+ implement 'filter' iterator to output key/value pairs

where value is not value_null().
+ use it in json::feature_generator to have more compact json

include/mapnik/feature_kv_iterator.hpp

@@ -23,12 +23,16 @@
 #ifndef MAPNIK_FEATURE_KV_ITERATOR_HPP
 #define MAPNIK_FEATURE_KV_ITERATOR_HPP
 
+// mapnik
+#include <mapnik/value.hpp>
+// boost
 #include <boost/tuple/tuple.hpp>
 #include <boost/iterator/iterator_facade.hpp>
-
+#include <boost/iterator/filter_iterator.hpp>
+#include <boost/variant.hpp>
+// stl
 #include <map>
-//#include <mapnik/feature.hpp>
+
-#include <mapnik/value.hpp>
 
 namespace mapnik {
 
@@ -57,6 +61,30 @@ private:
 
 };
 
+struct is_null : public boost::static_visitor<bool>
+{
+    bool operator() (value_null const& val) const
+    {
+        return true;
+    }
+
+    template <typename T>
+    bool operator() (T val) const
+    {
+        return false;
+    }
+};
+
+struct value_not_null
+{
+    bool operator() (feature_kv_iterator::value_type const& kv) const
+    {
+        return !boost::apply_visitor(is_null(),boost::get<1>(kv).base());
+    }
+};
+
+typedef boost::filter_iterator<value_not_null, feature_kv_iterator> feature_kv_iterator2;
+
 }
 
 #endif // MAPNIK_FEATURE_KV_ITERATOR_HPP

include/mapnik/json/feature_generator_grammar.hpp

@@ -27,6 +27,7 @@
 #include <mapnik/global.hpp>
 #include <mapnik/feature.hpp>
 #include <mapnik/json/geometry_generator_grammar.hpp>
+#include <mapnik/feature_kv_iterator.hpp>
 
 // boost
 #include <boost/spirit/include/karma.hpp>
@@ -47,26 +48,26 @@ struct is_container<mapnik::Feature const> : mpl::false_ {} ;
 template <>
 struct container_iterator<mapnik::Feature const>
 {
-    typedef mapnik::feature_kv_iterator type;
+    typedef mapnik::feature_kv_iterator2 type;
 };
 
 template <>
 struct begin_container<mapnik::Feature const>
 {
-    static mapnik::feature_kv_iterator
+    static mapnik::feature_kv_iterator2
     call (mapnik::Feature const& f)
     {
-        return f.begin();
+        return mapnik::feature_kv_iterator2(mapnik::value_not_null(),f.begin(),f.end());
     }
 };
 
 template <>
 struct end_container<mapnik::Feature const>
 {
-    static mapnik::feature_kv_iterator
+    static mapnik::feature_kv_iterator2
     call (mapnik::Feature const& f)
     {
-        return f.end();
+        return mapnik::feature_kv_iterator2(mapnik::value_not_null(),f.end(),f.end());
     }
 };