source: src/CodePatterns/Singleton.hpp@ 163eec

/*
 * Singleton.hpp
 *
 *  Created on: Mar 10, 2010
 *      Author: crueger
 */

#ifndef SINGLETON_HPP_
#define SINGLETON_HPP_

// include config.h
#ifdef HAVE_CONFIG_H
#include <config.h>
#endif

#include <boost/thread.hpp>

#include "CodePatterns/Assert.hpp"

/**
 * This template produces the generic singleton pattern using the CRTP idiom.
 *
 * <h1> Singleton Howto </h1>
 * <h2> Introduction </h2>
 *
 * A Singleton is a class of which there can only be a single Object in the programm. It is
 * described as an design-pattern in Gof:96 (the famous design-pattern book). In the
 * molecuilder there are so far several Singletons serving a wide range of purposes:
 *
 * - the World, which handles all atoms, molecules and bonds
 * - the ActionRegistry, which stores all created actions by name for later use
 * - the UIFactory, which is an AbstractFactory (another design-pattern from Gof:96) and
 *   handles all creation of gui elements to ensure a single type throughout the programm
 * - the logger and errorLogger classes, that can be used to output messages on the screen
 *   depending on certain conditions
 *
 * Because those classes can only be instantiated once you cannot simply call <code>new World()</code>
 * or <code>delete</code> on them. Rather they have to be constructed and accessed using the singleton
 * mechanism. This mechanism consists of four static functions (and a fifth that is used internally,
 * but we will get to that later). These functions are:
 *
 * - <code>Singleton& Singleton::getInstance()</code> : returns the instance of the singleton as
 *    a reference
 * - <code>Singleton* Singleton::getPointer()</code> : returns the instance of the singleton as a
 *    pointer
 * - <code>void Singleton::purgeInstance()</code> : destroys the single Instance of the singleton
 * - <code>Singleton& Singleton::resetInstance()</code> : resets the Singleton, i.e. destroys the
 *    old instance and creates a new one
 *
 * If you need the instance of the singleton it is usually fine just to use one off the accessor
 * functions (i.e. <code>getInstance()</code> or <code>getPointer()</code>. Any creation of the
 * Singleton is then handled by these functions, so that the same object will be returned whenever
 * one of these functions is called. This easy process is true for most singletons you will need
 * to use. The only special singleton is the UIFactory.
 *
 * <h3>Special functions of the UIFactory</h3>
 *
 * If you simply call the <code>getInstance()</code> method of the UIFactory class the program
 * will crash. This happens, because the UIFactory in itself is abstract and needs to know what
 * kind of user interface it should produce later on. You need to tell the class the type of UI
 * using the void <code>UIFactory::makeUserInterface(InterfaceTypes type)</code> method. This will
 * also take care of creating the sole instance, so that the accessor functions will work afterwards.
 * What this also means is, that you cannot <code>reset()</code> the UIFactory, because at that
 * point it wont know anymore what to construct. A sequence of <code>UIFactory::purgeInstance()</code>,
 * <code>UIFactory::makeUserInterface()</code> and <code>UIFactory::getInstance()</code> will work
 * though.
 *
 * In order to make life easier and propagate changes to the singleton mechanism to all those
 * classes, there is a simple framework class that can be used to make any other class a
 * singleton through inheritance. This class can be found in the Pattern directory.
 *
 * <h2>How to make a class Singleton</h2>
 *
 * Most of the time you will only need singletons that don't require additional
 * information for creation. So I will cover the basic case for constructing singletons
 * first and then explain what has to be changed to make it accept special parameters.
 * Singletons are created by inheriting from the <code>Singleton<class T></code> template
 * using the Curiously recurring template pattern (CRTP). What this means is, that the
 * class they inherit from carries the inheriting class as a template parameter. For example
 * <code>class MySingletonExaple : public Singleton<MySingletonExample>{...}</code>. If you
 * want to know more about this idiom have a look at the
 * <A HREF="http://en.wikipedia.org/wiki/Curiously_recurring_template_pattern">wikipedia
 * page for this idiom</A>, but don't worry if you don't quite get how this works for now, for
 * the use of the singleton framework this is not important.
 *
 * If you want to make a class a singleton you can use the following sequence of steps.
 *
 * - Inherit from the singleton pattern using the CRTP as above:<br>
 *   @code
 *     class MySingletonExaple : public Singleton<MySingletonExample>{ ...}
 *   @endcode
 * - Make constructor and destructor private to avoid creation or destruction from
 *   outside the class:<br>
 *    @code
 *      class MySingletonExaple : public Singleton<MySingletonExample>{
 *        private:
 *          MySingletonExample();
 *          ~MySingletonExample();
 *      ...}
 *    @endcode
 * - give the inherited class access to the class internals using a friend declaration:<br>
 *    @code
 *       class MySingletonExaple : public Singleton<MySingletonExample>{
 *         friend class Singleton<MySingletonExample>; // don't forget the template parameters here
 *         private:
 *           MySingletonExample();
 *           ~MySingletonExample();
 *       ...}
 *    @endcode
 *
 * - include the file "Singleton_impl.hpp" that carries the implementation details of
 *   the singleton functions in your implementation file of the class.
 * - make the compiler construct the template instantiations. For this you can use the defined
 *   keyword <code>CONSTRUCT_SINGLETON(name)</code> at any toplevel point in the implementation
 *   file:<br>
 *    @code
 *       void MySingletonExample::foo(){...}
 *       void MySingletonExample::bar(){...}
 *       CONSTRUCT_SINGLETON(MySingletonExample) // no ; after this
 *     @endcode
 *
 * <h3>Singleton with initialization parameters</h3>
 *
 * Sometimes it is necessary for a singleton to be passed some initilization parameters. For
 * example the UIFactory mentioned above needs to know what kind of user interface it has to
 * produce. Making a singleton that takes initialization parameters is only sligtly different
 * from the steps lined out above. Here are all the differences:
 *
 * - pass an extra <code>false</code> to the template to deactivate the standard instantiation
 *   mechanism
 * - write a method that handles the special parameters and instantiation. In this method you
 *   can use the <code>setInstance(T*)</code> method inherited from the singleton pattern to set
 *   the created instance. The <code>setInstance()</code> method will only work when the
 *   <code>false<code> template parameter has been set and produce errors otherwise.
 *
 */
template <class T, bool _may_create=true>
class Singleton
{
private:
  /**
   * simple auto_ptr that is used by Singleton template
   *
   * This ptr_t allows destruction of the object using a private destructor,
   * when only the Singleton pattern is friend with the class
   *
   * All methods have similar sematics to auto_ptr
   */
  class ptr_t {
  public:
    ptr_t();
    ptr_t(T* _content);
    ~ptr_t();
    T& operator*();
    T* get();
    void reset(T* _content);
    void reset();
    ptr_t& operator=(const ptr_t& rhs);
  private:
    mutable T* content;
  };

