Hi there, First of all thank you to Jonathan Turkanis for the iostreams library - it has been very useful. I am using a filter to calculate a hash on data as it is sent to an ostream, using symmetric_filter. The problem is that even when I flush the filtering_ostream, the extra bytes in the symmetric_filter destination buffer aren't flushed out to the sink until the filtering_ostream goes out of scope (or reset() or pop() is called). I found the following in the FAQ: "Note, however, that there is no guarantee that all data written to a filtering_stream will be forwarded to the final Sink until the stream is closed, unless all the Filters in the underlying chain are Flushable." My questions: 1) How does one make a symmetric_filter Flushable? 2) Is this the best way to do this sort of on-the-fly hash calculation? I've included example code below. Thank you in advance for any advice! Darren #include "boost/iostreams/filtering_stream.hpp" #include "boost/iostreams/filter/symmetric.hpp" #include <iostream> using namespace std; class MySymmetricFilter { public: typedef char char_type; bool filter(const char*& src_begin, const char* src_end, char*& dest_begin, char* dest_end, bool flush) { // just copy everything for (; src_begin!=src_end && dest_begin!=dest_end ; ++src_begin, ++dest_begin) *dest_begin = *src_begin; // do some hash calculation on bytes copied return false; } void close() {} }; struct MyFilter : public boost::iostreams::symmetric_filter<MySymmetricFilter> { typedef boost::iostreams::symmetric_filter<MySymmetricFilter> base_type; MyFilter(int bufferSize) : base_type(bufferSize) {} }; int main() { boost::iostreams::filtering_ostream fos; fos.push(MyFilter(40)); // buffer size == 40 fos.push(cout); fos << "The quick brown fox jumps over the lazy dog" << flush; // length == 43 cout << "123456789 "; flush(fos); // no effect? return 0; // prints "The quick brown fox jumps over the lazy 123456789 dog" // last 3 bytes "dog" are printed when fos goes out of scope } Darren Kessner Scientific Programmer Darren.Kessner@cshs.org Spielberg Family Center for Applied Proteomics Cedars-Sinai Medical Center http://www.sfcap.cshs.org/ IMPORTANT WARNING: This message is intended for the use of the person or entity to which it is addressed and may contain information that is privileged and confidential, the disclosure of which is governed by applicable law. If the reader of this message is not the intended recipient, or the employee or agent responsible for delivering it to the intended recipient, you are hereby notified that any dissemination, distribution or copying of this information is STRICTLY PROHIBITED. If you have received this message in error, please notify us immediately by calling (310) 423-6428 and destroy the related message. Thank You for your cooperation.