Two element array, shifted twice, is true?

SwaJime has asked for the wisdom of the Perl Monks concerning the following question:

So I'm stepping through some code in a cpan module ...

I have undef for @_, so I'm expecting to execute line 148 next. However, the debugger puts me on line 145. Why did it go to 145 instead of 148? Why would undef be treated as a true value?

  DB<13> n
Net::DBus::RemoteService::get_object(/usr/lib/perl5/Net/DBus/RemoteSer
+vice.pm:144):
144:        if (@_) {
  DB<13> l
144==>        if (@_) {
145:        my $interface = shift;
146:        return $self->{objects}->{$object_path}->as_interface($int
+erface);
147         } else {
148:        return $self->{objects}->{$object_path};
149         }
150     }
151
152:    1;
153
  DB<13> x @_
0  undef
  DB<14> n
Net::DBus::RemoteService::get_object(/usr/lib/perl5/Net/DBus/RemoteSer
+vice.pm:145):
145:        my $interface = shift;
  DB<14> n
Net::DBus::RemoteService::get_object(/usr/lib/perl5/Net/DBus/RemoteSer
+vice.pm:146):
146:        return $self->{objects}->{$object_path}->as_interface($int
+erface);
  DB<14> x $interface
0  undef
[download]

The method was invoked as $proxy = $service->get_object($self->{object_path}, $mdata->{interface}); and the innards of the function have shifted the parameters into variables. So @_ should have 0 elements, right?

Here is the whole method from /usr/lib/perl5/Net/DBus/RemoteService.pm

135     sub get_object {
136:        my $self = shift;
137:        my $object_path = shift;
138
139:        unless (defined $self->{objects}->{$object_path}) {
140:        $self->{objects}->{$object_path} = Net::DBus::RemoteObject
+->new($self,
141                                         $object_path);
142         }
143
144:        if (@_) {
145:        my $interface = shift;
146==>        return $self->{objects}->{$object_path}->as_interface($i
+nterface);
147         } else {
148:        return $self->{objects}->{$object_path};
149         }
150     }
[download]

Update: I just realized that the 1st parameter is the added class object. DUH :-( So it's a three element array. Back to this in a few after I rerun the debugger.

Comment on Two element array, shifted twice, is true? Select or Download Code

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Re: Two element array, shifted twice, is true? by dave_the_m (Monsignor) on Aug 16, 2016 at 18:25 UTC
`DB<13> x @_ 0 undef` [download] This shows that the array contains 1 element, and that element happens to be undefined. So `@a` in scalar context returns 1, which is true. Presumably `get_object()` is being called with 3 arguments. Dave.	[reply] [d/l] [select]
Re^2: Two element array, shifted twice, is true? by SwaJime (Scribe) on Aug 16, 2016 at 18:41 UTC
Yes, I had forgotten that since it is a class method, the class object is passed in as the first parameter. So the two explicit parameters are actually the second and third elements of @_. My third element was undef, which is a problem elsewhere in my code.	[reply]