Re: Constructors with multiple inheritance

[Simon Wright <simon@pushface.org>]

"Rego, P." <pvrego@gmail.com> writes:

>     procedure Test_Pkg_User is
>        Test_Obj : Test_Pkg.Test_Class_Ptr;
>     begin
>        Test_Obj := Test_Obj.Construct;
>        Put_Line ("Value = "&Integer'Image (Test_Obj.Info));
>     end Test_Pkg_User;
>
> The message I get is the following:
>     c:\tst>gnatmake test_pkg_user.adb
>     gcc -c test_pkg_rev623_user.adb
>     test_pkg_user.adb:12:16: warning: null value not allowed here
>     test_pkg_user.adb:12:16: warning: "Constraint_Error" will be raised at run time
>     gcc -c test_pkg.adb
>     gnatbind -x test_pkg_user.ali
>     gnatlink test_pkg_user.ali
>
> What am I still doing wrong? Thanks again.
>
> Note: Sure if I put in test_pkg_user.adb
>     Test_Obj : Test_Pkg.Test_Class_Ptr := new Test_Pkg.Test_Class;
> it returns me a build with no warning, but I think this way I allocate
> memory twice to the pointer (one in the call new Test_Pkg.Test_Class
> and the other in the construct method execution), right?

The declaration

   function Construct (T : access Test_Class) return Test_Class_Ptr;

requires T not to be null because of ARM2005 3.10(13.1/2) [1]

   "In addition, the anonymous access subtype defined by the
   access_definition for a controlling access parameter (see 3.9.2)
   excludes null."

Of course, as your code stands, even if you changed Construct to

   function Construct (T : Test_Class_Ptr) return Test_Class_Ptr;

the call would fail anyway because you don't check for T being null
before saying

      T_Ptr.Info := T.Info + 1;

[1] http://www.adaic.org/resources/add_content/standards/05rm/html/RM-3-10.html