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Singleton Object Management


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This is from my series of presentations on C++ and Design Pattern. This was first presented at Interra in 2005.

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Singleton Object Management

  1. 1. November 7, 2005 Singleton Object Management Dr. Partha Pratim Das Interra Systems (India) Pvt. Ltd. Resource Management Series
  2. 2. Agenda <ul><li>Backgrounder </li></ul><ul><ul><li>atexit() behavior </li></ul></ul><ul><ul><li>Object Lifetime </li></ul></ul><ul><li>Singleton Class </li></ul><ul><ul><li>What?, Why? & How? </li></ul></ul><ul><ul><li>A simple singleton </li></ul></ul><ul><ul><ul><li>How to enforce singleton discipline </li></ul></ul></ul><ul><ul><li>Meyer’s Singleton </li></ul></ul><ul><ul><ul><li>The Dead Reference Problem </li></ul></ul></ul><ul><ul><li>The Phoenix Singleton </li></ul></ul><ul><li>Q&A </li></ul>
  3. 3. Backgrounder Object Lifetime
  4. 4. atexit() Callback Behavior – Execute code after main() <ul><li>atexit() signature </li></ul><ul><ul><ul><li>int atexit(void (*pFunction)(void)); </li></ul></ul></ul><ul><ul><ul><li>// 0 is success </li></ul></ul></ul><ul><li>Use atexit() to register any function having the following signature </li></ul><ul><ul><ul><li>void CallbackFunction(void); </li></ul></ul></ul><ul><li>The registered function will be called from the runtime system after main() exits </li></ul><ul><li>If multiple functions are registered, then the calls will made be in the reverse order of registration (LIFO) </li></ul><ul><li>Used by: </li></ul><ul><ul><li>Compiler for </li></ul></ul><ul><ul><ul><li>Local Static Objects </li></ul></ul></ul><ul><ul><li>Application Programmer for any function call beyond main() </li></ul></ul>
  5. 5. atexit() Callback Behavior <ul><li>Note: </li></ul><ul><ul><li>Executing return from main() or direct call to exit(.) invokes all callbacks registered with atexit() before the control actually exits (global destructors are called). </li></ul></ul><ul><ul><li>Call to abort() bypasses callbacks. </li></ul></ul><ul><ul><li>atexit() registration from within a Callback function may have unspecified behavior. </li></ul></ul>
  6. 6. Object Lifetime <ul><li>Starts with Constructor execution </li></ul><ul><ul><li>Must follow Memory Allocation </li></ul></ul><ul><li>Ends with Destructor execution </li></ul><ul><ul><li>Must precede Memory De-allocation </li></ul></ul><ul><li>For Built-in Types (w/o Constructor / Destructor) the notion follows the same pattern though the compiler actually optimizes the creation / destruction processes. </li></ul>
  7. 7. Object Lifetime <ul><li>Automatic Object – Function / Block Scope </li></ul><ul><ul><li>Space allocated on stack when (just before) the control enters the scope </li></ul></ul><ul><ul><li>Object created (and initialized) when the control passes the declaration </li></ul></ul><ul><ul><li>Object destroyed when (just after) the control leaves the scope </li></ul></ul><ul><ul><li>Objects (within a scope) are destroyed in the reverse order of creations </li></ul></ul><ul><ul><li>Space de-allocated after all automatic objects have been destructed. </li></ul></ul>
  8. 8. Object Lifetime <ul><li>Non-static member Object – Class Scope </li></ul><ul><ul><li>Constructed in the initialization list of the Constructor of the Parent Object </li></ul></ul><ul><ul><ul><li>Before the first statement of the constructor executes </li></ul></ul></ul><ul><ul><li>Follows the lexical order of declarations in the class </li></ul></ul><ul><ul><li>Destructed by the Destructor of the Parent Object </li></ul></ul><ul><ul><ul><li>After the last statement of the destructor executes </li></ul></ul></ul>
  9. 9. Object Lifetime <ul><li>Free Store Object </li></ul><ul><ul><li>Lifetime controlled by the user </li></ul></ul><ul><ul><li>Constructor called by operator new </li></ul></ul><ul><ul><ul><li>Must follow Memory Allocation </li></ul></ul></ul><ul><ul><li>Destructor called by operator delete </li></ul></ul><ul><ul><ul><li>Must precede Memory De-allocation </li></ul></ul></ul>
  10. 10. Object Lifetime <ul><li>Static Object – Global / Class Scope </li></ul><ul><ul><li>Built-in Type – no constructor / destructor </li></ul></ul><ul><ul><ul><li>Object created (and initialized) implicitly when the program is loaded </li></ul></ul></ul><ul><ul><ul><ul><li>before first assembly instruction in main() </li></ul></ul></ul></ul><ul><ul><ul><ul><li>actually before any global object of user-defined type is constructed </li></ul></ul></ul></ul><ul><ul><ul><li>Object destroyed implicitly before the program is unloaded </li></ul></ul></ul><ul><ul><ul><ul><li>after last assembly instruction in main() </li></ul></ul></ul></ul><ul><ul><ul><ul><li>actually after all global objects of user-defined type are destructed </li></ul></ul></ul></ul><ul><ul><ul><li>Has no boundary for translation units – literally “load time” </li></ul></ul></ul><ul><ul><ul><li>Has no executable code in construction / destruction . </li></ul></ul></ul>
