C++; mirror and pass through from std::basic_istream<>?

Question

GIVEN:

A class MyStream derived of std::basic_istream<> contains a pointer subject to a std::basic_istream<> object. It shall respond to tellg() and read() with modified content from the corresponding responses of subject.

template <class T> MyStream :
   public std::basic_istream<typename T::char_type, typename T::traits_type> {

   std::basic::istream<...>*   subject;

   ...
};

PROBLEM: The functions tellg(), seekg() and read() as well as the status-flag functions are not virtual.

QUESTION: How can a MyStream object pass tell, seek, and read to subject, forward the response to the caller and modifiy the status flags so that they correspond to the flags of subject?

The trick is to use a different / derived [`streambuf`](https://en.cppreference.com/w/cpp/io/basic_streambuf) in your derived stream class. [SO: What exactly is streambuf? How do I use it?](https://stackoverflow.com/a/8117182/7478597) — Scheff's Cat, Jun 15 '19 at 08:20

score 1 · Accepted Answer · edited Jun 16 '19 at 01:43

1

Use something like this:

template <typename T> struct MyStream : public std::basic_istream<typename T::char_type, typename T::traits_type> {
   struct rdbuf_impl : public std::basic_streambuf<typename T::char_type, typename T::traits_type> {
       // overwrite what you need
       std::basic::istream<...>* subject;

       // for tellg passthru (an example)
       pos_type seekoff( off_type off, std::ios_base::seekdir dir,
                      std::ios_base::openmode which = ios_base::in | ios_base::out ) overwrite {
           return subject->pubseekoff(off, dir, which);
       }
   };
   MyStream(std::basic::istream<...>* subject) {
       auto v = new rdbuf_impl(subject);
       rdbuf(v); // set associated stream buffer 'v' in 'this'.
   }
};

EDIT: Let's consider tellg method. Looking at definition of class basic_istream::tellg (https://en.cppreference.com/w/cpp/io/basic_istream/tellg) - it is written, that tellg will call rdbuf()->pubseekoff(0, std::ios_base::cur, std::ios_base::in). Documentation for basic_streambuf::pubseekof (https://en.cppreference.com/w/cpp/io/basic_streambuf/pubseekoff) mentions, that it will call seekoff(off, dir, which). seekoff is virtual in basic_streambuf class and you can overwrite it.

edited Jun 16 '19 at 01:43

Frank-Rene Schäfer

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answered Jun 15 '19 at 08:23

Radosław Cybulski

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Would you, please, elaborate on your solution? How can I forward calls to `tellg()` etc to `subject`? How is the state of `subject` reflected in the status bits of a `MyStream`? – Frank-Rene Schäfer Jun 15 '19 at 09:58
Sorry, I undid the checkmark, because, I can still not see how the flags and `read()` function could be handled. It currently seems to be insanely complicated. Why would I need a buffer and cannot pass the data through without intermediate storage? – Frank-Rene Schäfer Jun 15 '19 at 21:52
You stated problem, that `tellg` and friends are not virtual and how to pass information around. I gave you a solution in form of an example solving `tellg` issue. Others are solved similarly. It is complicated due to a design of C++ library, which is like 20 years old, if not older. – Radosław Cybulski Jun 16 '19 at 08:35
It's a bitter pill, but I guess I have to swallow it--you're right. (There were marvelous library designs made 40 years ago, so no excuse to the I/O-library developers ... or maybe, I am missing the point of brilliance). – Frank-Rene Schäfer Jun 17 '19 at 08:50

C++; mirror and pass through from std::basic_istream<>?

1 Answers1