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Subject: Using Class wide types as factories, is this legit?
From: David Botton <david@botton.com>
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Xref: number.nntp.dca.giganews.com comp.lang.ada:189097
Date: 2014-09-22T22:19:32-07:00
List-Id: <comp.lang.ada>

I have a pattern I have used in the past to make Classwides act as factorie=
s for objects.

This worked in versions of GNAT about a year and a half ago. Is this a comp=
iler bug or bad Ada?

The key line is:

Row =A0: Active_Record'Class :=3D Template;

This is meant to act as a factory for new objects matching the Template cla=
ss by duplicating Template on assignment to Row.

Code:
   type Active_Record
     (Table_Name : access constant String;
      Connection : access          Gnoga.Server.Database.Connection'Class)
     is new Ada.Finalization.Controlled with
      record
         Is_New     : Boolean                               :=3D True;
         Fields     : Gnoga.Types.Data_Array_Type;
         Values     : Gnoga.Types.Data_Map_Type;
      end record;

=A0 =A0function Find_All
=A0 =A0 =A0(Template : Active_Record'Class;
=A0 =A0 =A0 Like =A0 =A0 : String :=3D "";
=A0 =A0 =A0 Order_By : String :=3D "")
=A0 =A0 =A0 return Active_Record_Array.Vector
=A0 =A0is
=A0 =A0 =A0 use Ada.Strings.Unbounded;
=A0 =A0 =A0 use Ada.Strings;

=A0 =A0 =A0 SQL =A0: Unbounded_String :=3D
=A0 =A0 =A0 =A0 To_Unbounded_String ("select * from ") & Template.Table_Nam=
e.all;
=A0 =A0begin
=A0 =A0 =A0 if Like /=3D "" then
=A0 =A0 =A0 =A0 =A0SQL :=3D SQL & " where " & Like;
=A0 =A0 =A0 end if;

=A0 =A0 =A0 if Order_By /=3D "" then
=A0 =A0 =A0 =A0 =A0SQL :=3D SQL & " order by " & Order_By;
=A0 =A0 =A0 end if;

=A0 =A0 =A0 declare
=A0 =A0 =A0 =A0 =A0RS =A0 : Gnoga.Server.Database.Recordset'Class :=3D
=A0 =A0 =A0 =A0 =A0 =A0Template.Connection.Query (To_String (SQL));

=A0 =A0 =A0 =A0 =A0Rows : Active_Record_Array.Vector;
=A0 =A0 =A0 =A0 =A0Row =A0: Active_Record'Class :=3D Template;
=A0 =A0 =A0 begin
=A0 =A0 =A0 =A0 =A0Row.Is_New :=3D False;

=A0 =A0 =A0 =A0 =A0while RS.Next loop
=A0 =A0 =A0 =A0 =A0 =A0 Row.Values :=3D RS.Field_Values;
=A0 =A0 =A0 =A0 =A0 =A0 Rows.Append (Row);
=A0 =A0 =A0 =A0 =A0end loop;

=A0 =A0 =A0 =A0 =A0RS.Close;
=A0 =A0 =A0 =A0 =A0return Rows;
=A0 =A0 =A0 end;
=A0 =A0end Find_All;


On run the following error appears:
db_active(14597,0x7fff775fb310) malloc: *** error for object 0xc00007fe659d=
040f: pointer being freed was not allocated
*** set a breakpoint in malloc_error_break to debug
Abort trap: 6


Using this work around removes the error, but limits the use of derived typ=
es of Active_Record being created by the assignment:

so replacing - Row =A0: Active_Record'Class :=3D Template;

with:
=A0=A0 =A0 =A0 =A0 Row =A0: Active_Record (Template.Table_Name, Template.Co=
nnection);

All works.