Re: Strings with discriminated records

[Simon Wright <simon@pushface.org>]

NiGHTS <nights@unku.us> writes:

> 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?

You need to read ARM7.6, in particular (17):
http://www.ada-auth.org/standards/rm12_w_tc1/html/RM-7-6.html#p17

Disregarding for the moment any "assignment" that might happen in
computing the result of Create, & looking at
   
   declare
      M : Nights.Message := Nights.Create ("Hello World");
   begin
      null;
   end;

First, the Create call makes an anonymous object.

Then, M is in theory finalized, but this is likely optimised away.

Then, the bits of the anonymous object are copied into M. This is a
"shallow copy". [***]

Then, the anonymous object is finalized, which frees
<anonymous>.Cstr. HOWEVER, M.Cstr still contains the same value, which
means it's pointing to deallocated memory.

Then, on exit from the declare block, M is finalized, and Finalize tries
to free M.Cstr again. Oops.

This is always going to happen if you make a shallow copy of something
which contains plain allocated memory.

As Dmitry said upthread, you need an Adjust, which makes a deep copy of
the plain allocated memory, immediately after [***] above:

   procedure Adjust (M : in out Message) is
      use Interfaces.C.Strings;
   begin
      M.Cstr := New_Char_Array (Value (M.Cstr));
   end Adjust;

> 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.

If the boolean was in Message, the same argument as above applies.

>          I'm perplexed. Not sure if I should ever trust
> Ada.Finalization.Controlled.

As with most things, not until you know how to use it.