[wip] on #3116

include/mapnik/attribute.hpp

@@ -36,12 +36,15 @@ namespace mapnik {
 struct attribute
 {
     std::string name_;
+    bool is_id_;
     explicit attribute(std::string const& name)
-        : name_(name) {}
+        : name_(name),
+          is_id_(name == "__id__") {}
 
     template <typename V ,typename F>
     V const& value(F const& f) const
     {
+        if (is_id_) return f.id();
         return f.get(name_);
     }
 

include/mapnik/attribute_collector.hpp

@@ -62,7 +62,7 @@ struct expression_attributes
 
     void operator() (attribute const& attr) const
     {
-        names_.emplace(attr.name());
+        if (attr.name() != "__id__") names_.emplace(attr.name());
     }
 
     template <typename Tag>

include/mapnik/expression_evaluator.hpp

@@ -77,6 +77,10 @@ struct evaluate
 
     value_type operator() (global_attribute const& attr) const
     {
+        if (attr.name == "@id")
+        {
+            return feature_.id();
+        }
         auto itr = vars_.find(attr.name);
         if (itr != vars_.end())
         {

src/parse_path.cpp

@@ -67,9 +67,17 @@ namespace path_processor_detail
         void operator() (attribute const& attr) const
         {
             // convert mapnik::value to std::string
-            value const& val = feature_.get(attr.name());
+            std::string const& name = attr.name();
+            if (name == "__id__")
+            {
+                std::clog << "TODO\n";
+            }
+            else
+            {
+                value const& val = feature_.get(name);
                 filename_ += val.to_string();                
             }
+        }
 
         std::string & filename_;
         feature_impl const& feature_;

test/visual/renderer.hpp

@@ -177,7 +177,7 @@ struct grid_renderer : renderer_base<mapnik::image_rgba8>
                 }
                 if (val < 0)
                 {
-                    throw std::runtime_error("grid renderer: feature id is negative.");
+                    val = std::abs(val);
                 }
 
                 val *= 100;