Re: Visibility and access to "public" attributes

[nospam@thanks.com.au (Don Harrison)]

Mike Card wrote:

[..]

Example of how to emulate Eiffel's look-but-dont-touch attributes:

:package Test1 is
:   type My_Type is tagged private;           -- attributes invisible
:   function A(Obj : My_Type) return Boolean; -- access function
:private
:   type My_Type is tagged
:      record
:         A : Boolean;                        -- attribute
:      end record;
:end Test1;
:
:
:package body Test1 is
:   function A(Obj : My_Type) return Boolean
:   is
:   begin
:      return Obj.A;
:   end A;
:end Test1;

I accept this more accurately emulates the Eiffel functionality, and.. 

:.. there is no restriction in Ada that prevents an access function
:from having the same name as a class' (tagged type's) attribute.

True.

:That said, I think it is fair to say that I believe the idea behind
:Ada's "all or nothing" approach is that you may want to ensure that
:a client never depends on some of a type's attributes. 

As you acknowledge below, this can be done in Eiffel by hiding all the 
attributes of the type.. (See slso my response to Jon). 

:I know that Eiffel can totally hide the attributes of an object (i.e.
:accessed via methods only) as well. When writing Eiffel programs, how
:do you decide when to make a type's attributes "read-only visible"
:and when to hide them?

You export them when they're needed (or may be needed) by clients; otherwise,
you hide them.


Mike subsequently wrote:

:Now, suppose you change the AIRCRAFT class to have jet engines and
:thus remove or change the Propeller_RPM attribute? Because it was
:*visible*, someone may have written code to read it and now if you
:change the spec of the class their code will break. It doesn't
:matter that they can't update the attribute! If they can refer to it
:at all and you remove the attribute from the class definition, their
:code is broken!

As others have pointed out, the problem here is that your design for the 
AIRCRAFT class, rather than the Eiffel mechanism, is deficient - it 
assumed the aircraft was propeller-driven.


Don.                     (Reverse to reply)
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Don Harrison             au.com.csa.syd@donh