Array Design Idea

[Sean Kelly]

Jb wrote:
> "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?

Steven Schveighoffer's proposal entitled "Proposal:  alternative GC idea 
that negates the need for T[new]" would address issue, though it has yet 
to receive any responses.


Sean