Re: [Boost-users] using filtering_istream for decompression

From: Robert Vinluan (rvinluan_at_[hidden])
Date: 2006-02-15 16:11:49

Hi Ed,

Couldn't you use the iostreams copy() algorithm?

So I guess your code would look something like:

        io::filtering_istream decompressFilter;
        decompressFilter.push(io::gzip_decompressor());
                  
        std::strstream streamOfCompressedData;
        streamOfCompressedData.write((char*)compressedHTML, 586);
        decompressFilter.push(streamOfCompressedData);
                                  
        std::ostrstream output;
        io::copy(decompressFilter, output);

I haven't actually tested this code snippet, but it should get you what 
you are looking for.  You probably still have to flush the data by 
calling sync().

Hope this helps.

--Rob

Ed Sykes wrote:
> Hi,
> 
> I'm trying to use a filtering istream with a gzip decompressor attached.
> I've got it all working but the one part that I am having trouble with
> is figuring out the most effective way of consuming the decompressed
> data. I'd like to be able to get hold of all the decompressed data in
> one go, rather than reading out a word at a time using the >> operator.
> It seems to me that the most logical way of doing it, given what I have
> read about the way streams work, would be to use rdbuf() to get the
> streambuffer and then send that to a stringstream. However when I try
> this the filtering_istream throws an exception of type
> boost::iostreams::gzip_error. I do know however that the data integrity
> is fine since I am able to use >> to push data into a std::string.
> 
> Instinctively I realise that the filtering_istream doesn't have an
> internal buffer as such and using the >> operator pulls data from the
> source, through the decompressor and into the type on the right hand
> side. This leaves me confused as to what the best way of getting the
> data out is if I can't use the filtering_istream as a typical stream
> inheriting from basic_istream. 
> 
> I think that perhaps I'm misunderstanding a fundamental principle of how
> to use the filtering_istream and I'm hoping that someone on the list can
> point me in the right direction of how I should achieve a read of all
> the decompressed data. So does anyone have any idea how I should
> proceed? I've included the code I have thus far below.
> 
> Thanks in advance, Ed.
