Re: deep copy or shallow copy?
On 12/08/2011 12:52 PM, Timon Gehr wrote:
On 12/08/2011 09:50 PM, RenatoL wrote:
snippet 1)
auto arr1 = [1,2,3];
auto arr2 = arr1;
arr1[1] = 22;
arr2[2] = 33;
foreach (i; 0..arr1.length) write(arr1[i], );
writeln();
foreach (i; 0..arr2.length) write(arr2[i], );
output:
1 22 33
1 22 33
OK
snippet 2)
int[3] arr1 = [1,2,3];
int[3] arr2 = arr1;
arr1[1] = 22;
arr2[2] = 33;
foreach (i; 0..arr1.length) write(arr1[i], );
writeln();
foreach (i; 0..arr2.length) write(arr2[i], );
output:
1 22 3
1 2 33
that's unclear to me... i agree with the behaviour of the
dynamic array... but if we have a static array we have a deep copy?
Both copies are 'shallow', but static arrays are value types.
'shallow' would be misleading for a fixed-length (static) array because
there is nothing else but the elements for fixed-length arrays:
void main()
{
int[3] a;
assert(cast(void*)a == cast(void*)a[0]);
}
Fixed-length array storage is similar to C arrays. These are different:
- they don't decay to a 'pointer to first element' when passed to
functions (being value types, the whole array is copied)
- a.length is a convenience, equivalent to a.sizeof / a[0].sizeof
So it is impossible to do anything shallow with them unless we
explicitly maintain a pointer to a fixed-length array ourselves.
Ali
Re: deep copy or shallow copy?
On 12/09/2011 09:32 PM, Ali Çehreli wrote:
On 12/08/2011 12:52 PM, Timon Gehr wrote:
On 12/08/2011 09:50 PM, RenatoL wrote:
snippet 1)
auto arr1 = [1,2,3];
auto arr2 = arr1;
arr1[1] = 22;
arr2[2] = 33;
foreach (i; 0..arr1.length) write(arr1[i], );
writeln();
foreach (i; 0..arr2.length) write(arr2[i], );
output:
1 22 33
1 22 33
OK
snippet 2)
int[3] arr1 = [1,2,3];
int[3] arr2 = arr1;
arr1[1] = 22;
arr2[2] = 33;
foreach (i; 0..arr1.length) write(arr1[i], );
writeln();
foreach (i; 0..arr2.length) write(arr2[i], );
output:
1 22 3
1 2 33
that's unclear to me... i agree with the behaviour of the
dynamic array... but if we have a static array we have a deep copy?
Both copies are 'shallow', but static arrays are value types.
'shallow' would be misleading for a fixed-length (static) array because
there is nothing else but the elements for fixed-length arrays:
It is not misleading since the array might be an array of references.
(and if it does not contain references, shallow and deep are the same
thing anyway)
void main()
{
int[3] a;
assert(cast(void*)a == cast(void*)a[0]);
}
Fixed-length array storage is similar to C arrays. These are different:
- they don't decay to a 'pointer to first element' when passed to
functions (being value types, the whole array is copied)
- a.length is a convenience, equivalent to a.sizeof / a[0].sizeof
So it is impossible to do anything shallow with them unless we
explicitly maintain a pointer to a fixed-length array ourselves.
Ali
You can always slice it, of course
int[3] a;
int[] b = a[]; // b now is a dynamic array that aliases a's contents
Re: deep copy or shallow copy?
On Friday, December 09, 2011 22:09:15 Timon Gehr wrote: On 12/09/2011 09:32 PM, Ali Çehreli wrote: So it is impossible to do anything shallow with them unless we explicitly maintain a pointer to a fixed-length array ourselves. Ali You can always slice it, of course int[3] a; int[] b = a[]; // b now is a dynamic array that aliases a's contents Though, of course, you have to be careful with that, since the static array then owns the memory for that dynamic array, and if the static array goes out of scope before the dynamic array does, then the dynamic array points to garbage and will result in bugs. - Jonathan M Davis
Re: deep copy or shallow copy?
