Re: How to do null comparision with pointers in ADA83?

[Adam Beneschan <adambeneschan@aol.com>]

On Friday, October 25, 2013 11:48:05 AM UTC-7, seshuch...@gmail.com wrote:

If AVL_Node_Ref is declared "type AVL_Node_Ref is private;" in package P, then the only places where the program can tell it's an access type are in the private part of P (after the complete declaration is seen), and in the body of P.  (And, in Ada 95+, in some places in child packages.)  Here, if you're in another package, it won't be able to tell that AVL_Node_Ref is an access type, even if you say "use P;".  It only knows that it's a private type.  You should be able to compare two AVL_Node_Refs, since "=" is still defined on private types.  But you can't compare an AVL_Node_Ref to null, since it's not allowed to know AVL_Node_Ref is an access type.

When a package declares a private type, it also has to decide what operations on that type it wants to provide for other packages to use.  Here, you probably want to declare a function like Is_Null in the public part of the package.  Now other packages can use that function without having to know anything about AVL_Node_Ref's full definition.

                             -- Adam