Checking objects: ref, isa and can
Perl can tell you what class an object belongs to and what it can do.
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Checking objects: ref, isa and can
ref($obj) returns the class name of a blessed reference. The methods isa and can, which every class inherits from UNIVERSAL, check whether an object is derived from a class and whether it has a given method. can returns a code reference that you can call, or a false value if the method does not exist.
Note:
Call isa and can as $obj->isa(Class) only on real objects; on unknown values use Scalar::Util::blessed first.
Example: Checking objects: ref, isa and can
use strict;
use warnings;
package Shape;
sub new { my $class = shift; return bless {}, $class; }
sub area { return 0; }
package Circle;
our @ISA = ('Shape');
sub radius { return 2; }
package main;
my $c = Circle->new;
print "ref: ", ref($c), "\n";
print "isa Circle: ", ($c->isa('Circle') ? "yes" : "no"), "\n";
print "isa Shape: ", ($c->isa('Shape') ? "yes" : "no"), "\n";
print "isa Dog: ", ($c->isa('Dog') ? "yes" : "no"), "\n";
print "can radius: ", ($c->can('radius') ? "yes" : "no"), "\n";
print "can area (inherited): ", ($c->can('area') ? "yes" : "no"), "\n";
print "can volume: ", ($c->can('volume') ? "yes" : "no"), "\n";
if (my $m = $c->can('radius')) {
print "radius via can: ", $c->$m(), "\n";
}
print "class string can: ", (Circle->can('new') ? "yes" : "no"), "\n";
# Output:
# ref: Circle
# isa Circle: yes
# isa Shape: yes
# isa Dog: no
# can radius: yes
# can area (inherited): yes
# can volume: no
# radius via can: 2
# class string can: yesLogin to try C/C++/Java/PHP code in the editor
Common Mistakes
- Comparing ref($obj) with eq and missing subclasses
- Calling a method on undef or an unblessed reference
- Forgetting that can returns a code reference, not the method's result
Chapter Summary
- ref returns the class of an object
- isa checks for the class or its ancestors
- can checks for a method
- can returns a code reference
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