// In header: <boost/log/sinks/basic_sink_frontend.hpp>
template<typename CharT> 
class basic_formatting_sink_frontend : public basic_sink_frontend {
public:
  // types
  typedef CharT                          char_type;       // Character type. 
  typedef std::basic_string< char_type > string_type;     // Formatted string type. 
  typedef basic_formatter< char_type >   formatter_type;  // Formatter function object type. 
  typedef formatter_type::stream_type    stream_type;     // Output stream type. 
  // member classes/structs/unions
  struct formatting_context {
    // member classes/structs/unions
    class cleanup_guard {
    public:
      // construct/copy/destruct
      explicit cleanup_guard(formatting_context &) noexcept;
      cleanup_guard(cleanup_guard const &) = delete;
      cleanup_guard & operator=(cleanup_guard const &) = delete;
      ~cleanup_guard();
    };
    // construct/copy/destruct
    formatting_context();
    formatting_context(unsigned int, std::locale const &, 
                       formatter_type const &);
    // public data members
    const unsigned int m_Version;    // Object version. 
    string_type m_FormattedRecord;    // Formatted log record storage. 
    stream_type m_FormattingStream;    // Formatting stream. 
    formatter_type m_Formatter;    // Formatter functor. 
  };
  // construct/copy/destruct
  explicit basic_formatting_sink_frontend(bool);
  // public member functions
  template<typename FunT> void set_formatter(FunT const &);
  void reset_formatter();
  std::locale getloc() const;
  void imbue(std::locale const &);
  // protected member functions
  formatter_type & formatter();
  template<typename BackendMutexT, typename BackendT> 
    void feed_record(record_view const &, BackendMutexT &, BackendT &);
  template<typename BackendMutexT, typename BackendT> 
    bool try_feed_record(record_view const &, BackendMutexT &, BackendT &);
};