comparison DEPENDENCIES/generic/include/boost/container/detail/singleton.hpp @ 102:f46d142149f5

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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 container_detail {
+
+// 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 container_detail
+} // namespace container
+} // namespace boost
+
+#include <boost/container/detail/config_end.hpp>
+
+#endif   //BOOST_CONTAINER_DETAIL_SINGLETON_DETAIL_HPP