Question

The POCO Library requires an istream for input and ostream for output to compress data using its zlib wrapper. I have data in a std::vector(unsigned char) and would like to compress this data into another std::vector(unsigned char). Is there any easy way to do this?

Answer 1

I don’t know that this is the most efficient way, but as a start I’d try this:

typedef unsigned char uc;
typedef vector<uc> v;
void Doit(const v& in, v& out)
{
   ostringstream outStream;
   DeflatingOutputStream compressor(outStream, DeflatingStreamBuf::STREAM_GZIP);
   copy(in.begin(), in.end(), ostream_iterator<uc>(compressor));
   compressor.close();
   string outStr(outStream.str());
   out.assign(outStr.begin(), outStr.end());
}

I suspect this copies the data twice, unnecessarily. First, the call to ostringstream::str() makes a copy, and next std::vector::assign() makes a copy.

@Alf P. Steinbach had an excellent suggestion — use Boost stream adapters. If you have boost::iostreams::filtering_ostream available to you, you could try this:

typedef unsigned char uc;
typedef vector<uc> v;

void Doit(const v& in, v& out)
{
   filtering_ostream outStream(back_inserter(out));
   DeflatingOutputStream compressor(outStream, DeflatingStreamBuf::STREAM_GZIP);
   std::copy(in.begin(), in.end(), ostream_iterator<uc>(compressor));
   compressor.close();
   outStream.flush();
}