boost/interprocess/smart_ptr/weak_ptr.hpp
////////////////////////////////////////////////////////////////////////////// // // This file is the adaptation for Interprocess of boost/weak_ptr.hpp // // (C) Copyright Peter Dimov 2001, 2002, 2003 // (C) Copyright Ion Gaztanaga 2006-2008. // 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/interprocess for documentation. // ////////////////////////////////////////////////////////////////////////////// #ifndef BOOST_INTERPROCESS_WEAK_PTR_HPP_INCLUDED #define BOOST_INTERPROCESS_WEAK_PTR_HPP_INCLUDED #include <boost/interprocess/detail/config_begin.hpp> #include <boost/interprocess/detail/workaround.hpp> #include <boost/interprocess/smart_ptr/shared_ptr.hpp> #include <boost/detail/no_exceptions_support.hpp> #include <boost/interprocess/allocators/allocator.hpp> #include <boost/interprocess/smart_ptr/deleter.hpp> #include <boost/pointer_to_other.hpp> //!\file //!Describes the smart pointer weak_ptr. namespace boost{ namespace interprocess{ //!The weak_ptr class template stores a "weak reference" to an object //!that's already managed by a shared_ptr. To access the object, a weak_ptr //!can be converted to a shared_ptr using the shared_ptr constructor or the //!member function lock. When the last shared_ptr to the object goes away //!and the object is deleted, the attempt to obtain a shared_ptr from the //!weak_ptr instances that refer to the deleted object will fail: the constructor //!will throw an exception of type bad_weak_ptr, and weak_ptr::lock will //!return an empty shared_ptr. //! //!Every weak_ptr meets the CopyConstructible and Assignable requirements //!of the C++ Standard Library, and so can be used in standard library containers. //!Comparison operators are supplied so that weak_ptr works with the standard //!library's associative containers. //! //!weak_ptr operations never throw exceptions. //! //!The class template is parameterized on T, the type of the object pointed to. template<class T, class A, class D> class weak_ptr { /// @cond private: // Borland 5.5.1 specific workarounds typedef weak_ptr<T, A, D> this_type; typedef typename boost::pointer_to_other <typename A::pointer, T>::type pointer; typedef typename detail::add_reference <T>::type reference; typedef typename detail::add_reference <T>::type const_reference; /// @endcond public: typedef T element_type; typedef T value_type; //!Effects: Constructs an empty weak_ptr. //!Postconditions: use_count() == 0. weak_ptr() : m_pn() // never throws {} // generated copy constructor, assignment, destructor are fine // // The "obvious" converting constructor implementation: // // template<class Y> // weak_ptr(weak_ptr<Y> const & r): m_px(r.m_px), m_pn(r.m_pn) // never throws // { // } // // has a serious problem. // // r.m_px may already have been invalidated. The m_px(r.m_px) // conversion may require access to *r.m_px (virtual inheritance). // // It is not possible to avoid spurious access violations since // in multithreaded programs r.m_px may be invalidated at any point. //!Effects: If r is empty, constructs an empty weak_ptr; otherwise, //!constructs a weak_ptr that shares ownership with r as if by storing a //!copy of the pointer stored in r. //! //!Postconditions: use_count() == r.use_count(). //! //!Throws: nothing. template<class Y> weak_ptr(weak_ptr<Y, A, D> const & r) : m_pn(r.m_pn) // never throws { //Construct a temporary shared_ptr so that nobody //can destroy the value while constructing this const shared_ptr<T, A, D> &ref = r.lock(); m_pn.set_pointer(ref.get()); } //!Effects: If r is empty, constructs an empty weak_ptr; otherwise, //!constructs a weak_ptr that shares ownership with r as if by storing a //!copy of the pointer stored in r. //! //!Postconditions: use_count() == r.use_count(). //! //!Throws: nothing. template<class Y> weak_ptr(shared_ptr<Y, A, D> const & r) : m_pn(r.m_pn) // never throws {} //!Effects: Equivalent to weak_ptr(r).swap(*this). //! //!Throws: