how to delete singleton object

Question

Assuming this implementation of a singleton pattern ( of course we should avoid Singleton: it is just question), I have just been thinking about static object being created.

Answer 1

I prefer to not use a pointer.

class Single
{
private:
   Single();

public:
   Single& Instance()
   {
      static Single the_instance;
      Return the_instance;
   }
};

This singleton will live from the time Instance() is called until the application is exiting and performing the destruction of static objects. In this case the destructor of the singleton object will be called.

In practice, even when using a pointer as in the original example, the memory will be reclaimed by the OS upon application exit. However in this case the object's destructor will not be called.

Answer 2

The classical singleton pattern does not describe the deletion aspect.

However, if I have to do it, I would start with a simple approach like the following (it is not fool-proof):

1) Similar to the static method for creating/retrieving the singleton object, say createObject(), there is a static method for destructing the singleton object, say destructObject()

2) There is a counter which counts the current number of objects in the system;

• it starts at 0
• on createObject() call, it increments by 1
• on deleteObject() call, it decrements by 1.
  • If it reaches 0, then the delete is called to actually destruct the object

Answer 3

Part of the Singleton design pattern is that it is indestructible.

EDIT:

There are 2 varieties of singletons with respect to destructibility:

1. Destructible (They die when the application does)
2. Indestructible (They die when the machine does)

Either way, if built properly, once the singleton instance is created, it stays. This one of the major criticisms of the Singleton design pattern.

Here are a few references that address the destructibility aspect of the pattern.

http://nicolabonelli.wordpress.com/2009/06/04/singleton-a-mirage-of-perfection/ http://www10.informatik.uni-erlangen.de/Teaching/Courses/SS2009/CPP/altmann.pdf http://sourcemaking.com/design_patterns/singleton http://sourcemaking.com/design_patterns/to_kill_a_singleton