Code Like Zell
A cheap abstract class in Ruby
Posted 29 March 2009.
When I create a class that I don’t want to be instantiated, I use the following pattern:
class MakeMeAbstract
def initialize
raise "Don't instantiate me!" if abstract_class?
puts "Instantiated!"
end
private
def abstract_class?
self.class == MakeMeAbstract
end
end
It can’t be instantiated:
MakeMeAbstract.new rescue puts "I'm abstract" # => "I'm abstract"
But it can be subclassed, and the subclass can be instantiated:
class Child < MakeMeAbstract
end
Child.new # => "Instantiated!"
This is a learning experience for me. When subclassing an abstract class I wonder if it would be better to use a mixin instead: I could overwrite mixed in methods just as easily as methods in a parent class. The structure changes from this:
class Bar
def status
print "is full"
end
end
class WetBar < Bar
def status
print "is wet, "
super
end
end
wet_bar = WetBar.new
wet_bar.status # => "is wet, is full"
To this:
module Bar
def status
print "is full"
end
end
class WetBar
include Bar
def status
print "is wet, "
super
end
end
wet_bar = WetBar.new
wet_bar.status # => "is wet, is full"
Input is welcome in the comments.
