How to have a constant field inside a object - oracle

Question

to test the code: https://dbfiddle.uk/?rdbms=oracle_21&fiddle=1be9f3d406df287afd874ad5dfc94cfc
I would like to have constant field inside a object. I can do that with a package but I haven't succeeded to do that with an object. Why?
create type a as object(
a integer
) --OK
create type as object(
b constant integer :=1
)
ORA-02302: invalid or missing type name

Answer 1

The CREATE TYPE syntax does not allow it:
object_type_def ::=
datatype ::=
Nowhere does it say that you can use the CONSTANT keyword.
As an alternative, you can create an attribute and set it to a value in a user-defined constructor:
CREATE TYPE test IS OBJECT(
a NUMBER,
pi NUMBER,
CONSTRUCTOR FUNCTION test(
SELF IN OUT NOCOPY test,
a NUMBER
) RETURN SELF AS RESULT
);
with the type body:
CREATE TYPE BODY test IS
CONSTRUCTOR FUNCTION test(
SELF IN OUT NOCOPY test,
a NUMBER
) RETURN SELF AS RESULT
IS
BEGIN
SELF.a := a;
SELF.pi := 3.14159;
RETURN;
END;
END;
/
Then:
SELECT test(1).a, test(1).pi
FROM DUAL;
Outputs:
TEST(1).A
TEST(1).PI
1
3.14159
However, that does not make the value a constant and it can still be set to something else:
DECLARE
v_obj TEST := TEST(1);
BEGIN
v_obj.pi := 4; -- For large circles!
DBMS_OUTPUT.PUT_LINE( v_obj.a || ', ' || v_obj.pi );
END;
/
Outputs:
1, 4
As long as you have a policy of never changing the value then it will be the default from the user-defined constructor; if someone breaks that policy then...
db<>fiddle here