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 // Copyright (C) 2000 Stephen Cleary
 // Copyright (C) 2008 Ion Gaztanaga
 //
 // Distributed under the Boost Software License, Version 1.0. (See
 // accompanying file LICENSE_1_0.txt or copy at
 // http://www.boost.org/LICENSE_1_0.txt)
 //
 // See http://www.boost.org for updates, documentation, and revision history.
 //
 // This file is a modified file from Boost.Pool
 
 //////////////////////////////////////////////////////////////////////////////
 //
 // (C) Copyright Ion Gaztanaga 2007-2013. Distributed under the Boost
 // Software License, Version 1.0. (See accompanying file
 // LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
 //
 // See http://www.boost.org/libs/container for documentation.
 //
 //////////////////////////////////////////////////////////////////////////////
 
 #ifndef BOOST_CONTAINER_DETAIL_SINGLETON_DETAIL_HPP
 #define BOOST_CONTAINER_DETAIL_SINGLETON_DETAIL_HPP
 
 #ifndef BOOST_CONFIG_HPP
 #  include <boost/config.hpp>
 #endif
 
 #if defined(BOOST_HAS_PRAGMA_ONCE)
 #  pragma once
 #endif
 
 #include <boost/container/detail/config_begin.hpp>
 #include <boost/container/detail/workaround.hpp>
 
 //
 // The following helper classes are placeholders for a generic "singleton"
 //  class.  The classes below support usage of singletons, including use in
 //  program startup/shutdown code, AS LONG AS there is only one thread
 //  running before main() begins, and only one thread running after main()
 //  exits.
 //
 // This class is also limited in that it can only provide singleton usage for
 //  classes with default constructors.
 //
 
 // The design of this class is somewhat twisted, but can be followed by the
 //  calling inheritance.  Let us assume that there is some user code that
 //  calls "singleton_default<T>::instance()".  The following (convoluted)
 //  sequence ensures that the same function will be called before main():
 //    instance() contains a call to create_object.do_nothing()
 //    Thus, object_creator is implicitly instantiated, and create_object
 //      must exist.
 //    Since create_object is a static member, its constructor must be
 //      called before main().
 //    The constructor contains a call to instance(), thus ensuring that
 //      instance() will be called before main().
 //    The first time instance() is called (i.e., before main()) is the
 //      latest point in program execution where the object of type T
 //      can be created.
 //    Thus, any call to instance() will auto-magically result in a call to
 //      instance() before main(), unless already present.
 //  Furthermore, since the instance() function contains the object, instead
 //  of the singleton_default class containing a static instance of the
 //  object, that object is guaranteed to be constructed (at the latest) in
 //  the first call to instance().  This permits calls to instance() from
 //  static code, even if that code is called before the file-scope objects
 //  in this file have been initialized.
 
 namespace boost {
 namespace container {
 namespace dtl {
 
 // T must be: no-throw default constructible and no-throw destructible
 template <typename T>
 struct singleton_default
 {
   private:
     struct object_creator
     {
       // This constructor does nothing more than ensure that instance()
       //  is called before main() begins, thus creating the static
       //  T object before multithreading race issues can come up.
       object_creator() { singleton_default<T>::instance(); }
       inline void do_nothing() const { }
     };
     static object_creator create_object;
 
     singleton_default();
 
   public:
     typedef T object_type;
 
     // If, at any point (in user code), singleton_default<T>::instance()
     //  is called, then the following function is instantiated.
     static object_type & instance()
     {
       // This is the object that we return a reference to.
       // It is guaranteed to be created before main() begins because of
       //  the next line.
       static object_type obj;
 
       // The following line does nothing else than force the instantiation
       //  of singleton_default<T>::create_object, whose constructor is
       //  called before main() begins.
       create_object.do_nothing();
 
       return obj;
     }
 };
 template <typename T>
 typename singleton_default<T>::object_creator
 singleton_default<T>::create_object;
 
 } // namespace dtl
 } // namespace container
 } // namespace boost
 
 #include <boost/container/detail/config_end.hpp>
 
 #endif   //BOOST_CONTAINER_DETAIL_SINGLETON_DETAIL_HPP

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