UNISA Chatter – Design Patterns in C++ Part 11: Patterns … Singleton versus Monostate

See UNISA – Summary of 2010 Posts for a list of related UNISA posts. This post is focused on my final preparations for the exam next week and chatter around the Singleton and Monostate patterns I have been having with fellow students.

… loaned from book “Software Engineers on their way to Pluto”, showing the aftermath of an exciting year in terms of module COS3114

My final view … I hope it covers everything for the exam 🙂

The following mind map is my final summary for the exam preparations … the time has now come to dig through the assignments and previous exams and wrap my head around the information to be in a position to walk into the exam with confidence next Friday. One of the discussions (chatter) not shown on the mind map is the comparison of a singleton and a monostate pattern, which forms the remainder of this post.

Singleton versus Monostate … one (primarily mine) of the views

Let’s look at the code example for both patterns:

Singleton Pseudocode

   1: public class SingletonPattern
   2: {
   3:   private static SingletonPattern _instance = 0;
   4:   private SingletonPattern(){} // private constructor
   5:   public static SingletonPattern GetInstance()
   6:   {
   7:     // introduce locks when running in a multi-threaded environment
   8:     if ( _instance == 0 )
   9:     {
  10:       _instance = new SingletonPattern();
  11:     }
  12:     return _instance;
  13:   }
  14: }

Monostate Pseudocode

   1: public class MonostatePattern
   2: {
   3:   private static int sharedValue = 0;
   4:   public MonostatePattern(){} // public constructor
   5:   //getter and setters
   6:   public void SetSharedValue (int newValue)
   7:   {
   8:     sharedValue = newValue;
   9:   }
  10:   public ing GetSharedValue()
  11:   {
  12:     return sharedValue;
  13:   }
  14: }

Comparing Apples with Apples




Overview Limits the instantiation to the class to one instance, which implies that it enforces the structure of a singularity. Limits the data variables (state) contained within the class to one instance, without enforcing any structural constraints.
Constructor private … ensuring that the class cannot be instantiated, other than by itself. public … allowing the instantiation of more than one instance of the class, both by itself and class consumers.
  • Can be applied to any class
  • Can be created through derivation (sub classing)
  • Class is only created once and only when needed.
  • Users are unaware that they are working with a Monostate pattern … the pattern is transparent.
  • Polymorphism is possible by overriding class members, which are not static.
  • Lifetime, construction and destruction is controllable.
  • Undefined lifetime … there is no way of destroying a singleton properly, especially with C++ where it could lead to null pointers when dereferencing.
  • Each call GetInstance() processes the if(){} logic, which is inefficient.
  • Users are aware that they are working with a Singleton because of having to call GetInstance(). The pattern is therefore not transparent.
  • Cannot be applied to just any class.
  • Due to multiple constructions and destructions the pattern is not as efficient as the Singleton.
  • Due to multiple instantiations, space allocation is worse (more space) compared to the Singleton.

So, when should I use which? If you need polymorphic derivations or logic, or a controlled lifetime of the objects, you should consider the Monostate Pattern. In all other cases, I recommend the Singleton.

… that’s it for this series. Fellow students, GOOD LUCK with the exam next week! May the exam be as interesting and slightly less challenging as the course this year 🙂

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