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 Hu Chunming
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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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