References to const

[Jonathan M Davis]

Andrei Alexandrescu wrote:

> 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

I would like to make the suggestion that Rebindable be discussed in the section of the documentation that discusses const and invariant ( http://www.digitalmars.com/d/2.0/const3.html ).  It would make the situation clearer.  Now it _is_ a part of phobos rather than a part of D itself, so I don't know if you want to discuss it in depth in the Language section of D's documentation, but I think that it would be quite helpful for those learning D.  As it is, I don't find it entirely clear from that page that a const reference is const in the sense that both the reference and what it references are const, though arguably it follows from the transivity of const.  Personally, I had to play around with some code to figure out in what way a const reference was const.

In any case, I thank anyone who's worked on the documentation.  While it certainly isn't perfect, I've found it to be a big help.

- Jonathan M Davis