Re: Strings with discriminated records

[Jere <jhb.chat@gmail.com>]

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.