Zlib Filters

Overview
Acknowledgments
Headers
Synopsis
Reference
namespace boost::iostreams::zlib
Class zlib_params
Class template zlib_compressor
Class template zlib_decompressor
Class zlib_error
Examples
Installation

Overview

The class templates basic_zlib_compressor and basic_zlib_decompressor perform compression and decompression in the ZLIB format ([Deutsch1]) using Jean-loup Gailly's and Mark Adler's zlib compression library ([Gailly]).

The zlib Filters are DualUseFilters so that they may be used for either input or output. Most commonly, however, the compression Filters will be used for output and the decompression Filters for input.

Acknowledgments

The zlib Filters were influences by the work of Jeff Garland ([Garland]) and Jonathan de Halleux ([de Halleux]).

Thanks to Jean-loup Gailly and Mark Adler for making their excellent library available to the public with a Boost-compatible license.

Headers

<boost/iostreams/filter/zlib.hpp>

Synopsis

namespace boost { namespace iostreams {

namespace zlib {
                    
    // Compression levels

extern const int no_compression;
extern const int best_speed;
extern const int best_compression;
extern const int default_compression;

    // Compression methods

extern const int deflated;

    // Compression strategies

extern const int default_strategy;
extern const int filtered;
extern const int huffman_only;

    // Status codes

extern const int stream_error;
extern const int version_error;
extern const int data_error;
extern const int buf_error;

}   // End namespace boost::iostreams::zlib

struct zlib_params;

template<typename Alloc = std::allocator<char> >
struct basic_zlib_compressor;

template<typename Alloc = std::allocator<char> >
struct basic_zlib_decompressor;

typedef basic_zlib_compressor<>   zlib_compressor;
typedef basic_zlib_decompressor<> zlib_decompressor;

class zlib_error;

} } // End namespace boost::io

Reference

Namespace boost::iostreams::zlib

The namespace boost::iostreams::zlib contains integral constants used to configure zlib Filters and to report errors. The constants have the following interpretations. (See [Gailly] for additional details.)

ConstantInterpretation
no_compression Compression-level constant specifying that no compression should be performed. Equal to Z_NO_COMPRESSION.
best_speed Compression-level constant requesting the fasted compression. Equal to Z_BEST_SPEED.
best_compression. Compression-level constant requesting the best compression ratio. Equal to Z_BEST_COMPRESSION.
default_compression Default compression level. Equal to Z_DEFAULT_COMPRESSION.
deflated Compression method constant; currently the only supported method. Equal to Z_DEFLATED.
default_strategy Compression strategy constant. Equal to Z_DEFAULT_STRATEGY.
filtered Compression strategy constant. Equal to Z_FILTERED.
huffman_only Compression strategy constant. Equal to Z_HUFFMAN_ONLY.
stream_error Error code used to indicate that one of the members of zlib_params is invalid; may also indicate an internal Iostreams library error. Equal to Z_STREAM_ERROR.
version_error Error code used to indicate that data was compressed with an incompatible version of zlib. Equal to Z_VERSION_ERROR.
data_error Error code used to indicate that a compressed datastream is corrupted. Equal to Z_DATA_ERROR.
buf_error Error code indicating an internal Iostreams library error. Equal to Z_BUF_ERROR.

Class zlib_params

Description

Encapsulates the parameters used to configure basic_zlib_compressor and basic_zlib_decompressor.

Synopsis

struct zlib_params {

    // Non-explicit constructor
    zlib_params( int level           = zlib::default_compression,
                 int method          = zlib::deflated,
                 int window_bits     = default value, 
                 int mem_level       = default value, 
                 int strategy        = zlib::default_strategy,
                 bool noheader       = false );
    int  level;
    int  method;
    int  window_bits;
    int  mem_level;
    int  strategy;
    bool noheader;
};

zlib_params::zlib_params

    zlib_params( int level           = zlib::default_compression,
                 int method          = zlib::deflated,
                 int window_bits     = default value, 
                 int mem_level       = default value, 
                 int strategy        = zlib::default_strategy,
                 bool noheader       = false );

Constructs a zlib_params object, where the parameters have the following interpretations:
level- Compression level. Must be equal to zlib::default_compression or a value in the range 0-9. The value 0 yields no compression, while 9 yields the best compression ratio. Affects compression only.
method- Compression method. Must equal zlib::deflated. Affects compression only.
window_bits- The base two logarithm of the window size. Must be in the range 8-15; defaults to 15.
mem_level- Specifies the amount of memory to be used. Must be in the range 1-9; defaults to 8. Affects compression only.
strategy- Must be zlib::default_strategy, zlib::filtered or zlib::huffman_only. Affects compression only.
noheader- True if the ZLIB header and trailing ADLER-32 checksum should be omitted (see [Deutsch1]). This results in compression according to the deflate specification (see [Deutsch2]).

