Re: [boost] [smart_ptr] enable_shared_from_this enhancement (patch)

From: Frank Mori Hess (frank.hess_at_[hidden])
Date: 2008-03-17 17:31:52

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Here's a proof-of-concept patch to svn trunk for enable_shared_from_this.hpp 
and shared_ptr.hpp.  After applying the patch, you should be able to use 
shared_from_this() successfully from a constructor.  It's only a 
proof-of-concept because it includes some quick-and-dirty nastiness (it makes 
the shared_count in shared_ptr public!).  

- -- 
Frank
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Index: boost/enable_shared_from_this.hpp
===================================================================
--- boost/enable_shared_from_this.hpp	(revision 43677)
+++ boost/enable_shared_from_this.hpp	(working copy)
@@ -25,7 +25,10 @@
 {
 protected:
 
-    enable_shared_from_this()
+    enable_shared_from_this():
+      _internal_shared_this(static_cast<T*>(this), &_internal_shared_this_deleter, detail::ignore_shared_from_this_tag()),
+      _internal_weak_this(_internal_shared_this),
+      _owned(false)
     {
     }
 
@@ -58,6 +61,16 @@
         return p;
     }
 
+    bool owned() const
+    {
+      return _owned;
+    }
+
+    static void _internal_shared_this_deleter(const enable_shared_from_this *obj)
+    {
+        detail::shared_count().swap(obj->_user_deleter);
+    }
+
 //  Note: No, you don't need to initialize _internal_weak_this
 //
 //  Please read the documentation, not the code
@@ -65,7 +78,10 @@
 //  http://www.boost.org/libs/smart_ptr/enable_shared_from_this.html
 
     typedef T _internal_element_type; // for bcc 5.5.1
+    mutable shared_ptr<_internal_element_type> _internal_shared_this;
+    mutable detail::shared_count _user_deleter;
     mutable weak_ptr<_internal_element_type> _internal_weak_this;
+    mutable bool _owned;
 };
 
 } // namespace boost
Index: boost/shared_ptr.hpp
===================================================================
--- boost/shared_ptr.hpp	(revision 43677)
+++ boost/shared_ptr.hpp	(working copy)
@@ -90,10 +90,18 @@
 
 // enable_shared_from_this support
 
-template<class T, class Y> void sp_enable_shared_from_this( shared_count const & pn, boost::enable_shared_from_this<T> const * pe, Y const * px )
+struct ignore_shared_from_this_tag {};
+
+template<class T, class Y> void sp_enable_shared_from_this( shared_count & pn, boost::enable_shared_from_this<T> const * pe, Y const * px)
 {
-    if(pe != 0) pe->_internal_weak_this._internal_assign(const_cast<Y*>(px), pn);
+    if(pe != 0)
+    {
+        pe->_user_deleter = pn;
+        pn.swap(pe->_internal_shared_this.pn);
+        pe->_internal_shared_this.reset();
+        pe->_owned = true;
 }
+}
 
 #ifdef _MANAGED
 
@@ -104,7 +112,7 @@
     template<class T> sp_any_pointer( T* ) {}
 };
 
-inline void sp_enable_shared_from_this( shared_count const & /*pn*/, sp_any_pointer, sp_any_pointer )
+inline void sp_enable_shared_from_this( shared_count & /*pn*/, sp_any_pointer, sp_any_pointer )
 {
 }
 
@@ -115,7 +123,7 @@
 # pragma set woff 3506
 #endif
 
-inline void sp_enable_shared_from_this( shared_count const & /*pn*/, ... )
+inline void sp_enable_shared_from_this( shared_count & /*pn*/, ... )
 {
 }
 
@@ -186,6 +194,13 @@
         boost::detail::sp_enable_shared_from_this( pn, p, p );
     }
 
+    // constructor that doesn't trigger enable_shared_from_this code, needed
+    // for enable_shared_from_this internal implementation
+    template<class Y, class D> shared_ptr(Y * p, D d, detail::ignore_shared_from_this_tag tag):
+      px(p), pn(p, d)
+    {}
+
+
     // As above, but with allocator. A's copy constructor shall not throw.
 
     template<class Y, class D, class A> shared_ptr( Y * p, D d, A a ): px( p ), pn( p, d, a )
@@ -480,6 +495,8 @@
 #endif
 
     T * px;                     // contained pointer
+/*yikes! made pn public to get this hack working */
+public:
     boost::detail::shared_count pn;    // reference counter
 
 };  // shared_ptr

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