覆盖 istream get(char)

Question

I'm implementing a few very specific I/O streams, one of them, an input stream that has data available only sometimes (think about some kind of socket that you poll for data while it is open). My streams are based on a class that inherits from basic_iostream and internally use a buffer that inherits from basic_filebuf. So my filebuf has a special way of signalling that there's no data available when underflow is called. I thought it's a good idea, because technically it's not EOF, unless the channel really gets closed. This status basically says - "there's no data now, but the channel is open, so you can keep asking, it might arrive eventually".

The problem is traits_type::eof() is checked inside get(char) function which does not exactly look like it was supposed to be overriden (maybe I'm wrong).

But anyway - what would be the best way of signalling such state? If such state occurs, I would know about it in my buffer's underflow method...

Answer 1

For this propose you can use a user defined char_traits , and put it as the template parameter to your basic_filebuf ie something like:

#include <fstream>
#include <limits>

struct my_char_traits : public std::char_traits<char> 
{
  typedef std::char_traits<char>::int_type int_type;
  static constexpr int_type eof() noexcept 
  {
    return std::numeric_limits<int_type>::min();
  }
};

class my_buff: std::basic_filebuf<char, my_char_traits>
{
};