Question

According to this

http://perldoc.perl.org/UNIVERSAL.html

I shouldn’t use UNIVERSAL::isa() and should instead use $obj->isa() or CLASS->isa().

This means that to find out if something is a reference in the first place and then is reference to this class I have to do

eval { $poss->isa('Class') } 

and check $@ and all that gumph, or else

use Scalar::Util 'blessed'; blessed $ref && $ref->isa($class); 

My question is why? What’s wrong with UNIVERSAL::isa called like that? It’s much cleaner for things like:

my $self = shift if UNIVERSAL::isa($_[0], __PACKAGE__) 

To see whether this function is being called on the object or not. And is there a nice clean alternative that doesn’t get cumbersome with ampersands and potentially long lines?

Answer 1

The primary problem is that if you call UNIVERSAL::isa directly, you are bypassing any classes that have overloaded isa. If those classes rely on the overloaded behavior (which they probably do or else they would not have overridden it), then this is a problem. If you invoke isa directly on your blessed object, then the correct isa method will be called in either case (overloaded if it exists, UNIVERSAL:: if not).

The second problem is that UNIVERSAL::isa will only perform the test you want on a blessed reference just like every other use of isa. It has different behavior for non-blessed references and simple scalars. So your example that doesn’t check whether $ref is blessed is not doing the right thing, you’re ignoring an error condition and using UNIVERSAL‘s alternate behavior. In certain circumstances this can cause subtle errors (for example, if your variable contains the name of a class).

Consider:

use CGI;  my $a = CGI->new();  my $b = 'CGI';  print UNIVERSAL::isa($a,'CGI');  # prints 1, $a is a CGI object. print UNIVERSAL::isa($b,'CGI');  # Also prints 1!! Uh-oh!! 

So, in summary, don’t use UNIVERSAL::isa… Do the extra error check and invoke isa on your object directly.