[Boost-users] enable_shared_from_this2

Subject: [Boost-users] enable_shared_from_this2
From: Peter Soetens (peter.soetens_at_[hidden])
Date: 2011-02-17 06:55:50

Hi list,

I was in the need for a shared_from_this() in my constructor, so I chose this 
implementation for my class. Apart from the header being 
there and one mailing list thread, there's nothing that points to the 
existence of this header.

Since we want to keep compatibility with Boost 1.38, I tried to backport the 
enable_shared_from_this2 class to that version. I was 
wondering why the weak_ptr::_empty() method was added to the weak_ptr API and 
why weak_ptr::expired() was not used instead ? I tried 
to emulate the same behavior by adding a 'bool' to the 
enable_shared_from_this2 class. Which works for my tests, but is there a 
pitfall to 
that solution ? Could I just replace this logic with weak_this_.expired() ?

diff --git a/rtt/internal/enable_shared_from_this2.hpp 
b/rtt/internal/enable_shared_from_this2.hpp
index a5bfcff..cb659f0 100644
--- a/rtt/internal/enable_shared_from_this2.hpp
+++ b/rtt/internal/enable_shared_from_this2.hpp
@@ -53,11 +53,13 @@ template< class T > class enable_shared_from_this2
 {
 protected:
 
-    enable_shared_from_this2()
+    enable_shared_from_this2() : weak_init(false)
     {
     }
 
-    enable_shared_from_this2( enable_shared_from_this2 const & )
+    // note: the copy constructor and operator= don't do anything, ie, they 
don't copy
+    // refcount status from the other object.
+    enable_shared_from_this2( enable_shared_from_this2 const & ) : 
weak_init(false)
     {
     }
 
@@ -75,6 +77,7 @@ private:
 
     mutable weak_ptr<T> weak_this_;
     mutable shared_ptr<T> shared_this_;
+    mutable bool weak_init; // we deviate from the boost 1.40 implementation 
because weak_this_._empty() is not available in older 
versions.
 
 public:
 
@@ -94,10 +97,11 @@ private:
 
     void init_weak_once() const
     {
-        if( weak_this_._empty() )
+        if( !weak_init )
         {
             shared_this_.reset( static_cast< T* >( 0 ), 
detail::esft2_deleter_wrapper() );
             weak_this_ = shared_this_;
+            weak_init = true;
         }
     }
 
@@ -111,6 +115,7 @@ public: // actually private, but avoids compiler template 
friendship issues
         if( weak_this_.use_count() == 0 )
         {
             weak_this_ = shared_ptr<T>( *ppx, py );
+            weak_init = true;
         }
         else if( shared_this_.use_count() != 0 )
         {

Thanks for any feedback,
Peter