c++ Using reinterpret_cast to cast unique_ptr<Derived>* to unique_ptr<Base>* for creating a transformable tree structure

Question

I am currently writing a program that needs to manipulate a tree structure (abstract syntax tree).
In the tree a node owns its children as unique_ptr and looks like:

struct Node {
  // to replace node itself in the tree:
  // points to unique_ptr that owns this node
  // a node can be stored in different unique_ptr types
  //  -> for example: NodeY could be stored in unique_ptr<NodeY> or unique_ptr<NodeX>)
  //  ->   thus self has to be of type unique_ptr<Node>*
  unique_ptr<Node> *self;
  // ...
};

struct NodeX: Node {
  unique_ptr<Node> child1;
  unique_ptr<NodeY> childY;
};

struct NodeY: Node {
  unique_ptr<NodeX> child1;
  unique_ptr<NodeY> child2;
  vector<unique_ptr<NodeY>> otherChildren;
};

struct NodeZ: NodeY {
  // ...
};
// and a lot of other nodes with different child types ...

While changing the tree it should be possible to replace a node in the tree.
For that purpose i store a self pointer to the owning unique_ptr in each node. A replacement action would look like:

// could replace node:
void visitNodeY(NodeY *node) {
  if (node->someCondition) {
     // replace
     auto newNode = make_unique<NodeZ>();
     unique_ptr<Node> *self = node->self; 
     // replace with make_unique<NodeA>() would break inheritance hierarchy, but i will not do it :)
     *self = move(newNode); // replace and delete old node
     node = self.get();     // get new address
     node->self = self;     // self still points to same address, only contend of unique_ptr has changed
  }
}

Now the problem is to set the self pointer after constructing a node.
To achieve that i am using reinterpret_cast :

void createNodeX_children(NodeX *nodex) {
  // create childY
  nodex->childY = make_unique<NodeY>();
  // ...
  // is that save?
  nodex->childY->self = reinterpret_cast<unique_ptr<Node>*>(&nodex->childY);
}

My Question is now: is it save using reinterpret_cast in this way, as long i don't break the inheritance hierarchy like mentioned above?

Answer 1

Don't reinterpret_cast . You can use std::unique_ptr 's constructors to transfer ownership polymorphically.