How can I map bytes to a matrix of structures?

[teo]

On Fri, 09 Sep 2011 17:43:04 +0200, Timon Gehr wrote:

> On 09/09/2011 05:19 PM, teo wrote:
>> Here is an example of what I am after:
>>
>> struct DATA
>> {
>>    ubyte D1;
>>    ubyte D2;
>>    ubyte D3;
>>    ubyte D4;
>> }
>>
>> void main()
>> {
>>    ubyte[16] a = [ 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08,
>>    0x01,
>> 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08 ];
>>    auto b = (cast(DATA*)a.ptr)[0 .. 4];
>>    auto c = (cast(DATA[]*)b.ptr)[0 .. 2][0 .. 2];
>> }
>>
>> I need to have a DATA[2][2]. That code compiles but gives me a
>> segmentation fault.
> 
> If you actually want a dynamic DATA[2][] array of length 2, this works:
> auto b=(*(cast(DATA[2][2]*)a.ptr))[];
> 
> Otherwise:
> 
> A simple reinterpret cast should do:
> auto b=*(cast(DATA[2][2]*)a.ptr);
> 
> but note that this copies the data, because static arrays have value
> semantics.
> 
> If you want to have refer the new array to the same location, you can
> use a union.
> 
> void main(){
>      union Myunion{
> 	ubyte[16] a = [ 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 
0x01,
> 			0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08 ];
> 	DATA[2][2] b;
>      }
>      Myunion myunion;
>      assert(*(cast(DATA[2][2]*)myunion.a.ptr)==myunion.b);
> }

Thank you Timon for the good explanation.

Just one more question (I suspect the answer will be no, but let me ask): 
is it possible to directly cast to ubyte[][]?