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路径: \\game3dprogramming\materials\GameFactory\GameFactoryDemo\references\boost_1_35_0\boost\pool\detail\singleton.hpp
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// Copyright (C) 2000 Stephen Cleary // // 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. #ifndef BOOST_POOL_SINGLETON_HPP #define BOOST_POOL_SINGLETON_HPP // The following code might be put into some Boost.Config header in a later revision #ifdef __BORLANDC__ # pragma option push -w-inl #endif // // 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
::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 details { namespace pool { // T must be: no-throw default constructible and no-throw destructible template
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
::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
::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
::create_object, whose constructor is // called before main() begins. create_object.do_nothing(); return obj; } }; template
typename singleton_default
::object_creator singleton_default
::create_object; } // namespace pool } // namespace details } // namespace boost // The following code might be put into some Boost.Config header in a later revision #ifdef __BORLANDC__ # pragma option pop #endif #endif
singleton.hpp
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