[nem-en] Initonly modifier for immutable fields

[Kamil Skalski]

Try this program with and without 'readonly' modifier:

struct S {
  public string y;

  public S (string a, string b) {
    y = b;
  }

  public void Modify (string i) {
    y = i;
  }
}


class A {
  public /*readonly*/ S z;

  public A(S a) {
    z = a;
  }
}


class M {
    public static void Main(string[] args) {

      A x = new A (new S ("b"));
      System.Console.WriteLine (x.z.y);
      x.z.Modify ("aaaaaaaaa");
      System.Console.WriteLine (x.z.y);
    }
}


The outputs can be quite suprising for anyone ;)

On 7/9/06, Sandro Magi <smagi at springairsystems.com> wrote:
> Kamil Skalski wrote:
> > - initonly field containing internally mutable value types:
> >
> > The most ugly change of behaviour I observed when running and
> > debugging our test suite is
> > how .NET forces such code to be compiled to:
> >
> > [Record]
> > struct S {
> >  public mutable muta : string;
> >
> >  public Modify (val : string) : void
> >  {
> >    muta = val;
> >  }
> > }
> >
> > [Record]
> > class A {
> >  public mystruct : S;
> > }
> >
> > def x = A (S ("first"));
> > System.Console.WriteLine (x.mystruct.muta);
> > x.mystruct.Modify ("second");
> > System.Console.WriteLine (x.mystruct.muta);
> >
> > will print:
> > first
> > first
> >
> > Since 'mystruct' is now initonly, we can only read its *copy*, we
> > cannot access a pointer (address) to it and effectively every
> > operation on it is always performed on a copy not on the original
> > struct.
> > If you try to assign directly to muta by
> > x.mystruct.muta = "second";
> >
> > compiler will detect its inability to modify structure in place and
> > signal error.
>
> Are you sure this is the reason? In C# structs are always immutable, so
> it seems to me that the above example should have always printed
>
> first
> first
>
> The above example would work in C# if S were a class.
>
> Sandro
>
> _______________________________________________
> https://nemerle.org/mailman/listinfo/devel-en
>


-- 
Kamil Skalski
http://nazgul.omega.pl