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 2006-2012.
// 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/intrusive/pointer_traits.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::intrusive::
pointer_traits<typename A::pointer>::template
rebind_pointer<T>::type pointer;
typedef typename ipcdetail::add_reference
<T>::type reference;
typedef typename ipcdetail::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
{ ipcdetail::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 ipcdetail::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;
ipcdetail::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