219 lines
7.4 KiB
C++
219 lines
7.4 KiB
C++
#ifndef BOOST_SERIALIZATION_SINGLETON_HPP
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#define BOOST_SERIALIZATION_SINGLETON_HPP
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/////////1/////////2///////// 3/////////4/////////5/////////6/////////7/////////8
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// singleton.hpp
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//
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// Copyright David Abrahams 2006. Original version
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//
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// Copyright Robert Ramey 2007. Changes made to permit
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// application throughout the serialization library.
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//
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// Copyright Alexander Grund 2018. Corrections to singleton lifetime
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//
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// Distributed under the Boost
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// Software License, Version 1.0. (See accompanying
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// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
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//
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// The intention here is to define a template which will convert
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// any class into a singleton with the following features:
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//
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// a) initialized before first use.
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// b) thread-safe for const access to the class
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// c) non-locking
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//
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// In order to do this,
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// a) Initialize dynamically when used.
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// b) Require that all singletons be initialized before main
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// is called or any entry point into the shared library is invoked.
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// This guarentees no race condition for initialization.
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// In debug mode, we assert that no non-const functions are called
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// after main is invoked.
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//
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// MS compatible compilers support #pragma once
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#if defined(_MSC_VER)
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# pragma once
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#endif
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#include <boost/assert.hpp>
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#include <boost/config.hpp>
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#include <boost/noncopyable.hpp>
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#include <boost/serialization/force_include.hpp>
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#include <boost/serialization/config.hpp>
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#include <boost/archive/detail/auto_link_archive.hpp>
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#include <boost/archive/detail/abi_prefix.hpp> // must be the last header
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#ifdef BOOST_MSVC
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# pragma warning(push)
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# pragma warning(disable : 4511 4512)
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#endif
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namespace boost {
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namespace serialization {
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//////////////////////////////////////////////////////////////////////
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// Provides a dynamically-initialized (singleton) instance of T in a
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// way that avoids LNK1179 on vc6. See http://tinyurl.com/ljdp8 or
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// http://lists.boost.org/Archives/boost/2006/05/105286.php for
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// details.
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//
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// Singletons created by this code are guaranteed to be unique
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// within the executable or shared library which creates them.
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// This is sufficient and in fact ideal for the serialization library.
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// The singleton is created when the module is loaded and destroyed
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// when the module is unloaded.
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// This base class has two functions.
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// First it provides a module handle for each singleton indicating
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// the executable or shared library in which it was created. This
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// turns out to be necessary and sufficient to implement the tables
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// used by serialization library.
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// Second, it provides a mechanism to detect when a non-const function
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// is called after initialization.
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// Make a singleton to lock/unlock all singletons for alteration.
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// The intent is that all singletons created/used by this code
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// are to be initialized before main is called. A test program
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// can lock all the singletons when main is entered. Thus any
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// attempt to retrieve a mutable instance while locked will
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// generate an assertion if compiled for debug.
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// The singleton template can be used in 2 ways:
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// 1 (Recommended): Publicly inherit your type T from singleton<T>,
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// make its ctor protected and access it via T::get_const_instance()
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// 2: Simply access singleton<T> without changing T. Note that this only
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// provides a global instance accesible by singleton<T>::get_const_instance()
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// or singleton<T>::get_mutable_instance() to prevent using multiple instances
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// of T make its ctor protected
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// Note on usage of BOOST_DLLEXPORT: These functions are in danger of
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// being eliminated by the optimizer when building an application in
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// release mode. Usage of the macro is meant to signal the compiler/linker
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// to avoid dropping these functions which seem to be unreferenced.
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// This usage is not related to autolinking.
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class BOOST_SYMBOL_VISIBLE singleton_module :
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public boost::noncopyable
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{
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private:
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BOOST_DLLEXPORT bool & get_lock() BOOST_USED {
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static bool lock = false;
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return lock;
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}
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public:
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BOOST_DLLEXPORT void lock(){
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get_lock() = true;
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}
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BOOST_DLLEXPORT void unlock(){
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get_lock() = false;
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}
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BOOST_DLLEXPORT bool is_locked(){
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return get_lock();
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}
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};
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static inline singleton_module & get_singleton_module(){
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static singleton_module m;
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return m;
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}
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namespace detail {
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// This is the class actually instantiated and hence the real singleton.
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// So there will only be one instance of this class. This does not hold
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// for singleton<T> as a class derived from singleton<T> could be
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// instantiated multiple times.
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// It also provides a flag `is_destroyed` which returns true, when the
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// class was destructed. It is static and hence accesible even after
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// destruction. This can be used to check, if the singleton is still
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// accesible e.g. in destructors of other singletons.
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template<class T>
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class singleton_wrapper : public T
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{
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static bool & get_is_destroyed(){
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// Prefer a static function member to avoid LNK1179.
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// Note: As this is for a singleton (1 instance only) it must be set
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// never be reset (to false)!
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static bool is_destroyed_flag = false;
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return is_destroyed_flag;
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}
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public:
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singleton_wrapper(){
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BOOST_ASSERT(! is_destroyed());
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}
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~singleton_wrapper(){
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get_is_destroyed() = true;
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}
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static bool is_destroyed(){
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return get_is_destroyed();
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}
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};
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} // detail
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template <class T>
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class singleton {
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private:
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static T * m_instance;
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// include this to provoke instantiation at pre-execution time
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static void use(T const &) {}
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static T & get_instance() {
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BOOST_ASSERT(! is_destroyed());
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// use a wrapper so that types T with protected constructors can be used
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// Using a static function member avoids LNK1179
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static detail::singleton_wrapper< T > t;
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// note that the following is absolutely essential.
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// commenting out this statement will cause compilers to fail to
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// construct the instance at pre-execution time. This would prevent
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// our usage/implementation of "locking" and introduce uncertainty into
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// the sequence of object initialization.
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// Unfortunately, this triggers detectors of undefine behavior
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// and reports an error. But I've been unable to find a different
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// of guarenteeing that the the singleton is created at pre-main time.
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if (m_instance) use(* m_instance);
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return static_cast<T &>(t);
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}
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protected:
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// Do not allow instantiation of a singleton<T>. But we want to allow
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// `class T: public singleton<T>` so we can't delete this ctor
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BOOST_DLLEXPORT singleton(){}
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public:
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BOOST_DLLEXPORT static T & get_mutable_instance(){
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BOOST_ASSERT(! get_singleton_module().is_locked());
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return get_instance();
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}
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BOOST_DLLEXPORT static const T & get_const_instance(){
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return get_instance();
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}
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BOOST_DLLEXPORT static bool is_destroyed(){
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return detail::singleton_wrapper< T >::is_destroyed();
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}
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};
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// Assigning the instance reference to a static member forces initialization
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// at startup time as described in
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// https://groups.google.com/forum/#!topic/microsoft.public.vc.language/kDVNLnIsfZk
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template<class T>
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T * singleton< T >::m_instance = & singleton< T >::get_instance();
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} // namespace serialization
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} // namespace boost
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#include <boost/archive/detail/abi_suffix.hpp> // pops abi_suffix.hpp pragmas
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#ifdef BOOST_MSVC
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#pragma warning(pop)
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#endif
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#endif // BOOST_SERIALIZATION_SINGLETON_HPP
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