Re: Strings with discriminated records

[NiGHTS <nights@unku.us>]

On Sunday, May 27, 2018 at 9:44:22 PM UTC-4, Jere wrote:
> On Sunday, May 27, 2018 at 7:08:03 PM UTC-4, NiGHTS wrote:
> > On Sunday, May 27, 2018 at 2:07:37 PM UTC-4, Simon Wright wrote:
> > > 
> > > > I've expanded my example to include a strange run-time crash I am getting.
> > > >
> > > > Type Message (Length : Positive) is new Ada.Finalization.Controlled with record
> > > >     Text  :  String (1 .. Length);
> > > >     Cstr  :  Interfaces.C.Strings.chars_ptr;
> > > > end record;
> > > >        
> > > > function Create (Value : String) return Message is
> > > > begin
> > > >     return (
> > > >         Ada.Finalization.Controlled with Length => Value'Length, 
> > > >         Text => Value, 
> > > >         Cstr => Interfaces.C.Strings.New_String (Value) 
> > > >     );
> > > > end Create; 
> > > >
> > > > procedure Finalize (
> > > >     M : in out Message
> > > > ) is
> > > > begin
> > > >     Interfaces.C.Strings.Free ( M.Cstr ); -- Crashes here
> > > > end;
> > > >
> > > > declare
> > > >     M : Message := Create ("Hello World");
> > > > begin
> > > >     null;
> > > > end; 
> > > >
> > > > Why does my program crash on Finalization?
> > > 
> > > Maybe Finalize is getting called twice? You should set M.Cstr to
> > > Null_Ptr after freeing it.
> > 
> > It doesn't work. On each automatic call of Finalize, the object data is reset. Its very strange.
> 
> It won't work unless you have an appropriate Adjust.  Since your object is
> controlled (and not limited controlled), the compiler is making temporaries,
> each with their own copy of the pointer (someone else mentioned this above).
> Each temporary will be finalized and each will have their own copy of
> the pointer, so nulling it only nulls that objects copy and not the others.
> 
> As stated above by someone else, you'll either have to do a clone with
> separately allocated memory or some sort of reference count that only 
> allows finalize to call free when the last copy calls finalize (and
> ignores the others calls).
> 
> Dmitry has an example of how to do this with an intrusive reference
> count with his handles/objects library (see his website).
> 
> Making the type limited controlled is also a solution since it cannot
> make temporaries for one like it can a regular controlled object.

Yes, I understand now. I will do some tests and see what works for me. Thank you.