OUT Parameter Trick
Raja asked a question but unfortunately, I was buried in the final aspects of the write of the new Oracle Database 12c PL/SQL Programming book. He wanted to know how to pass an object type as an OUT
-only mode parameter from a procedure.
That’s a great question, and it’s actually simple once you understand the difference between Oracle object types and other data types. Oracle object types must always be initiated before you use them, which means you must initialize any OUT
-only mode parameters at the top of your execution section, like this:
1 2 3 4 5 6 7 8 9 10 11 12 | CREATE OR REPLACE PROCEDURE reset_troll ( pv_troll OUT TROLL_OBJECT ) IS /* Troll default name. */ lv_troll_name VARCHAR2(20) := 'Bert'; BEGIN /* Initialize the incoming parameter by allocating memory to it. */ pv_troll := troll_object(); /* Set the name to something other than the 'Tom' default value. */ pv_troll.set_troll(lv_troll_name); END reset_troll; / |
Line 7 shows you the trick, initialize the incoming parameter because there isn’t an incoming parameter for an OUT
-only mode parameter. The calling parameter to an OUT
-only mode parameter is only a reference where PL/SQL will copy the internal object reference. While the calling parameter has been initialized, the reference to the call parameter’s object is where the internal object will be copied. The local program must first ensure a new memory location for a new instance of the object type before it can act on or return an object instance to the external reference. More or less, the internal object is copied to the calling object instance’s memory location when the procedure completes its execution.
Here’s the source code for the troll_object
object type and body:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 | CREATE OR REPLACE TYPE troll_object IS OBJECT ( troll VARCHAR2(20) , CONSTRUCTOR FUNCTION troll_object RETURN SELF AS RESULT , CONSTRUCTOR FUNCTION troll_object ( troll VARCHAR2 ) RETURN SELF AS RESULT , MEMBER FUNCTION get_troll RETURN VARCHAR2 , MEMBER PROCEDURE set_troll (troll VARCHAR2) , MEMBER FUNCTION to_string RETURN VARCHAR2) INSTANTIABLE NOT FINAL; / CREATE OR REPLACE TYPE BODY troll_object IS /* Default no-argument constructor. */ CONSTRUCTOR FUNCTION troll_object RETURN SELF AS RESULT IS troll TROLL_OBJECT := troll_object('Tom'); BEGIN SELF := troll; RETURN; END troll_object; /* Single argument constructor. */ CONSTRUCTOR FUNCTION troll_object (troll VARCHAR2) RETURN SELF AS RESULT IS BEGIN SELF.troll := troll; RETURN; END troll_object; /* A getter function. */ MEMBER FUNCTION get_troll RETURN VARCHAR2 IS BEGIN RETURN SELF.troll; END get_troll; /* A setter procedure. */ MEMBER PROCEDURE set_troll (troll VARCHAR2) IS BEGIN SELF.troll := troll; END set_troll; /* A function that returns the formatted object type's contents. */ MEMBER FUNCTION to_string RETURN VARCHAR2 IS BEGIN RETURN 'Hello '||SELF.troll; END to_string; END; / |
You can test the reset_troll procedure with the following anonymous block:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 | /* Enable printing from a PL/SQL block. */ SET SERVEROUTPUT ON SIZE UNLIMITED /* Anonymous testing block. */ DECLARE lv_troll TROLL_OBJECT := troll_object('Bill'); BEGIN dbms_output.put_line('--------------------'); /* Prints 'Hello William' */ dbms_output.put_line(lv_troll.to_string()); dbms_output.put_line('--------------------'); reset_troll(lv_troll); /* Prints 'Hello Bert' */ dbms_output.put_line(lv_troll.to_string()); dbms_output.put_line('--------------------'); END; / |
If you remark out line 7 from the reset_troll procedure, you’d raise the following exception by the call on line 10 because the local object hasn’t been instantiated (given life). It means there’s no memory location allocated for the instantiated (instance of an object type).
-------------------- Hello Bill -------------------- DECLARE * ERROR at line 1: ORA-30625: method dispatch ON NULL SELF argument IS disallowed ORA-06512: at "VIDEO.RESET_TROLL", line 10 ORA-06512: at line 8 |
Hope this helps those trying to solve the same problem.