Ticket #4590: boost_iostreams_filtering_ostream.cpp

// Test case for flushing a filtering_ostream based on the
// back_inserter example in
// <http://www.boost.org/doc/libs/1_44_0/libs/iostreams/doc/tutorial/container_sink.html>
// augmented by a transparent output filter simplified from the
// example given in
// <http://www.boost.org/doc/libs/1_44_0/libs/iostreams/doc/concepts/output_filter.html>.
//
// Flushing a filtering_ostream using the flush() member function
// stopped working in Boost 1.44. The string the back_inserter writes
// to stays empty. A valid workaround seems to call strict_sync(), as
// it leads to the expected result. The member function sync()
// documented in
// <http://www.boost.org/doc/libs/1_44_0/libs/iostreams/doc/classes/filtering_stream.html>
// does not seem to exist.
//

#include <cassert>
#include <string>
#include <boost/iostreams/categories.hpp>
#include <boost/iostreams/operations.hpp>
#include <boost/iostreams/device/back_inserter.hpp>
#include <boost/iostreams/filtering_stream.hpp>

#include <iostream>

namespace io = boost::iostreams;

// Simple output filter forwarding just forwarding the put chars to
// the sink.
struct transparent_output_filter {
  typedef char                   char_type;
  typedef io::output_filter_tag  category;

  template<typename Sink>
  bool put(Sink& snk, char c)
  {
    return io::put(snk, c);
  }
};

int main(int argc, char *argv[])
{
  using namespace std;

  string result;

  io::filtering_stream<io::output>  out;

  // Transparent filter just forward the characters to the sink. If it
  // is left out from the filter chain, out.flush() works correctly
  // and result has the expected value.
  out.push(transparent_output_filter());

  // Sink appends characters to string result.
  out.push(io::back_inserter(result));

  // Write test string to filtering_ostream.
  out << "Hello World!";

  // The flush only works in Boost 1.44 if there's only the sink
  // device in the chain. Until Boost 1.43 this used to work.
  out.flush();

  // The documented sync() member function leads to a compile error
  // "‘class
  // boost::iostreams::filtering_stream<boost::iostreams::output,
  // char, std::char_traits<char>, std::allocator<char>,
  // boost::iostreams::public_>’ has no member named ‘sync’".
  //out.sync();

  // Calling strict_sync() instead of flush() or sync() results in the
  // characters to be appended to the result string also in Boost 1.44.
  //out.strict_sync();

  // Check result.
  cout << "result = \"" << result << "\"" << endl;
  assert(result == "Hello World!");

  return 0;
}