+ add get_optional to mapnik::feature_impl

returns -> boost::optional<mapnik::value const&>
+ use get_optional in feature_kv_iterator to avoid
  throwing exceptions

include/mapnik/feature.hpp

@@ -159,16 +159,11 @@ public:
     value_type const& get(context_type::key_type const& key) const
     {
         context_type::map_type::const_iterator itr = ctx_->mapping_.find(key);
-        if (itr != ctx_->mapping_.end()
+        if (itr != ctx_->mapping_.end())
-            && itr->second < data_.size())
+            return get(itr->second);        
-        {
-            return data_[itr->second];
-        }
         else        
-        {
             throw std::out_of_range(std::string("Key does not exist: '") + key + "'");    
     }
-    }
     
     value_type const& get(std::size_t index) const
     {
@@ -177,6 +172,13 @@ public:
         throw std::out_of_range("Index out of range");
     }
     
+    boost::optional<value_type const&> get_optional(std::size_t index) const
+    {
+        if (index < data_.size())
+            return boost::optional<value_type const&>(data_[index]);
+        return boost::optional<value_type const&>();
+    }
+    
     std::size_t size() const
     {
         return data_.size();

src/feature_kv_iterator.cpp

@@ -22,6 +22,7 @@
 
 #include <mapnik/feature_kv_iterator.hpp>
 #include <mapnik/feature.hpp>
+#include <boost/optional.hpp>
 
 namespace mapnik {
 
@@ -44,7 +45,9 @@ bool feature_kv_iterator::equal( feature_kv_iterator const& other) const
 feature_kv_iterator::value_type const& feature_kv_iterator::dereference() const
 {
     boost::get<0>(kv_) = itr_->first;
-    boost::get<1>(kv_) = f_.get(itr_->first);
+    boost::optional<mapnik::value const&> val = f_.get_optional(itr_->second);
+    if (val) boost::get<1>(kv_) = *val;
+    else boost::get<1>(kv_) = value_null();
     return kv_;
 }