ensuring data-consistency of object - boost-python

Question

I'm having a problem with data-consistencies of objects e.g. how to deal with objects that get deleted while the user still has a reference to them.
simple pseudo-code example
node = graph.getNode(name)
node.destroy() < -- node gets destroyed
#or
graph.destroyNode(name)
node.getName() #<-- should complain that we're trying to reference an object that does not exist any more
a simple pseudo-code example would be
struct Node
{
/*...*/
};
typedef boost::shared_ptr<Node> NodePtr;
struct Graph
{
std::map<std::string,NodePtr> nodeMap;
NodePtr getNode(std::string name);
void removeNode(std::string name);
/*...*/
};
typedef boost::shared_ptr<Graph> GraphPtr;
// wrapper arround the the getNode function
object Graph_getNode( object obj, const std::string& key )
{
GraphPtr graphPtr = extract< GraphPtr >(obj);
return boost::python::api::object(graphPtr->getNode(key));
};
class_< Node,boost::noncopyable, NodePtr >( "Node", "node", no_init )
/*....*/
class_< Graph, bases<Node>, boost::noncopyable, GraphPtr >( "Graph", "graph", no_init )
.def("getNode", Graph_getNode, "get a node if it exists")
/*....*/
Are there any functions that I could define which are run on a Node-object every time it gets used where I could check if it is still valid ?
I hope this information is sufficient to understand my problem,
... thanks for reading!
Seb

Answer 1

As Node is exposed as being held by a boost::shared_ptr, the instance's lifetime can be affected by both C++ and Python. Python will keep the C++ Node object alive as long as a handle to the Python object holding the boost::shared_ptr exists.
Therefore, in the following code, lets start with the precondition that a Node named spam is held in graph. The code is annotated with the reference count for the C++ Node instance, as well as the Python Node instance.
# The node named 'spam' is held in
# graph's nodeMap.
# cpp count: 1; python count: 0
# Boost.Python creats a Python Node object
# that holds boost::shared_ptr<Node>.
node = graph.getNode('spam') # cpp count: 2; python count: 1
# Removes the boost::shared_ptr<Node> for
# 'spam' from graph.nodeMap.
graph.destroyNode('spam') # cpp count: 1; python count: 1
# The C++ Node is still alive because the
# Python Node holds a boost::shared_ptr<Node>
node.getName() # cpp count: 1; python count: 1
# When Python no longer has a handle to the
# Python Node object, it will be garbage
# collected, which in turn will destroy
# the held boost::shared_ptr<Node>
node = None # cpp count: 0; python count: 0
If node.getName() should not operate successfully on a destroyed node, then consider explicitly managing state. For example, node.destroy() would set state within node, and member-functions would check state before performing operations. If the state indicates the node has been destroyed, then raise an exception or handle it appropriately.
Here is a basic example demonstrating the lifetime:
#include <map>
#include <string>
#include <boost/make_shared.hpp>
#include <boost/python.hpp>
#include <boost/shared_ptr.hpp>
class Node
{
public:
Node(std::string name)
: name_(name)
{
std::cout << "Node()" << std::endl;
}
~Node()
{
std::cout << "~Node()" << std::endl;
}
std::string name() { return name_; }
private:
std::string name_;
};
class Graph
{
public:
// #brief Add node by name.
void add_node(std::string name)
{
nodes_.insert(make_pair(name, boost::make_shared<Node>(name)));
}
/// #brief Destroy node by name.
void destroy_node(std::string name)
{
nodes_.erase(name);
}
// #brief Get node by name.
boost::shared_ptr<Node> get_node(std::string name)
{
nodes_type::iterator result = nodes_.find(name);
return (result != nodes_.end())
? result->second
: boost::shared_ptr<Node>();
}
private:
typedef std::map<std::string, boost::shared_ptr<Node> > nodes_type;
nodes_type nodes_;
};
BOOST_PYTHON_MODULE(example)
{
namespace python = boost::python;
python::class_<Node, boost::shared_ptr<Node>,
boost::noncopyable>("Node", python::no_init)
.def("name", &Node::name)
;
python::class_<Graph, boost::noncopyable>("Graph")
.def("add_node", &Graph::add_node)
.def("get_node", &Graph::get_node)
.def("destroy_node", &Graph::destroy_node)
;
}
And its usage in Python:
>>> from example import Graph
>>> graph = Graph()
>>> graph.add_node('spam')
Node()
>>> node = graph.get_node('spam')
>>> print node.name()
spam
>>> graph.destroy_node('spam')
>>> print node.name()
spam
>>> graph = None
>>> node = None
~Node()
Note how Node's destructor was only invoked once Python no longer had a handle to the object.