Subclassing Curiosity

[Kevin Bingham]

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, 
http://www.perlmonks.org/, which is highly touted by our very own Tom 
Anderson (http://tomacorp.com/, We're not a corporation). On Perl Monks you 
should get an answer right quick.

Kudos!
-Kevin

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:
>
>#!/usr/bin/perl
>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 $dbh2)."\n";
>
>__END__
>
>The output of the above code looks like this:
>$obj is a: MyDBI
>DBI subclass 'MyDBI=HASH(0x813c1a0)::db' isn't setup, ignored at MyDBI.pl 
>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
>understanding?
>
>TIA
>Sebastian