Re: Strings with discriminated records

[NiGHTS <nights@unku.us>]

On Sunday, May 27, 2018 at 2:25:02 PM UTC-4, Dmitry A. Kazakov wrote:
> On 2018-05-27 19:11, NiGHTS 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?
> 
> Because it is wrong.
> 
> You have a controlled object which gets copied all the time. It means 
> that the pointer Cstr is shared by all copies and is freed multiple 
> times. Either
> 
> 1. You must override Adjust and make a new string for the result.
> 
> or
> 
> 2. You could deploy some reference counting schema cloning the content 
> when the string at the pointer is updated. This is how dynamic strings 
> are usually implemented.
> 
> P.S. If you want C-compatible strings use char_array, it is exactly the 
> thing you need.
> 
> -- 
> Regards,
> Dmitry A. Kazakov
> http://www.dmitry-kazakov.de

I confirmed with an Ada.Text_IO.Put_Line() that Finalize was called three times. I then disabled Finalize and created an explicitly called Finalize2 function and it worked fine. It's weird though because my program only elaborated the object once, so why did it finalize three times? 

I tried wrapping the body of finalize in a condition to force it to run only once, yet it still managed to run it again ignoring the boolean. I'm perplexed. Not sure if I should ever trust Ada.Finalization.Controlled.