avoid creating default initialised values if key doesn't exist

include/mapnik/attribute.hpp

@@ -25,10 +25,15 @@
 #ifndef MAPNIK_ATTRIBUTE_HPP
 #define MAPNIK_ATTRIBUTE_HPP
 
+// mapnik
+#include <mapnik/value.hpp>
+// stl
 #include <string>
 
 namespace mapnik {
 
+static mapnik::value _null_value;
+
 struct attribute
 {
     std::string name_;
@@ -36,9 +41,15 @@ struct attribute
         : name_(name) {}
     
     template <typename V ,typename F>
-    V value(F const& f) const
+    V const& value(F const& f) const
     {
-        return f[name_];
+        typedef typename F::const_iterator const_iterator;
+        const_iterator itr = f.find(name_);
+        if (itr != f.end())
+        {
+            return itr->second;
+        }
+        return _null_value;
     }
     std::string const& name() const { return name_;}
 };

include/mapnik/feature.hpp

@@ -66,6 +66,7 @@ private:
     std::map<std::string,value> props_;
 public:
     typedef std::map<std::string,value>::iterator iterator;
+    typedef std::map<std::string,value>::const_iterator const_iterator;
     explicit feature(int id)
         : properties(props_),
           id_(id),
@@ -146,7 +147,7 @@ public:
     {
         return props_;
     }
-    
+
     iterator begin()
     {
         return props_.begin();
@@ -156,7 +157,22 @@ public:
     {
         return props_.end();
     }
+
+    const_iterator begin() const
+    {
+        return props_.begin();
+    }
        
+    const_iterator end() const
+    {
+        return props_.end();
+    }
+    
+    const_iterator find(std::string const& key) const
+    {
+        return props_.find(key);
+    }
+    
     std::string to_string() const
     {
         std::stringstream ss;

include/mapnik/metawriter.hpp

@@ -47,11 +47,22 @@ struct placement;
 class metawriter_property_map
 {
 public:
+    typedef std::map<std::string, UnicodeString> property_map;
+    typedef property_map::const_iterator const_iterator;
+    
     metawriter_property_map() {}
     UnicodeString const& operator[](std::string const& key) const;
     UnicodeString& operator[](std::string const& key) {return m_[key];}
+    std::map<std::string, UnicodeString>::const_iterator find(std::string const& key) const
+    {
+        return m_.find(key);
+    }
+    std::map<std::string, UnicodeString>::const_iterator end() const
+    {
+        return m_.end();
+    }
 private:
-    std::map<std::string, UnicodeString> m_;
+    property_map m_;
     UnicodeString not_found_;
 };