Inability to dup/~ for const arrays of class objects

[Ali Çehreli]

On 05/28/2013 11:13 PM, Jonathan M Davis wrote:

 > On Tuesday, May 28, 2013 22:58:39 Ali Çehreli wrote:

 > To make matters worse, what happens when you have a const object 
which has a
 > reference as a member variable? e.g.
 >
 > class C
 > {
 >      class D d;
 > }
 >
 > const C c;

Thanks for doing the thinking for me. :) I could not think deeper than 
one level yesterday. :)

 > Because of the transitivity of const, d must be fully const, and yet 
with your
 > suggestion, it isn't.

We know the phrase "turtles all the way down". It makes sense. The 
interesting thing is, in the case of a class reference, at least due to 
syntax, it feels like it is "turtle one level up":

     const(C) c = new const(C);

The reason is, the syntax const(C) suggests that the object is const. 
(Compared to 'const C c = ...') However, although it looks like const 
qualifiying the object, it comes one step up affects the reference as well.

I wondered whether there were more turtles up: :)

void main()
{
     class A
     {
         int i;
         this(int i) { this.i = i; }
     }

     const(A) a1 = new const(A)(1);
     const(A) a2 = new const(A)(2);

     // The following is a compilation ERROR because it is not
     // possible to refer to another object in a non-mutating way:

     // a1 = a2;  <-- compilation ERROR

     // No problem at all! Define a wrapper for a const(A):
     class B
     {
         const(A) a;
         this(const(A) a) { this.a = a; }
     }

     B b1 = new B(a1);
     B b2 = new B(a2);

     // And use your const(A) through that wrapper:
     assert(b1.a.i == 1);

     // Maybe we should call this wrapper Rebindable ;)
     b1 = b2;

     // Yay! :)
     assert(b1.a.i == 2);
}

I always fail to remember std.typecons.Rebindable. :( It may be the 
solution to OP's issue:

   http://dlang.org/phobos/std_typecons.html#.Rebindable

Ali