How are 2D static arrays allocated?

[Jarrett Billingsley]

On Tue, Nov 4, 2008 at 11:41 AM, Saaa <empty at needmail.com> wrote:
>>
>> You can have a dynamic array that _points_ to the stack, but there is
>> no general mechanism for allocating a dynamic array directly on the
>> stack.  You might be able to do something with alloca, but stack space
>> is usually limited and you wouldn't be able to have very large arrays.
>>
>> Performance of dynamic arrays is the same no matter where their data
>> is.  Fixed-size 2D arrays are not faster _because_ they are on the
>> stack, they just happen to be allocated on the stack.  They are faster
>> (usually) because they don't need two pointer dereferences.  You can
>> allocated a fixed-size 2D array on the heap (well.. inside a struct or
>> class anyway) and it will be just as fast.
>
> Ok, so when I want a multimillion array with speed I should use a static
> array on the heap. That would be using new, right :)

You.. can't actually allocate a static array on the heap using new.
At least not directly.  It's kind of an embarrassing hole in the
syntax.  In fact, I'm not sure if you can even get a static array to
point onto the heap, since a static array "reference" is treated like
a value type when assigned to, so if you do something like:

int[3][4] b = (new int[3][4][](1))[0];

The weirdness on the right-hand-side is needed to get around the
compiler "helpfully" rewriting "new int[3][4]" as "new int[][](3, 4)".
 But this code will simply allocate a static array on the heap, and
then copy its contents onto the stack.  Dumb.

Other than using a struct/class that contains a static array and
allocating _that_ on the heap, I don't know of any way of getting it
on the heap.  But then you'd be double-dereferencing anyway since
you'd have to go through the struct pointer or class reference!  Sigh.