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