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Re^2: blessed confusion

by perl-diddler (Hermit)
on Sep 09, 2010 at 02:42 UTC ( #859452=note: print w/replies, xml ) Need Help??

in reply to Re: blessed confusion
in thread blessed confusion

Wait, wait, wait...'bless only once'...?? Um...I don't think so...

Each object's creation code must return a blessed reference to indicate that the returned 'object' is now part of that class (has been blessed into that class) -- meaning that the reference can be now called on any method in the new class.

W/o the blessing @ each level, you couldn't use the reference to call subclass methods, as it's not a pointer of any class (yet), until it's done being initialized into that class.

So something like 'cacheable' wouldn't be able to use the 'path' method of 'url' to derive a cacheable's path. Example:

package url (methods:new 'host' 'path') package url::cacheable sub new { $package=shift; $up=new url(@_); # host & path passed in args $up->path($cpath) if ($cpath=_canonical_path($up->path)) ne $path; bless $up, $package; }
So when $up comes back from 'new url', it's a ref to a url-blessed object and is used as such. But not until the path of the 'url' object has been tested as (and possibly set to) '_canonical_path', is it suitable to be blessed as a 'cachceable' object.

Am I missing something?

Replies are listed 'Best First'.
Re^3: blessed confusion
by chromatic (Archbishop) on Sep 09, 2010 at 06:06 UTC
    Am I missing something?

    Inside Perl 5, a reference only has one slot to associate it with a class. You can replace that association by blessing the reference into a different class, but there's only ever one class associated with a reference.

    I don't know what object system you're thinking of, but this is how Perl 5 has worked since the beginning. Test it and see:

    package Grandkid; use parent 'Kid'; sub new { bless {}, shift } package Kid; use parent 'Parent'; package Parent; sub inherited_method { 'Yep, inherited!' } package main; use Test::More 'no_plan'; my $gkid = Grandkid->new(); isa_ok( $gkid, 'Grandkid' ); isa_ok( $gkid, 'Kid' ); isa_ok( $gkid, 'Parent' ); is( $gkid->inherited_method(), 'Yep, inherited!', 'method inherited fr +om grandparent' ); done_testing();
Re^3: blessed confusion
by ikegami (Pope) on Sep 09, 2010 at 02:51 UTC

    W/o the blessing @ each level, you couldn't use the reference to call subclass methods,

    No, only the subclass's blessing is required. In fact, all the other blessings are removed by it. chromatic is simply suggesting that you bless it correctly the first time instead of correcting the incorrect blessing with a second blessing.

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