Data commit

Task/Singleton/Tcl/singleton-1.tcl

@@ -0,0 +1,21 @@
+package require TclOO
+
+# This is a metaclass, a class that defines the behavior of other classes
+oo::class create singleton {
+   superclass oo::class
+   variable object
+   unexport create ;# Doesn't make sense to have named singletons
+   method new args {
+      if {![info exists object]} {
+         set object [next {*}$args]
+      }
+      return $object
+   }
+}
+
+singleton create example {
+   method counter {} {
+      my variable count
+      return [incr count]
+   }
+}

Task/Singleton/Tcl/singleton-2.tcl

@@ -0,0 +1,14 @@
+% set a [example new]
+::oo::Obj20
+% set b [example new]  ;# note how this returns the same object name
+::oo::Obj20
+% expr {$a == $b}
+1
+% $a counter
+1
+% $b counter
+2
+% $a counter
+3
+% $b counter
+4