[原]ACE_Singleton类学习

winston

先看一下源代码。

1. template <class TYPE, class ACE_LOCK>
2. class ACE_Singleton : public ACE_Cleanup
3. {
4. public:
5.   /// Global access point to the Singleton.
6.   static TYPE *instance (void);
8.   /// Cleanup method, used by <ace_cleanup_destroyer> to destroy the
9.   /// ACE_Singleton.
10.   virtual void cleanup (void *param = 0);
12.   /// Dump the state of the object.
13.   static void dump (void);
15. protected:
16.   /// Default constructor.
17.   ACE_Singleton (void);
19.   /// Contained instance.
20.   TYPE instance_;
22. #if !defined (ACE_LACKS_STATIC_DATA_MEMBER_TEMPLATES)
23.   /// Pointer to the Singleton (ACE_Cleanup) instance.
24.   static ACE_Singleton<TYPE, ACE_LOCK> *singleton_;
25. #endif /* ACE_LACKS_STATIC_DATA_MEMBER_TEMPLATES */
27.   /// Get pointer to the Singleton instance.
28.   static ACE_Singleton<TYPE, ACE_LOCK> *&instance_i (void);
29. };

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实现文件：

1. // Singleton.cpp,v 4.54 2005/10/28 16:14:55 ossama Exp
3. #ifndef ACE_SINGLETON_CPP
4. #define ACE_SINGLETON_CPP
6. #include "ace/Singleton.h"
8. #if !defined (ACE_LACKS_PRAGMA_ONCE)
9. # pragma once
10. #endif /* ACE_LACKS_PRAGMA_ONCE */
12. #if !defined (__ACE_INLINE__)
13. #include "ace/Singleton.inl"
14. #endif /* __ACE_INLINE__ */
16. #include "ace/Object_Manager.h"
17. #include "ace/Log_Msg.h"
18. #include "ace/Framework_Component.h"
19. #include "ace/Guard_T.h"
21. ACE_RCSID (ace,
22.            Singleton,
23.            "Singleton.cpp,v 4.54 2005/10/28 16:14:55 ossama Exp")
26. ACE_BEGIN_VERSIONED_NAMESPACE_DECL
28. template <class TYPE, class ACE_LOCK> void
29. ACE_Singleton<TYPE, ACE_LOCK>::dump (void)
30. {
31. #if defined (ACE_HAS_DUMP)
32.   ACE_TRACE ("ACE_Singleton<TYPE, ACE_LOCK>::dump");
34. #if !defined (ACE_LACKS_STATIC_DATA_MEMBER_TEMPLATES)
35.   ACE_DEBUG ((LM_DEBUG,  ACE_LIB_TEXT ("instance_ = %x"),
36.               ACE_Singleton<TYPE, ACE_LOCK>::instance_i ()));
37.   ACE_DEBUG ((LM_DEBUG, ACE_END_DUMP));
38. #endif /* ACE_LACKS_STATIC_DATA_MEMBER_TEMPLATES */
39. #endif /* ACE_HAS_DUMP */
40. }
42. template <class TYPE, class ACE_LOCK> ACE_Singleton<TYPE, ACE_LOCK> *&
43. ACE_Singleton<TYPE, ACE_LOCK>::instance_i (void)
44. {
45. #if defined (ACE_LACKS_STATIC_DATA_MEMBER_TEMPLATES)
46.   // Pointer to the Singleton instance.  This works around a bug with
47.   // G++ and it's (mis-)handling of templates and statics...
48.   static ACE_Singleton<TYPE, ACE_LOCK> *singleton_ = 0;
50.   return singleton_;
51. #else
52.   return ACE_Singleton<TYPE, ACE_LOCK>::singleton_;
53. #endif /* ACE_LACKS_STATIC_DATA_MEMBER_TEMPLATES */
54. }
56. template <class TYPE, class ACE_LOCK> TYPE *
57. ACE_Singleton<TYPE, ACE_LOCK>::instance (void)
58. {
59.   ACE_TRACE ("ACE_Singleton<TYPE, ACE_LOCK>::instance");
61.   ACE_Singleton<TYPE, ACE_LOCK> *&singleton =
62.     ACE_Singleton<TYPE, ACE_LOCK>::instance_i ();
64.   // Perform the Double-Check pattern...
65.   if (singleton == 0)
66.     {
67.       if (ACE_Object_Manager::starting_up () ||
68.           ACE_Object_Manager::shutting_down ())
69.         {
70.           // The program is still starting up, and therefore assumed
71.           // to be single threaded.  There's no need to double-check.
72.           // Or, the ACE_Object_Manager instance has been destroyed,
73.           // so the preallocated lock is not available.  Either way,
74.           // don't register for destruction with the
75.           // ACE_Object_Manager:  we'll have to leak this instance.
77.           ACE_NEW_RETURN (singleton, (ACE_Singleton<TYPE, ACE_LOCK>), 0);
78.         }
79.       else
80.         {
81. #if defined (ACE_MT_SAFE) && (ACE_MT_SAFE != 0)
82.           // Obtain a lock from the ACE_Object_Manager.  The pointer
83.           // is static, so we only obtain one per ACE_Singleton
84.           // instantiation.
85.           static ACE_LOCK *lock = 0;
86.           if (ACE_Object_Manager::get_singleton_lock (lock) != 0)
87.             // Failed to acquire the lock!
88.             return 0;
90.           ACE_GUARD_RETURN (ACE_LOCK, ace_mon, *lock, 0);
92.           if (singleton == 0)
93.             {
94. #endif /* ACE_MT_SAFE */
95.               ACE_NEW_RETURN (singleton, (ACE_Singleton<TYPE, ACE_LOCK>), 0);
97.               // Register for destruction with ACE_Object_Manager.
98.               ACE_Object_Manager::at_exit (singleton);
99. #if defined (ACE_MT_SAFE) && (ACE_MT_SAFE != 0)
100.             }
101. #endif /* ACE_MT_SAFE */
102.         }
103.     }
105.   return &singleton->instance_;
106. }
108. template <class TYPE, class ACE_LOCK> void
109. ACE_Singleton<TYPE, ACE_LOCK>::cleanup (void *)
110. {
111.   delete this;
112.   ACE_Singleton<TYPE, ACE_LOCK>::instance_i () = 0;
113. }
115. #if !defined (ACE_LACKS_STATIC_DATA_MEMBER_TEMPLATES)
116. // Pointer to the Singleton instance.
117. template <class TYPE, class ACE_LOCK> ACE_Singleton<TYPE, ACE_LOCK> *
118. ACE_Singleton<TYPE, ACE_LOCK>::singleton_ = 0;

复制代码

class ACE_Singleton : public ACE_Cleanup 继承了ACE_Cleanup，这个类自动完成了对对象的清理工作。就是实现了
template <class TYPE, class ACE_LOCK> voidACE_Singleton<TYPE, ACE_LOCK>::cleanup (void *){  delete this;  ACE_Singleton<TYPE, ACE_LOCK>::instance_i () = 0;}
还有就是对程序的使用了ACE_Object_Manager 释放程序的实例。
ACE_Object_Manager::at_exit (singleton);
因此当你使用ACE_Singleton 大可以放心的使用，对象管理ACE经过自己的方法和类已经很好的解决了。
总之，不管他如何实现，我们完全就是直接简单的使用。
typedef ACE_Singleton<Class, ACE_Null_Mutex> instance;
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doublequick

ACE_Singleton对象的实例化和删除都处在你的程序、而不是平台的运行时环境的控制之下。和Boost的Singleton有所区别。:)