.

 Which does convey type information, and thus would
be superior to untyped macro definitions.