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Ruby Singleton Methods at Depth

A singleton method belongs to one object and to no other. This lesson builds them in two ways and lists what an object holds.

One dog learns a trick

def rex.trick defines trick on rex alone. Fido is a dog too, and fido answers false when asked about trick.

class Dog
end
rex = Dog.new
fido = Dog.new
def rex.trick
  puts "Roll over"
end
rex.trick
puts fido.respond_to?(:trick)

Built from a name with define_singleton_method

define_singleton_method takes the name greet as a symbol and a block for the body. The dog answers greet right away.

class Dog
end
dog = Dog.new
dog.define_singleton_method(:greet) do
  puts "Hello from this dog"
end
dog.greet

Listing singleton methods in order

The dog gains trick and greet in two different ways. singleton_methods lists both names, and sort prints them in a steady order.

class Dog
end
dog = Dog.new
def dog.trick
  puts "Roll over"
end
dog.define_singleton_method(:greet) do
  puts "Hello"
end
puts dog.singleton_methods.sort

Example

class Bird
end
pip = Bird.new
skye = Bird.new
def pip.song
  puts "Morning song"
end
pip.define_singleton_method(:call) do
  puts "Sharp call"
end
pip.song
pip.call
puts skye.respond_to?(:song)
puts pip.singleton_methods.sort

Key points

  • def followed by an object and a method name defines a method on that object alone.
  • define_singleton_method builds a singleton method from a name and a block.
  • singleton_methods lists the names of the singleton methods an object holds.
  • Sorting that list gives a steady order for printing.
  • Other objects of the same class answer false to respond_to? for the method.

In this example, pip gains song with a plain definition and call with define_singleton_method. Skye shares the Bird class and still answers false for song. The sorted list at the end names the two methods pip holds.

Note

A singleton method never spreads to the rest of its class.

Try it Yourself

Edit the code, then press Run.

Running is simulated

Exercise

Which method builds a singleton method from a name and a block?