adding infrastructure to register callbacks for settings

Using the Register class, one can register callbacks to manipulate options or check options afterwards.
These callbacks are stored in the Callbacks class (being a singleton) and should be called by the Settings class when appropriate.
The Settings class does not call them yet.

src/utility/settings.h

@@ -9,6 +9,7 @@
 #define SETTINGS_H_
 
 #include <iostream>
+#include <list>
 #include <boost/program_options.hpp>
 #include "src/exceptions/InvalidSettings.h"
 
@@ -118,7 +119,91 @@ namespace settings {
 	 */
 	Settings* instance();
 
-} // namespace parser
+
+	/*!
+	 *	@brief	Function type for functions registering new options.
+	 */
+	typedef void(*fptr_regOption)(bpo::options_description&);
+	
+	/*!
+	 *	@brief	Function type for function checking constraints on settings.
+	 */
+	typedef bool(*fptr_checkOption)(bpo::variables_map&);
+	
+	
+	/*!
+	 *	@brief This class handles callbacks for registering new options and
+	 *	checking constraints on them afterwards.
+	 *
+	 *	This class is also a singleton (like Settings) and is implemented much
+	 *	simpler as we don't need any custom initialization code.
+	 */
+	class Callbacks
+	{
+		private:
+			/*!
+			 *	@brief Stores register callbacks.
+			 */
+			std::list<fptr_regOption> reg_list;
+			/*!
+			 *	@brief Stores check callbacks.
+			 */
+			std::list<fptr_checkOption> check_list;
+			
+			/*!
+			 *	@brief Private constructor.
+			 */
+			Callbacks() {}
+			/*!
+			 *	@brief Private copy constructor.
+			 */
+			Callbacks(const Callbacks&) {}
+			/*!
+			 *	@brief Private destructor.
+			 */
+			~Callbacks() {}			
+		public:
+			/*!
+			 *	@brief Returns current instance to create singleton.
+			 *	@return current instance
+			 */
+			static Callbacks& getInstance()
+			{
+				static Callbacks instance;
+				return instance;
+			}
+		
+		/*!
+		 *	@brief Register class needs access to lists.
+		 */
+		friend class Register;
+	};
+	
+	/*!
+	 *	@brief Wrapper class to allow for registering callbacks during
+	 *	static initialization.
+	 */
+	class Register
+	{
+		public:
+			/*!
+			 *	@brief Registers given function as register callback.
+			 */
+			Register(const fptr_regOption ptr)
+			{
+				mrmc::settings::Callbacks::getInstance().reg_list.push_back(ptr);
+			}
+			
+			/*!
+			 *	@brief Registers given function as check callback.
+			 */
+			Register(const fptr_checkOption ptr)
+			{
+				mrmc::settings::Callbacks::getInstance().check_list.push_back(ptr);
+			}
+	};
+	
+} // namespace settings
 } // namespace mrmc
 
 #endif // SETTINGS_H_