> 
> 
> 
> #include "zlib.h"
> #include <boost/iostreams/filtering_streambuf.hpp>
> #include <boost/iostreams/filtering_stream.hpp>
> #include <boost/iostreams/filter/gzip.hpp> 
> #include <boost/iostreams/copy.hpp>
> #include <strstream>
> #include <string>
> 
> namespace io = boost::iostreams;
> 
> 	void DecompressIntoStringStream()
> 	{
> 		unsigned char compressedHTML[] = {
> 		0x1f, 0x8b, 
> 		0x08, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x03, 
> 		0xec, 0x7d, 0x7b, 0x77, 0xdb, 0xb6, 0xb2, 0xef, 
> 		0xdf, 0xf5, 0x5a, 0xfb, 0x3b, 0x30, 0xca, 0xae, 
> 		0x65, 0xd7, 0x7a, 0xbf, 0x6c, 0x4b, 0xa6, 0x7b, 
> 		0x1c, 0x3f, 0x92, 0xb4, 0x79, 0x78, 0xc7, 0x4e, 
> 		0xb3, 0x7b, 0x7b, 0x7b, 0xb2, 0x28, 0x92, 0xb2, 
> 		0x18, 0x53, 0x24, 0x43, 0x52, 0x72, 0x14, 0x27, 
> 		0x5f, 0xe3, 0x7e, 0x81, 0xf3, 0x45, 0xef, 0x6f, 
> 		0x06, 0x00, 0x09, 0x4a, 0xf2, 0x2b, 0xe9, 0x3e, 
> 		0xfb, 0xdc, 0xb5, 0x6e, 0xbb, 0x22, 0x93, 0x20, 
> 		0x1e, 0x83, 0xc1, 0x60, 0x30, 0x98, 0x19, 0x0c, 
> 		0xf6, 0x1e, 0x1d, 0xbd, 0x3e, 0x3c, 0xff, 0xfd, 
> 		0xf4, 0xd8, 0x78, 0x76, 0xfe, 0xf2, 0x85, 0x71, 
> 		0xfa, 0xf6, 0xc9, 0x8b, 0xe7, 0x87, 0x46, 0xa9,
> 		0x5a, 0xaf, 0xbf, 0x6b, 0x1f, 0xd6, 0xeb, 0x47, 
> 		0xe7, 0x47, 0xe2, 0x43, 0xa7, 0xd6, 0x68, 0x1a, 
> 		0xe7, 0xb1, 0x15, 0x24, 0x5e, 0xea, 0x85, 0x81, 
> 		0xe5, 0xd7, 0xeb, 0xc7, 0xaf, 0x4a, 0x46, 0x69, 
> 		0x9c, 0xa6, 0x51, 0xbf, 0x5e, 0xbf, 0xba, 0xba, 
> 		0xaa, 0x5d, 0xb5, 0x6b, 0x61, 0x7c, 0x51, 0x3f, 
> 		0x7f, 0x53, 0x1f, 0xa7, 0x13, 0xbf, 0x53, 0xf7, 
> 		0xc3, 0x30, 0x71, 0x6b, 0x4e, 0xea, 0x94, 0xf6, 
> 		0xd7, 0xf6, 0x28, 0xc9, 0xf0, 0xad, 0xe0, 0xc2, 
> 		0x2c, 0xb9, 0x41, 0xf5, 0xe9, 0x13, 0x4e, 0x73, 
> 		0x2d, 0x07, 0x9f, 0x12, 0x3b, 0xf6, 0xa2, 0x94, 
> 		0x3f, 0x4e, 0xad, 0x0b, 0xd7, 0xfc, 0x60, 0xcd, 
> 		0x2c, 0x91, 0xb6, 0xbf, 0x36, 0xb3, 0x62, 0x23, 
> 		0x08, 0xaf, 0xcc, 0xc0, 0xbd, 0x32, 0x8e, 0xac, 
> 		0xd4, 0xad, 0xa4, 0x4d, 0xb3, 0x51, 0x49, 0x5b, 
> 		0xf4, 0xd3, 0xa6, 0x9f, 0x0e, 0xfd, 0x74, 0xe9, 
> 		0xa7, 0x87, 0x9f, 0xf7, 0x9e, 0x4b, 0xcf, 0xe3, 
> 		0x08, 0x3f, 0x56, 0x88, 0x1f, 0xdb, 0x36, 0xcb, 
> 		0xe5, 0xca, 0xdc, 0x8f, 0xf0, 0x67, 0x80, 0xb2, 
> 		0xa8, 0xab, 0x76, 0xe1, 0xa6, 0xe7, 0xde, 0xc4, 
> 		0xdd, 0xd8, 0x1c, 0x70, 0xf5, 0x33, 0x37, 0x36, 
> 		0x4b, 0xcd, 0xd2, 0x60, 0x6d, 0xaf, 0xae, 0x5a, 
> 		0xdd, 0x4b, 0xbd, 0xd4, 0x77, 0xf7, 0x7f, 0xb7, 
