Template

• class behavioral pattern

GoF definition (p. 325):

"Define the skeleton of an algorithm in an operation, defeering some steps to subclasses. Template methods lets subclasses redefine certain steps of an algorithm without changing the algorithm's structure."

More succinctly, "a template method defines an algorithm in terms of abstract operations that subclasses override to provide concrete behavior" (GoF p. 326)

You should also draw a distinction between these abstract operations (which MUST be overriden) and "hook" operations (which MAY be overriden).

Pros:

• Fundamental technique for code reuse - factor the structure of algorithm into an abstract superclass.

Cons:

• Subclass writers must remember which operations need to be overriden. This is its own kind of dependency!

Example

Abstract class

class Animal
  attr_accessor :name

  # Here is our template method:
  def survive
    self.eat
    self.poop
    self.procreate
  end

  def eat
    raise NotImplementedError
  end

  def poop
    raise NotImplementedError
  end

  def procreate
    raise NotImplementedError
  end
end

Concrete classes

class Monkey < Animal
  def initialize
    @name = "monkey"
  end

  def eat
    puts "The #{@name} eats a banana."
  end

  def poop
    puts "The #{@name} defecates into its hand."
  end

  def procreate
    puts "The #{@name} hoots at a nearby female."
  end
end

class Turtle < Animal
  def initialize
    @name = "turtle"
  end
  def eat
    puts "The #{@name} lazily chews some lettuce."
  end

  def poop
    puts "The #{@name} relieves itself on the grass."
  end

  def procreate
    puts "The #{@name} stares intently at a female. She is far away."
  end
end

Implementation

george = Monkey.new
george.survive
slowpoke = Turtle.new
slowpoke.survive