Re: Can't hide parts of ADTs in private children

[Stephen Leake <Stephen.Leake@gsfc.nasa.gov>]

David Bowerman <davidb@syd.csa.com.au> writes:

> I am implementing an Abstract Data Type (Class) in a package.  The code
> goes something like this:
> 
>   package Class_X_Package is
> 
>     type Class_X is private;
>     type Class_X_Ptr is access Class_X;
> 
>     ...  operations on objects of type X
> 
>   private
> 
>     type Class_X is
>       record
>         Component_A : Type_A;
>         Component_B : Type_B;
>         ...
>       end record;
> 
>   end Class_X_Package;
> 
> The abstract data type (X) is complex, and Type_A, Type_B, etc hide a
> lot of structure within themselves.
> I would like to break the package down further, declare the types, and
> generally hide the details of, Type_A, Type_B, etc (never intended to be
> visible to the client) in private child packages.

Why do Type_A, Type_B belong in child packages, instead of in library
packages? In building an ADT, you can use facilities of a library,
while making it clear that the user of your ADT should not use the
library directly. Unless Type_A depends on something else in
Class_X_Package, just put it in its own library level package.

> But I can't do this because these types are referred to in the private
> part of the parent package spec, and this isn't allowed to 'with' its
> children.
> 
> Can anyone suggest another way of doing it.

Alternatively, make Class_X an access type, which allows you to defer
the full type definition to the package body, where you can 'with'
child packages.

-- Stephe