A Singleton Adapter uses the Adapter pattern to turn ordinary classes into Singletons optimized with the Double-Checked Locking optimization pattern.
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template<
class TYPE,
class ACE_LOCK>
class ACE_Singleton< TYPE, ACE_LOCK >
A Singleton Adapter uses the Adapter pattern to turn ordinary classes into Singletons optimized with the Double-Checked Locking optimization pattern.
This implementation is a slight variation on the GoF Singleton pattern. In particular, a single <ACE_Singleton<TYPE, ACE_LOCK> > instance is allocated here, not a <TYPE> instance. The reason for this is to allow registration with the ACE_Object_Manager, so that the Singleton can be cleaned up when the process exits. For this scheme to work, a (static) cleanup() function must be provided. ACE_Singleton provides one so that TYPE doesn't need to. If you want to make sure that only the singleton instance of <T> is created, and that users cannot create their own instances of <T>, do the following to class <T>: (a) Make the constructor of <T> private (or protected) (b) Make Singleton a friend of <T> Here is an example:
* class foo
* {
* friend class ACE_Singleton<foo, ACE_Null_Mutex>;
* private:
* foo () { cout << "foo constructed" << endl; }
* ~foo () { cout << "foo destroyed" << endl; }
* };
* typedef ACE_Singleton<foo, ACE_Null_Mutex> FOO;
* - Note
- The best types to use for ACE_LOCK are ACE_Recursive_Thread_Mutex and ACE_Null_Mutex. ACE_Recursive_Thread_Mutex should be used in multi-threaded programs in which it is possible for more than one thread to access the <ACE_Singleton<TYPE, ACE_LOCK>> instance. ACE_Null_Mutex can be used otherwise. The reason that these types of locks are best has to do with their allocation by the ACE_Object_Manager. Single ACE_Recursive_Thread_Mutex and ACE_Null_Mutex instances are used for all ACE_Singleton instantiations. However, other types of locks are allocated per ACE_Singleton instantiation.
Public Member Functions inherited from ACE_Cleanup