weak_ptr_example.cpp

struct Node {
  string name;
  weak_ptr<Node> parent;
  vector<shared_ptr<Node>> children;
  
  bool hasParent() {
    return parent.expired();
  }
  
  shared_ptr<Node> getParent() {
    return parent.lock(); 
    // If hasParent() then return shared_ptr from parent else nullptr
  }
  
  Node& addChild(shared_ptr<Node> node) {
    if(node.hasParent()) {
      throw "node already belongs to a parent";
    }
    children.emplace_back(node);
    node->parent = this;
    return *this;
  }
  
  Node& removeChild(unsigned i) {
    children[i].parent.reset(); // Remove parent;
    children.erase(&children[i]);
    return *this;
  }
  
  static shared_ptr<Node> create(string const& name) {
    return make_shared<Node>(name);
  }
};

shared_ptr<Node> createTree() {
  shared_ptr<Node> root = Node::create("root");
  shared_ptr<Node> foo = Node::create("foo");
  shared_ptr<Node> bar = Node::create("bar");
  shared_ptr<Node> baz = Node::create("baz");
  
  root
    ->addChild(foo) 
    ->addChild(bar);
    
  bar->addChild(baz);
    
  return root;
}

int main() {
  auto tree = createTree();
  auto baz = tree->children[1]->children[0];
  tree->removeChild(1); // Remove bar 
  ASSERT(baz.hasParent() == false); 
  // If parent would be a shared_ptr, 
  // then bar and baz would keep eachother alive forever.
  return 0;
}

weak_ptr can't be initialized with a raw pointer as it is on line #20. This example isn't compiling.