On Thu, 21 May 2009 04:51:16 -0400, MLT wrote:
> After a discussion on digitalmars.D I played with arrays a bit. Look at the
> following code:
> int[] a = [1,2,3,4,5,6,7,8,9] ;
> int[] b = a ;
> a ~= 10 ;
> b ~= 11 ;
> b[0] = 12 ;
> Stdout(b).newline ;
> Stdout(a).newline ;
>
> The result is:
> [12, 2, 3, 4, 5, 6, 7, 8, 9, 11]
> [12, 2, 3, 4, 5, 6, 7, 8, 9, 11]
>
> Which means that even though b was set only to a[0..10], after expanding b,
> also has direct access to element a[10]. But
> int[] a = [1,2,3,4,5,6,7,8,9] ;
> int[] b = a ;
> a ~= 10 ;
> b.length = b.length+1 ;
> b[0] = 11 ;
> Stdout(b).newline ;
> Stdout(a).newline ;
>
> Gives
> [11, 2, 3, 4, 5, 6, 7, 8, 9, 0]
> [11, 2, 3, 4, 5, 6, 7, 8, 9, 0]
>
> Now b is expanded in length, but a side effect is that a[10] is set to 0
> (i.e. initialized).
>
> In the end, I think b can only see things that happen to a[10] after it
> expanded, but not before. It seems there is no way for the following to work:
> int[] a = [1,2,3,4,5,6,7,8,9] ;
> int[] b = a ;
> a ~= 10 ;
>
> At this point, element a[10]=10 will never be visible to b. b can expand to
> it, but by doing that, a[10] will be overwritten.
>
> Is this on purpose? It could also be useful to have b.length=b.length+1 which
> only initializes memory that has not been initialized before.
Yes it is on purpose.
Here is what is happening ...
The contents of the array variable is actually a 2-element struct {addr,
length}. When you assign one array to another, that struct is what is
copied, not the array data itself.
int[] a = [1,2,3,4,5,6,7,8,9] ;
// Now 'a' contains {adr, 9}
int[] b = a ;
// Now 'b' contains {adr, 9}
a ~= 10 ;
// Now 'a' contains {adr, 10} -- The address doesn't change
-- because the buffer allocation
-- still enough room for another element.
b ~= 11 ;
// Now 'b' contains {adr, 10} -- The address doesn't change
-- because the buffer allocation
-- still enough room for another element
-- And that new element overwrote the '10'
-- appended to 'a'.
b[0] = 12 ;
// The element at address 'adr' is modified.
// As both 'a' and 'b' point to the same area
// it appears that updating 'b' changes 'a'.
The safe way to copy the data of an array is to do ...
int[] a = [1,2,3,4,5,6,7,8,9] ;
int[] b = a.dup ; // COPY a's data to to b.
a ~= 10 ;
b ~= 11 ;
b[0] = 12 ;
The result should now be:
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
[12, 2, 3, 4, 5, 6, 7, 8, 9, 11]
So remember, assigning one array to another is just creating an alias to
the original array. You end up with two arrays pointing to the same data
buffer.
--
Derek Parnell
Melbourne, Australia
skype: derek.j.parnell
