Inheritance in Perl 5 is very simple.
To do so, you simply declare a class array called @ISA.
This array will be used to store the name and parent class(es)
of the new species. Whenever the new class is used, it will
have this reference to its lineage and be able to check there
against method calls and properties.
For example, to define a subclass of
Cat, you could use
package NorthAmericanCat;
@NorthAmericanCat::ISA = ("Cat");
sub new {
[. . . Code goes here . . .]
}
Notice that in perl 5, package namespace references
are handled using the "::" notation to delimit the names.
For example, here NorthAmericanCat is separated from the ISA
variable name via "::". Another common inheritence idiom is
to make the generic abstract class be the first namespace
and then the specific concrete classes become the second namespace.
If we were following this convention, we would have called
the North American Cat class "Cat::NorthAmerican" rather than
"NorthAmericanCat" and ISA would be defined as @Cat::NorthAmerican::ISA
instead of "@NorthAmericanCat::ISA".
If this namespace stuff seems a little confusing, don't worry. Namespaces
are just labels for package names. They merely provide a way to flexibly
organize your classes and object names. This is similar to how you can
define a directory structure for Word processing and Spreadsheet documents
according to how complex and how many documents you have. If you have
one or two documents, you will probably dump them all in one directory.
But if you have many documents, you may organize them in a hierarchy that
makes sense to you.
Similarly, if your program is simple, you will likely just dump all the
package names into a flat namespace with no hierarchy. But as your
programs become larger and larger and more complex, you will likely
start noticing that you use "::" to separate hierarchical namespaces
more often.
Finally, note that
instantiation of an inherited class will work just the same
as it did without inheritance.
