Array Design Idea

[Jb]

"Sean Kelly" <sean at f4.ca> wrote in message 
news:fjk706$134g$1 at digitalmars.com...
> Craig Black wrote:
>> "Jb" <jb at nowhere.com> wrote in message 
>> news:fjk5g9$10lb$1 at digitalmars.com...
>>> "Craig Black" <cblack at ara.com> wrote in message 
>>> news:fjk2k6$pfl$1 at digitalmars.com...
>>>
>>>> There are three things that need to be stored for a resizable array: 
>>>> the pointer to the array, the size, and the capacity.  My array 
>>>> implementation, both the size and the capacity are stored on the heap 
>>>> with the array.  There is only one heap allocation, and the first 8 
>>>> bytes are reserved for the size and capacity values.  Further, there is 
>>>> no heap allocation if the array is empty.  If the array is empty, then 
>>>> the pointer is null, and the array is assumed to have a size and 
>>>> capacity of zero, so there is no need to store the size or capacity of 
>>>> an empty array.
>>> Thats how Delphi does dynamic arrays and strings. Well except that it 
>>> has length & refcount rather than length & capacity. Wouldnt it break 
>>> slicing though? You cant point halfway into an existing array and still 
>>> have the length & capacity at -4,-8 bytes?
>>
>> I still haven't wrapped my head around the slicing issue.  Obviously it's 
>> not an issue for C++ arrays.  Doesn't D already allocate the capacity of 
>> the array on the heap?  If this is so why doesn't this break slicing?
>
> D relies on the GC to tell it what the capacity for a block is. However, 
> because operations on slices should always reallocate, it is enough for 
> the GC to simply return 0 for the capacity if the supplied pointer is not 
> at the head of a memory block.

Except that a slice can somtimes point at the begining of a block. So slices 
dont always reallocate, although perhaps they should?