On Monday, 10 December 2012 at 19:46:18 UTC, Andrei Alexandrescu
wrote:
On 12/10/12 1:55 PM, Jacob Carlborg wrote:
On 2012-12-10 17:06, Dan wrote:
I think so. Here is a claim I think is true: gdup must do
full deep copy
to be safe and guarantee the transitive const/immutable. If
it is
implemented such that there are no casts, then the compiler
does its job
and ensures everything is good. If there are casts they need
to be
deemed safe - I have one cast to work around issues in
associative array
iteration.
I'm pretty sure it can't be done.
Just like Dan I thought it can be done but actually ownership
is impossible to establish in general. Consider:
class List(T) {
List next;
T payload;
...
}
versus
class Window {
Window parent;
...
}
It's pretty obvious from the name that duplicating a List would
entail duplicating its tail, whereas duplicating a Window would
entail just copying the reference to the same parent. However,
at the type level there's no distinction between the two
members.
Unless the tail is immutable, but in general yes.
Now, user-defined attributes could change the playfield
radically here. Consider we define things like @owned and
@foreign that would inform the gdup or deepdup function
appropriately:
class List(T) {
@owned List next;
T payload;
...
}
versus
class Window {
@foreign Window parent;
...
}
Now a generic deep duplication function has enough information
to duplicate things appropriately.
That is indeed a perfect use case for attributes.
