David, Thank you!
Suppose I will put class Edge into the Array.
How to define Edge hasZero constraint and provide a zeroValue for this
type?
class Edge {
var first: int;
var second: int;
public def this (f: int, s: int) {
first = f;
second = s;
}
}
Best Regards,
-----------------------------------------------------------------
Hai Chuan(Haichuan) Wang, 王海川
Research Staff Member, System Software, IBM Research - China
TEL: 86-21-60924401 FAX: 86-21-60923522
David P Grove <gro...@us.ibm.com> wrote on 2011-04-05 20:22:19:
> From: David P Grove <gro...@us.ibm.com>
> To: Mailing list for users of the X10 programming language <x10-
> us...@lists.sourceforge.net>
> Date: 2011-04-05 20:27
> Subject: Re: [X10-users] Create an generics Array[ET] with null as
> initial value.
>
>
> Hai Chuan Wang <wangh...@cn.ibm.com> wrote on 04/05/2011 05:11:39 AM:
> >
> > I'm writing a data structure with generics, and needs an internal array
> to
> > store data.
> > But the array should be initialized with null value at the beginning.
> >
> > public class Seq[ET] {
> > public var S:Array[ET](1){rail};
> > public var sz:int;
> >
> > public def this(s:int) {
> > sz = s;
> > S = new Array[ET](sz, null); //<==== problem here
> > }
> > }
> >
> > Because ET has no "hasZero" attribute. I don't know how to create the
> array
> > with null as initial value.
> >
> > new Array[ET](sz, null), will treat "null" as "(int)=>ET", and throw
NPE.
> > null cannot be casted to ET type, either.
> >
> > Anyone can give some suggestions?
>
> Array is intended to be type safe, and in X10 an unconstrained T could be
> instantiated on some type S such that sizeof(S) bytes of zero's is not a
> valid value of type S. So, Array has been written to prevent you from
> doing this.
>
> What you can do is:
> (a) add the {ET haszero} constraint to Seq which will prevent such
> instantiations and thus allow you to call the Array constructor that
> zero-initializes the backing storage.
> (b) force the user to provide an "initial value" in the constructor
> and use that to initialize the array.
> (c) instead of using Array, use ArrayList or IndexedMemoryChunk for
> your backing storage. Both of these sidestep this piece of the X10 type
> system. ArrayList does it by using an IMC under the covers, while not
> allowing clients to see any uninitialized values. So ArrayList is type
> safe, but has some constraints on how it can be used (in particular, you
> always have to densely use the indices elements from 0...size-1; you
can't
> have gaps). IndexedMemoryChunk is not typesafe.
> (d) You can make your backing array be an Array[Cell[T]]. This
> injects an extra-level of indirection, and will not perform as well as
the
> other solutions.
>
> Personally, I think (a) is the most viable solution. I think it is a
> fairly natural outcome of the design point taken by the X10 type system
> that many generic collection types should have the {T haszero} constraint
> on them to allow efficient implementation. Arguably, this change should
be
> made in the x10.util collection classes as well. This then forces clients
> that need to put types that don't satisfy the haszero constraint into
> Cells/Boxes themselves before inserting them into the collection. This
is
> cumbersome, but I would suggest is putting the programming burden where
it
> most likely belongs.
>
> --dave
>
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