make sure that the handler bitfield is always initialised

Add a new virtual function apply_start() to BaseHandler which
must forcably be called before running the handler through
osmium::apply (the C++ function).

Fixes #38.

lib/generic_handler.hpp

@@ -25,6 +25,7 @@ class BaseHandler : public osmium::handler::Handler {
     };
 
 public:
+virtual void apply_start() {};
 // handler functions
 virtual void node(const osmium::Node&) const = 0;
 virtual void way(const osmium::Way&) const = 0;
@@ -163,14 +164,7 @@ struct SimpleHandlerWrap: BaseHandler, wrapper<BaseHandler> {
         apply_object(osmium::io::File(cbuf, len, cfmt), locations, idx);
     }
 
-private:
-    void apply_object(osmium::io::File file, bool locations, const std::string &idx)
-    {
-        osmium::osm_entity_bits::type entities = osmium::osm_entity_bits::nothing;
-        BaseHandler::pre_handler handler = locations?
-                                            BaseHandler::location_handler
-                                            :BaseHandler::no_handler;
-
+    void apply_start() override {
         m_callbacks = osmium::osm_entity_bits::nothing;
         if (hasfunc("node"))
             m_callbacks |= osmium::osm_entity_bits::node;
@@ -182,6 +176,18 @@ struct SimpleHandlerWrap: BaseHandler, wrapper<BaseHandler> {
             m_callbacks |= osmium::osm_entity_bits::area;
         if (hasfunc("changeset"))
             m_callbacks |= osmium::osm_entity_bits::changeset;
+    }
+
+
+private:
+    void apply_object(osmium::io::File file, bool locations, const std::string &idx)
+    {
+        osmium::osm_entity_bits::type entities = osmium::osm_entity_bits::nothing;
+        BaseHandler::pre_handler handler = locations?
+                                            BaseHandler::location_handler
+                                            :BaseHandler::no_handler;
+
+        apply_start();
 
         if (m_callbacks & osmium::osm_entity_bits::area)
         {
@@ -202,6 +208,7 @@ struct SimpleHandlerWrap: BaseHandler, wrapper<BaseHandler> {
         apply(file, entities, handler, idx);
     }
 
+
     bool hasfunc(char const *name) {
         reference_existing_object::apply<SimpleHandlerWrap*>::type converter;
         PyObject* obj = converter( this );

lib/merged_input.hpp

@@ -16,6 +16,7 @@ namespace pyosmium {
 class MergeInputReader {
 public:
     void apply(BaseHandler& handler, bool simplify = true) {
+        handler.apply_start();
         if (simplify) {
             objects.sort(osmium::object_order_type_id_reverse_version());
             osmium::item_type prev_type = osmium::item_type::undefined;

lib/osmium.cc

@@ -13,11 +13,16 @@ void apply_reader_simple(osmium::io::Reader &rd, T &h) {
     osmium::apply(rd, h);
 }
 
+void apply_reader_simple(osmium::io::Reader &rd, BaseHandler &h) {
+    h.apply_start();
+    osmium::apply(rd, h);
+}
 
 template <typename T>
 void apply_reader_simple_with_location(osmium::io::Reader &rd,
                          osmium::handler::NodeLocationsForWays<T> &l,
                          BaseHandler &h) {
+    h.apply_start();
     osmium::apply(rd, l, h);
 }