WIP implementation draft

src/lib/opaque-holder.hpp

@@ -26,18 +26,18 @@
  ** 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
+ ** an inline buffer holding an object of the concrete subclass. Typically,
  ** 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
+ ** through a two-layer capsule; while the outer container exposes a neutral
+ ** interface, the inner container keeps track of the actual type by means
+ ** of 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
+ ** Using this approach is bound to 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
@@ -78,21 +78,106 @@ namespace lib {
   class OpaqueHolder
     : public BoolCheckable<OpaqueHolder<BA,siz> >
     {
-      char content_[siz];
       
-      typedef OpaqueHolder<BA,siz> _ThisType;
+      /** Inner capsule managing the contained object (interface) */
+      struct Buffer
+        {
+          char content_[siz];
+          virtual ~Buffer() {}
+          virtual bool isValid()  const { return false; }
+          virtual bool empty()    const { return true; }
+          
+          virtual void
+          clone (void* targetStorage)  const
+            {
+              new(targetStorage) Buffer();
+            }
+        };
       
       
+      /** concrete subclass managing a specific kind of contained object */
+      template<typename SUB>
+      struct Buff : Buffer
+        {
+          ~Buff()
+            {
+              get().~SUB();
+            }
+          
+          explicit
+          Buff (SUB const& obj)
+            {
+              REQUIRE (siz >= sizeof(SUB));
+              new(&content_) SUB (obj);
+            }
+          
+          Buff (Buff const& oBuff)
+            {
+              new(&content_) SUB (oBuff.get());
+            }
+      
+          Buff&
+          operator= (Buff const& ref)
+            {
+              if (&ref != this)
+                get() = ref.get();
+              return *this;
+            }
+          
+          
+          void
+          clone (void* targetStorage)  const
+            {
+              new(targetStorage) Buff(this->get());
+            }
+          
+          bool
+          empty()  const
+            {
+              return false;
+            }
+          
+          bool
+          isValid()  const
+            {
+              UNIMPLEMENTED ("maybe forward bool check to contained object...");
+            }
+          SUB&
+          get()  const
+            {
+              return *reinterpret_cast<SUB*> (&content_);
+            }
+        };
+      
+      
+      enum{ BUFFSIZE = sizeof(Buffer) };
+      
+      /** embedded buffer actually holding the concrete Buff object,
+       *  which in turn holds and manages the target object */
+      char storage_[BUFFSIZE];
+      
+      
+      
+      typedef OpaqueHolder<BA,siz> _ThisType;   /////TODO needed?
+      
+      Buffer&
+      buff()
+        {
+          return *reinterpret_cast<Buffer*> (&storage_);
+        }
+      
       
     public:
       OpaqueHolder()
-//      : created_(0)
-        { }
+        { 
+          new(&storage_) Buffer();
+        }
       
       template<class SUB>
       OpaqueHolder(SUB const& obj)
-//      : created_(0)
-        { }
+        { 
+          new(&storage_) Buff<SUB> (obj);
+        }
       
       ~OpaqueHolder() { clear(); }
       
@@ -100,13 +185,15 @@ namespace lib {
       void
       clear ()
         {
-          UNIMPLEMENTED ("delete contained object, if any");
+          buff().~Holder();
+          //////////////////////TODO sufficient?
         }
       
       
       OpaqueHolder (OpaqueHolder const& ref)
-//      : created_(must_be_empty (ref))
-        { }
+        {
+          ref.clone (storage_);
+        }
       
       OpaqueHolder&
       operator= (OpaqueHolder const& ref)

tests/lib/opaque-holder-test.cpp

@@ -91,6 +91,8 @@ namespace test{
   typedef OpaqueHolder<Base> Opaque;
   typedef vector<Opaque> TestList;
   
+  
+  
   /**********************************************************************************
    *  @test use the OpaqueHolder inline buffer to handle a family of classes
    *        through a common interface, without being forced to use heap storage
@@ -155,6 +157,20 @@ namespace test{
           ASSERT (5 == oo->getIt());
           VERIFY_ERROR (WRONG_TYPE, oo.get<D3>() );
           
+          D5 &rd5 (oo.get<D5>());
+          ASSERT (isSameObject (rd5, *oo));
+
+          // verify that self-assignment is properly detected...
+          oo = oo;
+          ASSERT (oo);
+          ASSERT (isSameObject (rd5, *oo));
+          oo = oo.get<D5>();
+          ASSERT (isSameObject (rd5, *oo));
+          ASSERT (oo);
+          oo = *oo;
+          ASSERT (isSameObject (rd5, *oo));
+          ASSERT (oo);
+          
           oo.clear();
           ASSERT (!oo);
           ASSERT (isnil(oo));