Array Design Idea

[Sean Kelly]

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.


Sean