Re: Why does Ada.Iterator_Interfaces specify Next as a function rather than a procedure?

["Randy Brukardt" <randy@rrsoftware.com>]

I think you're confusing the cursor and the iterator object. (Next has two 
parameters, after all.) I'm talking about the definition of the iterator 
object; that's where you keep any state necessary. The cursor just provides 
a reference to the current item, whatever that might be.

The instantiation takes the cursor type; it then is used to define the 
actual iterator type.

In almost all cases, the iterator object has to be separate from the 
underlying data structure; generally you define a function to create the 
initial iterator object. (There are aspects to automate this process, look 
at how the containers are structured if you want to try to use them.) One 
reason for this is the possibility that multiple tasks might iterate over 
the same read-only object.

                              Randy.






"Stephen Leake" <stephen_leake@stephe-leake.org> wrote in message 
news:ba860d9f-7e5f-431b-826b-76a6347db38e@googlegroups.com...
> On Wednesday, September 6, 2017 at 8:04:15 PM UTC-5, Stephen Leake wrote:
>> >      type State is record
>> >           -- The components of the state.
>> >      end record;
>> >
>> >      type My_Iterator is new Ada.Finalization.Controlled and
>> > Reversible_Iterator with record
>> >            My_State : access State; -- Writable state.
>> >            The_State : aliased State;
>> >      end record;
>
> I don't see how to do this; 'Reversible_Iterator' is defined in an 
> instantiation of Ada.Iterator_Interfaces, which takes the type My_Iterator 
> as a generic parameter.
>
> Just leaving that off still fails, if I try to make the contents of the 
> cursor private while exposing the Iterate function.
>
> Are you relying on some new Ada 202x syntax here?
>> >
>> >      procedure Initialize (Obj : in out My_Iterator) is
>> >      begin
>> >           Obj.My_State := The_State'Access; -- (*)
>> >      end Initialize;
>>
>> > This is a legitimate technique -- it is NOT erroneous in Ada
>> > 2012 -- so if your compiler has problems with it, file a bug report and 
>> > if
>> > they don't believe that, send me a proposed ACATS test!
>>
>> You are implying that my worry about the compiler assuming the input and 
>> output of Next must point to different items is not valid.
>>
>> I'll give it a try.
>>
>> -- Stephe
>