Re: OO in Ada

["Warren W. Gay VE3WWG" <ve3wwg@cogeco.ca>]

Matthew Heaney wrote:
> "John McCabe" <john.nospam@nospamassen.nospamdemon.co.uk> wrote in message
> news:3da2aafd.7023559@news.demon.co.uk...
...
>>>:)  However, if in a program 'use'ing the child package of 'Thick_Pen'
>>
> (and
>>>not mentioning the base package of 'Pen') I make a call to the routine
>>>'Draw' with an actual parameter of the type 'Pen' I get a compiler error
>>>message to the effect that 'Draw' is not visible.  In other words,
>>
> Thick_Pen
> 
>>>has not inherited the operation Draw for 'Pen'  Again,
>>>_there_is_no_inheritace_.
>>
>>There is an easy answer to this that I can't remember at the moment.
>>It's probably to do with using 'Class somewhere, but it may be that
>>you have an incorrect declaration somewhere that isn't actually a
>>creating a dispatching operation.

I believe that the problem referred to here is simply one
of a visibility. Either a 'use' or a fully qualified call is
required. If I understand correctly, you're saying you have
some parent package "Pen", and a child package "Thick_Pen",
and you use the package as in:

   use Pen.Thick_Pen;
   ...
   Draw(...); -- Compile error because Draw is defined in Pen


This is solved either by doing something like:

   use Pen, Pen.Thick_Pen;
   ...
   Draw(...);  -- OK now, because Pen.Draw is visible.


or qualifying by your call as in:

   use Pen.Thick_Pen;
   ...
   Pen.Draw(...);

Warren.
-- 
Warren W. Gay VE3WWG
http://home.cogeco.ca/~ve3wwg