Re: [Boost-users] [iostreams] How can I use the Stream Insertion operator (<<) with a filtering_streambuf?

Subject: Re: [Boost-users] [iostreams] How can I use the Stream Insertion operator (<<) with a filtering_streambuf?
From: Ken Appleby (ken.appleby_at_[hidden])
Date: 2013-07-26 13:06:47

On 26/07/2013 14:49, Larry Evans wrote:
> On 07/26/13 06:39, Frédéric Bron wrote:
>> This is how I do it with file_sink:
>>
>> boost::iostreams::filtering_ostream output_file;
>> output_file.push(boost::iostreams::bzip2_compressor(boost::iostreams::bzip2_params(9)));
>> output_file.push(boost::iostreams::file_sink("file.bz2",
>> std::ios_base::binary));
>> output_file<<"blalba";
>>
>> I think if you do just this, nothing happen until output_file is
>> destroyed. So enclose in { }.
>>
>> Frédéric
>>
> Thanks Frederic.
>
> However, I think this is not obvious.  I would have thought Ken's
> solution would have worked.  I would have also thought mine would
> have worked until I saw Ken's which indicated std::ios_base::binary
> was needed when the std::ofstream was created.
>
> Is there anywhere in the docs explaining why your method works
> and Ken's doesn't?
>
> -regards,
> Larry

Larry,

Passing the filtering_streambuf to a stream constructor should indeed 
work, as you expect.

The test program below works fine for me - boost 1_49 on mingw 4.4.

Ken Appleby

Soft Optics Ltd

-------------------

#include <iostream>
#include <fstream>

#include <boost/iostreams/filtering_streambuf.hpp>
#include <boost/iostreams/copy.hpp>
#include <boost/iostreams/filter/gzip.hpp>

int main()
{
     try
     {
         {
             std::ofstream myFile("hello.gz", std::ios_base::out | 
std::ios_base::binary);
 
boost::iostreams::filtering_streambuf<boost::iostreams::output> out;
             out.push(boost::iostreams::gzip_compressor());
             out.push( myFile );

             std::ostream os(&out);
             os << "text I want to be compressed and end up in the file...";
         }
         {
             std::ifstream myFile( "hello.gz", std::ios::in | 
std::ios::binary );
             boost::iostreams::filtering_streambuf< 
boost::iostreams::input > in;
             in.push( boost::iostreams::gzip_decompressor() );
             in.push( myFile );
             std::istream is( &in );
             std::string text;
             while ( is.good() )
             {
                 char ch = is.get();
                 if ( is.good() )
                     text.push_back( ch );
             }

             std::cout << "text from file is \"" << text << "\"" << 
std::endl << std::flush;
         }
     } catch ( std::exception& e )
     {
         std::cerr << e.what() << std::endl;
     }
}