source: code/branches/doc/src/libraries/util/Singleton.h @ 7332

/*
 *   ORXONOX - the hottest 3D action shooter ever to exist
 *                    > www.orxonox.net <
 *
 *
 *   License notice:
 *
 *   This program is free software; you can redistribute it and/or
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 *   as published by the Free Software Foundation; either version 2
 *   of the License, or (at your option) any later version.
 *
 *   This program is distributed in the hope that it will be useful,
 *   but WITHOUT ANY WARRANTY; without even the implied warranty of
 *   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 *   GNU General Public License for more details.
 *
 *   You should have received a copy of the GNU General Public License
 *   along with this program; if not, write to the Free Software
 *   Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
 *
 *   Author:
 *      Reto Grieder
 *   Co-authors:
 *      ...
 *
 */

/**
    @defgroup SingletonScope Singletons and Scope
    @ingroup Util
*/

/**
    @file
    @ingroup Util SingletonScope
    @brief Definition of the Singleton template that is used as base class for classes that allow only one instance.

    @anchor SingletonExample

    Classes that inherit from orxonox::Singleton follow the singleton pattern and thus
    allow only one instance of the class to exist. This istance is stored in a static
    variable called @c singletonPtr_s. orxonox::Singleton will access this variable, but
    it must be implemented in the deriving class.

    Example:
    @code
    class TestSingleton : public Singleton<TestSingleton>   // inherit from Singleton, pass the own class as template argument
    {
        friend class Singleton<TestSingleton>;              // friend declaration so Singleton can access singletonPtr_s

        public:
            TestSingleton();                                // public constructor because we may want to manage this singleton
                                                            //     with an orxonox::ScopedSingletonManager (see below)
            virtual ~TestSingleton();                       // public destructor

            void testFunction();                            // put your functions here

        private:
            int testValue_;                                 // put your variables here

            static TestSingleton* singletonPtr_s;           // static singleton instance pointer, used by the Singleton template
    };
    @endcode

    And don't forget to initialize the static singleton pointer in the source (*.cc) %file:
    @code
    TestSingleton* TestSingleton::singletonPtr_s = NULL;
    @endcode

    Usually a singleton gets created automatically when it is first used, but it will never
    be destroyed (unless the singleton explicitly deletes itself). To allow controlled
    construction and destruction, the singleton can be put within a virtual scope. This is
    done by registering the singleton class with orxonox::ScopedSingletonManager. To
    do so, the ManageScopedSingleton() macro has to be called:

    @code
    ManageScopedSingleton(TestSingleton, ScopeID::Graphics, false); // muste be called in a source (*.cc) file
    @endcode

    @b Important: If you call ManageScopedSingleton(), you don't have to initialize singletonPtr_s anymore,
    because that's already done by the macro.

    Now the singleton TestSingleton gets automatically created if the scope Graphics becomes
    active and also gets destroyed if the scope is deactivated.

    Note that not all singletons must register with a scope, but it's recommended.

    If a class inherits from orxonox::Singleton, it also inherits its functions. The most important
    function is orxonox::Singleton::getInstance() which returns a reference to the only instance
    of the singleton.

    Example:
    @code
    TestSingleton::TestSingleton()                          // implement the constructor
    {
        this->testValue_ = 15;
    }

    void TestSingleton::testFunction()                      // implement testFunction
    {
        COUT(0) << "My value is " << this->testValue_ << std::endl;
    }

    TestSingleton::getInstance().testFunction();            // prints "My value is 15"
    @endcode
*/

#ifndef __Util_Singleton_H__
#define __Util_Singleton_H__

#include "UtilPrereqs.h"

#include <cassert>
#include <cstring>

namespace orxonox
{
    /**
    @brief
        Base for singleton classes.

        Usage:
        Inherit publicly from Singleton<MyClass> and provide access to MyClass::singletonPtr_s.
        This can easily be done with a friend declaration.

        See @ref SingletonExample "this example" for an exemplary implementation.
    */
    template <class T>
    class Singleton
    {
    public:
        //! Returns a reference to the singleton instance
        static T& getInstance()
        {
            assert(T::singletonPtr_s != NULL);
            return *T::singletonPtr_s;
        }

        //! Tells whether the singleton has been created
        static bool exists()
        {
            return (T::singletonPtr_s != NULL);
        }

        //! Update method called by ClassSingletonManager (if used)
        void preUpdateSingleton(const Clock& time) { static_cast<T*>(T::singletonPtr_s)->preUpdate(time); }
        //! Empty update method for the static polymorphism
        void preUpdate(const Clock& time) { }
        //! Update method called by ClassSingletonManager (if used)
        void postUpdateSingleton(const Clock& time) { static_cast<T*>(T::singletonPtr_s)->postUpdate(time); }
        //! Empty update method for the static polymorphism
        void postUpdate(const Clock& time) { }

    protected:
        //! Constructor sets the singleton instance pointer
        Singleton()
        {
            assert(T::singletonPtr_s == NULL);
            T::singletonPtr_s = static_cast<T*>(this);
        }

        //! Destructor resets the singleton instance pointer
        ~Singleton()
        {
            assert(T::singletonPtr_s != NULL);
            T::singletonPtr_s = NULL;
        }

    private:
        Singleton(const Singleton& rhs); //!< Don't use (undefined)
    };
}

#endif /* __Util_Singleton_H__ */