[Boost-users] [Boost.Iostreams] Question about gzip codec and dual_use filter

From: Marc Viala Perso (marc.viala_at_[hidden])
Date: 2008-01-18 16:08:54

Hello,

I'm just wondering why zlib and bzip2 codec are "dual_use" filter but not
gzip? Can anybody enlighten me on this restriction? 

In fact, I have the following use case: 
I need to manage an "iostream" instance for interfacing with an external
library but I would like to be able to support the GZIP codec during
writing/reading operations, that is, I must deal with the following
"external" serializer engine:

        SerializerEngine ar(std::iostream& ios, OpeningMode mode)
        The SerializerEngine interface provides some read/write raw
operations.

For doing that, I've just tried to implement the following class:

class gzfstream: public
boost::iostreams::filtering_stream<boost::iostreams::dual_use>
{
public:
  template <class Path>
  gzfstream(const Path& file_path, std::ios_base::openmode mode):
    _file(file_path, mode)
  {
    if( _file.fail() ) return ;

    namespace fs = boost::filesystem ;
    namespace io = boost::iostreams ;
    
    const typename Path::string_type& ext = fs::extension(file_path) ;
    if( gz_extension_exists(ext) ) {
      const bool in_op  = (mode & std::ios::in)  != 0 ;
      const bool out_op = (mode & std::ios::out) != 0 ;
      if( in_op && out_op ) 
        throw std::logic_error("can't support input/output operation with
codec") ;
      if( out_op ) 	push(io::gzip_compressor()) ;
      else            	push(io::gzip_decompressor()) ;
    }
    push(_file) ;
  }  

  ~gzfstream()
  { reset() ; }
  
private:
  boost::filesystem::fstream _file ;

  bool gz_extension_exists(const std::string& ext)
  { return ext.size() == 3 &&  ext == ".gz" ; }

  bool gz_extension_exists(const std::wstring& ext)
  { return ext.size() == 3 &&  ext == L".gz" ; }
} ;

But this class doesn't compile only for GZIP codec because, as explained in
the documentation, GZIP doesn't support dual_use concept - contrary to the
ZLib and BZip2 codecs. Is there any issue to deal with my use case? 

Any help will be appreciate?

Best regards,

Marc VIALA