Question

I want to write a simple istream object, that would simply transform another istream.

I want to only implement readline (which would read a line from the original stream, would process it, and return the processed line), and have some generic code that upon read would use my read line, cache it, and give the required amount of bytes as output.

Is there any class that would allow me to do that?

For example

struct mystream : istreamByReadLine {
  istream& s;
  mystream(istream& _s):s(_s){}
  virtual string getline() {
    string line;
    getline(s,line);
    f(line);
    return line;
  }
}

class istreamByReadLine : istream {
  ... // implementing everything needed to be istream compatible, using my
  ... // getline() virtual method
}

Answer 1

Have you looked at boost.iostreams? It does most of the grunt work for you (possibly not for your exact use case, but for C++ standard library streams in general).