9.2 Invoking an Instance Method

The method arrow can be used on instances, as well as names of packages (classes). Let's get the sound that $tv_horse makes:

 my $noise = $tv_horse->sound; 

To invoke sound , Perl first notes that $tv_horse is a blessed reference, and thus an instance. Perl then constructs an argument list, similar to the way an argument list was constructed when you used the method arrow with a class name . In this case, it'll be just ($tv_horse) . (Later you'll see that arguments will take their place following the instance variable, just as with classes.)

Now for the fun part: Perl takes the class in which the instance was blessed, in this case Horse , and uses it to locate the subroutine to invoke the method, as if you had said Horse->sound instead of $tv_horse->sound . The purpose of the original blessing is to associate a class with that reference to allow the proper method (subroutine) to be found.

In this case, Horse::sound is found directly (without using inheritance), yielding the final subroutine invocation:

 Horse::sound($tv_horse) 

Note that the first parameter here is still the instance, not the name of the class as before. neigh is the return value, which ends up as the earlier $noise variable.

If Horse::sound had not been found, you'd wander up the @Horse::ISA list to try to find the method in one of the superclasses, just as for a class method. The only difference between a class method and an instance method is whether the first parameter is an instance (a blessed reference) or a class name (a string). ^[4]

^[4] This is perhaps different from other OOP languages with which you may be familiar.