  11. 11. Object Lifetime <ul><li>Static Object – Global / Class Scope </li></ul><ul><ul><li>User-Defined Types </li></ul></ul><ul><ul><ul><li>Space allocation is done at “load-time”. </li></ul></ul></ul><ul><ul><ul><li>Allocated space is zero-filled </li></ul></ul></ul><ul><ul><ul><li>Object created (and initialized) sequentially within a translation unit </li></ul></ul></ul><ul><ul><ul><li>Creation order between different translation units is arbitrary </li></ul></ul></ul><ul><ul><ul><ul><li>Some iostreams objects are properly initialized for use by the static constructors. These control text streams – cin, cout, cerr, clog </li></ul></ul></ul></ul><ul><ul><ul><li>Destroyed in the reverse order of creations – creation order is static </li></ul></ul></ul><ul><ul><ul><ul><li>iostreams objects can be used within the destructors called for static objects, during program termination. </li></ul></ul></ul></ul>
  12. 12. Object Lifetime <ul><li>Static Object – Function / Local Scope </li></ul><ul><ul><li>Built-in Type </li></ul></ul><ul><ul><ul><li>Same as static objects in Global / Class Scope </li></ul></ul></ul><ul><ul><li>User-Defined Types </li></ul></ul><ul><ul><ul><li>Object created (and initialized) when the control passes the declaration for the first time </li></ul></ul></ul><ul><ul><ul><li>Destroyed in the reverse order of creations (LIFO) </li></ul></ul></ul><ul><ul><ul><ul><li>after last assembly instruction in main() </li></ul></ul></ul></ul><ul><ul><ul><ul><li>before the global objects of user-defined type are destructed </li></ul></ul></ul></ul><ul><ul><ul><li>Uses atexit() – because the creation order is dynamic </li></ul></ul></ul>
  13. 13. Static Object Lifetime An Example
  14. 18. Output: VC 6.0
  15. 19. Output: VC 7.1
  16. 20. Singleton Objects What and Why
  17. 21. What is a Singleton Class? <ul><li>A class is a singleton if </li></ul><ul><ul><li>It has only one instance, and </li></ul></ul><ul><ul><li>Global point of access to the singular instance </li></ul></ul><ul><ul><li>Can be accessed anytime during the application </li></ul></ul><ul><li>“ Global point of access” – Implications </li></ul><ul><ul><li>Singleton object “owns” itself </li></ul></ul><ul><ul><li>No client step to create singletons </li></ul></ul><ul><ul><li>Creates and Destroys itself. </li></ul></ul><ul><li>Singleton is a Design Pattern </li></ul><ul><li>A singleton is an improved global variable </li></ul>
  18. 22. Lifetime Semantics for a Singleton <ul><li>A single object of a class stays throughout the lifetime of an application </li></ul><ul><ul><li>Created when the execution of the program “starts” and remains there till the application “ends” </li></ul></ul><ul><li>The Singleton class is instantiated at the time of first access and same instance is used thereafter till the application quits. </li></ul><ul><li>At no point during execution there is more than one instance of the class; but in between it may be created & destroyed several times. </li></ul><ul><ul><li>There are execution points where no object exists. </li></ul></ul>
  19. 23. Singleton Examples <ul><li>The office of the President of India is a Singleton. </li></ul><ul><ul><li>The constitution specifies the means by which a President is selected, limits the term of office, and defines the order of succession. </li></ul></ul><ul><ul><li>There can be at most one President at any given time. </li></ul></ul><ul><ul><li>There will be exactly one at any given time. </li></ul></ul><ul><ul><li>Regardless of the personal identity of the President, the title, “President of India&quot; is a global point of access that identifies the person in the office. </li></ul></ul>
  20. 24. Singleton Examples <ul><li>Purify license in a network can be a Singleton. </li></ul><ul><ul><li>A Singleton connection objects can ensure that only one connection can be made at any time. </li></ul></ul><ul><li>Printer can be a singleton in a network. </li></ul><ul><li>Keyboard, Display, Log, … </li></ul><ul><li>[MFC] – The global instance of the CWinApp-derived application class is the singleton. </li></ul><ul><li>In EDAObjects™ </li></ul><ul><ul><li>Memory Manager </li></ul></ul><ul><ul><li>Message Handler </li></ul></ul><ul><ul><li>Command Line Processor </li></ul></ul>
