Re: Streams Quandry in Ada95

[tmoran@acm.org]

>So how do I write a procedure that says:
>
>procedure Pump(In_Str, Out_Str:  Any_Stream_Derived_From Audio_Stream);

 procedure Pump(In_Str, Out_Str: in out Audio_Stream'Class);

>I can avoid the abstract problem by deriving from Audio_Stream and providing
>no-op primitives for the abstract primitives.  But if other streams derive from this,
>they may forget to override those no-op primitives (though I could cheat and
>raise exceptions... leading to run-time checks).
  Forget the no-ops, just make the primitives of the abstract Audio_Stream
themselves abstract.  Then anybody deriving a new non-abstract kind of
Audio_Stream will be *required by the compiler* to supply overrides for
the abstract primitives.
  Thus
type Raw_Sample_Type is range 0 .. 127;  -- or whatever
type Audio_Stream is abstract tagged null record;
function Get(Data : Audio_Stream) return Raw_Sample_Type is abstract;
procedure Put(Sample : in Raw_Sample_Type; Data : in out Audio_Stream)
  is abstract;
procedure Pump(In_Str, Out_Str: in out Audio_Stream'Class);
 ...
procedure Pump(In_Str, Out_Str: in out Audio_Stream'Class) is
  Sample : Raw_Sample_Type;
begin
  loop
    Sample := Get(In_Str);
    Put(Sample, Out_Str);
  end loop;
end Pump;

Is this the sort of thing you want to do?