Bypassing the scene of the graph

Hello, I have a SceneGraph consisting of a Node:

class Node { public: ... GLvoid addChild(Node* child) noexcept; GLboolean isExistChildren() const noexcept; GLvoid childrenForEach(std::function<GLvoid(Node* child)> callback) noexcept; ... private: Node* mChild = nullptr; Node* mNextNode = nullptr; } GLvoid Node::addChild(Node* child) noexcept { child->mNextNode = mChild; mChild = child; } GLboolean Node::isExistChildren() const noexcept { return mChild != nullptr; } GLvoid Node::childrenForEach(std::function<GLvoid(Node* child)> callback) noexcept { Node* iterator = mChild; while (iterator) { callback(iterator); iterator = iterator->mNextNode; } } 

At the moment, the detour is done as follows, and only ha is one level of the graph from the root:

 Node* mRootNode = ...; if (mRootNode->isExistChildren()) { mRootNode->childrenForEach([&shader](Node* child) { ... }); } 

I would like to write my custom iterator for this, so I don’t want to use callbacks, but I don’t know how to do without reference to the parent node (I don’t want to use extra memory for each node pointer) ... Can someone tell me how best to implement a workaround with minimum memory usage and greatest performance in this case?