Davor Josipovic Just another WordPress blog – rather tryout

11/08/2010

“Undefined reference to vtable” for QObject-derived classes in the implementation (.cpp) file.

Filed under: Programming,Qt — Tags: , — Davor @ 18:14

“undefined reference to vtable” seems to be a common problem with Qt’s meta-object system in combination with GCC.

The main cause due to GCC is described here: http://gcc.gnu.org/faq.html#vtables

But with QObject it gets a bit more complex. When you use the macro Q_OBJECT, you define some virtual methods which might trigger the above GCC error under certain circumstances. So make sure your use of the Q_OBJECT macro fulfills the following requirements:

  1. Make sure the Q_OBJECT macro is present in the definition of all QObject-derived classes
  2. Make sure you define your QObject-derived classes in your header files ONLY
  3. Make sure all of your header files are listed in your .pro file in the HEADERS-list
  4. Run qmake every time you add Q_OBJECT to one of your classes or modify your .pro file

(Source: http://www.theirishpenguin.com/2007/07/01/qobject-qmake-and-sadness-undefined-reference-to-vtable/)

Not fulfilling point 2:
You get the “undefined reference to vtable” error if you put your QObject-derived class in the implementation file because moc will not “add” the implementation of the virtual functions to the cpp file. Apparently you can enforce this by adding “#include cpp_file_name.moc” to your “cpp_file_name.cpp” file. Moc will detect this and will generate a “cpp_file_name.moc” file.

29/06/2010

PostgreSQL table audit in plpgsql

Filed under: Postgres,Programming — Tags: , — Davor @ 15:27

I recently had to implement a logging system for our database. Googling resulted in a few interesting ways to do this. The best I have found so far is from Lorenzo Alberton. I recommend you glance through that, and also an extension/improvement of it by James Gardner.

Now, Lorenzo’s implementation has a few problems. Most notably: not all of us want to install Tcl. So, I have “rewritten” it in plpgsql. This is possible since in the meantime the EXECUTE command was extended with the USING clause.

Some extra features are added also:

  • Support for multi-field PK’s.
  • Logging of PK’s.
  • Changes are detected by default type operators instead of textbased operators.

Note: I am using a slightly different naming convention, so you should adjust the names before using it with extra functions from James.

Note2: This is still a (working) tryout. Comments on improvement are welcome.

Update: fnc_audit(…) is used by trg_audit()

CREATE OR REPLACE FUNCTION private.tfc_audit()
  RETURNS TRIGGER AS
$BODY$ 
 
/*!
 Dynamic query exectution through plpgsql EXECUTE command makes this function possible.
 Some complications which can araise using this command are described here:
  - http://archives.postgresql.org/pgsql-general/2008-05/msg00314.php
*/ 
 
DECLARE 
 
_PK_NAMES CONSTANT VARCHAR[] := ARRAY(SELECT a.attname AS pk_name FROM pg_class c, pg_attribute a, pg_index i
     WHERE c.oid = i.indrelid
     AND a.attrelid = i.indexrelid
     AND a.attisdropped = FALSE /* such column names are like "........pg.dropped.1........" and are invalid */
     AND a.attnum > 0 /* i.e is not a system column like OID: they have (arbitrary) negative numbers */ 
     AND i.indisprimary = TRUE /* is pk */
     AND c.oid = TG_RELID /* regclass takes current schema into consideration! */); 
 
_FIELD_NAMES CONSTANT VARCHAR[] := ARRAY(SELECT a.attname AS pk_name FROM pg_class c, pg_attribute a
     WHERE c.oid = a.attrelid
     AND a.attisdropped = FALSE /* such column names are like "........pg.dropped.1........" and are invalid */
     AND a.attnum > 0 /* i.e is not a system column like OID: they have (arbitrary) negative numbers */ 
     AND c.oid = TG_RELID ); /* regclass takes current schema into consideration! */ 
 
_PK_VALUES VARCHAR[]; 
 
BEGIN 
 
-- this function works only when marked as AFTER trigger!
PERFORM assert(TG_WHEN = 'AFTER', 'Wrong execution of fnc_audit(): should only be executed AFTER an event!');
-- this function works only when marked as FOR EACH ROW trigger!
PERFORM assert(TG_LEVEL = 'ROW', 'Wrong execution of fnc_audit(): should only be executed FOR EACH ROW!'); 
 
