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Re: What does it mean to evaluate a VLA?

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On 12/18/2012 20:28, Keith Thompson wrote:
> N1370 6.5.3.4p2, discussing the sizeof operator, says:
>
>      If the type of the operand is a variable length array type, the
>      operand is evaluated; otherwise, the operand is not evaluated
>      and the result is an integer constant.
>
> What does it mean to "evaluate" the operand of sizeof when it's of a
> variable length array type?
>

I think it means to evaluate it as a void expression, for its side effects.

I would be startled if:

   size_t sz = sizeof (func1(), (int[func2()]){0});

didn't evaluate 'func1', which might influence the output of 'func2'. 
(Untested.)

> An example:
>
>      int n = 42;
>      volatile int vla[n];
>      printf("sizeof vla = %zu\n", sizeof vla);
>

I think the element type is 'volatile', not the array[C11 6.7.3p9].

It's still interesting that if the "as a void expression" theory above 
is correct, "nothing happens" for 'vla' here, because it's an array and 
an operand to 'sizeof', so not even a pointer value need be computed 
(else 'register' could go wrong).

But there's another situation where evaluation seems to happen despite 
apparent suppression:

   size_t sz = sizeof *( 0 ? 0 : (int(*)[myfunc()]) 0 );

The ternary's third operand "isn't evaluated," but because its type 
needs to be determined, those portions required for that determination 
are.  In this case, 'myfunc' is called.

I like to imagine that expressions are split into at least "type" and 
"value," and evaluation for the determination of one may be separate 
from evaluation for purposes of the other.  I think the non-VLA 'sizeof' 
case suppresses evaluations for value, but doesn't suppress those for type:

   size_t sz = sizeof (int[42 ? 1 : 3]);

- Shao Miller

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Thread

What does it mean to evaluate a VLA? Keith Thompson <kst-u@mib.org> - 2012-12-18 17:28 -0800
  Re: What does it mean to evaluate a VLA? Shao Miller <sha0.miller@gmail.com> - 2012-12-18 21:09 -0500
    Re: What does it mean to evaluate a VLA? Shao Miller <sha0.miller@gmail.com> - 2012-12-20 03:52 -0500
      Re: What does it mean to evaluate a VLA? Shao Miller <sha0.miller@gmail.com> - 2012-12-20 04:08 -0500
  Re: What does it mean to evaluate a VLA? Tim Rentsch <txr@alumni.caltech.edu> - 2012-12-20 20:43 -0800
    Re: What does it mean to evaluate a VLA? Shao Miller <sha0.miller@gmail.com> - 2012-12-21 05:01 -0500
      Re: What does it mean to evaluate a VLA? Tim Rentsch <txr@alumni.caltech.edu> - 2012-12-21 08:55 -0800

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