> 		0xc6, 0x61, 0xf8, 0xc8, 0x78, 0xfb, 0xab, 0xb1, 
> 		0x6e, 0x4d, 0xa2, 0x81, 0xf1, 0x3c, 0x76, 0xd1, 
> 		0x03, 0x67, 0xaf, 0x2e, 0x3e, 0xef, 0x4d, 0xdc, 
> 		0xd4, 0x32, 0x08, 0x01, 0x55, 0xf7, 0xe3, 0xd4, 
> 		0x9b, 0x99, 0xa5, 0xc3, 0x30, 0x48, 0xdd, 0x20, 
> 		0xad, 0x9e, 0xcf, 0x23, 0xb7, 0x64, 0xd8, 0xe2, 
> 		0xcd, 0x2c, 0xa5, 0xee, 0xa7, 0x94, 0x71, 0x32, 
> 		0x30, 0xec, 0xb1, 0x15, 0x27, 0x6e, 0x6a, 0x7a, 
> 		0x49, 0x58, 0xdd, 0xd9, 0xe9, 0xee, 0x56, 0x9b, 
> 		0x84, 0x88, 0xa5, 0x8a, 0x4e, 0x9f, 0x1f, 0x9e, 
> 		0x55, 0x5f, 0x58, 0x43, 0xd7, 0xcf, 0xab, 0x29, 
> 		0x6f, 0x70, 0x6a, 0xb3, 0xd6, 0x2c, 0x60, 0xdd, 
> 		0xb3, 0x63, 0x8b, 0xf1, 0x1e, 0x5b, 0xa9, 0x17, 
> 		0x5c, 0x24, 0xb3, 0x46, 0xab, 0x46, 0xc8, 0x2e, 
> 		0x19, 0xbe, 0x11, 0x1b, 0x1b, 0xf6, 0x67, 0xa3, 
> 		0x69, 0xf8, 0xf4, 0x13, 0xd0, 0x4f, 0x48, 0x3f, 
> 		0x33, 0xfc, 0x6c, 0x1a, 0x17, 0x6e, 0x60, 0xa4, 
> 		0xf1, 0xd4, 0x35, 0x46, 0x61, 0x9c, 0xd5, 0x38, 
> 		0xbd, 0xac, 0xcd, 0xa9, 0xd7, 0x35, 0x3b, 0x9c, 
> 		0x94, 0x6e, 0x2f, 0xaf, 0x0f, 0x7d, 0x9c, 0x58, 
> 		0x76, 0x11, 0x88, 0xa6, 0x0e, 0x44, 0x60, 0x34, 
> 		0x8c, 0x04, 0xff, 0x66, 0xf8, 0xe7, 0x1b, 0x8d, 
> 		0xfb, 0xb6, 0xbd, 0x58, 0x6c, 0xb3, 0xbc, 0x2f, 
> 		0x50, 0x15, 0x58, 0x13, 0xd7, 0x2c, 0x39, 0xae, 
> 		0x18, 0x2f, 0x90, 0x63, 0x8e, 0xa5, 0xd2, 0x0d, 
> 		0x83, 0x66, 0x3c, 0x0b, 0x27, 0xae, 0x71, 0x0a, 
> 		0xfa, 0x32, 0xfe, 0x8f, 0x71, 0xe6, 0x5a, 0xb1, 
> 		0x3d, 0x36, 0xd2, 0xb1, 0x6b, 0x5c, 0xb9, 0x43, 
> 		0x03, 0x63, 0xca, 0xcf, 0xb2, 0xe8, 0x3b, 0x24, 
> 		0x1d, 0x79, 0xb1, 0x6b, 0xa7, 0x61, 0x3c, 0xaf, 
> 		0x18, 0x89, 0x77, 0x11, 0x18, 0xd3, 0x88, 0xd1, 
> 		0x64, 0x19, 0xa3, 0xd8, 0x75, 0x0d, 0x99, 0xd1, 
> 		0x9d, 0x58, 0x9e, 0x6f, 0x58, 0xb6, 0x1d, 0x4e, 
> 		0x83, 0x94, 0x6b, 0x19, 0x79, 0xb2, 0x2a, 0x1f, 
> 		0xb4, 0x9a, 0xa4, 0xc8, 0x1d, 0x4e, 0x54, 0xee, 
> 		0x13, 0x2f, 0xb0, 0x02, 0xdb, 0xad, 0x18, 0x67};
> 
> 		io::filtering_istream decompressFilter;
> 		decompressFilter.push(io::gzip_decompressor());
> 		
> 		std::strstream streamOfCompressedData;
> 		streamOfCompressedData.write((char*)compressedHTML,
> 586);
> 		decompressFilter.push(streamOfCompressedData);
> 				
> 		std::stringstream output;
> 		try
> 		{
> 			output << decompressFilter.rdbuf();
> 		}
> 		catch(std::exception& exc)
> 		{
> 			exc;
> 			// dummy code to allow me to inspect exc:
> 			int i = 0;
> 			i;
> 		}
> 	}