boost::iostreams, gzip files and tellg

问题:

I have been using boost::iostreams for reading an uncompressed text file. In my application, I need multiple file handles (stored in a map) to efficiently buffer data for different parameters stored in this file. Also, if I read a line and it is for a parameter at a time later than I am currently interested in, I restore the stream position (recovered via tellg()) to where it was before I called getline() so I can still buffer this value at a future time. However, I now wish to read gzip compressed files, but otherwise perform identical operations as before. I have run across the following issues (discovered before, but the solution does not seem to work with my triplet of requirements). A short test main() that reproduces these issues follows:

#include <iostream>
#include <iomanip>
#include <fstream>
#include <string>
#include <boost/shared_ptr.hpp>// shared_ptr
#include <boost/iostreams/filtering_stream.hpp>// filtering_[io]stream
#include <boost/iostreams/filter/gzip.hpp>// gzip

//-----------------------------------------------------------------------------

int main( int argc, char** argv ){

    std::cout << std::scientific << std::setprecision(15) << std::endl;
    std::string fileName("test.txt.gz");
    bool gzipped(true);
    std::string line;

    // TEST 1
    boost::shared_ptr<std::ifstream> fileStream1;
    boost::shared_ptr<boost::iostreams::filtering_istream> fileFilter1;
    fileFilter1.reset( new boost::iostreams::filtering_istream );
    fileStream1.reset( new std::ifstream( fileName.c_str() ) );
    if(gzipped)
        fileFilter1->push( boost::iostreams::gzip_decompressor() );
    fileFilter1->push( *fileStream1 );
    while( std::getline( *fileFilter1, line ) ){
        //std::streampos strPos( fileFilter1->tellg() );// uncomment this line for run-time errors
        std::cout << line << std::endl;
    }
    std::cout << std::endl;

    // TEST 2
    boost::shared_ptr<std::ifstream> fileStream2;
    boost::shared_ptr<boost::iostreams::filtering_stream<boost::iostreams::input_seekable> > fileFilter2;
    fileFilter2.reset( new boost::iostreams::filtering_stream<boost::iostreams::input_seekable>() );
    fileStream2.reset( new std::ifstream( fileName.c_str() ) );
    //fileFilter2->push( boost::iostreams::gzip_decompressor() );// uncomment this line for compile-time errors
    fileFilter2->push( *fileStream2 );
    while( std::getline( *fileFilter2, line ) ){
        std::streampos strPos( fileFilter2->tellg() );
        std::cout << line << std::endl;
    }
    std::cout << std::endl;

    return 0;
}

The input file in this case can obviously contain whatever you like. Just make sure it has >1 lines of text to see the tellg() issue.

In TEST1, I can only imagine that the errors are caused by tellg()'s sentry creation and failbit modification (http://www.cplusplus.com/reference/istream/istream/sentry/)?!? This post (https://svn.boost.org/trac/boost/ticket/2449) suggests that:

boost::iostreams::filtering_stream<boost::iostreams::input_seekable>

will overcome the tellg() issue. However, TEST2 shows that I am unable to push a decompressor object onto this type of filter. I have not found a workaround for this.

This is my first time posting here, so apologies if my question has formatting issues!

Many thanks for any help you can provide, and any time you may spend musing!