- declared class as virtual

src/include/manifest.hpp

@@ -9,7 +9,7 @@ class Manifest{
   private:
     map<checksum_algorithms,string> manifest_algorithm_files;
   public:
-    map<string,string> get_checksum_file_pairs(checksum_algorithms algorithm);
+    virtual map<string,string> get_checksum_file_pairs(checksum_algorithms algorithm);
 };
 
 #endif