built-in array ptrEnd

[Steven Schveighoffer]

On Tue, 18 Sep 2012 08:02:29 -0400, Don Clugston <dac at nospam.com> wrote:

> On 17/09/12 18:40, bearophile wrote:
>> monarch_dodra:
>>
>>> IMO, this should really be built-in, in particular, since, in my
>>> understanding, an array is internally represented by the ptr and
>>> ptrEnd pair anyways.
>>
>> Currently this is not true, take a look at the ABI part in the D site.
>> Currently it's a pointer and length. Walter and/or Andrei discussed the
>> idea of turning them into two pointers, but I don't know if and why that
>> change was refused.
>
> Because it would be a mistake. You can efficiently get from (ptr,  
> length) to (ptr, endPtr) but the reverse is not true.

There is another reason to avoid this.

Note that if I have two consecutive blocks of memory:

0...4
and
4...8

If we define an array that points to the first block as a pointer to 0 and  
a pointer to 4, then that array also effectively points at the second  
block (4...8).  The way the GC works, it will not release the second block  
as long as you have a pointer to the first, even though the second pointer  
is not technically pointing at the block.

-Steve