Add context_destroyed to verifiables

Avoids destruction order fiasco as context can be destroyed before the
functions/verifiables

include/turtle/detail/function_impl_template.hpp

@@ -255,6 +255,11 @@ namespace detail
             context_ = &c;
         }
 
+        virtual void context_destroyed()
+        {
+            context_ = NULL;
+        }
+
         friend std::ostream& operator<<(
             std::ostream& s, const function_impl& impl )
         {

include/turtle/detail/group.hpp

@@ -22,6 +22,13 @@ namespace detail
     class group
     {
     public:
+        ~group()
+        {
+            for( verifiables_cit it = verifiables_.begin();
+                    it != verifiables_.end(); ++it )
+                (*it)->context_destroyed();
+        }
+
         void add( verifiable& v )
         {
             verifiables_.push_back( &v );

include/turtle/detail/object_impl.hpp

@@ -56,6 +56,8 @@ namespace detail
             if( children_.empty() )
                 detail::root::inst().remove( *this );
         }
+        virtual void context_destroyed()
+        {}
 
         virtual void serialize( std::ostream& s, const verifiable& v ) const
         {

include/turtle/detail/verifiable.hpp

@@ -25,6 +25,8 @@ namespace detail
         virtual bool verify() const = 0;
 
         virtual void reset() = 0;
+
+        virtual void context_destroyed() = 0;
     };
 }
 } // mock