format: implement generic format parser and printer

src/utility/comparators.cpp

@@ -81,8 +81,8 @@ bool LocaleBasedItemSorting::operator()(const MPD::Item &a, const MPD::Item &b)
 						result = m_cmp(a.playlist().path(), b.playlist().path());
 						break;
 					case MPD::Item::Type::Song:
-						result = m_cmp(a.song().toString(Config.browser_sort_format, Config.tags_separator),
-						               b.song().toString(Config.browser_sort_format, Config.tags_separator));
+						result = m_cmp(Format::stringify<char>(Config.browser_sort_format, &a.song()),
+						               Format::stringify<char>(Config.browser_sort_format, &b.song()));
 						break;
 				}
 				break;

src/utility/type_conversions.cpp

@@ -203,7 +203,7 @@ MPD::Song::GetFunction charToGetFunction(char c)
 		case 'P':
 			return &MPD::Song::getPriority;
 		default:
-			return 0;
+			return nullptr;
 	}
 }
 

src/utility/wide_string.cpp

@@ -18,7 +18,7 @@
  *   51 Franklin St, Fifth Floor, Boston, MA 02110-1301, USA.              *
  ***************************************************************************/
 
-#include <boost/locale/encoding.hpp>
+#include <boost/locale/encoding_utf.hpp>
 #include <cassert>
 #include "utility/wide_string.h"