How to get gzip_params from boost's gzip_decompressor() - c++

Question

I am using the boost gzip_decompressor() from the following link:
How can I read line-by-line using Boost IOStreams' interface for Gzip files?
Reading the gzip file works fine, but how do I read the gzip_params? I want to know the original file name that's stored in the gzip_params.file_name.

Answer 1

Excellent question.
The solution is to use component<N, T> to get a pointer to the actual decompressor instance:
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#include <iostream>
#include <fstream>
#include <boost/iostreams/filtering_stream.hpp>
#include <boost/iostreams/filter/gzip.hpp>
int main()
{
std::ifstream file("file.gz", std::ios_base::in | std::ios_base::binary);
try {
boost::iostreams::filtering_istream in;
using gz_t = boost::iostreams::gzip_decompressor;
in.push(gz_t());
in.push(file);
for(std::string str; std::getline(in, str); )
{
std::cout << "Processed line " << str << '\n';
}
if (gz_t* gz = in.component<0, gz_t>()) {
std::cout << "Original filename: " << gz->file_name() << "\n";
std::cout << "Original mtime: " << gz->mtime() << "\n";
std::cout << "Zip comment: " << gz->comment() << "\n";
}
}
catch(const boost::iostreams::gzip_error& e) {
std::cout << e.what() << '\n';
}
}
Preparing a sample file using
gzip testj.txt
mv testj.txt.gz file.gz
Prints
Processed line Hello world
Original filename: testj.txt
Original mtime: 1518987084
Zip comment: