Programming Perl, 3rd Edition

Managing Instance Data

Most classes create objects that are essentially just data structures with several internal data fields (instance variables) plus methods to manipulate them.

Perl classes inherit methods, not data, but as long as all access to the object is through method calls anyway, this works out fine. If you want data inheritance, you have to effect it through method inheritance. By and large, this is not a necessity in Perl, because most classes store the attributes of their object in an anonymous hash. The object's instance data is contained within this hash, which serves as its own little namespace to be carved up by whatever classes do something with the object. For example, if you want an object called $city to have a data field named elevation, you can simply access $city->{elevation}. No declarations are necessary. But method wrappers have their uses.

Suppose you want to implement a Person object. You decide to have a data field called "name", which by a strange coincidence you'll store under the key name in the anonymous hash that will serve as the object. But you don't want users touching the data directly. To reap the rewards of encapsulation, users need methods to access that instance variable without lifting the veil of abstraction.

For example, you might make a pair of accessor methods:

sub get_name {
    my $self = shift;
    return $self->{name};
}

sub set_name {
    my $self      = shift;
    $self->{name} = shift;
}

which leads to code like this:

$him = Person->new(); $him->set_name("Frodo"); ...