How to recursively print out all the nodes in a binary search tree using stringstream C++

Question

I would like to print the contents of all the nodes in my binary search tree by using a stringstream and recursion. The issue is that when I use this code, only the contents of the root are displayed. I know the reason is that every time I recursively call the function InOrder(BSTNode* bst_node), my stringstream variable is created again. What can I do to my code to fix this issue while still utilizing a stringstream for output?

This is my code:

string BSTree::InOrder(BSTNode* bst_node) {
  stringstream ss;
  if (root_ ==  NULL) {
    return "";
  } else {
    if (bst_node->GetLeftChild() != NULL) {
      InOrder(bst_node->GetLeftChild());
    }
    ss << bst_node->GetContents() << " ";
    if (bst_node->GetRightChild() != NULL) {
      InOrder(bst_node->GetRightChild());
    }
  }
  return ss.str();
}

Answer 1

Something like that maybe?

string BSTree::InOrder(BSTNode* bst_node)
{
  if (!bst_node)
    return "";
  ostringstream ss;
  ss << InOrder(bst_node->GetLeftChild());
  ss << bst_node->GetContents() << " ";
  ss << InOrder(bst_node->GetRightChild());
  return ss.str();
}

or, you can pass around the same instance of stringstream:

void BSTree::InOrderImpl(BSTNode* bst_node, ostringstream &ss)
{
  if (bst_node)
  {
    InOrderImpl(bst_node->GetLeftChild(), ss);
    ss << bst_node->GetContents() << " ";
    InOrderImpl(bst_node->GetRightChild(), ss);
  }
}

string BSTree::InOrder(BSTNode* bst_node)
{
  ostringstream ss;
  InOrder(bst_node, ss);
  return ss.str();
}