On 12/09/2011 01:18 PM, Jonathan M Davis wrote: On Friday, December 09, 2011 22:09:15 Timon Gehr wrote: On 12/09/2011 09:32 PM, Ali Çehreli wrote: So it is impossible to do anything shallow with them unless we explicitly maintain a pointer to a fixed-length array ourselves. Ali You can always slice it, of course int[3] a; int[] b = a[]; // b now is a dynamic array that aliases a's contents Though, of course, you have to be careful with that, since the static array then owns the memory for that dynamic array, and if the static array goes out of scope before the dynamic array does, then the dynamic array points to garbage and will result in bugs. - Jonathan M Davis That's news to me. Don't the static array elements belong to the runtime, managed by the garbage collector, and will be kept alive as long as the slice is alive? Ali
Re: deep copy or shallow copy?
On Friday, December 09, 2011 14:33:38 Ali Çehreli wrote:
On 12/09/2011 01:18 PM, Jonathan M Davis wrote:
On Friday, December 09, 2011 22:09:15 Timon Gehr wrote:
On 12/09/2011 09:32 PM, Ali Çehreli wrote:
So it is impossible to do anything shallow with them unless we
explicitly maintain a pointer to a fixed-length array ourselves.
Ali
You can always slice it, of course
int[3] a;
int[] b = a[]; // b now is a dynamic array that aliases a's contents
Though, of course, you have to be careful with that, since the static
array
then owns the memory for that dynamic array, and if the static array
goes out
of scope before the dynamic array does, then the dynamic array points
to
garbage and will result in bugs.
- Jonathan M Davis
That's news to me. Don't the static array elements belong to the
runtime, managed by the garbage collector, and will be kept alive as
long as the slice is alive?
Goodness no. The static array is on the stack, not on the heap. If you append
to a dynamic array which refers to a static array, then it'll reallocate that
memory onto the heap (leaving the original static array alone) so that the
dynamic array is then managed by the runtime, but the static array never is,
since it's on the stack, and as long as the dynamic array is a slice of the
static array, it's going to be pointing to the wrong thing if the static array
leaves scope.
So, slicing a static array to pass it to a function which isn't going to keep
the memory around isn't a big deal, but doing something like
int[] func()
{
int[5] a;
return a[];
}
is as bad as
int* func()
{
int a;
return a;
}
though at least in the second case, the compiler will give you an error. The
first probably should as well, but it doesn't currently. It _is_ escaping a
reference to a local variable though, which is a bug.
- Jonathan M Davis
Re: deep copy or shallow copy?
On 12/09/2011 02:58 PM, Jonathan M Davis wrote:
On Friday, December 09, 2011 14:33:38 Ali Çehreli wrote:
[...]
That's news to me. Don't the static array elements belong to the
runtime, managed by the garbage collector, and will be kept alive as
long as the slice is alive?
Goodness no. The static array is on the stack, not on the heap. If
you append
to a dynamic array which refers to a static array, then it'll
reallocate that
memory onto the heap (leaving the original static array alone) so
that the
dynamic array is then managed by the runtime, but the static array
never is,
since it's on the stack, and as long as the dynamic array is a slice
of the
static array, it's going to be pointing to the wrong thing if the
static array
leaves scope.
So, slicing a static array to pass it to a function which isn't going
to keep
the memory around isn't a big deal, but doing something like
int[] func()
{
int[5] a;
return a[];
}
is as bad as
int* func()
{
int a;
returna;
}
though at least in the second case, the compiler will give you an
error. The
first probably should as well, but it doesn't currently. It _is_
escaping a
reference to a local variable though, which is a bug.
- Jonathan M Davis
Thank you. Opened:
http://d.puremagic.com/issues/show_bug.cgi?id=7087
Ali
Re: deep copy or shallow copy?
On 12/08/2011 09:50 PM, RenatoL wrote: snippet 1) auto arr1 = [1,2,3]; auto arr2 = arr1; arr1[1] = 22; arr2[2] = 33; foreach (i; 0..arr1.length) write(arr1[i], ); writeln(); foreach (i; 0..arr2.length) write(arr2[i], ); output: 1 22 33 1 22 33 OK snippet 2) int[3] arr1 = [1,2,3]; int[3] arr2 = arr1; arr1[1] = 22; arr2[2] = 33; foreach (i; 0..arr1.length) write(arr1[i], ); writeln(); foreach (i; 0..arr2.length) write(arr2[i], ); output: 1 22 3 1 2 33 that's unclear to me... i agree with the behaviour of the dynamic array... but if we have a static array we have a deep copy? Both copies are 'shallow', but static arrays are value types.