  21. 25. Singleton Implementation How
  22. 26. Static Data + Static Function != Singleton <ul><li>Wrap the Singleton object and function that uses the object within a class </li></ul><ul><li>Make both static </li></ul><ul><li>Keep the object private and the method public </li></ul><ul><li>This is not a “good” singleton because </li></ul><ul><ul><li>static methods cannot be virtual </li></ul></ul><ul><ul><li>Initialization and Clean-up is difficult </li></ul></ul><ul><ul><li>There is no unique point of access </li></ul></ul>
  23. 27. Essence of Singleton Implementation <ul><li>To make a class singleton </li></ul><ul><ul><li>Make all constructors private </li></ul></ul><ul><ul><li>Provide a static method as a unique access point for the singleton. </li></ul></ul><ul><li>Examples follow … </li></ul>
  24. 28. A Simple Singleton
  25. 29. A Simple Singleton <ul><li>A simple – no-frills singleton is illustrated </li></ul><ul><li>Singleton is dynamically created </li></ul><ul><ul><li>a static creation may cause conflict in creation order </li></ul></ul><ul><li>Does not have a proper destruction point </li></ul><ul><ul><li>Cannot delete from within main() since some global objects may be using it </li></ul></ul><ul><ul><li>Hence the singleton leaks </li></ul></ul><ul><ul><ul><li>Is it a memory leak? </li></ul></ul></ul><ul><ul><ul><li>Is it a resource leak? </li></ul></ul></ul>
  26. 30. Meyer’s Singleton – A Solution <ul><li>Create the singleton as a local static object </li></ul><ul><li>Will be destroyed at exit </li></ul><ul><li>Does it solve all problems? </li></ul>Singleton& Singleton::Instance() { static Singleton theInstance; return theInstance; }
  27. 31. The Dead Reference Problem <ul><li>There are 3 singletons – </li></ul><ul><ul><li>Keyboard, </li></ul></ul><ul><ul><li>Display and </li></ul></ul><ul><ul><li>Log </li></ul></ul><ul><ul><ul><li>Error Reporting </li></ul></ul></ul><ul><ul><ul><li>Created on-demand </li></ul></ul></ul><ul><li>All are implemented by Meyer’s Singleton </li></ul><ul><li>Consider an exception scenario … </li></ul>
  28. 32. The Dead Reference Scenario <ul><li>Keyboard successfully created </li></ul><ul><li>Display fails to initialize </li></ul><ul><li>Log created </li></ul><ul><li>Error logged; application proceeds to exit </li></ul><ul><li>Log destroyed (LIFO order) </li></ul><ul><li>Keyboard fails to shutdown </li></ul><ul><li>Log::Instance() invoked for error reporting </li></ul><ul><ul><li>Returns a dead object!!! </li></ul></ul>
  29. 33. The Dead Reference Detection <ul><li>Maintain a flag with every singleton that tells if the singleton is alive. </li></ul><ul><li>If a dead reference is detected, an exceptions is raised. </li></ul><ul><li>The code follows … </li></ul>
  30. 34. Meyer’s Singleton with Dead Reference Detection
  31. 35. Phoenix Singleton <ul><li>Like Phoenix bird, this singleton rises repeatedly from its ashes </li></ul><ul><li>Outline </li></ul><ul><ul><li>Retrieve the Caracas (this a global allocation) </li></ul></ul><ul><ul><li>Reincarnate the singleton </li></ul></ul><ul><ul><li>Register a callback for destruction with atexit() </li></ul></ul>
  32. 36. Phoenix Singleton
  33. 37. More Singletons … <ul><li>Explicit Management of Lifetime with user-assigned priorities (Longevity Control) </li></ul><ul><li>Singletons under multi-threading </li></ul><ul><li>Singleton template </li></ul><ul><li>Singletons in Java, C# and .NET </li></ul>
  34. 38. Singletons in C++ References & Credits
  35. 39. References: Books <ul><li>Modern C++ Design: Generic Programming & Design Pattern Applied by Andrei Alexandrescu , Pearson Education 2001 </li></ul><ul><ul><li>Most of the material from “Implementing Singletons” chapter </li></ul></ul><ul><li>Exceptional C++ by Herb Sutter </li></ul><ul><ul><li>Discussion on Object Lifetime </li></ul></ul><ul><li>Effective C++ & More Effective C++: by Scott Meyers </li></ul><ul><ul><li>Many items relating to Object lifetime & Meyer’s Singleton </li></ul></ul>
  36. 40. References: Papers <ul><li>Object Lifetime Manager – A Complementary Pattern for Controlling Object Creation and Destruction by David L. Levine and Christopher D. Gill Douglas C. Schmidt , Design Patterns in Communications , (Linda Rising, ed.), Cambridge University Press, 2000. </li></ul>
  37. 41. Credits / Acknowledgements <ul><li>Debabrata Singha, ATOS Origin </li></ul><ul><ul><li>discussion on the “Object Lifetime Manager” paper. </li></ul></ul><ul><ul><li>understanding the atexit() behavior </li></ul></ul>
  38. 42. Thank You