TheSingleton module implements theSingleton pattern.

Usage¶↑

To useSingleton, include the module in your class.

classKlassincludeSingleton# ...end

This ensures that only one instance of Klass can be created.

a,b =Klass.instance,Klass.instancea==b# => trueKlass.new# => NoMethodError - new is private ...

The instance is created at upon the first call of Klass.instance().

classOtherKlassincludeSingleton# ...endObjectSpace.each_object(OtherKlass){}# => 0OtherKlass.instanceObjectSpace.each_object(OtherKlass){}# => 1

This behavior is preserved under inheritance and cloning.

Implementation¶↑

This above is achieved by:

• Making Klass.new and Klass.allocate private.

• Overriding Klass.inherited(sub_klass) and Klass.clone() to ensure that theSingleton properties are kept when inherited and cloned.

• Providing the Klass.instance() method that returns the same object each time it is called.

• Overriding Klass._load(str) to call Klass.instance().

• Overriding Klass#clone and Klass#dup to raise TypeErrors to prevent cloning or duping.

Singleton andMarshal¶↑

By default Singleton’s_dump(depth) returns the empty string. Marshalling by default will strip state information, e.g. instance variables from the instance. Classes usingSingleton can provide custom _load(str) and _dump(depth) methods to retain some of the previous state of the instance.

require'singleton'classExampleincludeSingletonattr_accessor:keep,:stripdef_dump(depth)# this strips the @strip information from the instanceMarshal.dump(@keep,depth)enddefself._load(str)instance.keep =Marshal.load(str)instanceendenda =Example.instancea.keep ="keep this"a.strip ="get rid of this"stored_state =Marshal.dump(a)a.keep =nila.strip =nilb =Marshal.load(stored_state)pa==b#  => truepa.keep#  => "keep this"pa.strip#  => nil