4.1 Inheritance
You've seen the use
of modules and how a Perl program can use all the code in a module by
simply loading it with the use command. This is a
simple and powerful method of software
reuse, by which software can be written once
but used many times by different programs.You've also seen how object-oriented Perl defines
classes in terms of modules, and how the use of classes, methods, and
objects provides a more structured way to reuse Perl software.There's another way to reuse Perl classes.
It's possible for a class to
inherit all the code and definitions of another
base
class. (This base class is sometimes called a
superclass
or a parent class.) The new derived
class (a.k.a. subclass)
can add more definitions or redefine certain definitions. Perl then
automatically uses the definitions of everything in the old class
(but treats them as if they were defined in this new derived class),
unless it finds them first in the derived class.In this chapter, I'll first develop a class
FileIO.pm, and then use the technique of
inheritance to develop another class SeqFileIO.pm
that inherits from FileIO.pm. This way of reusing
software by inheritance is extremely convenient when writing
object-oriented software. For instance, I make
SeqFileIO do a lot of its work simply by
inheriting the base class FileIO and then adding
methods that handle sequence file formats. I could use the same base
class FileIO to write a new class that specializes
in handling HTML files, microarray datafiles, SNP database files, and
so on. (See the exercises at the end of the chapter.)When inheriting a class, it is sometimes necessary to do a bit more
than just add new methods. In the SeqFileIO class,
I add some attributes to the object, and as a result the hash
%_attribute_properties also has to be changed. So
in the new class I define a new hash with that name, and as a result
the old definition from the base class is forgotten and the new,
redefined hash is used. As you read the new class, compare it with
the base class FileIO. Make note of what is new in
the class (e.g., the various put methods), what is
being redefined from the base class (e.g., the hash just mentioned),
and what is being inherited from the base class (e.g., the
new constructor.) This can help prepare you to
write your own class that uses inheritance.Occasionally, you want to invoke a method from a base class that has
been overridden. You can use the special SUPER class for that purpose. I
don't use that in the code for this chapter, but you
should be aware that it is possible to do. |
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