Does 'D' language supports 'C' like VLA?

[Steven Schveighoffer via Digitalmars-d]

On 4/13/15 12:53 PM, BS & LD wrote:
> As you know in 'C' you can create a variable-length-array using variably
> modified type and a run-time variable allocating a storage for it - the
> same way for any local (normally using the stack).
>
> However in 'D' I don't see such feature. Code like this fails:
>
> void main()
> {
>      size_t szArr = 3;
>
>      int[szArr] arr;
> }
>
> With this error message:
>
> error: variable szArr cannot be read at compile time
>       int[szArr] arr;
>
> Life example - http://melpon.org/wandbox/permlink/a6CzBhYk88FohKlf

Note, it's best to show when comparing C/C++ to D the C++ code and how 
you expect it to work too.

I experimented a bit with C++ to see what it will do:

#include <stdio.h>

int main(int argc, char *argv[])
{
     int arr[argc];
     int i;
     printf("i: %p, arr[0]: %p\n", &i, &arr[0]);
}

Which makes the array completely variable size depending on argc (a 
runtime variable). The results surprised me a bit:

$ ./testvararray
i: 0x7fff5aeadb5c, arr[0]: 0x7fff5aeadb30
$ ./testvararray a
i: 0x7fff5a955b4c, arr[0]: 0x7fff5a955b20
$ ./testvararray a b
i: 0x7fff5854eb4c, arr[0]: 0x7fff5854eb20
$ ./testvararray a b c
i: 0x7fff5fb12b3c, arr[0]: 0x7fff5fb12b10
$ ./testvararray a b c d
i: 0x7fff528e1b2c, arr[0]: 0x7fff528e1af0

So the code will move i around depending on arr size. It's consistent 
between runs as well, if you pass the same number of args. I didn't 
expect that, but I wasn't sure exactly what I expected :)

D doesn't do this, you have to know the size of the stack array at 
compile time. You can use alloca, which will give you some runtime 
allocation of stack, but it can be dangerous (as noted).

>
> Note:  I'm also amazed why 'D' compiler can't detect that 'szArr' is a
> constant anyway.

Value range propagation (the compiler understanding what values a 
variable can be at some point in time) only is inside one statement. It 
does not remember what szArr can be at a later statement.

-Steve