* This is also useful.

src/boost/weak_ptr.hpp

@@ -0,0 +1,193 @@
+#ifndef BOOST_WEAK_PTR_HPP_INCLUDED
+#define BOOST_WEAK_PTR_HPP_INCLUDED
+
+//
+//  weak_ptr.hpp
+//
+//  Copyright (c) 2001, 2002, 2003 Peter Dimov
+//
+//  Permission to copy, use, modify, sell and distribute this software
+//  is granted provided this copyright notice appears in all copies.
+//  This software is provided "as is" without express or implied
+//  warranty, and with no claim as to its suitability for any purpose.
+//
+//  See http://www.boost.org/libs/smart_ptr/weak_ptr.htm for documentation.
+//
+
+#include <boost/shared_ptr.hpp>
+
+#ifdef BOOST_MSVC  // moved here to work around VC++ compiler crash
+# pragma warning(push)
+# pragma warning(disable:4284) // odd return type for operator->
+#endif
+
+namespace boost
+{
+
+template<class T> class weak_ptr
+{
+private:
+
+    // Borland 5.5.1 specific workarounds
+    typedef weak_ptr<T> this_type;
+
+public:
+
+    typedef T element_type;
+
+    weak_ptr(): px(0), pn() // never throws in 1.30+
+    {
+    }
+
+//  generated copy constructor, assignment, destructor are fine
+
+
+//
+//  The "obvious" converting constructor implementation:
+//
+//  template<class Y>
+//  weak_ptr(weak_ptr<Y> const & r): px(r.px), pn(r.pn) // never throws
+//  {
+//  }
+//
+//  has a serious problem.
+//
+//  r.px may already have been invalidated. The px(r.px)
+//  conversion may require access to *r.px (virtual inheritance).
+//
+//  It is not possible to avoid spurious access violations since
+//  in multithreaded programs r.px may be invalidated at any point.
+//
+
+    template<class Y>
+    weak_ptr(weak_ptr<Y> const & r): pn(r.pn) // never throws
+    {
+        px = r.lock().get();
+    }
+
+    template<class Y>
+    weak_ptr(shared_ptr<Y> const & r): px(r.px), pn(r.pn) // never throws
+    {
+    }
+
+#if !defined(BOOST_MSVC) || (BOOST_MSVC > 1200)
+
+    template<class Y>
+    weak_ptr & operator=(weak_ptr<Y> const & r) // never throws
+    {
+        px = r.lock().get();
+        pn = r.pn;
+        return *this;
+    }
+
+    template<class Y>
+    weak_ptr & operator=(shared_ptr<Y> const & r) // never throws
+    {
+        px = r.px;
+        pn = r.pn;
+        return *this;
+    }
+
+#endif
+
+    shared_ptr<T> lock() const // never throws
+    {
+#if defined(BOOST_HAS_THREADS)
+
+        // optimization: avoid throw overhead
+        if(expired())
+        {
+            return shared_ptr<element_type>();
+        }
+
+        try
+        {
+            return shared_ptr<element_type>(*this);
+        }
+        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>();
+        }
+
+#else
+
+        // optimization: avoid try/catch overhead when single threaded
+        return expired()? shared_ptr<element_type>(): shared_ptr<element_type>(*this);
+
+#endif
+    }
+
+    long use_count() const // never throws
+    {
+        return pn.use_count();
+    }
+
+    bool expired() const // never throws
+    {
+        return pn.use_count() == 0;
+    }
+
+    void reset() // never throws in 1.30+
+    {
+        this_type().swap(*this);
+    }
+
+    void swap(this_type & other) // never throws
+    {
+        std::swap(px, other.px);
+        pn.swap(other.pn);
+    }
+
+    void _internal_assign(T * px2, detail::shared_count const & pn2)
+    {
+        px = px2;
+        pn = pn2;
+    }
+
+    template<class Y> bool _internal_less(weak_ptr<Y> const & rhs) const
+    {
+        return pn < rhs.pn;
+    }
+
+// Tasteless as this may seem, making all members public allows member templates
+// to work in the absence of member template friends. (Matthew Langston)
+
+#ifndef BOOST_NO_MEMBER_TEMPLATE_FRIENDS
+
+private:
+
+    template<class Y> friend class weak_ptr;
+    template<class Y> friend class shared_ptr;
+
+#endif
+
+    T * px;                     // contained pointer
+    detail::weak_count pn;      // reference counter
+
+};  // weak_ptr
+
+template<class T, class U> inline bool operator<(weak_ptr<T> const & a, weak_ptr<U> const & b)
+{
+    return a._internal_less(b);
+}
+
+template<class T> void swap(weak_ptr<T> & a, weak_ptr<T> & b)
+{
+    a.swap(b);
+}
+
+// deprecated, provided for backward compatibility
+template<class T> shared_ptr<T> make_shared(weak_ptr<T> const & r)
+{
+    return r.lock();
+}
+
+} // namespace boost
+
+#ifdef BOOST_MSVC
+# pragma warning(pop)
+#endif    
+
+#endif  // #ifndef BOOST_WEAK_PTR_HPP_INCLUDED