Boost C++ Libraries of the most highly regarded and expertly designed C++ library projects in the world. Herb Sutter and Andrei Alexandrescu, C++ Coding Standards

This is the documentation for an old version of Boost. Click here to view this page for the latest version.


// 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
// See 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
// See for documentation.


#  include <boost/config.hpp>

#  pragma once

#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
    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;


    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.

      return obj;
template <typename T>
typename singleton_default<T>::object_creator

} // namespace dtl
} // namespace container
} // namespace boost

#include <boost/container/detail/config_end.hpp>