Killing a singleton

Jan 08

Killing a singleton

I am reading “Pattern Hatching” by John Vlissides (http://www.research.ibm.com/designpatterns/pubs/ph-jun96.txt). I have come across the Singleton Pattern. The way to use a SingletonDestroyer (SD) class to “KILL a Singleton instance” is interesting.

In John’s approach, destructor is made protected so that the user is not allowed to delete the singleton instance explicitly, also as said Singleton class has the responsibility to construct and destruct its instance. For this reason a separate class SD is introduced, which is a friend of class Singleton. Class Singleton also has a static member of SD. So as the SD’s instance goes out of scope, it deletes the pointer of Singleton assigned to it. SD has public constructor and a setter function to set the Singleton pointer.

class SingletonDestroyer {
public:
SingletonDestroyer(Singleton* s= 0) { _singleton = s; }
~SingletonDestroyer() { delete _singleton; }

void SetSingleton(Singleton* s) { _singleton = s; }
private:
Singleton* _singleton;
};

class Singleton {
public:
static Singleton* Instance();
protected:
Singleton() { }

friend class SingletonDestroyer;
virtual ~Singleton() { }
private:
static Singleton* _instance;
static SingletonDestroyer _destroyer;
};

Having a public SD constructor give a chance to user to create the SD object explicitly, such as:

Singleton *pSingleton = Singleton::Instance();
SingetonDestroyer explicitSingletonDestroyer(pSingleton);

So the basic purpose of making the destructor of Singleton protected is defeated.

I have made a minor change in John’s approach to rectify this problem. Class SD is made the protected inner class of class MySingleton. This prevents the user from creating an instance of SD class explicitly and thus restricting the user to delete the Singleton pointer knowingly or unknowingly.

class CMySingleton
{
protected:
class SingletonDestroyer // Inner Class
{
public:
SingletonDestroyer () {}
~SingletonDestroyer () {
delete _pInstance; // deleting the singleton instance
}
private:
// Prevent users from making copies of a
// Destroyer to avoid double deletion:
SingletonDestroyer(const SingletonDestroyer&);
void operator=(const SingletonDestroyer&);
};

friend class SingletonDestroyer;
protected:
CMySingleton();
virtual ~CMySingleton();
public:
static CMySingleton* GetInstance ();
void DoSomething();
protected: //members
static CMySingleton *_pInstance;
static SingletonDestroyer _destroyer;
int _nSomeVal;
};

It still doesn’t help you if you need to delete your singleton *before* the end of the program.