how to return an array of c++ objects

I have a Graph object which contains a std::Map<int, Node*> nodes .
graph->getNodes() just returns this->nodes . It is this function I'm
attempting to wrap in a PHP extension. 

I have already exposed the Graph and Node objects to userspace and need to
return an array of Node objects. However, I'm getting a segfault with my
current code and can't figure out why. Can someone spot my mistake? Here's
the relevant code (please let me know if I forget something useful):

---code---
struct node_object {
        zend_object std;
        Node *node;
};
zend_class_entry *node_ce;

/* snip... */

PHP_METHOD(Graph, getNodes)
{
        Graph *graph;
        graph_object *obj =
(graph_object*)zend_object_store_get_object(getThis() TSRMLS_CC);
        graph = obj->graph;

        node_object* n;
        zval* node_zval;

        if (obj == NULL) {
                RETURN_NULL();
        }

        if (object_init_ex(node_zval, node_ce) != SUCCESS) {
                RETURN_NULL();
        }

        std::map<int, Node*> nodes = graph->getNodes();

        array_init(return_value);
        
        for (std::map<int, Node*>::iterator i = nodes.begin(); i !=
nodes.end(); ++i) {
                php_printf("X");
        
                n = (node_object*) zend_object_store_get_object(node_zval
TSRMLS_CC);
                n->node = i->second;
                add_index_zval(return_value, i->first, node_zval);
        }
        php_printf("]");
}

--- end code ---
--- output ---
$ php -r '$g = new Graph(); $g->getNodes();'
XX]Segmentation fault

--- end output ---

(As you can see, the two-node long map is being traversed, the function
exits, then the segfault.)

Thanks very much for your help.
John