Logical Const using a Mutable template

[Max Samukha]

On 11/30/2010 09:30 AM, Jesse Phillips wrote:
> This came up in discussion and I think the behavior is safe and usable when wanting to change class data in a const function.
>
> http://article.gmane.org/gmane.comp.lang.d.general/43476
>
> One limitation is that value types declared as Mutable (Mutable!(int)) can not be changed if the encapsulating class is declared immutable. For this reason a Mutable value can not be changed inside a const function. I think this is acceptable.
>
> Another limitation appears to be an issue with alias this. It is commented as a //FIXME in the code.
>
> https://gist.github.com/721066
>
> An example usage looks like:
>
>      class Inner { int n; }
>
>      class A {
>          Mutable!(int*) n;
>          Mutable!(int)  n2;
>          Mutable!(Inner) innerClass;
>          this() { n = new int; innerClass = new Inner; }
>
>          void foo( int num ) {
>              n2 = num;
>          }
>
>          void bar( int num ) const {
>              innerClass.n = num;
>          }
>      }
>
>      auto aImmu = new immutable(A);
>      auto aMu   = new A;
>      int i = 8;
>
>      *aImmu.n = i,
>      aImmu.bar(6),
>      *aMu.n = i,
>      aMu.n =&i,
>      aMu.n2 = i,
>      aMu.bar(6),
>      aMu.foo(6),
>

It is not enough to forward opAssign and opEquals. Any operator should 
be handled. For example, change the implementation of A.foo to

n2 += 1;

and you will get a segfault during compilation.

You could add more kludge with operator overloads but I don't think it 
is worth the effort. It makes more sense to wait until "alias this" bugs 
are fixed. On the other hand, we may wait forever since the way "alias 
this" should handle operators has never been specified.