substream from istream

Question

Suppose I have an ifstream which represents a large file containing lots of sub-files aggregated together. I want to be able to create a "sub" istream from the larger ifstream (given a size and offest) representing a part of the file so other code can read from that substream as if it was an independent istream.

Any ideas on how I might accomplish this?

EDIT - I would prefer to avoid boost.

Answer 1

This is an example of a streambuf "filter" that reads from a contained streambuf starting at a specified location and reading up to a specified size. You create substreambuf, passing your original streambuf in and substreambuf then translates access so that everything is read from the desired location in the underlying streambuf.

Most of the overhead involved in calling sgetc and snextc from underflow and uflow should optimize away. Many extraction operators work byte by byte, so there should not be additional overhead beyond maintaining the read position within the subsection and checking for the end of the subsection. Of course, reading large chunks of data will be less efficient with this class (although that could be fixed).

This still needs improvements like testing that the requested location is within the underlying streambuf.

class substreambuf : public std::streambuf
{
public:

    substreambuf(std::streambuf *sbuf, std::size_t start, std::size_t len) : m_sbuf(sbuf), m_start(start), m_len(len), m_pos(0)
    {
        std::streampos p = m_sbuf->pubseekpos(start);
        assert(p != std::streampos(-1));
        setbuf(NULL, 0);
    }

protected:

    int underflow()
    {
        if (m_pos + std::streamsize(1) >= m_len)
            return traits_type::eof();
        return m_sbuf->sgetc();
    }

    int uflow()
    {
        if (m_pos + std::streamsize(1) > m_len)
            return traits_type::eof();
        m_pos += std::streamsize(1);
        return m_sbuf->sbumpc();
    }

    std::streampos seekoff(std::streamoff off, std::ios_base::seekdir way, std::ios_base::openmode which = std::ios_base::in | std::ios_base::out)
    {
        std::streampos cursor;

        if (way == std::ios_base::beg)
            cursor = off;
        else if (way == std::ios_base::cur)
            cursor = m_pos + off;
        else if (way == std::ios_base::end)
            cursor = m_len - off;

        if (cursor < 0 || cursor >= m_len)
            return std::streampos(-1);
        m_pos = cursor;
        if (m_sbuf->pubseekpos(m_start + m_pos, std::ios_base::beg) == std::streampos(-1))
            return std::streampos(-1);

        return m_pos;
    }

    std::streampos seekpos(std::streampos sp, std::ios_base::openmode which = std::ios_base::in | std::ios_base::out)
    {
        if (sp < 0 || sp >= m_len)
            return std::streampos(-1);
        m_pos = sp;
        if (m_sbuf->pubseekpos(m_start + m_pos, std::ios_base::beg) == std::streampos(-1))
            return std::streampos(-1);
        return m_pos;
    }

private:
    std::streambuf *m_sbuf;
    std::streampos m_start;
    std::streamsize m_len;
    std::streampos m_pos;
};

It can be used like this

using namespace std;

void somefunc(ifstream &bigifs)
{
    substreambuf sbuf(bigifs.rdbuf(),100,100);
    //new istream with the substreambuf as its streambuf
    istream isub(&sbuf);

    //use isub normally
}

This was inspired by Filtering Streambufs

Answer 2

I've done something like this using the Boost.Iostreams library. Look under Tutorial|Writing Devices. The idea is to create a "device" class which implements the low-level interface (read/write/seek) and then instantiate an istream/ostream derived class using your device class to do the actual I/O.