I have implemented a stream that sends output to the debugger (using OutputDebugString() in Windows): class debug_output_sink : public boost::iostreams::sink { public: debug_output_sink(int) { } std::streamsize write(const char* s, std::streamsize n); // also tried with: // struct category : boost::iostreams::sink::category, boost::iostreams::flushable_tag { }; // bool flush() { return true; } }; When I use it directly, flush() works fine: stream_buffer debug_streambuf(debug_sink); ostream debug_stream(&debug_streambuf); debug_stream << "test\n"; debug_stream.flush(); // characters appear immediately However, when I put it in a filter, flushing the filter doesn't flush the stream: // scope { stream_buffer debug_streambuf(debug_sink); ostream debug_stream(&debug_streambuf); filtering_ostreambuf filtering_buf; filtering_buf.push(debug_stream); ostream os(&filtering_buf); os << "test"; os.flush(); // nothing filtering_buf.pubsync(); // nothing either filtering_buf.strict_sync(); // still nothing } // characters appear at last ! What I am missing here? Thanks, Jean-Louis Leroy