$self is a hashref and __PACKAGE__ is the package in the class that it's in. If this code is in package "Foo", then the '_init' key in self is auto-incremented. The first time this occurs, it will evaluate as false, allowing Foo::_init to function, but subsequent calls will fail.

nysus wrote:

But how does throwing a class, {__PACKAGE__}, on the right side of an object method call, $self->{_init}, create an attribute?

You're simply auto-vivifying a hash key with the name being equal to the current package name. You see, Perl has an interesting shortcut for complex data structures. In this case, the following are equivalent:

$self->{_init}->{__PACKAGE__};
$self->{_init}{__PACKAGE__};
[download]

Cheers,
Ovid

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_____________________________________________________
Jeff japhy Pinyan: Perl, regex, and perl hacker.
s++=END;++y(;-P)}y js++=;shajsj<++y(p-q)}?print:??;

Very interesting. I created the following package:

package Scrap::Foo;
use strict;

sub bar {
    my $dummy;
    $dummy->{_init}->{__PACKAGE__}++;
    return $dummy;
}
1
[download]

And my test code:

use strict;
use Scrap::Foo;
use Data::Dumper;
my $dummy = Scrap::Foo::bar();
print Dumper $dummy;
[download]

Generated this output:

$VAR1 = {
          '_init' => {
                       '__PACKAGE__' => '1'
                     }
        };
[download]

And the fix:

my $package = __PACKAGE__;
$dummy->{_init}->{$package}++;
[download]

Thanks for the info. Now who wants to tell Damian? :)

Cheers,
Ovid

Update: japhy's unary fix is not only shorter, but it should be faster as it doesn't involve creating a variable for no other reason than throwing it away.

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The "unary +" fix is shorter...

$foo->{+__PACKAGE__} = 1;
[download]

Hmm, patching a book isn't quite so easy!

_____________________________________________________
Jeff japhy Pinyan: Perl, regex, and perl hacker.
s++=END;++y(;-P)}y js++=;shajsj<++y(p-q)}?print:??;

That code simply increments a marker in that sub-hash. The key is the name of the current package (respecting inheritance). If _init() has already been called for the package, the key will already have been autovivified, and the value of the expression will be true.

Make sense?

It should work perfectly the first time! - toma

This one is not yet listed on the Manning site so I submitted it (before I saw this note).

Hrm. Are you sure, nysus, that this code helps prevent multiple inheritance? As I see it, the __PACKAGE__ key (barring the minor bug) would only prevent the object from being re-initialized in the same package/class, not by other classes. (And that's even assuming all other classes that might bless the object follow this convention!)

Please enlighten me if I am missing something, though! :-)

bbfu
Seasons don't fear The Reaper.
Nor do the wind, the sun, and the rain.
We can be like they are.

No, I'm not sure. Don't pay much heed to what I write. I'm a total OO newbie and I've been cramming Conway's book down my throat for the past few days. My ability to retain all the info in it is beginning to wane. It's time for me to practice with my own simple code and then come back to the book when my knowledge is more thorough.

$PM = "Perl Monk's";
$MCF = "Most Clueless Friar Abbot Bishop";
$nysus = $PM . $MCF;
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