// In header: <boost/log/sinks/text_multifile_backend.hpp>
template<typename FormatterT> 
class file_name_composer_adapter {
public:
  // types
  typedef filesystem::path                             result_type;       // Functor result type. 
  typedef result_type::string_type::value_type         native_char_type;  // File name character type. 
  typedef FormatterT                                   formatter_type;    // The adopted formatter type. 
  typedef basic_formatting_ostream< native_char_type > stream_type;       // Formatting stream type. 
  // construct/copy/destruct
  explicit file_name_composer_adapter(formatter_type const &, 
                                      std::locale const & = std::locale());
  file_name_composer_adapter(file_name_composer_adapter const &);
  file_name_composer_adapter & operator=(file_name_composer_adapter const &);
  // public member functions
  result_type operator()(record_view const &) const;
};