a mysterious probelm of pointer

[stereomatching]

I am building a simple tree by pointer
this tree would only have one child and one sibling
if you call addChild more than once on the same node, it would add a sibling
if you call addchild more than two times(assume it is three times) on the same node
it would add a sibling on the "next" pointer which point to "0"(it would traverse the tree)

Below is my code

template<typename T>
class node
{
  public:
    explicit node(T value_);
    void addChild(node<T>* Node);
    ~node();

  private:
    void addSibling(node<T>* Node);
    T value;
    node<T>* prev;
    node<T>* child;
    node<T>* next;
};

template<typename T>
node<T>::node(T value_)
:value(value_),
 prev(0),
 child(0),
 next(0)
 {}

template<typename T>
node<T>::~node()
{
  prev = 0;
  if(child)
    delete child;

  if(next)
    delete next;

  child = 0;
  next = 0;
}

int callTime = 0;
template<typename T>
void node<T>::addChild(node<T>* Node)
{
  ++callTime;

  cout<<"callTime = "<<callTime<<endl;
  if(child)
    cout<<"child->value = "<<child->value<<endl;
  else
    cout<<"child = 0"<<endl;
  
  [b]//the puzzle of mine, next would always point to "0"[/b]
  if(next)
    cout<<"next->value = "<<next->value<<endl;
  else
    cout<<"next address = 0"<<endl;

  if(!child)
    child = Node;
  else
    child->addSibling(Node);
}

template<typename T>
void node<T>::addSibling(node<T>* Node)
{
  [b]//the most mysterious way is, in this function body, next
  //would not always be zero?[/b]
  if(!next)
    next = Node;

  else
  {
    node<T>* ptr = next;
    while(ptr->next)
      ptr = ptr->next;

    if(!ptr->next)
    {
      ptr->next = Node;
      Node->prev = ptr;
    }
  }
}

void simpleTree()
{
  node<int>* root = new node<int>(1);
  root->addChild(new node<int>(2) );
  root->addChild(new node<int>(3) );
  root->addChild(new node<int>(4) );
  root->addChild(new node<int>(5) );
  root->addChild(new node<int>(6) );
  root->addChild(new node<int>(7) );
  root->addChild(new node<int>(8) );

  delete root;
}

But if I alter the code a little bit
just ignore the function addSibling then the next would not point to 0 again

template<typename T>
class node
{
  public:
    explicit node(T value_);
    void addChild(node<T>* Node);
    ~node();

  private:    
    T value;
    node<T>* prev;
    node<T>* child;
    node<T>* next;
};

template<typename T>
node<T>::node(T value_)
:value(value_),
 prev(0),
 child(0),
 next(0)
 {}

template<typename T>
node<T>::~node()
{
  prev = 0;
  if(child)
    delete child;

  if(next)
    delete next;

  child = 0;
  next = 0;
}

int callTime = 0;
template<typename T>
void node<T>::addChild(node<T>* Node)
{
  ++callTime;

  cout<<"callTime = "<<callTime<<endl;
  if(child)
    cout<<"child->value = "<<child->value<<endl;
  else
    cout<<"child = 0"<<endl;

  [b]//next would not point to "0" again, what is going on?[/b]
  if(next)
    cout<<"next->value = "<<next->value<<endl;
  else
    cout<<"next address = 0"<<endl;

  if(!child)
    child = Node;
  else
  {
    if(!next)
    next = Node;

  else
  {
    node<T>* ptr = next;
    while(ptr->next)
      ptr = ptr->next;

    if(!ptr->next)
    {
      ptr->next = Node;
      Node->prev = ptr;
    }  
  }
}

void simpleTree()
{
  node<int>* root = new node<int>(1);
  root->addChild(new node<int>(2) );
  root->addChild(new node<int>(3) );
  root->addChild(new node<int>(4) );
  root->addChild(new node<int>(5) );
  root->addChild(new node<int>(6) );
  root->addChild(new node<int>(7) );
  root->addChild(new node<int>(8) );

  delete root;
}

Although the result are fine, but I want to know why?
What is happening?Did I did any mistakes?
Thanks a lot

[stereomatching]

I solved it

child->addSibling(Node);

sould be

addSibling(Node);