Re: package dependence question

[Robert A Duff <bobduff@world.std.com>]

Carl Banks <oaf@psu.edu> writes:

> My question is, is there a better way to define such a doubly-
> traversible tree, so that different node types need not be defined in
> the same package?

There is no ideal solution to this problem in Ada 95 (although the ARG
is currently working on a language modification that will solve the
problem nicely).

One thing you can do is declare tagged type Node, and then "type
Node_Ptr is access all Node'Class;".  Make types A, B, and C derive from
Node.  Then B can have a component like "Parent: Node_Ptr;".  You lose
type checking -- the parent of a B is always an A, but the code doesn't
say so, except in a comment.

Another way to have mutually-recursive types is to create a dummy parent
type for one or more of the types.  Eg, if you have X and Y that point
to each other, you can declare Dummy_X and Dummy_Y as null records, and
point to Dummy_X'Class and Dummy_Y'Class.  Then X is derived from
Dummy_X, adding the actual record components.  Similarly for Y.

- Bob