logical const without casts!

[Steven Schveighoffer]

On Thu, 29 Sep 2011 13:06:55 -0400, Simen Kjaeraas  
<simen.kjaras at gmail.com> wrote:

> On Thu, 29 Sep 2011 16:54:24 +0200, Steven Schveighoffer  
> <schveiguy at yahoo.com> wrote:
>
>> I just thought of an interesting way to make a logical const object  
>> without casts.  It requires a little extra storage, but works without  
>> changes to the current compiler (and requires no casts).
> [snip]
>> What do people think about this?
>
> This is what I think about it:
>
> class A {
>      int n;
>      void delegate( ) dg;
> }
>
> pure
> A createAnA( int n ) {
>      A result = new A;
>      result.n = n;
>      result.dg = (){ result.n++; };
>      return result;
> }
>
> void main( ) {
>      immutable A tmp = createAnA( 3 );
>      assert( tmp.n == 3 );
>      tmp.dg();
>      assert( tmp.n == 3 );
> }

I agree this breaks immutability, and needs to be addressed.  I think  
probably implicit casting of delegates (or items that containe delegates)  
to immutable from strong-pure functions should be disallowed.

But it's not the pattern I described, and uses a relatively new trick  
(implicit immutable casting).  I'll file a bug for this case.

-Steve