References to const

[Andrei Alexandrescu]

Jonathan M Davis wrote:
> Okay, I'm wondering if there is any way in D to have a reference to
> const.  As far as I can tell, if you use const on a reference, you
> get a constant reference to constant data.
> 
> const A a1; const A a2 = new A(); A otherA = new A(); const A a3 =
> otherA;
> 
> So, a1 points permanently to null.  You can't change what it points
> to (and of course you can't change null).  a2 points to an A which
> was constructed on assignment.  a2 can't be made to point to anything
> else and you can't change the value that it points to.  a3 points to
> another the same A as otherA does, but it can't be made to point to a
> different A and it can't alter the value that it points to (while
> otherA can be altered to point to a different A and you can use it to
> alter the value that it points to).
> 
> What I'd _like_ to be able to do is have reference which I can alter
> such that it points to a different object but which does not allow me
> to alter what it points to.  In C++, you have
> 
> const A* a1;  //variable pointer to a constant object. A* const a2;
> //constant pointer to a variable object const A* const a3;
> //constant pointer to a constant object
> 
> From what I can tell, you can do 2 of those in D with pointers:
> 
> const (A)* a1;  //variable pointer to a constant object. const (A*)
> a3;  //constant pointer to a constant object
> 
> But I don't see how to get either a const pointer, so it lacks one of
> the three with pointers.  With references, however, it lacks 2 of the
> three from what I can tell.  It only has constant references to
> constant data.
> 
> I'm hoping that I just don't understand well enough how const works
> in D, but right now, I don't see any way to get a variable reference
> to constant data.  The missing constant pointer to variable data and
> constant reference to variable data might be nice, but they don't
> matter anywhere near as much to me.
> 
> Is there a way to have variable reference to constant data in D?  I
> just don't see it at the moment and it would be _really_ useful.

http://www.digitalmars.com/d/2.0/phobos/std_typecons.html#Rebindable

Andrei