从函数返回时错误提升boosting_ostream

Question

just a quick question regarding boost::iostream::filtering_ostream().

I have a function which (internally) creates a shared_ptr to a boost::iostream::filtering_ostream, and returns a shared pointer to a std::ostream.

Whenever I don't use a compressor in the function, everything seems to work fine, but as soon as I add a compressor, the output file becomes corrupt. If I write the text within the 'getOutputStreamComp' function, everything works then also.

The example below just writes some numbers to a file, just as a POC.

#include <iostream>
#include <string>
#include <fstream>
#include <boost/iostreams/filtering_stream.hpp>
#include <boost/iostreams/device/file.hpp>
#include <boost/iostreams/filter/gzip.hpp>
#include <boost/shared_ptr.hpp>

boost::shared_ptr<std::ostream> getOutputStream(const std::string& fileName)
{
    boost::shared_ptr<boost::iostreams::filtering_ostream> out(boost::shared_ptr<boost::iostreams::filtering_ostream>(new boost::iostreams::filtering_ostream()));
    out->push(boost::iostreams::file_sink(fileName),std::ofstream::binary);

    return out;
}

boost::shared_ptr<std::ostream> getOutputStreamComp(const std::string& fileName)
{
    boost::shared_ptr<boost::iostreams::filtering_ostream> out(boost::shared_ptr<boost::iostreams::filtering_ostream>(new boost::iostreams::filtering_ostream()));
    out->push(boost::iostreams::gzip_compressor());
    out->push(boost::iostreams::file_sink(fileName),std::ofstream::binary);

    return out;
}

int main(int argc, char** argv)
{
    boost::shared_ptr<std::ostream> outFile     = getOutputStream("test.txt");
    boost::shared_ptr<std::ostream> outFileComp = getOutputStreamComp("testcomp.txt.gz");

    // This file is fine.
    for (size_t i(0); i < 10000; ++i)
    {
        *outFile << "i: " << i << std::endl;
    }

    // This file is corrupt.
    for (size_t i(0); i < 10000; ++i)
    {
        *outFileComp << "i: " << i << std::endl;
    }
}

Any ideas that you might have would be gratefully received!

Thanks,

Dave

Answer 1

After fixing the typos ( *outFile and *outFileComp ), I cannot reproduce this: compiling with g++ 4.8.1/boost-1.53, it runs and produces two good files: test.txt with 10,000 lines (file size 78,890) and testcomp.txt.gz (file size 22,064) which unpacks with gunzip into the exact copy of test.txt .

Perhaps in your real program, you tried examining the file before the program ended (or at least before the last reference to the shared_ptr was gone)? A common gotcha of compressed filtered streams is that flushing outFileComp as you do with std::endl does not force the complete writeout of the compressed file so far.

Answer 2

Okay, so I found the issue, with help from Cubbi - the following line(which appears twice):

out->push(boost::iostreams::file_sink(fileName),std::ofstream::binary);

should have been:

out->push(boost::iostreams::file_sink(fileName,std::ofstream::binary));

The code was still compiling, so I was probably setting a flag somewhere in the filtering_ostream, which may have led to issues further down the line.

This fix allows the file to be created as expected.