How to solve the error "PLS-00363: expression cannot be used as an assignment target" in Oracle? - oracle

Question

I have the following plsql object :
CREATE OR REPLACE TYPE TestType AS OBJECT
(
firstname VARCHAR2(30),
lastname VARCHAR2(30),
MEMBER PROCEDURE dosomething,
MEMBER function downgrade return number,
CONSTRUCTOR FUNCTION TestType(fname VARCHAR2, lname VARCHAR2) RETURN SELF AS RESULT
)
/
CREATE OR REPLACE TYPE BODY TestType AS
CONSTRUCTOR FUNCTION TestType(fname VARCHAR2, lname VARCHAR2)
RETURN SELF AS RESULT
IS
BEGIN
SELF.firstname := fname;
SELF.lastname := lname;
RETURN;
END;
MEMBER function downgrade return number IS
BEGIN
self.dosomething; /* Compilation Error Here */
return 1;
END;
MEMBER PROCEDURE dosomething IS
BEGIN
null;
END;
END;
/
It's an example of a complex object I have. Basically from a member function, I need to call a member procedure. When doing so, I'm getting the compilation error : PLS-00363: expression cannot be used as an assignment target
I don't get why this error is raised.
Does anyone know what am I doing wrong please ?
Thanks.
Cheers,

Answer 1

Although the documentation says:
Member methods have a built-in parameter named SELF that denotes the object instance currently invoking the method.
SELF can be explicitly declared, but that is not necessary. ...
it also later says:
SELF is always the first parameter passed to the method.
In member functions, if SELF is not declared, its parameter mode defaults to IN.
In member procedures, if SELF is not declared, its parameter mode defaults to IN OUT. The default behavior does not include the NOCOPY compiler hint.
You're getting the error because your procedure call
self.dosomething;
is implicitly passing self to the procedure as IN OUT, but it's implicitly passed into the function as IN. That means that it isn't in the right mode for that procedure call - the mode means the procedure could modify self, while the function says it won't.
You can avoid the error by changing the mode in the function, by explicitly declaring the self argument, in both the type:
MEMBER function downgrade (self in out TestType) return number,
and type body:
MEMBER function downgrade (self in out TestType) return number IS
fiddle
If your real procedure won't modify self (maybe unlikely) then you could declare it's self IN instead.
It is often said that functions should only return values and shouldn't have side-effects, so arguably that would be more correct - someone calling your member function might not expect it to change anything.
So if dosomething is changing data then perhaps downgrade should also be a procedure, with an OUT argument instead of a return value. You then wouldn't need to declare self as both would be IN OUT by default.

Answer 2

This one compiles successfully:
MEMBER FUNCTION DOWNGRADE RETURN NUMBER IS
me TestType := SELF;
BEGIN
me.dosomething;
RETURN 1;
END;
But you should test it carefully, whether it is working as expected.
Otherwise you could change the procedure into a function and ignore the return value:
CREATE OR REPLACE TYPE TestType AS OBJECT
(
firstname VARCHAR2(30),
lastname VARCHAR2(30),
MEMBER FUNCTION dosomething RETURN NUMBER,
MEMBER FUNCTION DOWNGRADE RETURN NUMBER,
CONSTRUCTOR FUNCTION TestType(fname VARCHAR2, lname VARCHAR2) RETURN SELF AS RESULT
)
/
CREATE OR REPLACE TYPE BODY TestType AS
CONSTRUCTOR FUNCTION TestType(fname VARCHAR2, lname VARCHAR2)
RETURN SELF AS RESULT
IS
BEGIN
SELF.firstname := fname;
SELF.lastname := lname;
RETURN;
END;
MEMBER FUNCTION DOWNGRADE RETURN NUMBER IS
ret NUMBER;
BEGIN
ret := self.dosomething;
RETURN 1;
END;
MEMBER FUNCTION dosomething RETURN NUMBER IS
BEGIN
NULL;
RETURN NULL;
END;
END;
/
Another approach could be to create a STATIC PROCEDURE procedure instead of a MEMBER PROCEDURE