Learning Perl Objects, References amp;amp; Modules [Electronic resources] نسخه متنی

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.