cleanup and comments

src/rendergraph/examples/sg_example/customitem.h

@@ -11,7 +11,7 @@ class CustomItem : public QQuickItem {
 
   public:
     explicit CustomItem(QQuickItem* parent = nullptr);
-    ~CustomItem();
+    ~CustomItem() override;
 
   protected:
     QSGNode* updatePaintNode(QSGNode*, UpdatePaintNodeData*) override;

src/rendergraph/opengl/rendergraph/nodeinterface.h

@@ -8,10 +8,21 @@ template<class T_Node>
 class NodeInterface : public T_Node {
   public:
     void appendChildNode(std::unique_ptr<BaseNode> pNode) {
+        // Transfers ownership to this.
         BaseNode* pRawNode = pNode.release();
+        // Note: Ideally we would use unique_ptrs internally, but
+        // Qt uses raw pointers for QSGNode hierarchy. For simplicity
+        // we mimic this.
+
         T_Node::appendChildNode(pRawNode);
     }
+
     std::unique_ptr<BaseNode> detachChildNode(BaseNode* pNode) {
+        // After removeChildNode, the caller has the responsibility
+        // to deal with the child node. By returning a unique_ptr
+        // we preoprtly transfer ownership to the caller (which
+        // can result in deleting pNode if the caller doesn't
+        // take the unique_ptr).
         T_Node::removeChildNode(pNode);
         return std::unique_ptr<BaseNode>(pNode);
     }