It provides a method to create your own constructor, your own procedural logic to setup the object type:


ops$tkyte@ORA9IR2> create or replace type myType
  2  as object
  3  ( x int,
  4    y date,
  5    z varchar2 (25),
  6    constructor function myType return self as result,
  7    constructor function myType( some_number in number ) return self as result
  8  )
  9  /
 
Type created.
 
ops$tkyte@ORA9IR2>
ops$tkyte@ORA9IR2> create or replace type body myType
  2  as
  3
  4  constructor function myType return self as result
  5  is
  6  begin
  7     x := 55;
  8     y := sysdate;
  9     z := 'Hello World';
 10     return;
 11  end;
 12
 13  constructor function myType( some_number in number ) return self as result
 14  is
 15  begin
 16     x := some_number;
 17     y := sysdate+some_number;
 18     z := 'Hello World ' || some_number;
 19     return;
 20  end;
 21
 22  end;
 23  /
 
Type body created.
 
ops$tkyte@ORA9IR2>
ops$tkyte@ORA9IR2>
ops$tkyte@ORA9IR2>
ops$tkyte@ORA9IR2> declare
  2          x myType := myType();
  3          y myType := myType(2);
  4  begin
  5          dbms_output.put_line( x.x || ', ' || x.y || ', ' || x.z );
  6          dbms_output.put_line( y.x || ', ' || y.y || ', ' || y.z );
  7  end;
  8  /
55, 04-MAR-04, Hello World
2, 06-MAR-04, Hello World 2
 
PL/SQL procedure successfully completed.
 

ops$tkyte@ORA9IR2> create or replace TYPE copy_ca is record    ( x number )
  2  /
 
Warning: Type created with compilation errors.
 


because "is record" is "not valid"

create or replace type copy_ca as OBJECT (...... ) would be

record is PLSQL specific 

(always nice to have a script that runs!  varchar2, does not compile... c1 number; does not compile.....)

anyway  

my_a is a scalar object.

my_a.my_b is a TABLE (collection).  A.my_b(1).my_c would be valid.  A.my_b.my_c is not.  you are missing a subscript (and some initialization)

after fixing up script....


ops$tkyte@ORA9IR2> declare
  2  my_A A;
  3  begin
  4
  5  my_a := a( null, table1( b(null,null, null ) ) );
  6
  7  select c(sysdate,hiredate)
  8    bulk collect into my_A.my_B(1).my_C
  9    from emp;
 10  end;
 11  /
 
PL/SQL procedure successfully completed.
 

ops$tkyte@ORA9IR2> set describe depth all
ops$tkyte@ORA9IR2> desc a
 Name                                     Null?    Type
 ---------------------------------------- -------- ----------------------------
 C1                                                NUMBER
 MY_B                                              TABLE1
   C2                                              NUMBER
   C3                                              VARCHAR2(25)
   MY_C                                            TABLE2
     START_DATE                                    DATE
     END_DATE                                      DATE
 


my_a.my_b is a table of three thnigs.  You select 1 thing.


Not really sure what your ultimate goal here is, but the shape of what you are trying to select comes no where near the shape of the thing you are selecting INTO.