nothing. //! //!Notes: The implementation is free to meet the effects (and the //!implied guarantees) via different means, without creating a temporary. template<class Y> weak_ptr & operator=(weak_ptr<Y, A, D> const & r) // never throws { //Construct a temporary shared_ptr so that nobody //can destroy the value while constructing this const shared_ptr<T, A, D> &ref = r.lock(); m_pn = r.m_pn; m_pn.set_pointer(ref.get()); return *this; } //!Effects: Equivalent to weak_ptr(r).swap(*this). //! //!Throws: nothing. //! //!Notes: The implementation is free to meet the effects (and the //!implied guarantees) via different means, without creating a temporary. template<class Y> weak_ptr & operator=(shared_ptr<Y, A, D> const & r) // never throws { m_pn = r.m_pn; return *this; } //!Returns: expired()? shared_ptr<T>(): shared_ptr<T>(*this). //! //!Throws: nothing. shared_ptr<T, A, D> lock() const // never throws { // optimization: avoid throw overhead if(expired()){ return shared_ptr<element_type, A, D>(); } BOOST_TRY{ return shared_ptr<element_type, A, D>(*this); } BOOST_CATCH(bad_weak_ptr const &){ // Q: how can we get here? // A: another thread may have invalidated r after the use_count test above. return shared_ptr<element_type, A, D>(); } BOOST_CATCH_END } //!Returns: 0 if *this is empty; otherwise, the number of shared_ptr objects //!that share ownership with *this. //! //!Throws: nothing. //! //!Notes: use_count() is not necessarily efficient. Use only for debugging and //!testing purposes, not for production code. long use_count() const // never throws { return m_pn.use_count(); } //!Returns: Returns: use_count() == 0. //! //!Throws: nothing. //! //!Notes: expired() may be faster than use_count(). bool expired() const // never throws { return m_pn.use_count() == 0; } //!Effects: Equivalent to: //!weak_ptr().swap(*this). void reset() // never throws in 1.30+ { this_type().swap(*this); } //!Effects: Exchanges the contents of the two //!smart pointers. //! //!Throws: nothing. void swap(this_type & other) // never throws { detail::do_swap(m_pn, other.m_pn); } /// @cond template<class T2, class A2, class D2> bool _internal_less(weak_ptr<T2, A2, D2> const & rhs) const { return m_pn < rhs.m_pn; } template<class Y> void _internal_assign(const detail::shared_count<Y, A, D> & pn2) { m_pn = pn2; } private: template<class T2, class A2, class D2> friend class shared_ptr; template<class T2, class A2, class D2> friend class weak_ptr; detail::weak_count<T, A, D> m_pn; // reference counter /// @endcond }; // weak_ptr template<class T, class A, class D, class U, class A2, class D2> inline bool operator<(weak_ptr<T, A, D> const & a, weak_ptr<U, A2, D2> const & b) { return a._internal_less(b); } template<class T, class A, class D> inline void swap(weak_ptr<T, A, D> & a, weak_ptr<T, A, D> & b) { a.swap(b); } //!Returns the type of a weak pointer //!of type T with the allocator boost::interprocess::allocator allocator //!and boost::interprocess::deleter deleter //!that can be constructed in the given managed segment type. template<class T, class ManagedMemory> struct managed_weak_ptr { typedef weak_ptr < T , typename ManagedMemory::template allocator<void>::type , typename ManagedMemory::template deleter<T>::type > type; }; //!Returns an instance of a weak pointer constructed //!with the default allocator and deleter from a pointer //!of type T that has been allocated in the passed managed segment template<class T, class ManagedMemory> inline typename managed_weak_ptr<T, ManagedMemory>::type make_managed_weak_ptr(T *constructed_object, ManagedMemory &managed_memory) { return typename managed_weak_ptr<T, ManagedMemory>::type ( constructed_object , managed_memory.template get_allocator<void>() , managed_memory.template get_deleter<T>() ); } } // namespace interprocess } // namespace boost #include <boost/interprocess/detail/config_end.hpp> #endif // #ifndef BOOST_INTERPROCESS_WEAK_PTR_HPP_INCLUDED