  /**
   * This object handles the actual creation inside the singleton
   *
   * Using template specialization this will allways know what it can
   * do or cannot do at compile time
   */
  template<class creator_T, bool creator_may_create>
  struct creator_t {
    inline static creator_T* make();
    inline static void set(creator_T*&,creator_T*);
  };

  // specialization to allow fast creations

  /**
   * Specialized template that allows automatic construction only
   */
  template<class creator_T>
  struct creator_t<creator_T,true>{
    inline static creator_T* make(){
      return new creator_T();
    }

    inline static void set(creator_T*&,creator_T*){
      ASSERT(0, "Cannot set the Instance for a singleton of this type");
    }
  };

  /**
   * specialized template that allows setting only
   */
  template<class creator_T>
  struct creator_t<creator_T,false>{
    inline static creator_T* make(){
      ASSERT(0, "Cannot create a singleton of this type directly");
      return 0;
    }
    inline static void set(ptr_t& dest,creator_T* src){
      dest.reset(src);
    }
  };

  // this is used for creation
  typedef creator_t<T,_may_create> creator; //< the creator used

public:
  // make the state of this singleton accessible
  static const bool may_create=_may_create; //!< the type of singleton that we have

  /**
   * returns the instance of this Singleton as a reference
   *
   * If no Singleton exists at this point and we are allowed to create one
   * a new one is created and stored inside the singleton
   *
   * If no automatic creation is allowed, make sure to create an instance first
   * using the appropriate methods of the derived class. Otherwise this method
   * would fail.
   */
  static T& getInstance();

  /**
   * returns the instance of this Singleton as a reference
   *
   * If no Singleton exists at this point and we are allowed to create one
   * a new one is created and stored inside the singleton
   *
   * If no automatic creation is allowed, make sure to create an instance first
   * using the appropriate methods of the derived class. Otherwise this method
   * would fail.
   */
  static const T& getConstInstance();

  /**
   * returns the instance of this singleton as a pointer
   *
   * If no Singleton exists at this point and we are allowed to create one
   * a new one is created and stored inside the singleton.
   *
   * If no automatic creation is allowed, make sure to create an instance first
   * using the appropriate methods of the derived class. Otherwise this method
   * would fail.
   */
  static T* getPointer();

  /**
   * returns the instance of this singleton as a pointer
   *
   * If no Singleton exists at this point and we are allowed to create one
   * a new one is created and stored inside the singleton.
   *
   * If no automatic creation is allowed, make sure to create an instance first
   * using the appropriate methods of the derived class. Otherwise this method
   * would fail.
   */
  static const T* getConstPointer();

  /**
   * destroys the current instance of this singleton
   */
  static void purgeInstance();

  /**
   * destroys the current instance of the singleton and immidiately constructs
   * a new one. Similar to using <code>purgeInstance()</code> and <code>getInstance()</code>
   * but plays more nicely when observers are present. Especially the new instance is created
   * before the old one is destroyed so observers can switch their targets, when they are notified
   * of the destruction.
   *
   * If no automatic creation is allowed this method wont work.
   */
  static T&   resetInstance();

protected:
  /**
   * Method used to set the instance, when no automatic creation is allowed.
   *
   * Call this after the instantiation method in the derived class has created
   * it's instance and want's the singleton mechanism to keep it around for later
   * use.
   *
   * This method will always fail when automatic creation is enabled.
   */
  static void setInstance(T*);


  /**
   * empty constructor to allow creation of subclases
   */
  Singleton();

private:
  /**
   * the copy constructor is private to avoid accidental copying of Singletons, for example
   * when passing singletons to functions by value instead of by reference. If you
   * need copying of singletons call the default constructor in the copy constructor
   * of the derived object. The copyied object wont be known to the singleton mechanism.
   */
  Singleton(const Singleton&);

  static boost::recursive_mutex instanceLock; //!< a lock for the pointer of the instance
  static ptr_t theInstance; //!< the actual instance of the singleton
};

#endif /* SINGLETON_HPP_ */