Downcasting in Object PL/SQL Does not Work in Oracle 10g Enterprise Edition

Question

create type one_t as object (
x number, y date ) not final
create type two_t under one_t()
set serverout on
declare
super one_t;
sub two_t := two_t(1, sysdate);
begin
super := one_t(1, sysdate);
sub := new two_t(1, sysdate);
dbms_output.put_line('sub ' || sub.x || ' ' || sub.y );
select treat(super as two_t) into sub from dual;
dbms_output.put_line('super ' || super.x || ' ' || super.y );
if sub is null then
dbms_output.put_line('stupid');
end if;
if sub is of (two_t) then
dbms_output.put_line('stupid');
end if;
end;
The Other Piece Does not Even Compile
declare
super one_t;
sub two_t := two_t(1, sysdate);
begin
super := one_t(1, sysdate);
sub := treat(super as two_t);
dbms_output.put_line('sub ' || sub.x || ' ' || sub.y );
select treat(super as two_t) into sub from dual;
dbms_output.put_line('super ' || super.x || ' ' || super.y );
if sub is null then
dbms_output.put_line('stupid');
end if;
if sub is of (two_t) then
dbms_output.put_line('stupid');
end if;
end;
ORA-06502: PL/SQL: numeric or value error: cannot assign supertype instance to
subtype
ORA-06512: at line 6

Answer 1

You need use correct type when you create object.
Don't forget object in reality is a memory pointer,
so what if you don't allocate correct memory in creation,
you can't treat object as subtype.
And meanwhile, it doesn't depend on Oracle version -
your example doesn't work in 9.2.0.1 for example.
SQL> declare
  2  super one_t;
  3  sub two_t := two_t(1, sysdate);
  4  begin
  5  super := two_t(1, sysdate);
  6  sub := treat(super as two_t);
  7  dbms_output.put_line('sub ' || sub.x || ' ' || sub.y );
  8  select treat(super as two_t) into sub from dual;
  9  dbms_output.put_line('super ' || super.x || ' ' || super.y );
10  if sub is null then
11  dbms_output.put_line('stupid');
12  end if;
13  if sub is of (two_t) then
14  dbms_output.put_line('stupid');
15  end if;
16  end;
17  /
sub 1 08.06.05
super 1 08.06.05
stupid
&nbsp
PL/SQL procedure successfully completed.Rgds.