Serialization tree structure using boost::serialization - c++

Question

I have to serialize libkdtree++ in my program, the tree structures are briefly described as following:
struct _Node_base {
_Node_base * _M_parent, *_M_left, * _M_right;
template<Archive>
serialize(Archive &ar, const unsigned int version) {
ar & _M_left & _M_right;
}
}
template<typename V>
struct _Node : public _Node_base {
typedef V value_type;
value_type value;
template<Archive>
serialize(Archive &ar, const unsigned int version) {
ar.register_type(static_cast<_Node*>(NULL));
ar & boost::serialization::base_object<_Node_base>(*this);
ar & value;
}
}
struct Tree {
_Node * root;
template<Archive>
serialize(Archive &ar, const unsigned int version) {
ar & root;
}
}
This program reports "stream error".
But from the "serailzed file", it lacks the value fields for the children nodes of roots. Thus I think it is possible that BaseNode serialized _M_left and _M_right pointer. However since _Node_base have no idea about the value type of _Node, so it looks hard to add "ar.register_type" to _Node_base.serialize().

Answer 1

The pointer_conflict exception documentation states (sic):
pointer_conflict, // an attempt has been made to directly
// serialization::detail an object
// after having already serialzed the same
// object through a pointer. Were this permited,
// it the archive load would result in the
// creation of an extra copy of the obect.
I think the conflict occurs where each is serialised by a ptr in BaseNode::serialize and via the direct object, the *Node expression, in Node::serialize. However since the base_object function takes a reference and not a ptr I'm not sure how you would avoid this.
One possibility is to not serialize the parent ptr. Instead, after deserialization, do a walk of the tree and fix up the parent ptrs to point to the node parent. E.g. add the following method to BaseNode :
void fix (BaseNode* parent = 0)
{
this->parent = parent;
if (left != 0)
left->fix (this);
if (right != 0)
right->fix (this);
}
Then just call root->fix ()

Answer 2

The following solution for libkdtree++ & boost::serialization seems work:
// KDTree::_Node
friend class boost::serialization::access;
template<class Archive>
//void serialize(Archive & ar, const unsigned int version)
void save(Archive & ar, const unsigned int version) const
{
ar.register_type(static_cast< _Link_type>(NULL));
ar & boost::serialization::base_object<_Node_base>(*this);
_Link_type left = static_cast<_Link_type>(_M_left);
_Link_type right = static_cast<_Link_type>(_M_right);
ar & left & right;
ar & _M_value;
}
template<class Archive>
void load(Archive & ar, const unsigned int version)
{
ar.register_type(static_cast< _Link_type>(NULL));
ar & boost::serialization::base_object<_Node_base>(*this);
_Link_type left, right;
ar & left & right;
ar & _M_value;
if (left) {
left->_M_parent = this;
}
if (right) {
right->_M_parent = this;
}
_M_left = left;
_M_right = right;
}
BOOST_SERIALIZATION_SPLIT_MEMBER()