Re: Type extension questions

[comar@lang8.cs.nyu.edu (Cyrille Comar)]

ncohen@watson.ibm.com (Norman H. Cohen) writes:
: In article <3l0frd$b9t@lang8.cs.nyu.edu>, comar@cs.nyu.edu (Cyrille Comar)
: writes: 
: |> koehnema@enuxsa.eas.asu.edu (Harry Koehnemann) writes: 
: ...
: |> :                                                             What I
: |> : really want is private inheritance ala C++.
: |>
: |> Don't try to be too close to C++, this doesn't fit Ada's
: |> model. "Private inheritance" is meaningless in Ada since "Private" is
: |> a caracteristic of packages and inheritance a characteristic of types
: 
: No, there is a direct Ada analog to a C++ class with a private base
: class, namely a private type implemented as a tagged derived type: 
: 
:    package Base_Type_Package is
:       type Base_Type is tagged ...;
:       ...
:    end Base_Type_Package;
: 
:    with Base_Type_Package;
:    package Derived_Type_Package is
:       type Derived_Type is private;
:       procedure Some_Operation (X: in out Derived_Type);
:       ...
:    private
:       type Derived_Type is new Base_Type_Package.Base_Type with ...;
:    end Derived_Type_Package;
: 
: --
: Norman H. Cohen    ncohen@watson.ibm.com



No, This is NOT a direct analog! A private Child package of
Derived_Type_Package has full visibility over Drived_Type and
Base_Type and you CANNOT enforce private inheritance in this
context. Your statement is correct if and only if you are ready to
accept a non-sense rule such as 1 package = 1 class = 1 tagged type or
something like that.

I really think it is important to avoid the confusion between packages
which controls the privacy in Ada and C++ classes which also control
the provacy but are more related to tagged type than to packages...
-- 
------------------------------------------------------------------------
Cyrille Comar,                                  E-mail: comar@cs.nyu.edu
Gnat Project                                    US phone: (212) 998-3489