Subclassing Curiosity

Kevin Bingham kevin at
Tue May 7 17:13:48 CDT 2002

Hey Sebastian!

Sorry, I don't know the answer to this particular one, though somebody 
kicking around might. Though considering our meager mailing list, who knows 
when you'll get a response. I suggest posting your question to Perl Monks,, which is highly touted by our very own Tom 
Anderson (, We're not a corporation). On Perl Monks you 
should get an answer right quick.


At 04:48 PM 5/6/2002 -0700, Seb wrote:
>I have a curiosity with regard to subclassing DBI. I'm just not sure I
>get the principles involved. I've got a module that is a DBI subclass.
>In order to subclass DBI as "package Mine", one must also provide
>packages Mine::db and Mine::st, because the database and statement
>handles used in DBI are their own classes.
>My confusion comes in when I put a constructor in my main package, and
>try to use DBI methods on its returned object. (DBI has no constructor
>method with which to reference a DBI object, per se)
>So I have something like this:
>package MyDBI;
>use DBI;
>@ISA = qw(DBI);
>sub new {
>   my $class = shift;
>   my $self = {};
>   bless $self, $class;
>   return $self;
>package MyDBI::db;
>@ISA = qw(DBI::db);
>package MyDBI::st;
>@ISA = qw(DBI::st);
># now our main program
>package main;
>my $obj = new MyDBI;
># $obj is a MyDBI object, with DBI base methods
>print '$obj is a: '.(scalar ref $obj)."\n";
>my $dbh = $obj->connect('DBI:Pg:dbname=somedb', undef, undef);
># This generates a warning
># $dbh is a DBI::db object, NOT a MyDBI::db object
>print '$dbh is a: '.(scalar ref $dbh)."\n";
># On the other hand, if I use the module name, not my object...
>my $dbh2 = MyDBI->connect('DBI:Pg:dbname=somedb', undef, undef);
># No error, and $dbh2 is a proper MyDBI::db object
>print '$dbh2 is a: '.(scalar ref $dbh2)."\n";
>The output of the above code looks like this:
>$obj is a: MyDBI
>DBI subclass 'MyDBI=HASH(0x813c1a0)::db' isn't setup, ignored at 
>line 26
>$dbh is a: DBI::db
>$dbh2 is a: MyDBI::db
><disconnect errors snipped>
>Why does the first case ($obj->connect) not do what I expect it to,
>while the second case (MyDBI->connect) works fine?
>Is this just some basic scoping or inheritance thing that I'm not

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