Memory allocation in D (noob question)

[Steven Schveighoffer]

"mandel" wrote in message
> Robert Fraser Wrote:
>
>> mandel wrote:
>> > It probably is a noob question,
>> > but aren't array lengths just hidden size_t values
>> > that are passed around?
>> > Why do we need to allocate space for them, too?
>> >
>> > voif foo()
>> > {
>> >   size_t length;
>> >   char* ptr; //allocated memory of 2^n
>> >   //.. the same as..?
>> >   char[] data;
>> > }
>>
>> The extra space allocated isn't for the length (in fact, it's just a
>> byte I think); it's to make checking for array bounds errors possible
>> (since there's a byte of space that, if accessed, indicates an
>> overflow). I tmight be used for something else, too.
>
> Thanks, that answers my question.
> But I can't think how it could be used for array bounds errors checking 
> right now.
> Well, I guess there some ng post about this, somewhere.
> But the page allocation overhead looks ugly for a language like D.

Think of it this way:

int[] array1 = new int[5];
int[] array2 = new int[5];

imagine that array 1 and array 2 are now sequential in memory *AND* there is 
no extra byte separating them.

Now I create the valid array slices:

int[] array3 = array1[$..$];
int[] array4 = array2[0..0];

Note that both of these arrays are bit-for-bit identical (both have 0 length 
and the same ptr value).  Which one points to which piece of memory?  How is 
the GC to decide which memory gets collected?

These are the types of problems that the extra byte helps with.

I personally think there exists a way to fix this efficiently without adding 
the extra byte, but I can't think of one :)

Oh, and also, the size_t length is not stored in the allocated memory.  It's 
stored in the array structure, usually on the stack or inside a class 
instance.

I hope this helps your understanding of the issue.

-Steve