Transitive const sucks (ownership - aggregation v association)

[Bruce Adams]

James Dennett Wrote:

> Walter Bright wrote:
> > Janice Caron wrote:
> >> It's a solution which doesn't scale.
> >>
> >> And there goes encapsulation...
> > 
> > At least for this example, the same issue exists when attempting to do
> > "const-correctness" in C++.
> 
> No, it doesn't.  C++ doesn't hide the references from you
> with its syntax, but the const Rectangle having a reference
> to a Raster would *not* imply constness of that Raster in
> C++.  (Indeed, this is a fairly common situation, whether
> Raster is held by reference or some kind of pointer.)
> 

[snip]
> 
> -- James

This is a good point to emphasise. In object oriented programming
there is a clear difference between encapsulating something in an
aggregate and merely being associated with it. In the rectangle example a raster is clearly not "part of" a rectangle but
it is associated. 

An important concept here is ownership. In C++ I general find myself labelling pointers as owned and not-owned.
If you own a pointer you are responsible for deleting the object it points to. Some experimental static analysis tools are able to use pointers labelled this way to reason about memory allocation. Research
suggests (I will have to google around a bit before I can quote references) that adding something to a language to indicate ownership is warranted.
D with its garbage collection doesn't have this problem on the surface but there is still a distinction between something that is owned and something that is referred to. This is a distinction that is (as far as I can tell) currently impossible to represent except via comments. In D everything is a reference. Perhaps the solution is to make non-owned members (in the sense of association) pointers and reserve references (minus the pointer) for aggregation. A value keyword might help some but I think it would address the wrong problem.

 For members of an aggregate const should be transitive. If I cannot change this object then I should not be able to change any of its parts.
If there is a logger, cache or raster - Associated - with the class I may still be able to change it - depending on whether that particular associated is marked as const.

Regards,

Bruce.