-- table sanity check
PERFORM assert(TG_TABLE_NAME IS NOT NULL, 'TG_TABLE_NAME IS NULL');
PERFORM assert(_PK_NAMES IS NOT NULL AND array_length(_PK_NAMES, 1) IS NOT NULL, 'Table ' || TG_TABLE_NAME || ' can not be logged: it has no PK!');  -- FOR-loop will otherwise throw "upper bound of FOR loop cannot be null" 
 
-- skip changes on audit_table: otherwise endless loop
IF TG_TABLE_NAME = 'tbl_audits' THEN
 RETURN NULL; -- result is ignored since this is an AFTER trigger
END IF; 
 
-- find PK value
FOR i IN 1 .. array_length(_PK_NAMES, 1)
LOOP
 DECLARE
  tmp text;
  tmp_record record;
 BEGIN
  CASE TG_OP
  WHEN 'UPDATE', 'INSERT' THEN tmp_record = NEW;
  WHEN 'DELETE' THEN tmp_record = OLD;
  END CASE; 
 
  EXECUTE 'SELECT $1.' || quote_ident(_PK_NAMES[i]) INTO STRICT tmp USING tmp_record;
  _PK_VALUES[i] := tmp;
 END;
END LOOP; 
 
-- PK values sanity check
PERFORM assert(_PK_VALUES IS NOT NULL AND array_length(_PK_VALUES, 1) IS NOT NULL, '_PK_VALUES IS NULL'); -- FOR-loop will otherwise throw "upper bound of FOR loop cannot be null"
PERFORM assert(array_length(_PK_VALUES, 1) = array_length(_PK_NAMES, 1), '_PK_VALUES count not equal to _PK_NAMES count in table ' || TG_TABLE_NAME); 
 
-- find fields to be logged
FOR i IN 1 .. array_length(_FIELD_NAMES, 1)
LOOP
 -- field_name sanity check
 PERFORM assert(_FIELD_NAMES[i] IS NOT NULL, '_FIELD_NAMES[' || i || '] IS NULL'); 
 
 DECLARE
  _field_name CONSTANT VARCHAR := quote_ident(_FIELD_NAMES[i]);
  _distinct BOOLEAN;
  _old text;
  _new text;
 BEGIN 
 
 CASE TG_OP --fall through is unfortunately not supported
  WHEN 'UPDATE' THEN
   -- compare the two fields according to the type-default functions.
   -- note that <> returns false on NULL input and IS DISTINCT FROM does not, but only if both inputs are NULL!
   EXECUTE 'SELECT $1.' || _field_name || ' IS DISTINCT FROM $2.' || _field_name || ';' INTO STRICT _distinct USING OLD, NEW;
   CONTINUE WHEN NOT _distinct; 
 
   EXECUTE 'SELECT $1.' || _field_name || ';' INTO STRICT _new USING NEW;
   EXECUTE 'SELECT $1.' || _field_name || ';' INTO STRICT _old USING OLD;
  WHEN 'INSERT' THEN
   EXECUTE 'SELECT $1.' || _field_name || ';' INTO STRICT _new USING NEW;
  WHEN 'DELETE' THEN
   EXECUTE 'SELECT $1.' || _field_name || ';' INTO STRICT _old USING OLD;
  ELSE
   RAISE EXCEPTION 'Unhandled TG_OP in fnc_audit(): %', TG_OP;
 END CASE; 
 
 CONTINUE WHEN _new IS NULL AND _old IS NULL; 
 
 PERFORM private.fnc_audit(statement_timestamp()::TIMESTAMP WITHOUT TIME zone, SESSION_USER::VARCHAR, TG_TABLE_NAME::VARCHAR, _field_name, _PK_NAMES, _PK_VALUES, TG_OP::private.table_audit_mod_type, _old, _new);
 
 END;
END LOOP; 
 
RETURN NULL; -- result is ignored since this is an AFTER trigger
END; 
 
$BODY$
  LANGUAGE 'plpgsql' VOLATILE SECURITY DEFINER;

A specific function for inserting into tbl_audits is used:

CREATE OR REPLACE FUNCTION private.fnc_audit(_audi_timestamp TIMESTAMP WITHOUT TIME zone, _audi_user name, _audi_table name, _audi_field name, _audi_pk_name CHARACTER VARYING[], _audi_pk_value CHARACTER VARYING[], _audi_mod_type private.table_audit_mod_type, _audi_value_old text, _audi_value_new text)
  RETURNS void AS
$BODY$
DECLARE
BEGIN 
 
