Some object-oriented languages (notably C++) support
multiple inheritance, where a class can have more than one immediate
parent, inheriting functionality from each. Although powerful, this
technique can be dangerous, as the inheritance hierarchy can
become ambiguous.
Other languages, such as Java, support single inheritance. Here, a
class can have only one immediate parent. Although cleaner (and easier
to implement), single inheritance also has drawbacks---in the real
world things often inherit attributes from multiple sources (a ball is
both a
bouncing thing and a
spherical thing, for
example).
Ruby offers an interesting and powerful compromise, giving you the
simplicity of single inheritance and the power of multiple
inheritance. A Ruby class can
have only one direct parent, and so Ruby is a single-inheritance
language. However, Ruby classes can include the functionality of any
number of mixins (a mixin is like a partial class definition). This
provides a controlled multiple-inheritance-like capability with none
of the drawbacks. We'll explore mixins more beginning
on page 98.
So far in this chapter we've been looking at classes and their
methods. Now it's time to move on to the objects, such as the
instances of class
Song.
