Simple Zlib C++ String Compression and Decompression

Question

I need a simple compression and decompression of a std::string in C++. I looked at this site and the code is for Character array. What I want to implement are the two functions:

std::string original = "This is to be compressed!!!!";
std::string compressed = string_compress(original);
std::cout << compressed << std::endl;
std::string decompressed = string_decompress(compressed);
std::cout << decompressed << std::endl;

I had tried the boost compression as:

std::string CompressData(const std::string &data)
{
    std::stringstream compressed;
    std::stringstream decompressed;
    decompressed << data;
    boost::iostreams::filtering_streambuf<boost::iostreams::input> out;
    out.push(boost::iostreams::zlib_compressor());
    out.push(decompressed);
    boost::iostreams::copy(out, compressed);
    return compressed.str();
}

std::string DecompressData(const std::string &data)
{
    std::stringstream compressed;
    std::stringstream decompressed;
    compressed << data;
    boost::iostreams::filtering_streambuf<boost::iostreams::input> in;
    in.push(boost::iostreams::zlib_decompressor());
    in.push(compressed);
    boost::iostreams::copy(in, decompressed);
    return decompressed.str();
}

but the code sometimes gives Null characters in string ie \u0000. How do I handle if the compressed data contains these null characters. Is the return type string correct? How can I implement function string_compress and string_decompress using zlib?

Answer 1

You can do as @LawfulEvil suggested. Here is the code snippet that works :)

std::string original = "This is to be compressed!!!!";
std::string compressed_encoded = string_compress_encode(original);
std::cout << compressed_encoded << std::endl;
std::string decompressed_decoded = string_decompress_decode(compressed_encoded);
std::cout << decompressed_decoded << std::endl;

Using this as the base64 encode/decode library.

#include <sstream>
#include <boost/iostreams/filtering_streambuf.hpp>
#include <boost/iostreams/copy.hpp>
#include <boost/iostreams/filter/zlib.hpp>
#include <cpp-base64/base64.h>

std::string string_compress_encode(const std::string &data)
{
    std::stringstream compressed;
    std::stringstream original;
    original << data;
    boost::iostreams::filtering_streambuf<boost::iostreams::input> out;
    out.push(boost::iostreams::zlib_compressor());
    out.push(original);
    boost::iostreams::copy(out, compressed);

    /**need to encode here **/
    std::string compressed_encoded = base64_encode(reinterpret_cast<const unsigned char*>(compressed.c_str()), compressed.length());

    return compressed_encoded;
}

std::string string_decompress_decode(const std::string &data)
{
    std::stringstream compressed_encoded;
    std::stringstream decompressed;
    compressed_encoded << data;

    /** first decode  then decompress **/
    std::string compressed = base64_decode(compressed_encoded);

    boost::iostreams::filtering_streambuf<boost::iostreams::input> in;
    in.push(boost::iostreams::zlib_decompressor());
    in.push(compressed);
    boost::iostreams::copy(in, decompressed);
    return decompressed.str();
}

Answer 2

Compression makes use of all the values available for each byte, so it will appear as 'garbage' or 'weird' characters when attempting to view as ascii. Its expected. You'll need to encode the data for transmission / json packing to avoid nulls. I suggest base 64. Code to do that is available at the link below(which I didn't author so I won't copy here).

1. http://www.adp-gmbh.ch/cpp/common/base64.html
2. Binary data JSONCPP