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Object constructor quirk

with 5 comments

Never change something that works! Not really, but sometimes you feel that way. Especially, when you toast 5 to 10 minutes working through an undocumented behavior in PL/SQL. You’d think after writing it for 19 years, I’d have seen it all but not so.

I was working through a tried and true example from Chapter 14 of the Oracle Database 11g PL/SQL Programming book to prepare for teaching my class tomorrow, when I found this nice quirk. It took a few minutes to figure out what was happening, but here it is so you don’t have to look for it too. You can only use variable names that are attributes of the object type as formal parameters in object type constructors. If you try to vary it, you’ll trigger the following exception:

LINE/COL ERROR
-------- -----------------------------------------------------------------
4/11     PL/SQL: Item ignored
4/26     PLS-00307: too many declarations OF 'HELLO_THERE' MATCH this CALL
6/5      PL/SQL: Statement ignored
6/13     PLS-00320: the declaration OF the TYPE OF this expression IS incomplete OR malformed

All I did to trigger this exception was change the who variable to make it scope specific, like iv_who for instance variable, pv_who for parameter variable, and lv_who for local variable.

Broken Code ↓

This shows you the broken code and explains why it’s broken.

The broken code has an object type like the following. Interestingly enough, the object type will compile fine but the object body fails when the attribute variable name differs from a constructor function parameter value.

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CREATE OR REPLACE TYPE hello_there IS OBJECT
( iv_who VARCHAR2(20)
, CONSTRUCTOR FUNCTION hello_there
  RETURN SELF AS RESULT
, CONSTRUCTOR FUNCTION hello_there
  ( pv_who VARCHAR2 )
  RETURN SELF AS RESULT
, MEMBER FUNCTION get_who RETURN VARCHAR2
, MEMBER PROCEDURE set_who (pv_who VARCHAR2)
, MEMBER PROCEDURE to_string )
INSTANTIABLE NOT FINAL;
/

The broken code has an object body like the following. The difference between the parameter name in the constructor and the object type attribute name causes the PLS-00307 exception. It took some playing around to figure out what it was really complaining about.

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CREATE OR REPLACE TYPE BODY hello_there IS
 
  CONSTRUCTOR FUNCTION hello_there RETURN SELF AS RESULT IS
    hello HELLO_THERE := hello_there('Generic Object.');
  BEGIN
    SELF := hello;
    RETURN;
  END hello_there;
 
  CONSTRUCTOR FUNCTION hello_there (pv_who VARCHAR2) RETURN SELF AS RESULT IS
 
  BEGIN
    SELF.iv_who := pv_who;
    RETURN;
  END hello_there;
 
  MEMBER FUNCTION get_who RETURN VARCHAR2 IS
  BEGIN
    RETURN SELF.iv_who;
  END get_who;
 
  MEMBER PROCEDURE set_who (pv_who VARCHAR2) IS
  BEGIN
    SELF.iv_who := pv_who;
  END set_who;
 
  MEMBER PROCEDURE to_string IS
  BEGIN
    dbms_output.put_line('Hello '||SELF.iv_who);
  END to_string;
 
END;
/

Working Code ↓

This shows you the working code and explains why it works.

The working code has an object type like the following. You should notice that the only difference renames the pv_who in the overriding constructor’s parameter list to iv_who. While this doesn’t throw an exception creating the user-defined object type, it does throw an exception when you try to create the object body or implementation of the object type.

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CREATE OR REPLACE TYPE hello_there IS OBJECT
( iv_who VARCHAR2(20)
, CONSTRUCTOR FUNCTION hello_there
  RETURN SELF AS RESULT
, CONSTRUCTOR FUNCTION hello_there
  ( iv_who VARCHAR2 )
  RETURN SELF AS RESULT
, MEMBER FUNCTION get_who RETURN VARCHAR2
, MEMBER PROCEDURE set_who (pv_who VARCHAR2)
, MEMBER PROCEDURE to_string )
INSTANTIABLE NOT FINAL;
/

The working code has an object body like the following. Like the object type before, the pv_who as a formal parameter of the constructor now uses the same variable name as the attribute for the object type.

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CREATE OR REPLACE TYPE BODY hello_there IS
 
  CONSTRUCTOR FUNCTION hello_there RETURN SELF AS RESULT IS
    hello HELLO_THERE := hello_there('Generic Object.');
  BEGIN
    SELF := hello;
    RETURN;
  END hello_there;
 
  CONSTRUCTOR FUNCTION hello_there (iv_who VARCHAR2) RETURN SELF AS RESULT IS
 
  BEGIN
    SELF.iv_who := iv_who;
    RETURN;
  END hello_there;
 
  MEMBER FUNCTION get_who RETURN VARCHAR2 IS
  BEGIN
    RETURN SELF.iv_who;
  END get_who;
 
  MEMBER PROCEDURE set_who (pv_who VARCHAR2) IS
  BEGIN
    SELF.iv_who := pv_who;
  END set_who;
 
  MEMBER PROCEDURE to_string IS
  BEGIN
    dbms_output.put_line('Hello '||SELF.iv_who);
  END to_string;
 
END;
/

This is certainly interesting. I’ve no clue why such a limitation exists. The name of a parameter list member in the constructor should be independent from the attribute of a user object.

Written by maclochlainn

June 25th, 2009 at 11:57 pm

Posted in Objects,Oracle,pl/sql,sql

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