xo-alloc: UT for allocator interation + misc improvements

include/xo/allocutil/ObjectVisitor.hpp

@@ -0,0 +1,53 @@
+/** @file ObjectVisitor.hpp
+ *
+ **/
+
+#include "IAlloc.hpp"
+#include <cstdint>
+
+namespace xo {
+    namespace gc {
+        /** @class ObjectVisitor
+         *  @brief visit IObject* members of a T instance
+         *
+         *  Garbage collector relies on being able to navigate to
+         *  an IObject-instance to find+update embedded pointers to
+         *  other IObjects.
+         *
+         *  An IObject implemnetation must override
+         *    IObject::_forward_children(IAlloc * gc)
+         *  and call
+         *   gc->forward_inplace(&child_)
+         *  for each such child pointer.
+         *
+         *  For non-template classes this is straightforward
+         *  See for example
+         *    xo::obj::List in object/List.hpp
+         *
+         *  For a template class Foo<T> that contains T-instances,
+         *  need to handle case where T contains IObject pointers.
+         *  See for example the
+         *    xo::tree::RedBlackTree in ordinaltree/RedBlackTree.hpp
+         *
+         *  Use ObjectVisitor<T>::forward_children() to pick up
+         *  navigation code for such template arguments.
+         **/
+        template <typename T>
+        class ObjectVisitor {
+            //void forward_children(T & target, IAlloc * gc) { (void)target; (void)gc; }
+        };
+
+#define XO_TRIVIAL_OBJECT_VISITOR(TYPE)                       \
+        template <>                                           \
+        class ObjectVisitor<TYPE> {                           \
+        public:                                               \
+            static void forward_children(TYPE &, IAlloc *) {} \
+        }
+
+        XO_TRIVIAL_OBJECT_VISITOR(int32_t);
+        XO_TRIVIAL_OBJECT_VISITOR(double);
+
+    } /*namespace gc*/
+} /*namespace xo*/
+
+/* ObjectVisitor.hpp */