Constructors in Ada95

Constructors in Ada95

[Andy Askey]

Hello,
I am sure this has been asked/answered several hundred times but
nothing current appears in DejaNews.

I want to call a procedure each time an object is instantiated for a
specific class/type and pass the current instance into the procedures.
(I could call the procedure after the object is instantiated, but it
will be easier on the user of my class/type if the procedure is
automatically called after instantiation.)  I looked into using a
Controlled type, but I still cannot figure out how to work on a
specific object instance's data.

For example (possible syntax errors included)...


package pkg is
  type type1 is tagged private;
private
  type type1 is tagged
    record
      i : integer := 0;
    end record;
end pkg;

package body pkg is
  ...
begin
  Proc_To_Call( This_Current_Instance : in Type1'class);
end pkg;


Any tips will be useful as I cannot find any info about this in any of
my Ada books.  I have "Ada(95) as a Second Language", so if you know
of a section that discusses C++ style constructors, please help me out
with the page numbers.  Also, any info on the web that deals with this
would be helpful.  I do not mind reading, but I don't have a lot of
time to figure out what it is I should read to answer my immediate
question.

Thanx.
--
----------------------------------------------------
|                   Andy Askey                     |
|                Software Engineer                 |
|           Raytheon Electronic Systems            |
|   353 James Record Road, Huntsville, AL 35824    |
|   Phone: (256) 772-4846  Fax: (256) 772-4771     |
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----------------------------------------------------


--
Andy Askey
ajaskey@mindspring.com

Re: Constructors in Ada95

[Simon Wright]

Andy Askey <ajaskey@mindspring.com> writes:

> I am sure this has been asked/answered several hundred times but
> nothing current appears in DejaNews.

Perhaps because there's a pretty full discussion in the LRM/Rationale!
If you don't have access, try www.adapower.com ..

with Ada.Finalization;
> package pkg is
>   type type1 is tagged private;
  type Type1 is new Ada.Finalization.Controlled with private;
> private
>   type type1 is tagged
>     record
>       i : integer := 0;
>     end record;
  type Type1 is new Ada.Finalization.Controlled with record
    I : Integer := 0;
  end record;
  procedure Initialize (T : in out Type1);
> end pkg;
> 
> package body pkg is
>   ...
  procedure Initialize (T : in out Type1) is
  begin
    ..
  end Initialize;
> begin
>   Proc_To_Call( This_Current_Instance : in Type1'class);
  -- no, leave this out.
> end pkg;

Initialize gets called whenever an object of type Type1 is created.

See also Adjust and Finalize.

Re: Constructors in Ada95

[David]

In article <h414bsc31c83a96l429kfeflk8rsfbk7fq@4ax.com>,
  Andy Askey <ajaskey@mindspring.com> wrote:
> Hello,
> I am sure this has been asked/answered several hundred times but
> nothing current appears in DejaNews.
>
> I want to call a procedure each time an object is instantiated for a
> specific class/type and pass the current instance into the procedures.
> (I could call the procedure after the object is instantiated, but it
> will be easier on the user of my class/type if the procedure is
> automatically called after instantiation.)  I looked into using a
> Controlled type, but I still cannot figure out how to work on a
> specific object instance's data.
>
Here is a fairly complete example:

with Ada.Finalization;
package Constructor_Example is

   type String_Access is access all String;

   type Example
     (Initial_Name  : String_Access;
      Initial_Value : Integer)
   is new Ada.Finalization.Limited_Controlled with private;

   function Image (Object : Example) return String;

   procedure Copy (Object : in out Example; From : Example);

   procedure Set (Object : in out Example; Name : String);

   procedure Set (Object : in out Example; Value : Integer);

private

   type Example
     (Initial_Name  : String_Access;
      Initial_Value : Integer)
   is new Ada.Finalization.Limited_Controlled with record
      Name  : String_Access;
      Value : Integer;
   end record;

   procedure Initialize (Object : in out Example);

   procedure Finalize (Object : in out Example);

end Constructor_Example;

with Ada.Text_IO;
with Ada.Strings.Unbounded;
with Ada.Unchecked_Deallocation;
package body Constructor_Example is

   procedure Free is new Ada.Unchecked_Deallocation
     (Object => String,
      Name   => String_Access);

   ----------
   -- Copy --
   ----------

   procedure Copy (Object : in out Example; From : Example)
   is
   begin
      Free (Object.Name);
      Object.Name := new String '(From.Name.all);
      Object.Value := From.Value;
   end Copy;

   -----------
   -- Image --
   -----------

   function Image (Object : Example) return String is
      Unbounded_String : Ada.Strings.Unbounded.Unbounded_String;
   begin
      if Object.Name = null then
        Ada.Strings.Unbounded.Append
        (Source => Unbounded_String,
         New_Item => "Object is un-named;");
      else
        Ada.Strings.Unbounded.Append
        (Source => Unbounded_String,
         New_Item => Object.Name.all);
      end if;
      Ada.Strings.Unbounded.Append
        (Source => Unbounded_String,
         New_Item => " has the value" &
         Integer'Image (Object.Value));
      return Ada.Strings.Unbounded.To_String (Unbounded_String);
   end Image;

   ---------
   -- Set --
   ---------

   procedure Set (Object : in out Example; Name : String) is
   begin
      Free (Object.Name);
      Object.Name := new String '(Name);
   end Set;

   ---------
   -- Set --
   ---------

   procedure Set (Object : in out Example; Value : Integer) is
   begin
      Object.Value := Value;
   end Set;

   ----------------
   -- Initialize --
   ----------------

   procedure Initialize (Object : in out Example)
   is
   begin
      if Object.Initial_Name /= null then
         Object.Name := new String '(Object.Initial_Name.all);
      end if;
      Object.Value := Object.Initial_Value;
      Ada.Text_IO.Put_Line
        ("Initialized object """ & Image (Object) & """.");
   end Initialize;

   --------------
   -- Finalize --
   --------------

   procedure Finalize (Object : in out Example)
   is
   begin
      Ada.Text_IO.Put_Line
        ("Finalizing object """ & Image (Object) & """.");
      Free (Object.Name);
   end Finalize;

end Constructor_Example;

with Ada.Text_IO;
with Constructor_Example;
procedure Test_Constructor_Example is
   Instance_1_Initial_Name : aliased String := "Instance_1";
   Instance_1 : Constructor_Example.Example
     (Instance_1_Initial_Name'Unchecked_Access, 1234);
begin
   declare
   Instance_2 : Constructor_Example.Example
     (null, 4321);
   begin
      Ada.Text_IO.Put_Line (Constructor_Example.Image (Instance_2));
      Constructor_Example.Set (Instance_2, "INSTANCE_2");
      Constructor_Example.Set (Instance_2, 5678);
      Ada.Text_IO.Put_Line (Constructor_Example.Image (Instance_2));
      Constructor_Example.Copy (Instance_2, Instance_1);
      Ada.Text_IO.Put_Line
        ("Image of Instance_2, after Instance_1 has been copied to
it:");
      Ada.Text_IO.Put_Line (Constructor_Example.Image (Instance_2));
      Ada.Text_IO.Put_Line ("Leaving inner scope.");
   end;
   Ada.Text_IO.Put_Line (Constructor_Example.Image (Instance_1));
   Ada.Text_IO.Put_Line ("Leaving outer scope.");
end Test_Constructor_Example;







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