$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]
If you've dereferenced the first reference, you're allowed to drop the arrow notation for subsequent dereferences. Thus, $self->{_init} becomes a hashref on the fly by merely appending curly braces.

Cheers,
Ovid

Vote for paco!

Join the Perlmonks Setiathome Group or just click on the the link and check out our stats.

In reply to (Ovid) Re: Syntax for preventing repeat object initializations by Ovid
in thread Syntax for preventing repeat object initializations by nysus

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.