Why boost::iostream::filtering_ostream using boost::iostreams::zlib_compressor needs to be destroyed for the sink to be written?

Question

After spending quite some time debugging the issue today, I noticed the boost::iostream::filtering_ostream needs to be destroy for the sink to be written. Test code: #include #include #include struct ZlibOstream : boost::iostreams::filtering_ostream { ZlibOstream(std::ostream& os) { boost::iostreams::filtering_ostream::push(boost::iostreams::zlib_compressor{}); boost::iostreams::filtering_ostream::push(os); } }; int main() { std::ostringstream oss; #ifdef HAS_SCOPE { #endif ZlibOstream zlibOstream{oss}; zlibOstream << "This is a test string.\n"; #ifdef HAS_SCOPE } #endif return (oss.tellp() == 0); } Calling flush() doesn't do the trick and I don't need to do this when I remove the zlib_compressor. Result with coliru: https://coliru.stacked-crooked.com/a/7cd166d2d820e838 What would be the reason behind this behavior ? JavaScript questions and answers, JavaScript questions pdf, JavaScript question bank, JavaScript questions and answers pdf, mcq on JavaScript pdf, JavaScript questions and solutions, JavaScript mcq Test , Interview JavaScript questions, JavaScript Questions for Interview, JavaScript MCQ (Multiple Choice Questions)

Answer 1

This is in fact related to this question: Flushing a boost::iostreams::zlib_compressor. How to obtain a "sync flush"? You need a call to boost::iostreams::zlib_compressor::close for the flush to happen. You can achieve this by calling either pop() or reset() on the boost::iostream::filtering_ostream. Note, pop() as its name suggest pop the last filter in the chain and reset() clear the chain completely such that the filtering_ostream won't be usable afterward. Example: #include #include #include struct ZlibOstream : boost::iostreams::filtering_ostream { ZlibOstream(std::ostream& os) { boost::iostreams::filtering_ostream::push(boost::iostreams::zlib_compressor{}); boost::iostreams::filtering_ostream::push(os); } }; int main() { std::ostringstream oss; ZlibOstream zlibOstream{oss}; zlibOstream << "This is a test string.\n"; zlibOstream.reset(); // needed if you want to write to oss return oss.tellp(); }