CODEWITHSUNDEEP
c++bufferiostream
Ruben Verborgh
Ruben Verborgh

Reputation: 3665

How to implement seekg/seekpos on an in-memory buffer?

Combining this answer on in-memory streams with this answer on seekoff, I implemented an in-memory buffer as follows:

struct membuf : std::streambuf {
    membuf(char const* base, size_t size) {
        char* p(const_cast<char*>(base));
        this->setg(p, p, p + size);
    }
};
struct imemstream : virtual membuf, std::istream {
    imemstream(char const* base, size_t size) :
        membuf(base, size),
        std::istream(static_cast<std::streambuf*>(this)) {
    }

    std::iostream::pos_type seekoff(std::iostream::off_type off,
                                    std::ios_base::seekdir dir,
                                    std::ios_base::openmode which = std::ios_base::in) {
        if (dir == std::ios_base::cur) gbump(off);
        return gptr() - eback();
    }
};

However, as explained in the comments of the first aswer, we still need to get seekg/seekpos working. So how to correctly implement seekpos here?

PS This question goes in the same direction, but gives a more specific answer.

Upvotes: 2

Views: 1778

Answers (1)

Jan Michael Auer
Jan Michael Auer

Reputation: 731

Since this hasn't been answered, I'm posting the implementation proposed here. 

pos_type seekpos(pos_type sp, std::ios_base::openmode which) override {
  return seekoff(sp - pos_type(off_type(0)), std::ios_base::beg, which);
}

Also, the implementation of seekoff did not work for me in the case of other directions. Consider amending it by the following:

pos_type seekoff(off_type off,
                 std::ios_base::seekdir dir,
                 std::ios_base::openmode which = std::ios_base::in) override {
  if (dir == std::ios_base::cur)
    gbump(off);
  else if (dir == std::ios_base::end)
    setg(eback(), egptr() + off, egptr());
  else if (dir == std::ios_base::beg)
    setg(eback(), eback() + off, egptr());
  return gptr() - eback();
}

Upvotes: 12

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