See [Gailly] for additional details.

Class template basic_zlib_compressor

Description

Model of DualUseFilter which performs compression using zlib ([Gailly]).

Synopsis

template<typename Alloc = std::allocator<char> >
struct basic_zlib_compressor {
    typedef char                    char_type;
    typedef implementation-defined  category;

    basic_zlib_compressor( const zlib_params& = zlib::default_compression, 
                           std::streamsize buffer_size = default value );

    // DualUseFilter members.
};

typedef basic_zlib_compressor<> zlib_compressor;

Template Parameters

Alloc- A C++ standard library allocator type ([ISO], 20.1.5), used to allocate a character buffer and to configure zlib.

basic_zlib_compressor::basic_zlib_compressor

    basic_zlib_compressor( const zlib_params& = zlib::default_compression, 
                           std::streamsize buffer_size = default value );

Constructs an instance of basic_zlib_compressor with the given parameters and buffer size. Since a zlib_params object is implicitly constructible from an int representing a compression level, an int may be passed as the first constructor argument.

Class template basic_zlib_decompressor

Description

Model of DualUseFilter which performs decompression using zlib ([Gailly]).

Synopsis

template<typename Alloc = std::allocator<char> >
struct basic_zlib_decompressor {
    typedef char                    char_type;
    typedef implementation-defined  category;

    basic_zlib_decompressor( int window_bits = default value, 
                             std::streamsize buffer_size = 
                                 default value );
    basic_zlib_decompressor( const zlib_params&, 
                             std::streamsize buffer_size = 
                                 default value );

    // DualUseFilter members.
};

typedef basic_zlib_decompressor<> zlib_decompressor;

Template Parameters

Alloc- A C++ standard library allocator type ([ISO], 20.1.5), used to allocate a character buffer and to configure zlib.

basic_zlib_decompressor::basic_zlib_decompressor

    basic_zlib_decompressor( int window_bits = default value, 
                             std::streamsize buffer_size = 
                                 default value );
    basic_zlib_decompressor( const zlib_params&, 
                             std::streamsize buffer_size = 
                                 default value );

The first member constructs an instance of basic_zlib_decompressor with the given parameters and buffer size. The second member constructs an instance of basic_zlib_decompressor with the given window bits value and buffer size. Other parameters affecting decompression are set to default values.

Class zlib_error

Description

Used by the zlib Filters to report errors.

Synopsis

class zlib_error : public std::ios_base::failure {
public:
    zlib_error(int error);
    int error() const;
};

zlib_error::zlib_error

    zlib_error(int error);

Constructs an instance of zlib_error with the given error code from the namespace boost::iostreams::zlib.

zlib_error::error

    void error() const;

Returns an error code from the namespace boost::iostreams::zlib.

Examples

The following code decompresses data from a file and writes it to standard output. 
#include <fstream>
#include <iostream>
#include <boost/iostreams/filtering_streambuf.hpp>
#include <boost/iostreams/copy.hpp>
#include <boost/iostreams/filter/zlib.hpp>

int main() 
{
    using namespace std;

    ifstream file("hello.z", ios_base::in | ios_base::binary);
    filtering_streambuf<input> in;
    in.push(zlib_decompressor());
    in.push(file);
    boost::iostreams::copy(in, cout);
}

Installation

The zlib Filters depend on the third-party zlib library, which is not included in the Boost distribution. Prebuilt zlib binaries are available on most UNIX and UNIX-like systems, and will be found automatically by the Boost build system. Windows users can obtain prebuilt binaries at the zlib homepage. Users can also configure the Boost Iostream library to build zlib from the source code, which is available at the zlib homepage. For details on configuring the build system to find your zlib installation, please see Installation.

Revised 02 Feb 2008

© Copyright 2008 CodeRage, LLC
© Copyright 2004-2007 Jonathan Turkanis

Distributed under the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)