intrusive_ptr.hpp added (still experimental)

[SVN r13526]

include/boost/shared_ptr.hpp

@@ -68,6 +68,7 @@ template<> struct shared_ptr_traits<void>
 //
 
 template<typename T> class weak_ptr;
+template<typename T> class intrusive_ptr;
 
 template<typename T> class shared_ptr
 {
@@ -110,6 +111,11 @@ public:
     {
     }
 
+    template<typename Y>
+    shared_ptr(intrusive_ptr<Y> const & r): px(r.get()), pn(r.get()) // never throws
+    {
+    }
+
     template<typename Y>
     shared_ptr(shared_ptr<Y> const & r, detail::static_cast_tag): px(static_cast<element_type *>(r.px)), pn(r.pn)
     {