libdeng2: Trying out a DENG2_PRIVATE compatible with old compilers

std::auto_ptr<> with an undefined struct is not tolerated by some
compilers. Now DENG2_PRIVATE uses a custom PrivateAutoPtr class that
reinterprets the unknown instance type as an IPrivate when deleting.

doomsday/libdeng2/include/de/libdeng2.h

@@ -234,23 +234,52 @@
  */
 #define DENG2_PRIVATE(Var) \
     struct Instance; \
-    std::auto_ptr<Instance> Var;
+    de::PrivateAutoPtr<Instance> Var;
 
 #if defined(__cplusplus)
 namespace de {
 
+struct IPrivate {
+    virtual ~IPrivate() {}
+};
+
+template <typename InstType>
+class PrivateAutoPtr
+{
+public:
+    PrivateAutoPtr(InstType *p = 0) : ptr(p) {}
+    ~PrivateAutoPtr() { reset(); }
+
+    InstType &operator * () const { return *ptr; }
+    InstType *operator -> () const { return ptr; }
+    void reset(InstType *p = 0) {
+        delete reinterpret_cast<IPrivate *>(ptr);
+        ptr = p;
+    }
+    InstType *get() const {
+        return ptr;
+    }
+    InstType *release() {
+        InstType *p = ptr;
+        ptr = 0;
+        return p;
+    }
+private:
+    InstType *ptr;
+};
+
 /**
  * Utility template for defining private implementation data (pimpl idiom). Use
  * this in source files, not in headers.
  */
-template <typename Type>
-struct Private {
-    Type &self;
-    Type *thisPublic;
-    typedef Private<Type> Base;
-
-    Private(Type &i) : self(i), thisPublic(&i) {}
-    Private(Type *i) : self(*i), thisPublic(i) {}
+template <typename PublicType>
+struct Private : public IPrivate {
+    PublicType &self;
+    PublicType *thisPublic;
+    typedef Private<PublicType> Base;
+
+    Private(PublicType &i) : self(i), thisPublic(&i) {}
+    Private(PublicType *i) : self(*i), thisPublic(i) {}
 };
 
 template <typename FromType, typename ToType>