Transparently handle first argument to Perl Package subroutines

It’s always baffled me that Perl subroutines behave differently when called from inside the Package vs. outside.

The use case may be obvious but I’ll say it: you write a small utility routine that you want to call as $pack->func(1); and from within the Package as func(1);;

The problem of course is that when called as $pack->func(1); the first arg will the object itself and when called as func(1); from within the package the first arg will be “1.”

There’s a very simple solution:
shift if ((ref $_[0]) eq __PACKAGE__);

Here’s a simple example:


#!/usr/bin/perl -w
#
package Pack;

sub new {
    my $c = shift;

    my $self = {};
    bless $self, $c;
    return $self;
}

sub func {
    shift if ((ref $_[0]) eq __PACKAGE__);
    my $a = shift;

    print "passed in: $a\n";
}

sub call_func {
    print "calling func from inside ", __PACKAGE__, ":\n";
    func(2);
}
1;

my $p = new Pack;

print "calling func from main:\n";
$p->func(1);

$p->call_func();

Try commenting the “shift” line in sub func() and see how it behaves differently.

Leave a Reply

Your email address will not be published. Required fields are marked *