need to factor out a special holder class to solve the problem with the empty check

src/lib/error.hpp

@@ -121,6 +121,9 @@ namespace lumiera {
     LUMIERA_ERROR_DECLARE (EXTERNAL );    ///< failure in external service the application relies on
     LUMIERA_ERROR_DECLARE (ASSERTION);    ///< assertion failure
     
+    /* generic error situations */
+    LUMIERA_ERROR_DECLARE (WRONG_TYPE);   ///< runtime type mismatch 
+    
 /** Macro for creating derived exception classes properly 
  *  integrated into Lumiera's exception hierarchy. Using
  *  this macro assures that the new class will get the full

src/lib/exception.cpp

@@ -64,6 +64,9 @@ namespace lumiera {
     LUMIERA_ERROR_DEFINE (EXTERNAL , "failure in external service"); 
     LUMIERA_ERROR_DEFINE (ASSERTION, "assertion failure");
 
+    /* some further generic error situations */
+    LUMIERA_ERROR_DEFINE (WRONG_TYPE, "runtime type mismatch"); 
+    
   } // namespace error
   
   LUMIERA_ERROR_DEFINE (EXCEPTION, "generic Lumiera exception"); 

src/lib/opaque-holder.hpp

@@ -0,0 +1,157 @@
+/*
+  OPAQUE-HOLDER.hpp  -  buffer holding an object inline while hiding the concrete type 
+ 
+  Copyright (C)         Lumiera.org
+    2009,               Hermann Vosseler <Ichthyostega@web.de>
+ 
+  This program is free software; you can redistribute it and/or
+  modify it under the terms of the GNU General Public License as
+  published by the Free Software Foundation; either version 2 of the
+  License, or (at your option) any later version.
+ 
+  This program is distributed in the hope that it will be useful,
+  but WITHOUT ANY WARRANTY; without even the implied warranty of
+  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+  GNU General Public License for more details.
+ 
+  You should have received a copy of the GNU General Public License
+  along with this program; if not, write to the Free Software
+  Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
+ 
+*/
+
+/** @file opaque-holder.hpp
+ ** Helper allowing type erasure while holding the actual object inline.
+ ** Controlling the actual storage of objects usually binds us to commit
+ ** to a specific type, thus ruling out polymorphism. But sometimes, when
+ ** we are able to control the maximum storage for a family of classes, we
+ ** can escape this dilemma by using the type erasure pattern combined with
+ ** an inline buffer holding the object of the concrete subclass. Frequently
+ ** this situation arises when dealing with functor objects.
+ ** 
+ ** This template helps building custom objects and wrappers based on this
+ ** pattern: it provides an buffer for the target objects and controls access
+ ** through a two-layer capsule; while the outer container provides a neutral
+ ** interface, the inner container keeps track of the actual type using a
+ ** vtable. OpaqueHolder can be empty; but re-accessing the concrete
+ ** object requires knowledge of the actual type, similar to boost::any
+ ** (but the latter uses heap storage). 
+ ** 
+ ** Using this approach is bound by specific stipulations regarding the
+ ** properties of the contained object and the kind of access needed.
+ ** When, to the contrary, the contained types are \em not related
+ ** and you need to re-discover their concrete type, then maybe
+ ** a visitor or variant record might be a better solution.
+ ** 
+ ** @see opaque-holder-test.cpp
+ ** @see function-erasure.hpp usage example
+ ** @see variant.hpp 
+ */
+
+
+#ifndef LIB_OPAQUE_HOLDER_H
+#define LIB_OPAQUE_HOLDER_H
+
+
+#include "lib/error.hpp"
+#include "lib/bool-checkable.hpp"
+
+
+
+namespace lib {
+  
+  using lumiera::error::LUMIERA_ERROR_WRONG_TYPE;
+  
+  
+  /**
+   * Inline buffer holding and owning an object while concealing the
+   * concrete type. Access to the contained object is similar to a
+   * smart-pointer, but the object isn't heap allocated. OpaqueHolder
+   * may be created empty, which can be checked by a bool test.
+   * The whole compound is copyable if and only if the contained object
+   * is copyable.
+   */
+  template
+    < class BA
+    , size_t siz = sizeof(BA) 
+    >
+  class OpaqueHolder
+    : public BoolCheckable<OpaqueHolder<BA,siz> >
+    {
+      char content_[siz];
+      
+      typedef OpaqueHolder<BA,siz> _ThisType;
+      
+      
+      
+    public:
+      OpaqueHolder()
+//      : created_(0)
+        { }
+      
+      template<class SUB>
+      OpaqueHolder(SUB const& obj)
+//      : created_(0)
+        { }
+      
+      ~OpaqueHolder() { clear(); }
+      
+      
+      void
+      clear ()
+        {
+          UNIMPLEMENTED ("delete contained object, if any");
+        }
+      
+      
+      OpaqueHolder (OpaqueHolder const& ref)
+//      : created_(must_be_empty (ref))
+        { }
+      
+      OpaqueHolder&
+      operator= (OpaqueHolder const& ref)
+        {
+          UNIMPLEMENTED ("copy operation");
+          return *this;
+        }
+        
+      
+      BA&
+      operator* ()  const  // never throws
+        {
+          ASSERT (!empty());
+          return (BA&) content_;
+        }
+      
+      BA* 
+      operator-> ()  const // never throws
+        {
+          ASSERT (!empty());
+          return (BA*) &content_;
+        }
+      
+      template<class SUB>
+      SUB& get()  const
+        {
+          UNIMPLEMENTED ("downcast to concrete type");
+//          return (SUB*) &content_; 
+        }
+      
+      bool empty() const
+        {
+          UNIMPLEMENTED ("check for empty container");
+        }
+      
+      bool isValid() const
+        {
+          UNIMPLEMENTED ("empty check and forward to contained object");
+        }
+      
+      
+    };
+  
+  
+  
+  
+} // namespace lib
+#endif

src/lib/scoped-holder.hpp

@@ -53,8 +53,8 @@
  */
 
 
-#ifndef LIB_SCOPEDHOLDER_H
-#define LIB_SCOPEDHOLDER_H
+#ifndef LIB_SCOPED_HOLDER_H
+#define LIB_SCOPED_HOLDER_H
 
 #include <boost/scoped_ptr.hpp>
 

src/proc/asset/unknown.cpp

@@ -60,7 +60,7 @@ namespace asset
                                     );
   }
   
-  LUMIERA_ERROR_DEFINE (ORIG_NOT_FOUND, "Media refered by placeholder not found");
+  LUMIERA_ERROR_DEFINE (ORIG_NOT_FOUND, "Media referred by placeholder not found");