[Boost-users] [iostreams] Closing a filtering_ostream

From: Paul Giaccone (paulg_at_[hidden])
Date: 2005-10-04 11:35:34

How do you close a filtering_ostream object once you have finished 
writing to it?

In my program below, I would expect the file "hello.bz2" to have been 
written once the close() function has been called, but it is not the 
case.  The contents are not actually written until I exit the scope in 
which the filtering_ostream object is defined (at which point the 
destructor presumably does the job).  (The file "some_text.txt" contains 
just the text "hello world!" so I would expect it not to be written 
until the file is closed.)

I would like to be able to do this explicitly and not have to rely on 
the destruction of the object - could you explain how can this be done?

Another question: what should the second parameter to close() be?  I've 
used BOOST_IOS::trunc here, but is there some other more appropriate value?

Thanks,

Paul Giaccone

#include <fstream>
#include <iostream>
#include <boost/iostreams/copy.hpp>
#include <boost/iostreams/filter/bzip2.hpp>
#include <boost/iostreams/device/file.hpp>
#include <boost/iostreams/filtering_stream.hpp>

int main(void)
{
    {
    boost::iostreams::filtering_ostream out;
    out.push(boost::iostreams::bzip2_compressor());
    out.push(boost::iostreams::file_sink("hello.bz2", BOOST_IOS::trunc));
    std::ifstream input_stream("some_text.txt");
    boost::iostreams::copy(input_stream, out);
    input_stream.close();

    if (!boost::iostreams::flush(out))
    {
        std::cerr << "failed to write file" << std::endl;
    }

    boost::iostreams::close(out, BOOST_IOS::_Dummy_enum_val);
    }    //exiting scope closes the stream; close() seems not to

    boost::iostreams::filtering_istream in;
    in.push(boost::iostreams::bzip2_decompressor());
    in.push(boost::iostreams::file_source("hello.bz2"));
    boost::iostreams::copy(in, std::cout);
    std::cout.flush();

    return 0;
}