INSERT INTO private.tbl_audits(audi_timestamp, audi_user, audi_table, audi_field, audi_pk_name, audi_pk_value, audi_mod_type, audi_value_old, audi_value_new)
 VALUES (_audi_timestamp, _audi_user, _audi_table, _audi_field, _audi_pk_name, _audi_pk_value, _audi_mod_type, _audi_value_old, _audi_value_new); 
 
END;
$BODY$
  LANGUAGE 'plpgsql' VOLATILE;

TG_OP is stored as an ENUM (= more efficient) so we need a new type:

CREATE TYPE private.table_audit_mod_type AS ENUM ('INSERT', 'UPDATE', 'DELETE', 'TRUNCATE');

Here is a very important helper function:

CREATE OR REPLACE FUNCTION assert(BOOLEAN, CHARACTER VARYING)
  RETURNS void AS
$BODY$
BEGIN
 IF NOT $1 OR $1 IS NULL THEN
   IF $2 IS NOT NULL THEN 
     RAISE EXCEPTION 'Assert failure: %', $2;
   END IF;
   RAISE NOTICE 'Assert. Message is null';
 END IF; 
END;$BODY$
  LANGUAGE 'plpgsql' VOLATILE;

And finaly the audit table:

CREATE TABLE private.tbl_audits
(
  audi_timestamp TIMESTAMP WITHOUT TIME zone NOT NULL,
  audi_user CHARACTER VARYING(30) NOT NULL,
  audi_table CHARACTER VARYING(45) NOT NULL,
  audi_field CHARACTER VARYING(30) NOT NULL,
  audi_pk_name CHARACTER VARYING(30)[] NOT NULL,
  audi_pk_value CHARACTER VARYING(60)[] NOT NULL,
  audi_mod_type private.table_audit_mod_type NOT NULL,
  audi_value_old text,
  audi_value_new text,
  CONSTRAINT tbl_audits_check_pk CHECK (array_length(audi_pk_name, 1) = array_length(audi_pk_value, 1))
);

A more storage-efficient way to store logged data would be to make an audi_changeset column which consists of an array of changed fieldnames, old values and new values for each PK during a statement execution. It would be equivalent to the following view:

CREATE OR REPLACE VIEW vew_audit_changesets AS
 SELECT tbl_audits.audi_timestamp, tbl_audits.audi_user, tbl_audits.audi_table, tbl_audits.audi_pk_name, tbl_audits.audi_pk_value, tbl_audits.audi_mod_type, ARRAY[array_agg(tbl_audits.audi_field), array_agg(tbl_audits.audi_value_old)::CHARACTER VARYING[], array_agg(tbl_audits.audi_value_new)::CHARACTER VARYING[]] AS audi_changeset
   FROM tbl_audits
  GROUP BY tbl_audits.audi_timestamp, tbl_audits.audi_user, tbl_audits.audi_table, tbl_audits.audi_pk_name, tbl_audits.audi_pk_value, tbl_audits.audi_mod_type;

fnc_audit() is easily adjustable to store logs to such a table. But I wonder whether this is worth the fuss? Most of the queries run on vew_audit_changesets would first require the “unnesting of array audi_changeset to sets” (i.e. a structure which resembles tbl_audits). Why? Because SQL is set-oriented, not array-oriented. In other words, it seems easy to convert from tbl_audits structure to vew_audit_changesets structure, but how does one go from vew_audit_changesets structure to tbl_audits structure, and is this conversion efficient? Is it worth to save logs in a vew_audit_changesets-like table?

11/05/2010

IsDirty() class member for QDataWidgetMapper

Filed under: C++,Programming,Qt — Tags: , — Davor @ 15:57

Here is something I wanted to share. It’s a pity something similar isn’t implemented by the Trolls. But then again, it doesn’t require much coding either.

    bool isDirty() const {
        Q_ASSERT(orientation() == Qt::Horizontal);
        Q_ASSERT(rootIndex() == QModelIndex());
        for(int i = 0; i < model()->columnCount(); i++) {
            QWidget *mapWidget = mappedWidgetAt(i);
            if (mapWidget){
                QByteArray p = mappedPropertyName(mapWidget);
                QModelIndex idx = model()->index(currentIndex(), i);
                if (idx.data(Qt::EditRole) != mapWidget->property(p))
                    return true;
            }
        }
        return false;
    }

One use is to check whether an update is required before going to the next record…

PS Other way one could achieve the same thing by adjusting the source code because the “mappings”-container is in the private implementation of QDataWidgetMapper.

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