Revert previous fix for #89 and add a new improved fix that enhances the

value class to have a proper null type and do comparisions SQL style so
that null is neither equal nor not equal to other things.

bindings/python/mapnik_feature.cpp

@@ -47,6 +47,11 @@ namespace boost { namespace python {
             {
                return ::PyUnicode_FromWideChar((wchar_t*)s.getBuffer(),implicit_cast<ssize_t>(s.length()));
             }
+            
+            PyObject * operator() (mapnik::value_null const& s) const
+            {
+               return NULL;
+            }
       };
       
       struct mapnik_value_to_python

include/mapnik/feature.hpp

@@ -120,11 +120,6 @@ namespace mapnik {
             return props_.end();
          }
           
-         bool exists(const std::string &k) const
-         {
-            return props_.find(k) != props_.end();
-         }
-
          std::string to_string() const
          {
             std::stringstream ss;

include/mapnik/value.hpp

@@ -39,7 +39,11 @@
 
 namespace mapnik  {
    
-   typedef boost::variant<bool,int,double,UnicodeString> value_base;
+   struct value_null
+   {
+   };
+   
+   typedef boost::variant<value_null,bool,int,double,UnicodeString> value_base;
    
    namespace impl {
       struct equals
@@ -72,6 +76,60 @@ namespace mapnik  {
 	    {
 	       return  lhs == rhs;
 	    }
+
+            bool operator() (value_null, value_null) const
+	    {
+	       return false;
+	    }
+      };
+      
+      struct not_equals
+	 : public boost::static_visitor<bool>
+      {
+            template <typename T, typename U>
+	    bool operator() (const T &, const U &) const
+	    {
+	       return true;
+	    }
+	
+            template <typename T>
+            bool operator() (T lhs, T rhs) const
+	    {
+	       return lhs != rhs;
+	    }
+            
+            bool operator() (int lhs, double rhs) const
+	    {
+	       return  lhs != rhs;
+	    }
+	
+            bool operator() (double lhs, int rhs) const
+	    {
+	       return  lhs != rhs;
+	    }
+	
+            bool operator() (UnicodeString const& lhs, 
+                             UnicodeString const& rhs) const
+	    {
+	       return  lhs != rhs;
+	    }
+
+            bool operator() (value_null, value_null) const
+	    {
+	       return false;
+	    }
+
+            template <typename T>
+            bool operator() (value_null, const T &) const
+	    {
+	       return false;
+	    }
+
+            template <typename T>
+            bool operator() (const T &, value_null) const
+	    {
+	       return false;
+	    }
       };
 
       struct greater_than
@@ -103,6 +161,11 @@ namespace mapnik  {
 	    {
 	       return  lhs > rhs;
 	    }
+
+            bool operator() (value_null, value_null) const
+	    {
+	       return false;
+	    }
       };
     
       struct greater_or_equal
@@ -134,6 +197,11 @@ namespace mapnik  {
 	    {
 	       return lhs >= rhs;
 	    }
+
+            bool operator() (value_null, value_null) const
+	    {
+	       return false;
+	    }
       };
     
       struct less_than
@@ -166,6 +234,11 @@ namespace mapnik  {
 	    {
 	       return lhs < rhs;
 	    }
+
+            bool operator() (value_null, value_null) const
+	    {
+	       return false;
+	    }
       };
 
       struct less_or_equal
@@ -199,6 +272,11 @@ namespace mapnik  {
 	    {
 	       return lhs <= rhs;
 	    }
+
+            bool operator() (value_null, value_null) const
+	    {
+	       return false;
+	    }
       };
     
       template <typename V>
@@ -386,6 +464,11 @@ namespace mapnik  {
 	       ss << std::setprecision(16) << val;
 	       return ss.str();
             }
+            
+            std::string operator() (value_null const& val) const
+            {
+	       return "";
+            }
        };
 
       struct to_unicode : public boost::static_visitor<UnicodeString>
@@ -411,6 +494,11 @@ namespace mapnik  {
 	       out << std::setprecision(16) << val;
                return UnicodeString(out.str().c_str());
             }
+            
+            UnicodeString operator() (value_null const& val) const
+            {
+	       return UnicodeString("");
+            }
       };
       
       struct to_expression_string : public boost::static_visitor<std::string>
@@ -455,6 +543,11 @@ namespace mapnik  {
 	       ss << std::setprecision(16) << val;
 	       return ss.str();
             }
+
+            std::string operator() (value_null const& val) const
+	    {
+               return "null";
+            }
       };
    }
 
@@ -468,7 +561,7 @@ namespace mapnik  {
         
       public:
 	 value ()
-            : base_(0) {}
+            : base_(value_null()) {}
 	
 	 template <typename T> value(T _val_)
             : base_(_val_) {}
@@ -480,7 +573,7 @@ namespace mapnik  {
 
 	 bool operator!=(value const& other) const
 	 {
-	    return !(boost::apply_visitor(impl::equals(),base_,other.base_));
+	    return boost::apply_visitor(impl::not_equals(),base_,other.base_);
 	 }
 	
 	 bool operator>(value const& other) const

src/agg_renderer.cpp

@@ -456,8 +456,6 @@ namespace mapnik
                                   proj_transform const& prj_trans)
    {
       typedef  coord_transform2<CoordTransform,geometry2d> path_type;
-      if (feature.exists(sym.get_name()))
-      {
       UnicodeString text = feature[sym.get_name()].to_unicode();
       boost::shared_ptr<ImageData32> const& data = sym.get_image();
       if (text.length() > 0 && data)
@@ -507,7 +505,6 @@ namespace mapnik
          }
       }
    }
-   }
    
    template <typename T>
    void  agg_renderer<T>::process(line_pattern_symbolizer const& sym,
@@ -669,8 +666,6 @@ namespace mapnik
    {
       typedef  coord_transform2<CoordTransform,geometry2d> path_type;
       
-      if (feature.exists(sym.get_name()))
-      {
       UnicodeString text = feature[sym.get_name()].to_unicode();
       if ( text.length() > 0 )
       {
@@ -725,6 +720,5 @@ namespace mapnik
          }
       }
    }
-   }
    template class agg_renderer<Image32>;
 }

src/cairo_renderer.cpp

@@ -750,8 +750,6 @@ namespace mapnik
    {
       typedef coord_transform2<CoordTransform,geometry2d> path_type;
 
-      if (feature.exists(sym.get_name()))
-      {
       UnicodeString text = feature[sym.get_name()].to_unicode();
       boost::shared_ptr<ImageData32> const& data = sym.get_image();
 
@@ -796,7 +794,6 @@ namespace mapnik
          }
       }
    }
-   }
 
    template <typename T>
    void  cairo_renderer<T>::process(line_pattern_symbolizer const& sym,
@@ -927,8 +924,6 @@ namespace mapnik
    {
       typedef coord_transform2<CoordTransform,geometry2d> path_type;
 
-      if (feature.exists(sym.get_name()))
-      {
       UnicodeString text = feature[sym.get_name()].to_unicode();
 
       if (text.length() > 0)
@@ -983,6 +978,5 @@ namespace mapnik
          }
       }
    }
-   }
    template class cairo_renderer<Cairo::Surface>;
 }