I am still not clear about the implementation. Is sub foo {} in a different package or in Obj.pm?
Both. The actual foo functionality is contained within Foo::foo, while Obj::foo is the exact code from my example - it just references its internal Foo instance and passes calls off to its foo method.

Here's a complete runnable demonstration: 

#!/usr/bin/perl

use strict;
use warnings;

my $obj = Obj->new;
if ($obj->foo) {
  print "\$obj->foo called Foo::foo\n";
} else {
  die "Foo class was not touched!\n";
}

package Foo;

sub new { return bless { }; }

sub foo {
  print "In Foo::foo, where the work gets done.\n";
  return 1;
}

package Obj;

sub new {
  return bless { _foo => Foo->new };
}

sub foo {
  my $self = shift;
  print "In Obj::foo and about to pass the buck.\n";
  return $self->{_foo}->foo(@_);
}

[download]
The print in Obj::foo isn't necessary, of course, and you normally wouldn't do anything there other than delegating the call to the aggregated object. I just threw it in to demonstrate the control flow more clearly.

It may also be worth noting that, if Obj and Foo were in their own separate modules (instead of everything being in one file as in my example), applications would only need to use Obj to gain access to the functionality of $obj->foo. The implementation of Obj::foo and the existence of an internal Foo instance are completely opaque to them, so they would not need to use Foo unless they want to create their own independent Foo instances outside of their Objs.

In reply to Re^3: Designing an OO program with multiple inherited classes by dsheroh
in thread Designing an OO program with multiple inherited classes by punkish

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.