Re: Ada OO Mechanism

[Matthew Heaney <matthew_heaney@acm.org>]

Hyman Rosen <hymie@prolifics.com> writes:

> Also, I stated in an earlier message that I thought not allowing T'Class
> variables in records was not orthogonal, but understandable in light of
> the above. Now, suppose I need a function that returns a pair of values,
> one of which is a T'Class and the other a boolean. Is there a way to do
> this? 

Not really, no.  Here are some ideas:

1) return a record containing a class-wide component allocated on the
   heap

  type RT is
    record
      O : T_Class_Access;  
      B : Boolean;
    end record;

  function Func_Ret_RT return RT;


2) Break in the function apart into two functions:

  function Func_Ret_T_Tic_Class return T'Class;

  function Func_Ret_Boolean return Boolean;

This doesn't preserve atomicity, however.  (But that's not always an
issue anyway.)


3) Embed the Boolean value in state of the object:

  function Func_Ret_T_Tic_Class return T'Class;

  function Func_Ret_Boolean (O : T) return Boolean;


> I can't return a record containing the pair, because I can't have
> such a record. 

That is correct.  Although, if you follow approach 1), you can put the
class-wide value on the heap.


> I assume that a procedure with a pair of out parameters won't work,
> because the T'Class variable which receives the output must be
> initialized when declared, and then its tag can't change.

Yes, that is correct.


> So, is there a way?

See the three bullets above.