Rosetta Example: Singleton

For details see https://rosettacode.org/wiki/Singleton

A Singleton Class

package req nx

nx::Class create Singleton {
  #
  # We overload the system method "create". In the modified method we
  # save the created instance in the instance variable named
  # "instance"
  #
  :variable instance:object

  :public object method create {args} {
    return [expr {[info exists :instance] ? ${:instance} : [set :instance [next]]}]
  }
}

Demonstrating the behavior in a shell:

Calling Singleton new multiple times returns always the same object:

% expr {[Singleton new] eq [Singleton new]}
1

A Singleton Meta-class

Alternatively, we can follow a more generic approach and define a metaclass which allows one to define several application classes as singletons. The metaclass has the most general metaclass nx::Class as superclass. In contrary to the example obove, the create method is not defined as a class method, but it will be inherited to its instances (to the application classes).

nx::Class create Singleton -superclass nx::Class {
  #
  # We overload the system method "create". In the modified method we
  # save the created instance in the instance variable named
  # "instance"
  #
  :variable instance:object

  :public method create {args} {
    return [expr {[info exists :instance] ? ${:instance} : [set :instance [next]]}]
  }
}

Create an application class named Counter as a singleton:

% Singleton create Counter
::Counter

Calling Counter new multiple times returns always the same object:

% expr {[Counter new] eq [Counter new]}
1